Hansen et al paper: 3+ meters SLR by 2100

[AbruptSLR]

This is a re-post from the Antarctic folder:

The linked reference compares paleo ice core data with a coupled climate model projections to that Southern Ocean deep convection can drive Antarctic multidecadal warming events.  The projections indicate that such convection driven warming events must be preconditioned by: (1) heat accumulation in the depth Southern Ocean (which is occurring now); (2) changes in wind and/or sea ice patterns (which have been projected in the near future); and (3) fast sea-ice-albedo feedback.
While Hansen et al (2015) projects this type of deep convective ocean behavior, most other AR5 model projections do not; thus it is possible that in the future that: (a) initially the forecast increase in snow will fall more on sea ice than on land, where it will contribute to SLR; and (b) after a few decades of the warming event the sea ice extent will be reduced and the increased precipitation will fall more frequently as rain (rather than snow) that contributing the hydrofracturing and abrupt SLR.

J.B. Pedro, T. Martin, E. J. Steig, M. Jochum, W. Park & S.O. Rasmussen (20 February 2016), "Southern Ocean deep convection as a driver of Antarctic warming events", Geophysical Research Letters, DOI: 10.1002/2016GL067861

http://onlinelibrary.wiley.com/doi/10.1002/2016GL067861/abstract

Abstract: "Simulations with a free-running coupled climate model show that heat release associated with Southern Ocean deep convection variability can drive centennial-scale Antarctic temperature variations of up to 2.0 °C. The mechanism involves three steps: Preconditioning: heat accumulates at depth in the Southern Ocean; Convection onset: wind and/or sea-ice changes tip the buoyantly unstable system into the convective state; Antarctic warming: fast sea-ice–albedo feedbacks (on annual–decadal timescales) and slow Southern Ocean frontal and sea-surface temperature adjustments to convective heat release (on multidecadal–century timescales) drive an increase in atmospheric heat and moisture transport toward Antarctica. We discuss the potential of this mechanism to help drive and amplify climate variability as observed in Antarctic ice-core records."

Caption for second image: "Figure S2. Map showing the surface-air-temperature (SAT) anomaly during stage 2 (cf. Figure 3d). Circles mark locations of ice-core records. Color-coding of the circles depicts the maximum lagged correlation coefficient of modeled local SAT with SAT over the convection area (black cross in Weddell Sea). Lower panels show time series of modeled SAT anomalies at selected ice-core sites (red) together with the SAT anomaly over the convection region (black). Note different y-axis scaling for red lines."

[AbruptSLR]

Per the linked NASA website, the draft Hansen et al. (2015) paper has now been accepted for publication by Atmos. Chem. Phys. and will soon be Hansen et al (2016):

Hansen, J., M. Sato, P. Hearty, R. Ruedy, M. Kelley, V. Masson-Delmotte, G. Russell, G. Tselioudis, J. Cao, E. Rignot, I. Velicogna, E. Kandiano, K. von Schuckmann, P. Kharecha, A.N. LeGrande, M. Bauer, and K.-W. Lo, 2016: Ice melt, sea level rise and superstorms: Evidence from paleoclimate data, climate modeling, and modern observations that 2°C global warming is highly dangerous. Atmos. Chem. Phys., in press, doi:10.5194/acpd-15-20059-2015

http://pubs.giss.nasa.gov/abs/ha04710s.html

[sidd]

That Pedro paper has Rasmussen as co author, and I have referred to another paper by Rasmussen in the AMOC thread. There is much to be gained by reading those two in parallel. The second paper is

DOI:10.1038/srep20535

[AbruptSLR]

In the linked article, Peter Gleick associates rapid Arctic warming (AA) and reduced Arctic Sea Ice Extent to extreme weather events.  As the cool ocean water pool of Greenland ice sheet meltwater is contributing to these North Atlantic 'bomb cyclones', these extreme weather events may be the tip of the iceberg for the 'storms of my grandchildren' described by Hansen as both NH & SH ice sheet melting accelerates:

http://www.independent.co.uk/environment/climate-change/arctic-warming-rapidly-increasing-temperatures-are-possibly-catastrophic-for-planet-climate-a6896671.html

“Massive storms, sometimes called ‘bomb cyclones’, are created when warm air from the Atlantic and cold air from the Arctic combine. Just this season, massive flooding associated with one of these storms struck the United Kingdom producing record rainfall.
“There may be connections between out-of-season strong tornados in the central US and these new storm patterns. The central US was hit by unusual numbers of tornados in December 2015 causing billions in damage and many deaths.
“Part of the science on this suggests that as the Arctic warms faster, the difference in temperature between the mid-latitudes and the Arctic region decreases. This, in turn, affects storm tracks and the location and strength of the jet stream.”
The reduced amount of white ice also means less energy from the Sun is reflected back into space. Water, which is darker than ice, absorbs more energy and increases the rate of warming.
Dr Gleick said one of the reasons he was interested in Arctic sea ice was the ongoing drought in California."


See also:
https://en.wikipedia.org/wiki/Explosive_cyclogenesis

[AbruptSLR]

I believe that the Artic Amplification and ENSO/PDO interactions described by Robert Scribbler in the following linked article, provides color to many of the extreme weather projections that Hansen has projected with continued global warming:

http://robertscribbler.com/2016/02/25/as-a-titanic-el-nino-begins-to-fade-what-fresh-trouble-will-a-record-warm-world-bring/

Extract: "Though we may see these two events — the failure of El Nino to provide heavy rains to the US West Coast, and the massive northward pulses of storms, heat and moisture hitting the North Atlantic — as unrelated, the twain patterns appear to be linked to an ongoing polar amplification. Overall, heat within the Arctic has tended to weaken the Northern Hemisphere Jet Stream over these two zones. And even during El Nino, when the Jet would have typically strengthened, we have continued to see high amplitude wave patterns forming over these regions.

But as El Nino weakens and the Equator cools, the Jet Stream would tend to slow even more. Such an atmospheric state would tend to further exaggerate already significant Jet Stream wave patterns — transferring still more low-Latitude heat poleward. In addition, the ocean gyres tend to speed up as El Nino fades or transitions to La Nina. The result is an amplified pulse of warmer waters emerging from southern Latitudes and entering the Arctic.

It’s for these combined reasons — tendency to amplify south to north atmospheric heat transfer into the Arctic post El Nino and tendency to flush warmer waters toward Arctic Ocean zones during the same period that it appears we are entering a high risk time for potential new sea ice melts and possible related Greenland land ice melts during 2016 and 2017.

...

Finally, extreme above average sea surface temperatures are predicted to intensify over the Barents and Greenland seas through to end of Summer 2016. This is an area to watch. The added ocean heat would tend to pull the Jet Stream northward over Eastern Europe and Western Russia — generating risk of heatwaves and drought for this region even as Central Asia fell under risk of floods. Long range CFS precipitation and temperature model runs for Europe have not yet picked up this risk. However, given the intensity of heat predicted for Barents sea surfaces and the related tendency of warmth over oceans and in the far north to influence the formation of blocking patterns, heat domes, and high amplitude troughs, it’s worth keeping a weather eye on the situation."

[Lennart van der Linde]

Hansen on his paper that will be published soon:
http://csas.ei.columbia.edu/2016/03/22/ice-melt-sea-level-rise-and-superstorms-the-threat-of-irreparable-harm/

He ends with:
"One final point. This is a complex story, but one with important practical implications.  I find that the public sometimes misinterprets our science discussions, how research is done.  Skepticism is the lifeblood of science.  You can be sure that many scientists, indeed most scientists, will find some aspects in our long paper that they would interpret differently.  That’s entirely normal.  It takes time for conclusions to be agreed upon and details sorted out.

So after you have talked to a scientist about this topic, ask him or her a final question. Do you agree that we have reached a dangerous situation?  Do you think we may be approaching a point of no return, a situation in which our children inherit a climate system undergoing changes that are out of their control, changes that will cause them irreparable harm? That’s the bottom line."

[Wipneus]

Thanks, actually it is published today:

Received: 11 Jun 2015 – Published in Atmos. Chem. Phys. Discuss.: 23 Jul 2015
Revised: 17 Feb 2016 – Accepted: 18 Feb 2016 – Published: 22 Mar 2016

http://www.atmos-chem-phys.net/16/3761/2016/acp-16-3761-2016.html

[Lennart van der Linde]

Thanks, actually it is published today:

As soon as soon can be 

[Laurent]

What the gardian says about it :
Climate guru James Hansen warns of much worse than expected sea level rise
https://www.theguardian.com/science/2016/mar/22/sea-level-rise-james-hansen-climate-change-scientist

Hansen is also conservative ! Thought he is right there is not enough data to be sure, the trend is less than a 10 year doubling. Should we wait for more data ? ... personally I would say no !

[Anne]

Video abstract

[AbruptSLR]

What the gardian says about it :
Climate guru James Hansen warns of much worse than expected sea level rise
https://www.theguardian.com/science/2016/mar/22/sea-level-rise-james-hansen-climate-change-scientist

Hansen is also conservative ! Thought he is right there is not enough data to be sure, the trend is less than a 10 year doubling. Should we wait for more data ? ... personally I would say no !

It is not very comforting to realize how conservative Hansen has been with regards to:

(a) ECS likely being 4.1C instead of the 3.0C assumed by Hansen.
(b) Not assuming an aggressive clean-up of anthropogenic aerosol emissions.
(c) Not assuming a continuation of the current high anthropogenic methane emissions.
(d) Not assuming a relatively rapid acceleration on non-linear positive feedback mechanisms.

I realize that if Hansen we less conservative, fewer people would listen to him, so I am not criticizing him; but that doesn't mean that we are all safer, just because we close our eyes to the full range of risks.

[Richard Rathbone]

I hardly think a 10 year doubling time can be described as not rapid.

[jai mitchell]

When we reach <1,000 Km^2 of SIE in Sept of 2025 (at latest?) we will find arctic regional warming to be unprecedented in the holocene.  This will rapidly accelerate Greenland melt and AMOC freshwater lensing shutdown much more rapidly than the rate in the Hansen et al paper.

[sidd]

Mann is more confident that I. From the Guardian article:


"I’m unconvinced that we could see melting rates over the next few decades anywhere near his exponential predictions, and everything else is contingent upon those melting rates being reasonable."

