Sea Level Rise and Social Cost of Carbon

[AbruptSLR]

The linked reference emphasizes that climate change is not just a 21st century problem, but rather is a problem that will last for millennia as indicated by the attached image about SLR:

Peter U. Clark, Jeremy D. Shakun, Shaun A. Marcott, Alan C. Mix, Michael Eby, Scott Kulp, Anders Levermann, Glenn A. Milne, Patrik L. Pfister, Benjamin D. Santer, Daniel P. Schrag, Susan Solomon, Thomas F. Stocker, Benjamin H. Strauss, Andrew J. Weaver, Ricarda Winkelmann, David Archer, Edouard Bard, Aaron Goldner, Kurt Lambeck, Raymond T. Pierrehumbert & Gian-Kasper Plattner (2016), "Consequences of twenty-first-century policy for multi-millennial climate and sea-level change", Nature Climate Change, doi:10.1038/nclimate2923

http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate2923.html

Abstract: "Most of the policy debate surrounding the actions needed to mitigate and adapt to anthropogenic climate change has been framed by observations of the past 150 years as well as climate and sea-level projections for the twenty-first century. The focus on this 250-year window, however, obscures some of the most profound problems associated with climate change. Here, we argue that the twentieth and twenty-first centuries, a period during which the overwhelming majority of human-caused carbon emissions are likely to occur, need to be placed into a long-term context that includes the past 20 millennia, when the last Ice Age ended and human civilization developed, and the next ten millennia, over which time the projected impacts of anthropogenic climate change will grow and persist. This long-term perspective illustrates that policy decisions made in the next few years to decades will have profound impacts on global climate, ecosystems and human societies — not just for this century, but for the next ten millennia and beyond."

Caption: " Past and future global average sea level (upper chart) and sea level change (lower chart), from 20,000 years ago to 10,000 years in the future. Future projections show four different emissions scenarios (figures on right-hand side show total cumulative carbon emissions). Inset maps show current (left) and future projected (right) ice coverage of Greenland and Antarctic ice sheets. Source: Clark et al. (2016)."

[Sigmetnow]

The Guardian writes about the study ASLR exerpts in Reply #500 above:

Sea-level rise 'could last twice as long as human history'
Research warns of the long timescale of climate change impacts unless urgent action is taken to cut emissions drastically

Huge sea-level rises caused by climate change will last far longer than the entire history of human civilisation to date, according to new research, unless the brief window of opportunity of the next few decades is used to cut carbon emissions drastically.

Even if global warming is capped at governments’ target of 2C - which is already seen as difficult - 20% of the world’s population will eventually have to migrate away from coasts swamped by rising oceans. Cities including New York, London, Rio de Janeiro, Cairo, Calcutta, Jakarta and Shanghai would all be submerged.

http://www.theguardian.com/environment/2016/feb/08/sea-level-rise-could-last-twice-as-long-as-human-history

[AbruptSLR]

The linked reference discusses how the period from 2002 to 2014 was dominated by La Nina behavior that lead to a decade of precipitation storing water on land (see image) thus slowing the observed rate of sea level rise by 0.71 ± 0.20 millimeters per year.  What the reference does not speculate on is that now that we are in a positive PDO phase this pattern is likely to flip resulting in a new trend line for SLR of about 3.34 + 0.71 = 4.05mm/year plus or minus (to get back to neutral) + 0.71 = 4.76 mm/year plus or minus for the duration of the positive PDO phase assuming that glacial and ice sheet contributions remain constant (which is not very likely):

J. T. Reager, A. S. Gardner, J. S. Famiglietti, D. N. Wiese, A. Eicker & M.-H. Lo (12 Feb 2016), "A decade of sea level rise slowed by climate-driven hydrology", Science, Vol. 351, Issue 6274, pp. 699-703, DOI: 10.1126/science.aad8386


http://science.sciencemag.org/content/351/6274/699


Abstract: "Climate-driven changes in land water storage and their contributions to sea level rise have been absent from Intergovernmental Panel on Climate Change sea level budgets owing to observational challenges. Recent advances in satellite measurement of time-variable gravity combined with reconciled global glacier loss estimates enable a disaggregation of continental land mass changes and a quantification of this term. We found that between 2002 and 2014, climate variability resulted in an additional 3200 ± 900 gigatons of water being stored on land. This gain partially offset water losses from ice sheets, glaciers, and groundwater pumping, slowing the rate of sea level rise by 0.71 ± 0.20 millimeters per year. These findings highlight the importance of climate-driven changes in hydrology when assigning attribution to decadal changes in sea level."