[AbruptSLR]

I hardly think a 10 year doubling time can be described as not rapid.

If you think that Pollard, DeConto & Alley 2015, paleo-analysis is relevant to modern conditions the a 10-year doubling time for the WAIS is likely conservative (assuming both cliff failures and hydrofracting starts before 2050):

http://www.sciencedirect.com/science/article/pii/S0012821X14007961

Abstract
"Geological data indicate that global mean sea level has fluctuated on 10³ to 10⁶ yr time scales during the last ∼25 million years, at times reaching 20 m or more above modern. If correct, this implies substantial variations in the size of the East Antarctic Ice Sheet (EAIS). However, most climate and ice sheet models have not been able to simulate significant EAIS retreat from continental size, given that atmospheric CO2 levels were relatively low throughout this period. Here, we use a continental ice sheet model to show that mechanisms based on recent observations and analysis have the potential to resolve this model–data conflict. In response to atmospheric and ocean temperatures typical of past warm periods, floating ice shelves may be drastically reduced or removed completely by increased oceanic melting, and by hydrofracturing due to surface melt draining into crevasses. Ice at deep grounding lines may be weakened by hydrofracturing and reduced buttressing, and may fail structurally if stresses exceed the ice yield strength, producing rapid retreat. Incorporating these mechanisms in our ice-sheet model accelerates the expected collapse of the West Antarctic Ice Sheet to decadal time scales, and also causes retreat into major East Antarctic subglacial basins, producing ∼17 m global sea-level rise within a few thousand years. The mechanisms are highly parameterized and should be tested by further process studies. But if accurate, they offer one explanation for past sea-level high stands, and suggest that Antarctica may be more vulnerable to warm climates than in most previous studies."

[sidd]

" ... sea level has fluctuated on 103 to 106 yr time scales ..."

I think that reads 1000 to 1000000 yr time scales (the exponentiation caret seems to be missing )

[AbruptSLR]

" ... sea level has fluctuated on 103 to 106 yr time scales ..."

I think that reads 1000 to 1000000 yr time scales (the exponentiation caret seems to be missing )

sidd,

Of course you are right, and I have edited the original post.

Thanks,
ASLR

But while I am posting, let me add that if ECS really is 4.1C (as Sherwood 2014 indicates), and if Asia substantially reduces its aerosol emissions over then next couple of decades, and if we keep using methane as a bridge to a sustainable future; then cliff failures and hydrofracturing of key marine glaciers in the WAIS before 2050 is very likely.

[AbruptSLR]

I think it is nice to have a more complete reference and a couple of images from the paper when possible.  The first image shows Hansen et al 2016's latest climate response function that indicates that the effective ECS (from 2000 to 2100) will likely be higher than the actual ECS due to activation of non-linear positive feedback mechanisms.  The second image shows that all of the AIS might (or might not) already be following a 5-year doubling time for ice mass loss rate.

James Hansen, Makiko Sato, Paul Hearty, Reto Ruedy, Maxwell Kelley, Valerie Masson-Delmotte, Gary Russell, George Tselioudis, Junji Cao, Eric Rignot, Isabella Velicogna, Blair Tormey, Bailey Donovan, Evgeniya Kandiano, Karina von Schuckmann, Pushker Kharecha, Allegra N. Legrande, Michael Bauer, and Kwok-Wai Lo (2016), "Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 °C global warming could be dangerous", Atmos. Chem. Phys., 16, 3761-3812, doi:10.5194/acp-16-3761-2016

http://www.atmos-chem-phys.net/16/3761/2016/acp-16-3761-2016.html

Abstract: "We use numerical climate simulations, paleoclimate data, and modern observations to study the effect of growing ice melt from Antarctica and Greenland. Meltwater tends to stabilize the ocean column, inducing amplifying feedbacks that increase subsurface ocean warming and ice shelf melting. Cold meltwater and induced dynamical effects cause ocean surface cooling in the Southern Ocean and North Atlantic, thus increasing Earth's energy imbalance and heat flux into most of the global ocean's surface. Southern Ocean surface cooling, while lower latitudes are warming, increases precipitation on the Southern Ocean, increasing ocean stratification, slowing deepwater formation, and increasing ice sheet mass loss. These feedbacks make ice sheets in contact with the ocean vulnerable to accelerating disintegration. We hypothesize that ice mass loss from the most vulnerable ice, sufficient to raise sea level several meters, is better approximated as exponential than by a more linear response. Doubling times of 10, 20 or 40 years yield multi-meter sea level rise in about 50, 100 or 200 years. Recent ice melt doubling times are near the lower end of the 10–40-year range, but the record is too short to confirm the nature of the response. The feedbacks, including subsurface ocean warming, help explain paleoclimate data and point to a dominant Southern Ocean role in controlling atmospheric CO2, which in turn exercised tight control on global temperature and sea level. The millennial (500–2000-year) timescale of deep-ocean ventilation affects the timescale for natural CO2 change and thus the timescale for paleo-global climate, ice sheet, and sea level changes, but this paleo-millennial timescale should not be misinterpreted as the timescale for ice sheet response to a rapid, large, human-made climate forcing. These climate feedbacks aid interpretation of events late in the prior interglacial, when sea level rose to +6–9 m with evidence of extreme storms while Earth was less than 1 °C warmer than today. Ice melt cooling of the North Atlantic and Southern oceans increases atmospheric temperature gradients, eddy kinetic energy and baroclinicity, thus driving more powerful storms. The modeling, paleoclimate evidence, and ongoing observations together imply that 2 °C global warming above the preindustrial level could be dangerous. Continued high fossil fuel emissions this century are predicted to yield (1) cooling of the Southern Ocean, especially in the Western Hemisphere; (2) slowing of the Southern Ocean overturning circulation, warming of the ice shelves, and growing ice sheet mass loss; (3) slowdown and eventual shutdown of the Atlantic overturning circulation with cooling of the North Atlantic region; (4) increasingly powerful storms; and (5) nonlinearly growing sea level rise, reaching several meters over a timescale of 50–150 years. These predictions, especially the cooling in the Southern Ocean and North Atlantic with markedly reduced warming or even cooling in Europe, differ fundamentally from existing climate change assessments. We discuss observations and modeling studies needed to refute or clarify these assertions."

[Sigmetnow]

The New York Times on the Hansen paper:

Scientists Warn of Perilous Climate Shift Within Decades, Not Centuries

The nations of the world agreed years ago to try to limit global warming to a level they hoped would prove somewhat tolerable. But leading climate scientists warned on Tuesday that permitting a warming of that magnitude would actually be quite dangerous.

The likely consequences would include killer storms stronger than any in modern times, the disintegration of large parts of the polar ice sheets and a rise of the sea sufficient to begin drowning the world’s coastal cities before the end of this century, the scientists declared.

“We’re in danger of handing young people a situation that’s out of their control,” said James E. Hansen, the retired NASA climate scientist who led the new research. The findings were released Tuesday morning by a European science journal, Atmospheric Chemistry and Physics.

http://www.nytimes.com/2016/03/23/science/global-warming-sea-level-carbon-dioxide-emissions.html

[Richard Rathbone]

The important thing about this paper is that it is demonstrating that sea level rise on the multi-meter scale cannot be ruled out on a 50-year timescale and its a pretty thorough debunking of those (e.g. see IPCC AR5) that think it can be. Their GCM models aren't good enough, and their data isn't good enough.

From my reading of the paper, I don't think it can be ruled out on a 25-year timescale, but it won't take much more data to do that and Hansen is probably wise not to push his headline number shorter than 50 years. You can see very fast exponentials in noisy data if you go looking for them, but you are claiming the signal will appear out of the noise in the near future when you do and you end up looking rather silly when it doesn't. (e.g. Wadhams and his SIPN prediction)

[AbruptSLR]

The important thing about this paper is that it is demonstrating that sea level rise on the multi-meter scale cannot be ruled out on a 50-year timescale and its a pretty thorough debunking of those (e.g. see IPCC AR5) that think it can be. Their GCM models aren't good enough, and their data isn't good enough.

From my reading of the paper, I don't think it can be ruled out on a 25-year timescale, but it won't take much more data to do that and Hansen is probably wise not to push his headline number shorter than 50 years. You can see very fast exponentials in noisy data if you go looking for them, but you are claiming the signal will appear out of the noise in the near future when you do and you end up looking rather silly when it doesn't. (e.g. Wadhams and his SIPN prediction)

The authors of this paper (including Eric Rignot) are fully aware that current ice sheet models are inadequate to demonstrate based on physics that multi-meter SLR will occur by 2100; nevertheless, their assumption of different abrupt ice sheet mass loss scenarios are not based merely on exponential curve fitting to noisy data (that is just a tip of the iceberg) but rather they base these assumed ice mass loss scenarios on: (a) paleo-evidence; (b) expert understanding of both physics and models; and (c) an understanding of just how much AR5 science errs on the side of least drama.  With CO2 emissions (let alone CO2-e emissions) well over ten times those of the PETM; with evidence that the collapse of the Amundsen Sea Embayment marine glaciers is unavoidable at this point; and with Hansen et al (2016) limited climate model results; it is clear to me that we are only discussing how soon and how fast the WAIS collapse will occur; not whether we can stop it.

The ACME state-of-the-art ESM is tasked with making preliminary assessments of ice sheet mass loss by the end of 2017, and their final assessment is due by the end of 2024, which should shine more light on this complex topic; and until then I like to point-out that we all remain at risk; while you appear to lean towards a wait-and-see approach.

Previously, I have pointed out that Hansen et al (2016) also ignore the risks that ECS may be 4.1C (per Sherwood 2014), that aerosol clean-up may be more aggressive than the IPCC forcing scenarios assume; that methane emissions are currently rising at the extreme high end of the RCP 8.5 scenario; and that non-linear positive feedbacks could accelerate faster than they assume.  However, there are many other risks factors that could lead to a partial collapse of the WAIS this century including:
(a) The ice shelves in the Amundsen Sea Embayment are retreating rapidly due to the warm CDW, any may well present large areas of marine glacial faces (much larger than for Jakobshavn) to cliff failures by 2035.
(b) Surface ice melt of coastal portions of the WAIS could accelerate significantly by 2050, to the point that hydrofracturing could become a significant contributor to ice mass loss in the WAIS by that time.
(c) Before 2050 it is likely that more snow will fall on the Antarctic sea ice than predicted by AR5 models; while after 2050 there will likely be a higher chance of coastal rainfall (that could contribute to hydrofracturing) than considered by the AR5 models.

[Bruce Steele]

ASLR, Thank you for providing the open source link to the new Hansen et al paper.
It covers many topics you have highlighted over the last three years here on the ASIB. 
 The slowdown or potential shutdown of the North Atlantic Deep Water Formation 
and similar responses for the Antarctic Deep Water Formation with extreme Sea level rise ( 5 meters ) results in some rather counterintuitive results. 
 1. Globally averaged surface air temperatures may reverse current positive trends and go negative .
 2. Antarctic ice melt may increase in spite of decreasing surface temperatures because the fresh water lens will cap warm water transport to the surface where it otherwise would cool and instead direct it toward the base of the ice sheets and promote additional melt.

 It should be stressed that a concurrent slowdown or cessation of deep water formation in the North Atlantic and southern ocean bottom water formation would also shut down both oxygen transport into bottom waters as well as shutting down the major physical process transporting carbon into the deep ocean. 
So if I understand properly the potential results ,atmospheric CO2 would continue  to increase do to a breakdown in the ocean carbon sinks even as surface atmospheric temperatures drop. The temperature drops might slow down or reverse melt in the northern hemisphere but the increase in atmosphere CO2 would remain for thousands of years none the less. These are positive feedback responses I hadn't imagined before reading this new paper. 
 I haven't finished the entire paper ( 52 pages ) yet but the carbon sink responses  are my ideas and not something I have seen covered in the paper. A shutdown in deep and bottom water processes holds an enormously dangerous threat to all terrestrial and ocean lifeforms ...  Rapid sea level rise and the resultant fresh water lens formation is therefor an existential risk to a vast majority of life on the planet. Makes coastal city inundation a secondary rather than a primary threat of SLR.    

[AbruptSLR]

It should be stressed that a concurrent slowdown or cessation of deep water formation in the North Atlantic and southern ocean bottom water formation would also shut down both oxygen transport into bottom waters as well as shutting down the major physical process transporting carbon into the deep ocean.
So if I understand properly the potential results ,atmospheric CO2 would continue  to increase do to a breakdown in the ocean carbon sinks even as surface atmospheric temperatures drop. The temperature drops might slow down or reverse melt in the northern hemisphere but the increase in atmosphere CO2 would remain for thousands of years none the less. These are positive feedback responses I hadn't imagined before reading this new paper.
 I haven't finished the entire paper ( 52 pages ) yet but the carbon sink responses  are my ideas and not something I have seen covered in the paper. A shutdown in deep and bottom water processes holds an enormously dangerous threat to all terrestrial and ocean lifeforms ...  Rapid sea level rise and the resultant fresh water lens formation is therefor an existential risk to a vast majority of life on the planet. Makes coastal city inundation a secondary rather than a primary threat of SLR.   

Bruce,

Additionally, while dimethyl sulphide (DMS) is currently helping to fight global warming by contributing to negative forcing associated with cloud formation; with continued ocean acidification, the Southern Ocean in-particular will have decreased DMS emissions, which will lead to reduced cloud cover in Antarctica; which will lead to an acceleration in surface ice melting; which will lead to an acceleration of hydrofracturing induced calving events for Antarctic marine glaciers.

Best,
ASLR

[AbruptSLR]

With regards to my last post I provide the following references on DMS & the Southern Ocean, showing up to a 40% reduction in DMS emissions from the Southern Ocean due to global warming:

S. Kloster, K. D. Six, J. Feichter, E. Maier-Reimer, E. Roeckner, P. Wetzel, P. Stier & M. Esch (25 July 2007), "Response of dimethylsulfide (DMS) in the ocean and atmosphere to global warming", JGR, DOI: 10.1029/2006JG000224


http://onlinelibrary.wiley.com/doi/10.1029/2006JG000224/abstract

Abstract: "A global coupled ocean-atmosphere modeling system is applied in a transient climate simulation to study the response to global warming of Dimethylsulfide (DMS) in the ocean, the DMS flux to the atmosphere, and the resulting DMS concentrations in the atmosphere. The DMS production and consumption processes in the ocean are linked to plankton dynamics simulated in the marine biogeochemistry model HAMOCC5.1, embedded in an ocean general circulation model (MPI-OM). The atmospheric model ECHAM5 is extended by the microphysical aerosol model HAM, treating the sulfur chemistry in the atmosphere and the evolution of microphysically interacting internally and externally mixed aerosol populations. For future conditions (2000–2100) we assume greenhouse gas concentrations, aerosol and aerosol precursor emissions according to the SRES A1B scenario. We analyzed the results in terms of simulated changes between the period 1861–1890 and 2061–2090. For the global annual mean DMS sea surface concentration and the DMS flux we found a reduction by 10%. The DMS burden in the atmosphere is reduced by only 3%, owing to a longer lifetime of DMS in the atmosphere in a warmer climate (+7%). Regionally the response and the underlying mechanisms are quite inhomogeneous. The largest reduction in the DMS sea surface concentration is simulated in the Southern Ocean (−40%) caused by an increase in the summer mixed layer depth, leading to less favorable light conditions for phytoplankton growth. In the mid and low latitudes DMS sea surface concentrations are predominantly reduced due to nutrient limitation of the phytoplankton growth through higher ocean stratification and less transport of nutrients into the surface layers."

Amrani, A., Said-Ahmad, W., Shaked, Y., & Kiene, R. P. 2014 - In Press Sulfur isotope homogeneity of oceanic DMSP and DMS. Proceedings of the National Academy of Sciences, vol. 110 no. 46, 18413–18418, doi: 10.1073/pnas.1312956110


http://www.pnas.org/content/110/46/18413.full

Significance: "Oceanic emissions of volatile dimethyl sulfide (DMS) represent the largest natural source of biogenic sulfur to the global atmosphere, where it mediates aerosol dynamics and may affect climate. Sulfur isotope ratios (34S/32S) offer a way to estimate oceanic DMS contribution to aerosols. We used a unique method for the analysis of 34S/32S of DMS and its precursor, dimethylsulfoniopropionate (DMSP), in a range of marine environments. Surface water collected from six different ocean provinces revealed a remarkable consistency in 34S/32S ratios of DMS and DMSP ranging between +18.9 and +20.3‰. The 34S/32S of oceanic DMS flux to the atmosphere is thus relatively constant and distinct from anthropogenic sources of atmospheric sulfate, thereby enabling estimation of the DMS contribution to aerosols."

[AbruptSLR]

I very much enjoyed the following linked Slate article about Hansen et al 2016, particularly the extracted quote about Eric Rignot's thoughts on the matter of the true risks of abrupt sea level rise, ASLR:


http://www.slate.com/blogs/the_slatest/2016/03/22/james_hansen_sea_level_rise_climate_warning_passes_peer_review.html


Extract: "Hansen’s co-author Eric Rignot said in an email that the exploration of such extreme scenarios was justified “because they are not unlikely, and they are more likely than the more conservative scenarios branded by [United Nations] reports.”

“Ice sheet loss is non linear by nature,” said Rignot. “You push the ice sheet one way, they do not react; you push them more, they start reacting; you keep pushing and they fall apart. … If we get there, we won't be able to fix it.”

Kim Cobb, a climate scientist specializing in ancient climate change, agrees that the Hansen study is useful mostly because it explores the worst-case scenario. “My bet is on non-linearities kicking in that we cannot yet measure adequately,” Cobb said in an email. “In that way I think it’s important to explore the upper limits.”"

[Michael Hauber]

The possibility of rapid sea level rise hinges on the assumption of an exponential increase in melting rate.  While such a melting rate cannot be ruled out, the evidence for such an assumption is very thin, and should be considered a worst case scenario.  It is unreasonable to claim that this scenario is likely based on the evidence presented by Hansen in this paper, which is more of a 'what if' exercise exploring the consequences of rapid sea level rise rather than a convincing argument for rapid sea level rise.

Hansen himself states that 'the empirical data are too brief to imply a characteristic time for ice sheet loss or to confirm our hypothesis...cause exponential ice mass loss up to several meters of sea level'.

The only real evidence for the possibility of rapid sea level rise provided in this paper is a reference to Blanchon et al Which Hansen claims provides evidence of around 3 meters of sea level rise within a period decades at a sea level comparable to modern sea levels.  This paper studies sea level on a reef at one location.  The possibility of a sea level rise due to a local tectonic shift cannot be ruled out, although the location is very stable, so presumably such a possibility is low.  When discussing the speed of the sea level jump this paper states:

'Although the precision of these ages precludes any direct measurement of the rise rate involved in the jump, it was most likely similar to rates that caused ecologically sudden demise and back-stepping of Caribbean reefs during the last deglaciation'.

These rates exceeded 3.6  meters/year, and were during the last deglaciation, at sea levels much lower than today's sea level, and when there were much larger ice sheets to fuel rapid sea level rise.  (I point this out to support my comment that there is one location at which we have measured sea level rise at a rapid rate for sea levels similar to today)

[Lennart van der Linde]

These rates exceeded 3.6 meters/year, and were during the last deglaciation, at sea levels much lower than today's sea level, and when there were much larger ice sheets to fuel rapid sea level rise. (I point this out to support my comment that there is one location at which we have measured sea level rise at a rapid rate for sea levels similar to today)

Of course you mean 3.6 meters/century. Hansen et al themselves also point out there was more ice to melt during deglaciations than during interglacials. They also point out that the forcing now is much greater than back then, which seems to be their main reason for thinking SLR may become as rapid as during the last deglaciation.

[Lennart van der Linde]

Also see this new comment by Hansen on scientific reticence;
http://csas.ei.columbia.edu/2016/03/24/dangerous-scientific-reticence/

'A criticism of our paper that may warrant response is that the ice melt rates that we assumed were “unrealistic”. In fact it is certain that multi-meter per century melt rates have occurred many times in Earth’s history, spurred by much weaker forcings than the human-made forcing. We presented evidence in our paper that rapid sea level rise even occurred in late-Eemian, when there was less ice available for melt than today. Just this week a paper was published showing that the fastest natural increase of greenhouse gas climate forcing in the past 66 million years was at least 10 times slower than the human-made change. Unfortunately, the melt rates we talk about for the next several decades are very realistic, and we are already seeing expected response to current melt rates.'