[Sleepy]

La Ninas are getting weaker and weaker, since 82-83. The PDO for January was 1.53 and we're now up to 25 consecutive moths with positive values. The first page at Aviso shows +3.35 mm/year (06 February 2016) but the real graph and data files were not updated. Image attached.
The latest SSW over the Arctic contributed to a huge impact on ASIE. Glaciers are now calving all year round. Etc, etc, etc.

It's more and more like travelling down a one-way road, without a turning space.

[AbruptSLR]

La Ninas are getting weaker and weaker, since 82-83. The PDO for January was 1.53 and we're now up to 25 consecutive moths with positive values. The first page at Aviso shows +3.35 mm/year (06 February 2016) but the real graph and data files were not updated.

Sleepy,

Thanks for the updated Aviso image, and I note that the +3.35 mm/year is a linear regression over their entire data set.  Further, I note that in my Reply #502, I have corrected my math to show a PDO adjusted rate of +4.76mm/year. 

Finally, I remind readers that Scribbler has a discussion on the rapid acceleration of SLR from 2009 to October 2015 (at a rate of 5 mm/yr):

http://robertscribbler.com/2016/02/04/rapid-acceleration-in-sea-level-rise-from-2009-through-october-2015-global-oceans-have-risen-by-5-millimeters-per-year/

Extract: "Rapid Acceleration in Sea Level Rise — From 2009 Through October 2015, Global Oceans Have Risen by 5 Millimeters Per Year."

Best,
ASLR

[wili]

Apologies if this has already been posted somewhere:

http://www.carbonbrief.org/sea-level-research-says-only-a-brief-window-to-cut-emissions?utm_source=Daily+Carbon+Briefing&utm_campaign=585c34b9ac-cb_daily&utm_medium=email&utm_term=0_876aab4fd7-585c34b9ac-303429069

Sea level research says only a ‘brief window’ to cut emissions

As global temperatures rise, scientists know that sea levels will follow suit. Today, global sea level is the topic of two new papers, both published in Nature Climate Change.

The first looks at Antarctica’s ice shelves – known as “gatekeepers of the ice” – and how much ice each one can afford to lose before opening the gates to more glacier flow into the oceans.

The second study focuses on how our greenhouse gas emissions today could lead to tens of metres of sea level rise that persist for hundreds of thousands of years.
Rapid thinning

An ice shelf forms when glaciers on land reach the coast and flow into the ocean. If the ocean is cold enough, this ice doesn’t melt. Instead it forms a permanently floating sheet of ice.

Ice shelves surround three-quarters of Antarctica. They can be as much as two kilometers thick and the largest – the Ross ice shelf – is roughly the same size as Spain.

These floating tongues of ice provide the important function of holding back – or “buttressing” – the flow of ice from the glaciers behind them into the ocean.

But many of Antarctica’s ice shelves are thinning rapidly. When an ice shelf thins, retreats or even collapses, it is less able to hold back the glaciers behind it, allowing more ice to flow from land and into the sea, and adding to sea levels. After the collapse of Larsen B ice shelf in West Antarctica in 2002, for example, some glaciers started flowing eight times faster.

The new paper estimates how much of each of Antarctica’s ice shelves can be lost without saying goodbye to this buttressing effect.

[GeoffBeacon]

Kopp et. al Temperature-driven global sea-level variability in the Common Era

Not as bad as we thought

Semiempirical 21st century projections largely reconcile differences between Intergovernmental Panel on Climate Change projections and semiempirical models.

Or is it?