[AbruptSLR]

In the Antarctic folder I have provided a considerable amount of evidence to indicate that the risk of abrupt SLR from the WAIS this century is very real, and Hansen et al 2016's scenarios only represent a rough (but reasonable) approximation of these risks.  The following article by Scribbler summarizes the SLR scenario presented in the Hansen et al 2016 paper; but no one doubts that further work in the area of ASLR is still needed and researchers such as Rignot, Alley, DeConto and Pollard (to name a few) are hot on this trail of such non-linear research (not to mention the efforts of the ACME program that may take until 2024 before it has a chance of catching-up to the more approximate insights of the Hansen et al 2016 paper):

http://robertscribbler.com/2016/03/23/dr-james-hansen-human-warming-pushing-seas-to-exponentially-rise-by-several-meters-this-century/

Extract: "Grounding glaciers and ice shelves are, at first, weakened by slow but ramping melt rates. Eventually, the glaciers and shelves collapse due to the weakening process of melt which leads to a surge of previously buttressed ice sliding out into the oceans. As more fresh melt water expands over the ocean surface, it traps heat into deeper layers of the water column near the submerged glacial faces. So initial melt produces an amplifying feedback that delivers more ocean heat to the ice and, in turn, results in more ice rushing out into the North Atlantic or the Southern Ocean.
It is this mechanism that Hansen and colleagues fear will come into play over the course of the 21st Century. Their paper identifies a risk that such a mechanism could set up 5, 10, or 20 year melt doubling times for Greenland, West Antarctica or both this Century. A new perspective from some of the world’s top scientists that assumes the risk of non linear melt is high enough to present a major concern. As an example, under a 10 year doubling time, the current approximate 3 mm per year sea level rise would double to 6 mm per year by 2026, 12 mm per year by 2036, 2.4 cm per year by 2046, and nearly 5 cm per year by 2056.
Doubling times in non linear events often don’t fit a pure exponential curve — instead tending to follow a series of spikes and recessions with major transitional events coming at the end of any ‘curve.’ But Hansen’s particular perspective is useful given the fact that current rates of sea level rise do not appear to be following a linear pattern and due to the fact that the mechanism for large, Heinrich Event type glacial melt spikes is becoming more supported in the observational science."

[AbruptSLR]

It is all too easy for some commenters on this forum to forget the many central contributions that James Hansen has made towards recognizing climate change risks. The linked "The Guardian" article acknowledges his many contributions and points-out that now that Hansen et al (2016) paper exists in the peer-reviewed canon of literature that will be considered by AR6, is it now up to skeptics to apply the scientific method to determine whether this seminal paper adds to Hansen's storied climate career, or not.

http://www.theguardian.com/environment/planet-oz/2016/mar/24/has-veteran-climate-scientist-james-hansen-foretold-the-loss-of-all-coastal-cities-with-latest-study

Extract: "“You can be sure that many scientists – indeed most scientists – will find some aspects in our long paper that they would interpret differently,” Hansen says.

But he also commented that these disagreements shouldn’t be misinterpreted.

"I find that the public sometimes misinterprets our science discussions – how research is done. Scepticism is the lifeblood of science. It takes time for conclusions to be agreed upon and details sorted out. "

And so it’s in this spirit that I think Hansen’s paper should be taken.
Hansen and his co-authors have laid out their science and are proposing an idea that will live, die or evolve through the scientific method."

Whether or not March 2016 becomes another seminal moment in Hansen’s storied climate career only time, and a failure to reign in fossil fuel emissions, will likely tell."

[AbruptSLR]

First, I point out that Hansen et al 2016 is conservative in presenting GRACE satellite ice mass loss from the AIS, when they could have presented GRACE data for ice mass loss from the WAIS, such as the first attached image through December 2012, that shows a much higher rate of ice mass loss than that for all of the AIS.

Second, I provide the following quote from Hansen & Sato 2012, referring to the second attached image (which is H&S 2012 Figure 7), explaining the “wave” of a 6C effective ECS pulse:


Quote form Hansen & Sato 2012: "The equilibrium climate sensitivity for a positive (warming) from the Holocene state depends on the magnitude of the forcing. Hansen et al. (2008) conclude that the mean sensitivity over the entire range from the Holocene to a climate just warm enough to lose the Antarctic ice sheet is almost 6°C for doubled CO2, but most of the surface albedo feedback in that range is caused by loss of the Antarctic ice sheet. The decreasing amplitude of glacial-interglacial temperature oscillations between the late Pleistocene and Pliocene (Fig. 4b) suggests that the sensitivity is smaller as climate warms from the Holocene toward a Pliocene-like climate."

Therefore, it is reasonable to conclude that the 6 C "wave" on H&S Fig 7 is associated with the Antarctic ice sheet.  This is due to the fact that an abrupt ice mass loss from the WAIS cools the Southern Ocean, resulting in reduced radiation into space and consequently a pulse of higher effective ECS

Third, I note that while H&S 2012 assumes that the “fast feedback” ECS was only 3C, the following references indicate that it is likely well over 4C (see the third attached image from Sherwood et al 2014).  Also, the fourth image (from a Nature article about Sherwood et al 2014) shows that the high fast feedback ECS is due to increased deep atmospheric convection in the tropics that decrease cloud cover over the equatorial region.  Therefore, if both Hansen et al 2016 and Sherwood et al 2014 are correct, then the Polar Oceans will be relatively cool due to ice sheet mass loss, while the equatorial region will be warming rapidly, resulting in a severe temperature gradient from the equator to the poles (cause severe storms) & also causing a rapid expansion of the Hadley Cells poleward:


Sherwood et al (2014), which found that ECS cannot be less than 3C, and is likely currently in the 4.1C range. 


Sherwood, S.C., Bony, S. and Dufresne, J.-L., (2014) "Spread in model climate sensitivity traced to atmospheric convective mixing", Nature; Volume: 505, pp 37–42, doi:10.1038/nature12829

http://www.nature.com/nature/journal/v505/n7481/full/nature12829.html


In research with relevance to the Sherwood et.al. (2014) reference on the influence of tropical atmospheric convective mixing on equilibrium climate sensitivity, ECS, the linked reference (Tomassini et.al. 2014) projects values of ECS from "… a little over 3 °C to more than 10 °C". 

Tomassini, L., Voigt, A. and Stevens, B. (2014), "On the connection between tropical circulation, convective mixing, and climate sensitivity", Q.J.R. Meteorol. Soc.. doi: 10.1002/qj.2450

http://onlinelibrary.wiley.com/doi/10.1002/qj.2450/abstract


Furthermore, Tian (2015) indicates that the double-ITCZ bias constrains ECS to its high end (around 4.0C):

Tian, B. (2015), "Spread of model climate sensitivity linked to double-Intertropical Convergence Zone bias", Geophys. Res. Lett., 42, doi:10.1002/2015GL064119.


http://onlinelibrary.wiley.com/doi/10.1002/2015GL064119/abstract

[Laurent]

The second graph (1C Hansen & Sato 2012 ECS Effective vs Fast Feedbacks.png) is a bit too simple, what is the x axis at the max of the big bump for example. Do we have something similar from some other scientists ?

[Richard Rathbone]

The possibility of rapid sea level rise hinges on the assumption of an exponential increase in melting rate.  While such a melting rate cannot be ruled out, the evidence for such an assumption is very thin, and should be considered a worst case scenario.  It is unreasonable to claim that this scenario is likely based on the evidence presented by Hansen in this paper, which is more of a 'what if' exercise exploring the consequences of rapid sea level rise rather than a convincing argument for rapid sea level rise.

Hansen himself states that 'the empirical data are too brief to imply a characteristic time for ice sheet loss or to confirm our hypothesis...cause exponential ice mass loss up to several meters of sea level'.

The only real evidence for the possibility of rapid sea level rise provided in this paper is a reference to Blanchon et al Which Hansen claims provides evidence of around 3 meters of sea level rise within a period decades at a sea level comparable to modern sea levels.  This paper studies sea level on a reef at one location.  The possibility of a sea level rise due to a local tectonic shift cannot be ruled out, although the location is very stable, so presumably such a possibility is low.  When discussing the speed of the sea level jump this paper states:

'Although the precision of these ages precludes any direct measurement of the rise rate involved in the jump, it was most likely similar to rates that caused ecologically sudden demise and back-stepping of Caribbean reefs during the last deglaciation'.

These rates exceeded 3.6  meters/year, and were during the last deglaciation, at sea levels much lower than today's sea level, and when there were much larger ice sheets to fuel rapid sea level rise.  (I point this out to support my comment that there is one location at which we have measured sea level rise at a rapid rate for sea levels similar to today)

The exponential is not used because Hansen has evidence that feedback mechanisms will result in exponential response, but because it is a mathematically convenient forcing function that encompasses a wide variety of accelerating responses. You don't actually have to run the model again if you decide a different form is appropriate, just adjust the timings. As he puts it, you should view the progression of the model climate as a response to freshening sea water rather than time. If you think sufficient melt happens by 2040 to match his model in 2050, then the world in 2040 will look rather like his model in 2050.

There's indirect evidence for paleo rapid sea level rise as well as direct evidence. Thats where the boulders come in. Fast melt produces regional temperature changes of the sort which can be expected to drive storms of the sort that toss boulders like that where they have been tossed. Its also why there's much more attention to Greenland in the published version compared to the discussion version. The discussion version ignored Greenland on the basis that the amount of ice at risk of sudden collapse there was trivial compared to the Antarctic, but to get the climate response to drive the superstorms to toss the boulders, North Atlantic freshening has to be included. While I don't think the Antarctic melt he assumes is worst case (and the paper is all the stronger for it) I do think he is pushing the bounds for Greenland melt and the possibility for NADW shutdown remains rather weaker than the publicity surrounding the paper implies.

[AbruptSLR]

The second graph (1C Hansen & Sato 2012 ECS Effective vs Fast Feedbacks.png) is a bit too simple, what is the x axis at the max of the big bump for example. Do we have something similar from some other scientists ?