However, both semi-empirical and process model-based projections may un-derestimate GSL rise if new processes not active in the calibration period and not well represented in process models [e.g., marine ice sheet instability in Antarctica (32)] become major factors in the 21st century.

The New York Times reported

One of the authors of the new paper, Dr. Rahmstorf, had previously published estimates suggesting the sea could rise as much as five or six feet by 2100. But with the improved calculations from the new paper, his latest upper estimate is three to four feet.

That means Dr. Rahmstorf’s forecast is now more consistent with calculations issued in 2013 by the Intergovernmental Panel on Climate Change, a United Nations body that periodically reviews and summarizes climate research. That body found that continued high emissions might produce a rise in the sea of 1.7 to 3.2 feet over the 21st century.

[Lennart van der Linde]

Also see Mengel et al 2016:
http://www.pnas.org/content/early/2016/02/17/1500515113

And Stefan Rahmstorf who connects the dots at RealClimate:
http://www.realclimate.org/index.php/archives/2016/02/millennia-of-sea-level-change/

He also points to:
"two further new papers, also appearing in PNAS this week, by Gasson et al. and by Levy et al.. These papers look at the stability of the Antarctic Ice Sheet during the early to mid Miocene, between 23 and 14 million years ago. What is most relevant here is the advances in modelling the Antarctic ice sheet by including new mechanisms describing the fracturing of ice shelves and the breakup of large ice cliffs. The improved ice sheet model is able to capture the highly variable Antarctic ice volume during the Miocene; the bad news is that it suggests the Antarctic Ice Sheet can decay more rapidly than previously thought."

Gasson et al 2016 and Levy et al 2016 are being discussed in the Antarctica folder under the Potential WAIS Collapse thread. Gasson et al seem to find that about 10m of SLR in two centuries may be likely once WAIS collapse gets going, so maybe from around 2050 or so?

[Lennart van der Linde]

Correction, I didn't read carefully enough: Gasson et al don't find 10m in two centuries, but in four, with about 6m in the first two and about 4m in the next two.

[tombond]

Sea level rise for the whole of the 20th century was about 140mm (5.5 inches). 
See chart at
http://www.theguardian.com/environment/climate-consensus-97-per-cent/2016/feb/24/earth-is-warming-is-50x-faster-than-when-it-comes-out-of-an-ice-age

NASA data shows that for the 21st century to November 2015 the sea level rise is already 64mm (2.5 inches) with 36mm (say 1.5 inches) occurring in just the past 5 years (since January 1st 2011) suggesting a large acceleration in sea level rise is currently under way.

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

[sidd]

neato keen.

http://noc.ac.uk/news/sea-level-mapped-from-space-gps-reflections

[sidd]

one grain of sand at a time

http://www.mcclatchydc.com/news/nation-world/world/article61804767.html#emlnl=Evening_Newsletter

not so neato keen

[AbruptSLR]

Per the linked Aviso data, mean sea level increased over 1cm in 2015; which is about 3 times faster than the trend line value of +3.35mm/yr:

ftp://ftp.aviso.oceanobs.com/pub/oceano/AVISO/indicators/msl/MSL_Serie_MERGED_Global_IB_RWT_GIA_Adjust.txt


2015.001879 7.395465e-02
2015.029027 7.380310e-02
2015.056175 7.431604e-02
2015.083322 7.562385e-02
2015.110470 7.701932e-02
2015.137617 7.763227e-02
2015.164765 7.724165e-02
2015.191912 7.645551e-02
2015.219060 7.611650e-02
2015.246207 7.652439e-02
2015.273355 7.720411e-02
2015.300503 7.741302e-02
2015.327650 7.688440e-02
2015.354798 7.610278e-02
2015.381945 7.587687e-02
2015.409093 7.662098e-02
2015.436240 7.799320e-02
2015.463388 7.920639e-02
2015.490535 7.970721e-02
2015.517683 7.960347e-02
2015.544831 7.948334e-02
2015.571978 7.983224e-02
2015.599126 8.061223e-02
2015.626273 8.138412e-02
2015.653421 8.180773e-02
2015.680568 8.199674e-02
2015.707716 8.234852e-02
2015.734863 8.308320e-02
2015.762011 8.400241e-02
2015.789159 8.458516e-02
2015.816306 8.434915e-02
2015.843454 8.344596e-02
2015.870601 8.267728e-02
2015.897749 8.262383e-02
2015.924896 8.329785e-02
2015.952044 8.447426e-02

[AbruptSLR]

The linked (open access) reference provides support for the position that as Greenland ice sheet sustains surface melting, the surface darkens, which results in a positive feedback for more melting; which results in accelerating SLR:

Tedesco, M., Doherty, S., Fettweis, X., Alexander, P., Jeyaratnam, J., and Stroeve, J.: The darkening of the Greenland ice sheet: trends, drivers, and projections (1981–2100), The Cryosphere, 10, 477-496, doi:10.5194/tc-10-477-2016, 2016.


http://www.the-cryosphere.net/10/477/2016/

Abstract. The surface energy balance and meltwater production of the Greenland ice sheet (GrIS) are modulated by snow and ice albedo through the amount of absorbed solar radiation. Here we show, using space-borne multispectral data collected during the 3 decades from 1981 to 2012, that summertime surface albedo over the GrIS decreased at a statistically significant (99 %) rate of 0.02 decade−1 between 1996 and 2012. Over the same period, albedo modelled by the Modèle Atmosphérique Régionale (MAR) also shows a decrease, though at a lower rate ( ∼ −0.01 decade−1) than that obtained from space-borne data. We suggest that the discrepancy between modelled and measured albedo trends can be explained by the absence in the model of processes associated with the presence of light-absorbing impurities. The negative trend in observed albedo is confined to the regions of the GrIS that undergo melting in summer, with the dry-snow zone showing no trend. The period 1981–1996 also showed no statistically significant trend over the whole GrIS. Analysis of MAR outputs indicates that the observed albedo decrease is attributable to the combined effects of increased near-surface air temperatures, which enhanced melt and promoted growth in snow grain size and the expansion of bare ice areas, and to trends in light-absorbing impurities (LAI) on the snow and ice surfaces. Neither aerosol models nor in situ and remote sensing observations indicate increasing trends in LAI in the atmosphere over Greenland. Similarly, an analysis of the number of fires and BC emissions from fires points to the absence of trends for such quantities. This suggests that the apparent increase of LAI in snow and ice might be related to the exposure of a "dark band" of dirty ice and to increased consolidation of LAI at the surface with melt, not to increased aerosol deposition. Albedo projections through to the end of the century under different warming scenarios consistently point to continued darkening, with albedo anomalies averaged over the whole ice sheet lower by 0.08 in 2100 than in 2000, driven solely by a warming climate. Future darkening is likely underestimated because of known underestimates in modelled melting (as seen in hindcasts) and because the model albedo scheme does not currently include the effects of LAI, which have a positive feedback on albedo decline through increased melting, grain growth, and darkening.



See also:

https://www.washingtonpost.com/news/energy-environment/wp/2016/03/03/greenlands-vast-ice-sheet-is-getting-darker-heres-why-thats-really-bad-news/

Edit, see also:

https://eos.org/articles/faster-merging-snow-crystals-speed-greenland-ice-sheet-melting

Extract: "A new study has found that the warming of Greenland is speeding changes to the crystalline structure of the fallen snow there in such a way that the snowpack more readily absorbs solar energy. This transformation, in turn, is contributing to more warming of the vast Greenland ice sheet, the researchers report, which further accelerates the transformation to more-heat-absorbent snow."

[sidd]

One of the keys to Greenland albedo loss might be contained in this sentence:

"Changes in the abundances of light-absorbing algae and other organic material with warmer temperatures may also be contributing to declining albedo, particularly for the ice, but this is an essentially unstudied source of darkening."