Laurent,
Figure 7 from Hansen & Sato (2012) is based on empirical interpretation of paleo-evidence discussed in Hansen et al (2011), see both references below.

Hansen, J.E., and M. Sato, 2012: Paleoclimate implications for human-made climate change. In Climate Change: Inferences from Paleoclimate and Regional Aspects. A. Berger, F. Mesinger, and D. Šijački, Eds. Springer, pp. 21-48, doi:10.1007/978-3-7091-0973-1_2.

Hansen, J., M. Sato, P. Kharecha, and K. von Schuckmann, 2011: Earth's energy imbalance and implications. Atmos. Chem. Phys., 11, 13421-13449, doi:10.5194/acp-11-13421-2011.

To help understand the implication of this early (2011 & 2012) work, I provide the following link to a SkS article about these two papers, & the following extract.  Note that SkS comfortingly assumes that it will take a very long time to lose the AIS; however, they are too naïve about how quickly the WAIS can be lost; which in my opinion brings us to Hansen et al 2016.  If large portions of the WAIS could be lost (with cliff failures & hydrofracturing beginning by 2050) then the bi-hemisphere see-saw could affect the Arctic a few decades later; which might (taken together with ASI loss & GIS darkening) increase the planetary imbalance sufficiently to cause the 6C ESS value before the end of this century (which would in my book make this value an effective ECS value).  Hansen et al 2016, emphasize that other ESM projections do not adequately account for this effect; thus hopefully, now that Hansen et al 2016 has entered the canon of peer-reviewed literature, other ESM modelers will introduce hosing and the correct mixing of water (e.g. the AABW) into the deep ocean:

http://www.skepticalscience.com/hansen-and-sato-2012-climate-sensitivity.html

Extract: "This initial state dependency is illustrated by the more complex shape of the upper curve in Figure 1 above.  For example, during a cooling event to a glacial period like the LGM, the long-term Earth System Sensitivity is approximately 6°C for an equivalent forcing to a doubling (or in this case halving) of CO2.  This is primarily due to the increase in the Earth's reflectivity as large ice sheets form.
During a period like the Holocene while warming to a Pliocene-like climate, slow feedbacks (such as reduced ice and increased vegetation cover) increase the sensitivity to around 4.5°C for doubled CO2.  However, a climate warm enough to lose the entire Antarctic ice sheet would have a long-term sensitivity of close to 6°C.  Fortunately it would take a very long time to lose the entire Antarctic ice sheet.
Note also that the Earth System Sensitivity is deduced from various past climate change events like the Paleocene–Eocene Thermal Maximum (PETM), but the qualitative estimates of longer-term climate sensitivity are less precise than the HS12 fast feedback sensitivity estimates.  Hence the authors note that Figure 1 above is a schematic."

Best,
ASLR

[AbruptSLR]

The exponential is not used because Hansen has evidence that feedback mechanisms will result in exponential response, but because it is a mathematically convenient forcing function that encompasses a wide variety of accelerating responses. You don't actually have to run the model again if you decide a different form is appropriate, just adjust the timings. As he puts it, you should view the progression of the model climate as a response to freshening sea water rather than time. If you think sufficient melt happens by 2040 to match his model in 2050, then the world in 2040 will look rather like his model in 2050.

There's indirect evidence for paleo rapid sea level rise as well as direct evidence. Thats where the boulders come in. Fast melt produces regional temperature changes of the sort which can be expected to drive storms of the sort that toss boulders like that where they have been tossed. Its also why there's much more attention to Greenland in the published version compared to the discussion version. The discussion version ignored Greenland on the basis that the amount of ice at risk of sudden collapse there was trivial compared to the Antarctic, but to get the climate response to drive the superstorms to toss the boulders, North Atlantic freshening has to be included. While I don't think the Antarctic melt he assumes is worst case (and the paper is all the stronger for it) I do think he is pushing the bounds for Greenland melt and the possibility for NADW shutdown remains rather weaker than the publicity surrounding the paper implies.

Richard,
While I agree with everything that you say, I would like to point-out another reason that Hansen uses time-to-doubling (exponential) approximations of ice sheet mass loss is that:

(a) Here uses these math formulae for both the GIS, the EAIS and the WAIS, all of which have fundamentally different physical situations and which will each follow different scenarios/pathways to ice mass loss; and

(b) It is premature for Hansen et al (2016) [i.e. if Eric Rignot] to cite specific scenarios/pathways for the GIS, EAIS and WAIS as the state of ice sheet modeling is the least refined of all climate change models.  If they provide some example scenarios (as proof of plausibility) the skeptics would have a field day citing the smallest discrepancy to such example scenarios and they would then demand that we throw the baby out with the bathwater.  Thus Hansen et al (2016) wisely leaves debates of the esoterica of ice sheet modeling to other future papers (such as those likely to come from the ACME project, and from Pollard, DeConto & Alley).

Furthermore, I agree that possible scenarios leading to significant ice mass loss from the GIS are more tortured than those for the WAIS; which means that more science needs to be completed before higher confidence statements about the timing of ice mass loss from the GIS can be made.  For example, in the first linked article Jason Box makes lucky prediction in early 2012 that within a decade the entire surface of the GIS would experience some surface melting; which then happened a few months later.  However, the second linked article shows that after 2012 the rate of decline of albedo for the GIS has slowed, resulting in less surface ice melting.  Nevertheless, surface ice melting from the GIS is a complex result of numerous fluctuating mechanisms including: (a) atmospheric telecommunication with the ASIE; (b) aerosol deposition on the ice; (c) bacterial growth on the ice surface; (d) local weather, etc. 

We will need to learn more about all of this surface melt contributions (together with calving ice mass loss contributions) before making confident upper bound projections for ice mass loss from the GIS (let along from the AIS)

http://www.rollingstone.com/feature/greenland-melting

Extract: In early 2012, Box predicted there would be surface melting across the entirety of Greenland within a decade. Again, many scientists dismissed this as alarmist claptrap. If anything, Box was too conservative – it happened a few months later. He also believes that the climate community is underestimating how much sea levels could rise in the coming -decades. When I ask him if he thinks the high-end projections of six feet are too low, he doesn’t hesitate: “Shit, yeah.” "


http://darksnow.org/greenland-ice-albedo-decline/

Best,
ASLR

[Richard Rathbone]

While the superstorms and NADW shutdown are gathering the publicity, I actually think the most important messages from the paper's paleo analogue is that a 1 degree rise over preindustrial (i.e. where we are now) is dangerous (i.e. there could be several meters of sea level rise in the pipeline and it could happen on human timescales) and that paleo can only provide lower limits for how fast ice sheets can respond. However fast the sea level rises in the future, it will be faster than anything seen for comparable climates in the past because the CO2 forcing has been faster, and CO2 forcing governed paleo response rates.

[AbruptSLR]

While the superstorms and NADW shutdown are gathering the publicity, I actually think the most important messages from the paper's paleo analogue is that a 1 degree rise over preindustrial (i.e. where we are now) is dangerous (i.e. there could be several meters of sea level rise in the pipeline and it could happen on human timescales) and that paleo can only provide lower limits for how fast ice sheets can respond. However fast the sea level rises in the future, it will be faster than anything seen for comparable climates in the past because the CO2 forcing has been faster, and CO2 forcing governed paleo response rates.

Richard,

Once again I agree with all of your observations, and once again I would like to add that:
 
(a) All of the paleo-evidence cited by Hansen et al (2016) was available before their paper, but this data did not convince the extant ESM modelers to account for ASLR; and

(b) What Hansen et al (2016) did was to connect all of these numerous disconnected lines of evidence via a privately financed climate model (the private is critical so that management could not suppress the scientific message) that was sufficiently sophisticated to connect the dots (from storms in the North Atlantic to ice mass loss in the WAIS), to create a consistent storyline that made the evidence for ASLR plausible;

(c) As Hansen et al (2016) has now demonstrated the plausibility of ASLR, other more sophisticated programs like ACME will be under pressure to evaluate such cases.

In this same many the recent application of "dynamical systems theory" as discussed in the following linked references; have recently allowed for the understanding of complex paleo-storylines including time lags, that support the position that the current effective ECS may be as high as 4.35C (but is masked both by lag times and by aerosol impacts):

Egbert H. van Nes, Marten Scheffer, Victor Brovkin, Timothy M. Lenton, Hao Ye, Ethan Deyle and George Sugihara (2015), "Causal feedbacks in climate change", Nature Climate Change, doi:10.1038/nclimate2568

http://www.nature.com/nclimate/journal/v5/n5/full/nclimate2568.html


Abstract: "The statistical association between temperature and greenhouse gases over glacial cycles is well documented, but causality behind this correlation remains difficult to extract directly from the data. A time lag of CO2 behind Antarctic temperature—originally thought to hint at a driving role for temperature—is absent at the last deglaciation, but recently confirmed at the last ice age inception and the end of the earlier termination II . We show that such variable time lags are typical for complex nonlinear systems such as the climate, prohibiting straightforward use of correlation lags to infer causation. However, an insight from dynamical systems theory now allows us to circumvent the classical challenges of unravelling causation from multivariate time series. We build on this insight to demonstrate directly from ice-core data that, over glacial–interglacial timescales, climate dynamics are largely driven by internal Earth system mechanisms, including a marked positive feedback effect from temperature variability on greenhouse-gas concentrations."

While the linked (open access) reference has many appropriate qualifying statements and disclaimers, it notes that the AR5 paleo estimates of ECS were linear approximations that change when non-linear issues are considered.  In particular the find for the specific ECS, S[CO2,LI], during the Pleistocence (ie the most recent 2 million years) that:
"During Pleistocene intermediate glaciated climates and interglacial periods, S[CO2,LI] is on average ~ 45 % larger than during Pleistocene full glacial conditions."

Therefore, researchers such as James Hansen who relied on paleo findings that during recent full glacial periods ECS was about 3.0C, did not know that during interglacial periods this value would be 45% larger, or 4.35C.