[AbruptSLR]

The linked (open access) reference estimates that over 13 million continental US citizens may need to relocated due to SLR by 2100:

Mathew E. Hauer, Jason M. Evans and Deepak R. Mishra (14 MARCH 2016) , "Millions projected to be at risk from sea-level rise in the continental United States", Nature Climate Change,  DOI: 10.1038/NCLIMATE2961


http://www.nature.com/articles/nclimate2961.epdf?referrer_access_token=8UOrGZsIIykrT2EH_Auk1NRgN0jAjWel9jnR3ZoTv0NW5dzPCV1LQTM2JMQvXgeV5kcoIiVItcAo6QabUR9-178DTC5AmyL7sqoUXtYx2FydBJB3NZXi69rwMlAJSFnb4PbI1CrMlUnNDDLj1lRtE1FdsgdlaP7hfzAT8rce5yP_2UibeTtvtA4ujTyZbUPByzMHTNjjTGJZ8QEiLwK-0PCDzSfXGVWpXdchFnhL0A8Zlt-JZPnhT-fRsFjU3uw_&tracking_referrer=www.nytimes.com

Abstract: "Sea-level rise (SLR) is one of the most apparent climate change stressors facing human society.  Although it is known that many people at present inhabit areas vulnerable to SLR, few studies have accounted for ongoing population growth when assessing the potential magnitude of future impacts. Here we address this issue by coupling a small-area population projection with a SLR vulnerability assessment across all United States coastal counties. We find that a 2100 SLR of 0.9 m places a land area projected to house 4.2 million people at risk of inundation, whereas 1.8 m affects 13.1 million people—approximately three times larger than indicated by current populations. These results suggest that the absence of protective measures could lead to US population movements of a magnitude similar to the twentieth century Great Migration of southern African-Americans. Furthermore, our population projection approach can be readily adapted to assess other hazards or to model future per capita economic impacts."

See also:
http://www.nytimes.com/2016/03/15/science/rising-sea-levels-global-warming-climate-change.html?_r=0

Extract: " For the study, the authors combined future population estimates with predicted sea-level rise, using data from the National Oceanic and Atmospheric Administration, to demonstrate that millions are at risk: 4.2 million if seas rise by three feet; 13.1 million with a six-foot increase, a high-end estimate."

[Sigmetnow]

Bacteria could be speeding up the darkening of Greenland's ice
Greenland’s ice is melting, and scientists have discovered a photosynthesising microbe they believe to be responsible for accelerating the process

A single species of bacteria could be about to accelerate the melting of Greenland. A photosynthesising microbe from a genus called Phormidesmis has been identified as the guilty party behind the darkening of Greenland.

It glues soot and dust together to form a grainy substance known as cryoconite. As the surface darkens, the Greenland ice becomes less reflective, more likely to absorb summer sunlight and more likely to melt.

And, Dr Arwyn Edwards, a biologist at Aberystwyth University tells the Microbiology Society’s annual conference in Liverpool today, cryoconite holes now pockmark 200,000 square kilometres of the Greenland ice sheet.
...
Scientists have known about the cryoconite phenomenon for more than a century – the word was coined during an expedition to Greenland by Nils Adolf Erik Nordenskiöld in 1870 – but not the identity of the culprit. Britain’s Natural Environment Research Council has now put £2.4m into such research and working in Svalbard, Edwards and colleagues identified the species that triggers the process of microbial change on the ice sheet. They confirmed its presence in western Greenland and on the Petermann Glacier in north-west Greenland.

“If we recognize ice species as a living landscape we can see that the microbes themselves are able to change the glacial surface,” Edwards said. “It’s only recently that we’ve begun to understand that these cryoconite holes are dynamic, changing in size and shape. Microbes are capable of ecosystem engineering and respond to changes in their environment all the time.”

https://www.theguardian.com/science/2016/mar/23/bacteria-speeding-up-darkening-greenlands-ice-climate-change

[Steven]

New paper by Robert DeConto and David Pollard:

Contribution of Antarctica to past and future sea-level rise

which suggests that "Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated".

Abstract.

Polar temperatures over the last several million years have, at times, been slightly warmer than today, yet global mean sea level has been 6–9 metres higher as recently as the Last Interglacial (130,000 to 115,000 years ago) and possibly higher during the Pliocene epoch (about three million years ago). In both cases the Antarctic ice sheet has been implicated as the primary contributor, hinting at its future vulnerability.

Here we use a model coupling ice sheet and climate dynamics—including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine-terminating ice cliffs—that is calibrated against Pliocene and Last Interglacial sea-level estimates and applied to future greenhouse gas emission scenarios. Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated. In this case atmospheric warming will soon become the dominant driver of ice loss, but prolonged ocean warming will delay its recovery for thousands of years.

[AbruptSLR]

which suggests that "Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated".

That is 1.05 +/-0.3m relative to 2000, which is already 0.2m above 1880 (it is nice that scientists keep re-zeroing so that they stay current).  So relative to 1880 we could be talking about up to 1.55m of SLR (without considering steric, glacial & GIS contributions).

See also:
http://theconversation.com/what-does-the-science-really-say-about-sea-level-rise-56807

[jai mitchell]

I find it interesting that their model actually shows net negative current SLR contributions from Antarctica and only positive contributions starting in 2050.  I guess the model is looking at a 3C rise in Antarctica Deep Water current starting sometime in 2035 or so.  Since recent satellite analysis shows net mass loss this seems to be a significant under-estimation.

[AbruptSLR]

I find it interesting that their model actually shows net negative current SLR contributions from Antarctica and only positive contributions starting in 2050.  I guess the model is looking at a 3C rise in Antarctica Deep Water current starting sometime in 2035 or so.  Since recent satellite analysis shows net mass loss this seems to be a significant under-estimation.

As noted by sidd in the Antarctic folder, DeConto & Pollard (2016) know that their currently published findings are preliminary & incomplete as indicated by the following quote:


"In particular, the model lacks two-way coupling between the ice sheet and the ocean. This is especially relevant for RCP8.5, in which >1 Sv of freshwater and icebergs would be supplied to the Southern Ocean during peak retreat (Extended Data Fig. 8 ). Rapid calving and ice-margin collapse also implies ice mélange in restricted embayments that could provide buttressing and a negative feedback on retreat. The loss of ice mass would also have a strong effect on relative sea level at the margin owing to gravitational and solid-earth deformation effects [48], which could affect MISI and MICI dynamics because of their strong dependency on bathymetry. Future simulations should include coupling with Earth models that account for these processes."

[Michael Hauber]

RCP8.5 can only happen if the current explosive growth in renewable energy slams into a brick wall, and if there are no consequence of climate change that reduce our economic growth between now and 2100.  Under the much more reasonable RCP 4.5 the projection is for 40cm by 2100 (RCP4.5 still has more than half our energy from fossil fuels by 2100).  Add something for Greenland, and the IPCC 50cm for all other factors besides ice sheet collapse.  Perhaps we'll just barely reach 1 meter of SLR by 2100...

Under RCP 2.6 there is basically 0 contribution to SLR from Antarctica, even out to 2500.

[sidd]

Mr. Mitchell has hit upon an important point. The DeConto models were initialized in 1950 and spun up with climate data. They show mass gain in antarctica until mid century, they are incorrect in that respect, we already see mass waste. But they are useful in the sense it shows 3/4 meter from antarctica in the last 50 years of the century, thats 15mm/yr. So, on my part, i wonder how many years to subtract, and how many mm/yr i want to add to their current model. And I do wonder if the melt layer feedback is already operating. They are smart people, they will put in the Hansen melt layer feedback soon, i am sure.

[AbruptSLR]

RCP8.5 can only happen if the current explosive growth in renewable energy slams into a brick wall, and if there are no consequence of climate change that reduce our economic growth between now and 2100.  Under the much more reasonable RCP 4.5 the projection is for 40cm by 2100 (RCP4.5 still has more than half our energy from fossil fuels by 2100).  Add something for Greenland, and the IPCC 50cm for all other factors besides ice sheet collapse.  Perhaps we'll just barely reach 1 meter of SLR by 2100...

Under RCP 2.6 there is basically 0 contribution to SLR from Antarctica, even out to 2500.