Köhler, P., de Boer, B., von der Heydt, A. S., Stap, L. B., and van de Wal, R. S. W. (2015), "On the state dependency of the equilibrium climate sensitivity during the last 5 million years", Clim. Past, 11, 1801-1823, doi:10.5194/cp-11-1801-2015.


http://www.clim-past.net/11/1801/2015/cp-11-1801-2015.html
http://www.clim-past.net/11/1801/2015/cp-11-1801-2015.pdf


Abstract: "It is still an open question how equilibrium warming in response to increasing radiative forcing – the specific equilibrium climate sensitivity S – depends on background climate. We here present palaeodata-based evidence on the state dependency of S, by using CO2 proxy data together with a 3-D ice-sheet-model-based reconstruction of land ice albedo over the last 5 million years (Myr). We find that the land ice albedo forcing depends non-linearly on the background climate, while any non-linearity of CO2 radiative forcing depends on the CO2 data set used. This non-linearity has not, so far, been accounted for in similar approaches due to previously more simplistic approximations, in which land ice albedo radiative forcing was a linear function of sea level change. The latitudinal dependency of ice-sheet area changes is important for the non-linearity between land ice albedo and sea level. In our set-up, in which the radiative forcing of CO2 and of the land ice albedo (LI) is combined, we find a state dependence in the calculated specific equilibrium climate sensitivity, S[CO2,LI], for most of the Pleistocene (last 2.1 Myr). During Pleistocene intermediate glaciated climates and interglacial periods, S[CO2,LI] is on average ~ 45 % larger than during Pleistocene full glacial conditions. In the Pliocene part of our analysis (2.6–5 Myr BP) the CO2 data uncertainties prevent a well-supported calculation for S[CO2,LI], but our analysis suggests that during times without a large land ice area in the Northern Hemisphere (e.g. before 2.82 Myr BP), the specific equilibrium climate sensitivity, S[CO2,LI], was smaller than during interglacials of the Pleistocene. We thus find support for a previously proposed state change in the climate system with the widespread appearance of northern hemispheric ice sheets. This study points for the first time to a so far overlooked non-linearity in the land ice albedo radiative forcing, which is important for similar palaeodata-based approaches to calculate climate sensitivity. However, the implications of this study for a suggested warming under CO2 doubling are not yet entirely clear since the details of necessary corrections for other slow feedbacks are not fully known and the uncertainties that exist in the ice-sheet simulations and global temperature reconstructions are large."

Extracts: "…. important feedbacks of the climate system are not incorporated into all models. For example, when coupling a climate model interactively to a model of stratospheric chemistry, including ozone, the calculated transient warming on a 100-year timescale differs by 20% from results without such an interactive coupling (Nowack et al., 2015).

…

A major restriction of any geological-data-based estimate of climate sensitivity is that there was no period in Earth’s history during which the atmospheric CO2 concentration and global temperature changed as rapidly as today. Therefore, in all these data-based approaches (including our study here), ECS defined as global equilibrium temperature rise in response to a doubling of atmospheric CO2 can only be roughly estimated.

…

Slow feedbacks are of interest in a more distant future (Zeebe, 2013) but are not yet considered in climate simulations using fully coupled climate models underlying the fifth assessment report of the IPCC (Stocker et al., 2013)."

The linked reference could not make it more clear that paleo-evidence from inter-glacial periods indicates that ECS is meaningfully higher than 3C and that climate models are commonly under predicting the magnitude of coming climate change.  Furthermore, these finding concur with those of Köhler et al (2015) which indicates that inter-glacial values for specific ECS was about 45% higher than during glacial periods.


Dana L. Royer (2016), "Climate Sensitivity in the Geologic Past", Annual Review of Earth and Planetary Sciences, Vol. 44


http://www.annualreviews.org/doi/abs/10.1146/annurev-earth-100815-024150?src=recsys


Abstract: "The response of temperature to CO2 change (climate sensitivity) in the geologic past may help inform future climate predictions. Proxies for CO2 and temperature generally imply high climate sensitivities: ≥3 K per CO2 doubling during ice-free times (“fast-feedback” sensitivity) and ≥6 K during times with land ice (Earth-system sensitivity). Climate models commonly under predict the magnitude of climate change and have fast-feedback sensitivities close to 3 K. A better characterization of feedbacks in warm worlds boosts climate sensitivity to values more in line with proxies and produce climate simulations that better fit geologic evidence. As CO2 builds in our atmosphere, we should expect both slow (e.g., land ice) and fast (e.g., vegetation, clouds) feedbacks to elevate the long-term temperature response over that predicted from the canonical fast-feedback value of 3 K. Because temperatures will not decline for centuries to millennia, climate sensitivities that integrate slower processes do have relevance for current climate policy."


Best,
ASLR

[Lennart van der Linde]

And now see how referee Peter Thorne continues the review process:
http://icarus-maynooth.blogspot.nl/2016/03/on-hansen-et-al.html?m=1

He seems to miss/ignore Hansen et al's argument that the forcing now is much stronger than in the past, as did Drijfhout et al in their comment on the discussion paper of Hansen et al.

[AbruptSLR]

And now see how referee Peter Thorne continues the review process:
http://icarus-maynooth.blogspot.nl/2016/03/on-hansen-et-al.html?m=1

He seems to miss/ignore Hansen et al's argument that the forcing now is much stronger than in the past, as did Drijfhout et al in their comment on the discussion paper of Hansen et al.

Lennart,
Just to add to your comment that Peter Thorne, ignores that current rate of increase of CO₂ radiative forcing is over 10 times that during the PETM, I provide a few extra comments about the following extract from Thorne that multi-meter SLR "…. shall be a multi-century process".  The following linked information indicates that as early as 2012 such esteemed authorities as the U.S. NRC & NOAA, have formally recognized the risk of multi-meter SLR this century (so I have no idea what Thorne is talking about):

http://icarus-maynooth.blogspot.nl/2016/03/on-hansen-et-al.html?m=1

Extract: "That said, West Antarctic is unstable, and we almost certainly have passed a tipping point that will see eventual c.8m rise but the scientific literature generally suggests this shall be a multi-century process."


First, the following link discusses the findings of the December 2013 NRC report on abrupt climate change; indicating that there is a real but "unknown" risk that the WAIS might collapse this century (which might contribute between 3 to 4m to SLR on top of, contributions from the GIS & the EAIS and from mountain glaciers):

http://news.nationalgeographic.com/news/2013/12/131203-abrupt-climate-change-science-early-warning-report/

The linked article includes the following quote, which precisely supports the position that I have taken in my various posts in this folder:

"An abrupt slide of the vast West Antarctic Ice Sheet into the ocean would suddenly sink coasts worldwide under 10 to 13 feet (3 to 4 meters) of water. The report rates the risk of this calamity as "unknown" although probably low for this century.
"Unknown means we should be studying this question intently, not pretending it isn't there," White says."

Second, the linked NOAA 2012 SLR guidance document (with a free access pdf) that cites an "Highest Scenario" with a SLR of 2m by 2100, and they acknowledge that the possibility exists that SLR could exceed this "limit".  I also note that it would be difficult to achieve a SLR of 2m by 2100 without a partial collapse of the WAIS starting no later than 2070:

http://cpo.noaa.gov/sites/cpo/Reports/2012/NOAA_SLR_r3.pdf

Quote: "Our Highest Scenario is an upper limit for SLR by 2100, but the possibility exists that SLR could exceed this limit beyond this timeframe (Pfeffer et al 2008).

…..

Most of the ice loss in Antarctica has come from the West Antarctic ice sheet (WAIS; Rignot et al. 2008).  A significant portion of the WAIS is floating at or grounded below sea level, as are relatively smaller parts of the ice sheets in East Antarctica and Greenland.  Floating ice shelves support land-based ice sheets. Current and future ocean warming below the surface make ice shelves susceptible to catastrophic collapse, which in turn can trigger increased ice discharge to the ocean (Rignot et al. 2004, Scambos et al. 2004, Jacobs et al. 2011, Joughlin and Alley 2011, Yin et al. 2011).  Better understanding of how the polar ice sheets will respond to further changes in climatic conditions over the 21st century requires continued development of physical models (Price et al. 2011)."
Best,
ASLR

[sidd]

I have read, more than once, the argument that repeats of previous catastrophic SLR in this interglacial, say MWP1A, were unlikely since less ice remains to melt. Hansen demolishes this in his treatment of late Eemian SLR. The Eemian was a few tenths of  degree warmer than Holocene, sea level rose by 6-9m above present, paused, and then jumped again. Presumably, late Eemian had even less ice than we have now. And there is evidence that 2-3 meter late Eemian SLR was very quick indeed, perhaps as quick as MWP1A:

"... on ecological(decadal) timescales ..." (Blanchon, 2009, quoted in Hansen as well)

The paper is still too long.

[AbruptSLR]

For what its worth the following extract comes from the linked 2014 article of a Washington Post interview with Eric Rignot:

http://www.washingtonpost.com/national/health-science/research-casts-alarming-light-on-decline-of-west-antarctic-ice-sheets/2014/12/04/19efd3e4-7bbe-11e4-84d4-7c896b90abdc_story.html

Extract: "So how fast could the loss of West Antarctica unfold? Velicogna’s co-author, Eric Rignot of UC-Irvine, suggested that in his view, within 100 to 200 years, one-third of West Antarctica could be gone.
Rignot noted that the scientific community “still balks at this” — particularly the 100-year projection — but said he thinks observational studies are showing that ice sheets can melt at a faster pace than model-based projections take into account."

Also, NRC (2013) consider abrupt SLR to be at least a 1m of SLR within an approximately 30-year period (or a rate of SLR of 33 mm/yr over such a 30-year period by 2100), and NRC 2013 singles out the WAIS as a possible specific source for such abrupt global sea level rise.

[tombond]

Hansen has continually warned that land ice melt will not be linear and that abrupt sea level rise is possible later this century.

Plotting observations from NASA for global mean sea level rise from the site address below, shows that sea level rise has averaged 6mm per year for the past five years up to November 2015 (date of latest observation data).  This is twice the annual average for the previous 20 years (3mm) and suggests a large acceleration in sea level rise is currently under way.

See 'sea level' then ‘source files’ at http://climate.nasa.gov/

Any simple risk analysis for coastal infrastructure development should seriously consider this data and plan accordingly.

[ritter]

While the superstorms and NADW shutdown are gathering the publicity, I actually think the most important messages from the paper's paleo analogue is that a 1 degree rise over preindustrial (i.e. where we are now) is dangerous (i.e. there could be several meters of sea level rise in the pipeline and it could happen on human timescales) and that paleo can only provide lower limits for how fast ice sheets can respond. However fast the sea level rises in the future, it will be faster than anything seen for comparable climates in the past because the CO2 forcing has been faster, and CO2 forcing governed paleo response rates.

+1
We can argue the hows and whys and detailed specifics, but to me the takeaway is that it is happening at current temperatures and it is happening far faster than anyone expected a decade ago. Waiting for certainty in this case will end up with a lot of very wet feet.

[AbruptSLR]

For those who do not regularly look at the Antarctic folder, I provide the following relevant linked research about both paleo and possible future ASLR:


Johannes Sutter, Malte Thoma, Klaus Grosfeld, Paul Gierz, and Gerrit Lohmann (2015), "Learning from the past: Antarctic Eemian ice sheet dynamics as an analogy for future warming", Geophysical Research Abstracts, Vol. 17, EGU2015-13255-2, EGU General Assembly 2015


http://meetingorganizer.copernicus.org/EGU2015/EGU2015-13255-2.pdf


Abstract: "Facing considerable warming during this century the stability of the West Antarctic Ice Sheet is under increasing scrutiny. Recent observations suggest that the marine ice sheet instability of the WAIS has already started. We investigate the dynamic evolution of the Antarctic Ice Sheet during the last interglacial, forcing a state of the art 3D ice sheet model with Eemian boundary conditions. We elucidate the role of ocean warming and surface mass balance on the coupled ice sheet/shelf and grounding line dynamics. Special focus lies on an ice sheet modeling assessment of Antarctica’s potential contribution to global sea level rise during the Eemian. The transient model runs are forced by time slice experiments of a fully coupled atmosphere-ocean global circulation model, as well as different sets of sea level and bedrock reconstructions. The model result show strong evidences for a severe ice-sheet retreat in West Antarctica, leading to substantial contribution to global sea level from the Southern Hemisphere.  Additionally we compare future warming scenarios of West Antarctic Ice Sheet dynamics to our paleo ice sheet modeling studies."

&

Calibrating an ice sheet model using high-dimensional binary spatial data
Authors: Won Chang, Murali Haran, Patrick Applegate, David Pollard
(Submitted on 8 Jan 2015 (v1), last revised 2 Jul 2015 (this version, v4))

http://arxiv.org/abs/1501.01937
http://arxiv.org/pdf/1501.01937v4.pdf


Abstract: Rapid retreat of ice in the Amundsen Sea sector of West Antarctica may cause drastic sea level rise, posing significant risks to populations in low-lying coastal regions. Calibration of computer models representing the behavior of the West Antarctic Ice Sheet is key for informative projections of future sea level rise. However, both the relevant observations and the model output are high-dimensional binary spatial data; existing computer model calibration methods are unable to handle such data. Here we present a novel calibration method for computer models whose output is in the form of binary spatial data. To mitigate the computational and inferential challenges posed by our approach, we apply a generalized principal component based dimension reduction method. To demonstrate the utility of our method, we calibrate the PSU3D-ICE model by comparing the output from a 499-member perturbed-parameter ensemble with observations from the Amundsen Sea sector of the ice sheet. Our methods help rigorously characterize the parameter uncertainty even in the presence of systematic data-model discrepancies and dependence in the errors. Our method also helps inform environmental risk analyses by contributing to improved projections of sea level rise from the ice sheets.

&

Won Chang, Murali Haran, Patrick Applegate, David Pollard (October 7, 2015), "Improving Ice Sheet Model Calibration Using Paleoclimate and Modern Data"

http://arxiv.org/pdf/1510.01676.pdf

Abstract: "Human-induced climate change may cause significant ice volume loss from the West Antarctic Ice Sheet (WAIS). Projections of ice volume change from ice-sheet models and corresponding future sea-level rise have large uncertainties due to poorly constrained input parameters. In most future applications to date, model calibration has utilized only modern or recent (decadal) observations, leaving input parameters that control the long-term behavior of WAIS largely unconstrained. Many paleo-observations are in the form of localized time series, while modern observations are non-Gaussian spatial data; combining information across these types poses nontrivial statistical challenges. Here we introduce a computationally efficient calibration approach that utilizes both modern and paleo-observations to generate better-constrained ice volume projections.

Using fast emulators built upon principal component analysis and a reduced dimension calibration model, we can efficiently handle high-dimensional and non-Gaussian data. We apply our calibration approach to the PSU3D-ICE model which can realistically simulate long-term behavior of WAIS. Our results show that using paleo observations in calibration significantly reduces parametric uncertainty, resulting in sharper projections about the future state of WAIS. One benefit of using paleo observations is found to be that unrealistic simulations with overshoots in past ice retreat and projected future regrowth are eliminated."

&

David Pollard, Robert DeConto, Won Chang, Patrick Applegate and Murali Haran (Dec 18, 2015), "Modeling of past and future variations of the Antarctic Ice Sheet with Large Ensembles" AGU Fall Meeting, Paper 60833.

https://agu.confex.com/agu/fm15/meetingapp.cgi/Paper/60833

Abstract: "Recent observations of thinning and retreat of the Pine Island and Thwaites Glaciers identify the Amundsen Sea Embayment (ASE) sector of West Antarctica as particularly vulnerable to future climate change. To date, most future modeling of these glaciers has been calibrated using recent and modern observations. As an alternate approach, we apply a hybrid 3-D ice sheet-shelf model to the last deglacial retreat of Antarctica, making use of geologic data from ~20,000 years BP to present, focusing on the ASE but including other sectors of Antarctica. 
Following several recent ice-sheet studies, we use Large Ensemble statistical methods, performing sets of ~600 runs over the last 30,000 years with systematically varying model parameters. Objective scores for each run are calculated using modern data and past reconstructed grounding lines, relative sea level records, cosmogenic elevation-age data and uplift rates. Two types of statistical methods are used to analyze the Large-Ensemble
results: simple averaging weighted by the aggregate score, and more advanced Bayesian emulation and calibration methods that rigorously account for some of the uncertainties in the model and observations. 

Results for best-fit parameter ranges and envelopes of equivalent sea-level rise with the simple averaging method agree quite well with the more advanced techniques, but only for a Large Ensemble with dense (Full Factorial) parameter sampling. Runs are extended into the future using RCP scenarios, with drastic retreat mechanisms of hydrofracturing and structural ice-cliff failure. In most runs this produces grounding-line retreat into the West Antarctic interior, and into East Antarctic basins for RCP8.5, and the Large Ensemble analysis provides sea-level-rise envelopes with well defined parametric uncertainty bounds." 

&

R. Levy, D. Harwood, F. Florindo, F. Sangiorgi, R. Tripati, H. von Eynatten, E. Gasson, G. Kuhn, A. Tripati, R. DeConto, C. Fielding, B. Field, N. Golledge, R. McKay, T. Naish, M. Olney, D. Pollard, S. Schouten, F. Talarico, S. Warny, V. Willmott, G. Acton, K. Panter, T. Paulsen, and M. Taviani (2016), "Antarctic ice sheet sensitivity to atmospheric CO2variations in the early to mid-Miocene", Proceedings of the National Academy of Sciences, pp. 201516030, doi: 10.1073/pnas.1516030113


http://www.pnas.org/content/early/2016/02/17/1516030113.abstract?sid=054ec02a-cd04-4970-9a0b-91d9ea9d0fb7

Significance: "New information from the ANDRILL-2A drill core and a complementary ice sheet modeling study show that polar climate and Antarctic ice sheet (AIS) margins were highly dynamic during the early to mid-Miocene. Changes in extent of the AIS inferred by these studies suggest that high southern latitudes were sensitive to relatively small changes in atmospheric CO2 (between 280 and 500 ppm). Importantly, reconstructions through intervals of peak warmth indicate that the AIS retreated beyond its terrestrial margin under atmospheric CO2 conditions that were similar to those projected for the coming centuries."

Abstract: " Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23–14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3–4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2. These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene."

&

E. Gasson, R.M. DeConto, D. Pollard, and R.H. Levy (2016), "Dynamic Antarctic ice sheet during the early to mid-Miocene", Proceedings of the National Academy of Sciences, pp. 201516130, doi: 10.1073/pnas.1516130113

http://www.pnas.org/content/early/2016/02/17/1516130113

Significance: "Atmospheric concentrations of carbon dioxide are projected to exceed 500 ppm in the coming decades. It is likely that the last time such levels of atmospheric CO2 were reached was during the Miocene, for which there is geologic data for large-scale advance and retreat of the Antarctic ice sheet. Simulating Antarctic ice sheet retreat is something that ice sheet models have struggled to achieve because of a strong hysteresis effect. Here, a number of developments in our modeling approach mean that we are able to simulate large-scale variability of the Antarctic ice sheet for the first time. Our results are also consistent with a recently recovered sedimentological record from the Ross Sea presented in a companion article."

Abstract: "Geological data indicate that there were major variations in Antarctic ice sheet volume and extent during the early to mid-Miocene. Simulating such large-scale changes is problematic because of a strong hysteresis effect, which results in stability once the ice sheets have reached continental size. A relatively narrow range of atmospheric CO2 concentrations indicated by proxy records exacerbates this problem. Here, we are able to simulate large-scale variability of the early to mid-Miocene Antarctic ice sheet because of three developments in our modeling approach. (i) We use a climate–ice sheet coupling method utilizing a high-resolution atmospheric component to account for ice sheet–climate feedbacks. (ii) The ice sheet model includes recently proposed mechanisms for retreat into deep subglacial basins caused by ice-cliff failure and ice-shelf hydrofracture. (iii) We account for changes in the oxygen isotopic composition of the ice sheet by using isotope-enabled climate and ice sheet models. We compare our modeling results with ice-proximal records emerging from a sedimentological drill core from the Ross Sea (Andrill-2A) that is presented in a companion article. The variability in Antarctic ice volume that we simulate is equivalent to a seawater oxygen isotope signal of 0.52–0.66‰, or a sea level equivalent change of 30–36 m, for a range of atmospheric CO2 between 280 and 500 ppm and a changing astronomical configuration. This result represents a substantial advance in resolving the long-standing model data conflict of Miocene Antarctic ice sheet and sea level variability."

[Bruce Steele]

" internal Southern Ocean effects of ocean surface freshening and cooling seem 
to be well underway " ( from the last paragraph before the summary )
 Is there the possibility that a disconnect between northern hemispheric heating ,as we are currently experiencing, and southern hemispheric cooling or neutral conditions  would mean the northern hemisphere would need to be 4 degrees C above preindustrial before the earth pasted the 2 degree threshold ?  It would seem to me we would be crossing positive feedback triggers before a 2 degree average was ever crossed if that is the case.
 ??    

[AbruptSLR]

" internal Southern Ocean effects of ocean surface freshening and cooling seem
to be well underway " ( from the last paragraph before the summary )
 Is there the possibility that a disconnect between northern hemispheric heating ,as we are currently experiencing, and southern hemispheric cooling or neutral conditions  would mean the northern hemisphere would need to be 4 degrees C above preindustrial before the earth pasted the 2 degree threshold ?  It would seem to me we would be crossing positive feedback triggers before a 2 degree average was ever crossed if that is the case.
 ??   

Bruce,

There is a bipolar seesaw between heating and cooling in the different hemispheres; however, I do not believe that it happens as fast as you are postulating*.  I believe that the primary reason that the Southern Ocean has been freshening for several decades now is that the ozone hole accelerated the westerly winds over the Southern Ocean thus increasing the circumpolar ocean currents sufficiently that the Coriolis Effect drove circumpolar deep water, CDW, over the continental shelf and to the grounding lines for multiple key ice shelves and marine glacier. Furthermore, I believe that the second most important reason for the freshening is that global warming has intensified the magnitude of the El Nino / La Nina oscillations of the ENSO cycle, thus telecommunicating** more oceanic and atmospheric energy from the Pacific Ocean to the West Antarctic. However, this is topic is already covered by extensive earlier posts in this thread, so, if you are interested, search this thread for the keywords: "seesaw" and/or "bipolar", and/or "interhemispheric".  Also, if you are interested, you could search this thread for the keywords: "hydrate" and/or "methane".

Best,
ASLR

* The linked reference discusses paleo findings (from the last ice age) about how long (about 200 years) it takes for Antarctic to notice abrupt warming (or cooling) in the Northern Hemisphere:

WAIS Divide Project Members (2015), "Precise interpolar phasing of abrupt climate change during the last ice age", Nature, Volume: 520, Pages: 661–665, doi:10.1038/nature14401


http://www.nature.com/nature/journal/v520/n7549/full/nature14401.html


Abstract: "The last glacial period exhibited abrupt Dansgaard–Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard–Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard–Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard–Oeschger dynamics."

See also a discussion of this paper by Eric Steig at:

http://www.realclimate.org/

**The linked article reviews paleo-data w.r.t the bipolar see-saw mechanism associate past abrupt climate change events.  The paper states: "Our Antarctic high resolution data also suggest possible teleconnections between changes in low latitude atmospheric circulation and Antarctic without any Greenland temperature fingerprint."  This last statement highlights the risk that a modern increase in ENSO intensity (due to anthropogenic forcing) could trigger Antarctic ice mass loss independently of what Greenland's Ice Sheet does:

A. Landais, V. Masson-Delmotte, B. Stenni, E. Selmo, D.M. Roche, J. Jouzel, F. Lambert, M. Guillevic, L. Bazin, O. Arzel, B. Vinther, V. Gkinis and T. Popp  (15 April 2015) "A review of the bipolar see–saw from synchronized and high resolution ice core water stable isotope records from Greenland and East Antarctica", Quaternary Science Reviews, Volume 114, Pages 18–32, doi:10.1016/j.quascirev.2015.01.031


http://www.sciencedirect.com/science/article/pii/S0277379115000554

Abstract: "Numerous ice core records are now available that cover the Last Glacial cycle both in Greenland and in Antarctica. Recent developments in coherent ice core chronologies now enable us to depict with a precision of a few centuries the relationship between climate records in Greenland and Antarctica over the millennial scale variability of the Last Glacial period. Stacks of Greenland and Antarctic water isotopic records nicely illustrate a seesaw pattern with the abrupt warming in Greenland being concomitant with the beginning of the cooling in Antarctica at the Antarctic Isotopic Maximum (AIM). In addition, from the precise estimate of chronological error bars and additional high resolution measurements performed on the EDC and TALDICE ice cores, we show that the seesaw pattern does not explain the regional variability in Antarctic records with clear two step structures occurring during the warming phase of AIM 8 and 12. Our Antarctic high resolution data also suggest possible teleconnections between changes in low latitude atmospheric circulation and Antarctic without any Greenland temperature fingerprint."

[AbruptSLR]

As I have the general impression that many who look at this thread do not look at the Antarctic folder, I re-post the following information indicating that: (a) Most of the current freshening of the Southern Ocean comes from meltwater from ice shelves (which do not affect SLR) from West Antarctica; and (b) Ice mass loss from the key Thwaites Glacier are currently continuing to accelerate:

The accompanying images come from the linked World Meteorological Organization's Polar Space Task Group (PSTG) reports.  The second link provides the first two images.  The first image indicates that thru 2012 most of the ice mass loss in Antarctic Ice Shelves has been concentrated in West Antarctica; which means that the associated marine glaciers are losing buttressing faster than other Antarctic marine glaciers.  The second image indicates that from 2010 to 2013 the WAIS contributed 0.45mm/yr to SLR.  The last two images come from the third link.  The third image indicates that the ice velocities for the PIG accelerated during the period when the groundling was retreating down the negative slope of the seafloor and then stabilized thru August 2015.  The fourth image shows that while the grounding line for the Thwaites Glacier has not yet reached the negative slope of the seafloor; nevertheless, its ice velocities have accelerated since 2006 as it has progressively lost buttressing from the Thwaites Ice Tongue:

http://www.wmo.int/pages/prog/sat/pstg_en.php


http://www.wmo.int/pages/prog/sat/meetings/documents/PSTG-5_Doc_06_EC-PHORS-ppt.pdf


http://www.wmo.int/pages/prog/sat/meetings/documents/PSTG-5_Doc_13-01_BScheuchl-Ice-Sheets-Final.pdf

[AbruptSLR]

The linked article notes that while the Hansen et al (2016) paper is not "the final word on climate change", it documents several valid issues that were overlooked by all of the model projections used in AR5.  Furthermore, now that it is in the peer-reviewed canon of literature, it is already being cited in litigation in an effort to force policy makers to better protect their citizenry.


https://www.washingtonpost.com/news/energy-environment/wp/2016/03/22/we-had-all-better-hope-these-scientists-are-wrong-about-the-planets-future/

Extract: "However, when it comes to both the melt rates for Greenland and Antarctica, and also these cool ocean patches, we have a very limited time span of observations. It is far from clear, yet, that Hansen’s interpretation of them will prevail, and the new study also suggests closely observing these areas in coming years.
Stratification, the key idea in the new paper, means that warm ocean water would potentially reach the base of ice sheets that sit below sea level, melting them from below (and causing more ice melt and thus, stratification). It also means, in Hansen’s paper, a slowdown or even eventual shutdown of the overturning circulation in the Atlantic ocean, due to too much freshening in the North Atlantic off and around Greenland, and also a weakening of another overturning circulation in the Southern Ocean.

…

The paper contains many ideas and departures, but the key one is its suggestion of the possibility of greater sea level rise in this century than forecast by the U.N. Intergovernmental Panel on Climate Change.
“The models that were run for the IPCC report did not include ice melt,” Hansen said at a news conference regarding the new paper Monday. “And we also conclude that most models, ours included, have excessive small scale mixing, and that tends to limit the effect of this freshwater lens on the ocean surface from melting of Greenland and Antarctica.”
There is a great deal at stake. Hansen has cited the paper in court proceedings in a case playing out in Oregon, where a series of young plaintiffs, including his granddaughter Sophie, are suing the United States for violating their constitutional rights by allowing fossil fuel burning."

[AbruptSLR]

The linked article highlights that all of the Hansen et. al. (2016) projections could come to pass if we reach the 2C limit (which at this point is almost unavoidable before 2050).  Furthermore, the article joins other media articles in stating that: “We had all better hope these scientists are wrong about the planet’s future.”  While, I am in favor of hope, I am not in favor of Pollyannaish thinking; and while efforts like the ACME project does reflect a serious mainstream effort, it seems ridiculous to me that the AR5 projections did not include hosing in their runs seeing as the 2012 NOAA SLR guidelines (published well before the AR5 cut-off date) explicitly warned of the possibility of over 2m of SLR by 2100.  To me, it is a sad statement that Hansen (et. al.) almost had to mount a military-style campaign to get his (their) paper published; which hopefully will force reticent mainstream AR6 researchers to vet his (their) findings in time that their work can be included in AR6.  For those who are not aware of some of the background issues associated with this matter, I believe that the majority of my posts in the various threads of this forum present information relevant to this serious matter, & you can find them by opening the individual threads and searching for AbruptSLR:


Extract: "Hansen’s paper isn’t the first to spell out a scenario for climate doom. What makes it so harrowing, though, is that it says all these consequences would follow the global average temperature rising a relatively small amount: only two degrees Celsius.

...

 The Times and the Post both covered its release on Tuesday, the latter with the headline: “We had all better hope these scientists are wrong about the planet’s future.”


And indeed, we had better. Hansen and his team’s predictions are grave.

...

Now that Hansen’s findings have been published, they will be tested and vetted and re-checked. Notably, they do not carry the imprimatur of, say, the UN’s Intergovernmental Panel on Climate Change, which releases consensus reports about the best available science. In fact, that is part of the point: Hansen and his team believe they’ve found mechanisms that more popular climate models, including those used by UN teams, don’t take into account."

[Richard Rathbone]

And now see how referee Peter Thorne continues the review process:
http://icarus-maynooth.blogspot.nl/2016/03/on-hansen-et-al.html?m=1

He seems to miss/ignore Hansen et al's argument that the forcing now is much stronger than in the past, as did Drijfhout et al in their comment on the discussion paper of Hansen et al.

The designated reviewers of the original paper deserve a great deal of sympathy. It was a horribly written mess and Thorne deserves a great deal of credit for struggling through it and staying with the process till something much better got published. While I am not impressed by his understanding of some of the physics, I am very much impressed by how he conducted himself as a reviewer.