Michael,

First we are currently following RCP 8.5 80-90%CL pathway.  Second, collapse of the key marine Antarctic glaciers mostly triggered by the subsurface ocean temperature offshore of West Antarctica circa 2040; which in-turn is driven by what is happening in the South Pacific, South Atlantic and Indian Oceans in the 2020 to 2030 timeframe.  Third, AR5 ESLD w.r.t. such issues as ECS and aerosol forcing and ice-climate interaction. Fourth, the preliminary findings of DeConto & Pollard (2016) show over twice the GMSL by 2100 as projected by AR5 for RCP 8.5.

Therefore, referring to AR5 projections should not given anyone any sense of comfort.

Best,
ASLR

[Steven]

I find it interesting that their model actually shows net negative current SLR contributions from Antarctica and only positive contributions starting in 2050.  I guess the model is looking at a 3C rise in Antarctica Deep Water current starting sometime in 2035 or so.  Since recent satellite analysis shows net mass loss this seems to be a significant under-estimation.

That is discussed in the paper:

The CCSM4 simulations providing the model’s sub-ice-shelf melt
rates (Extended Data Fig. 5) underestimate the penetration of warm
Circum-Antarctic Deep Water into the Amundsen and Bellingshausen
seas observed in recent decades. As a result, the model fails to capture
recent, 21st-century thinning and grounding-line retreat along the
southern Antarctic Peninsula and the Amundsen Sea Embayment.
Correcting for the ocean-model cool bias along this sector of coastline
improves the position of Pine Island and Thwaites grounding lines
relative to observations (Fig. 4h) and increases GMSL rise by 9 cm
at 2100 (mainly due to the accelerated retreat of Pine Island Glacier),
but the correction has little effect on longer timescales (Extended Data
Table 1).

(This quote is from page 4 of the DeConto & Pollard paper.)

For the high emissions scenario (RCP8.5) their estimate for the Antarctic contribution to global mean sea level rise over the time period 2000-2100 is 1.05 +/- 0.30 m, without adjustment, and 1.14 +/- 0.36 m when adjusted as indicated in the above quote.  See Table 1 of the paper:

[Steven]

And I do wonder if the melt layer feedback is already operating.

I think that is unlikely.  E.g. see:

“The scale of the new freshwater input is small compared with snowfall or even rainfall in the Southern Ocean,” said Ted Scambos, senior research scientist at NSIDC. The Antarctic system has always experienced about 2,000 gigatons of melt annually, and the recent increases have only added about 150 gigatons. On top of that, the Southern Ocean annually receives about 20,000 to 40,000 gigatons of precipitation.

http://nsidc.org/icelights/2016/03/24/antarctic-sea-ice-an-update/

(This link is mainly about sea ice, but the "150 gigatons" per year refers to the loss of Antarctic land ice.)

[jai mitchell]

Under the much more reasonable RCP 4.5 the projection is for 40cm by 2100

RCP 4.5 is technically impossible now, the revisions to ECS and climate feedbacks make even RCP 6.0 nearly impossible at this point. 

[ritter]

Under the much more reasonable RCP 4.5 the projection is for 40cm by 2100

RCP 4.5 is technically impossible now, the revisions to ECS and climate feedbacks make even RCP 6.0 nearly impossible at this point.

Jai,

Can you expand on this a little, please?

[sidd]

Re: melt feedback

As is pointed out by Fogwill(2015) DOI: 10.1002/2015EF000306 and others, the location of the extra melt input is important, it is not as though the melt is dispersed evenly over the Southern oceans. So a comparison to the volume of total southern ocean precipitation or total annual melt from all icebergs from antactica may not be appropriate.

[AbruptSLR]

Here are two plots showing that we are well above the RCP 8.5 80% CL (for all GHGs) pathway.  Who knows what the future will hold.

[Michael Hauber]

Here are two plots showing that we are well above the RCP 8.5 80% CL (for GHG) pathway.  Who knows what the future will hold.

So you believe that the recent trends in total CO2 in the atmosphere is the best judge of the future Co2 pathways then?  In that case why don't you believe that current total SLR is the best judge of future SLR pathways?  Current SLR is just over 3mm a year, and has not sped up significantly over the last few decades (but has sped up over longer time frames).

Do you believe that we cannot get to the 40% or so level of renewable energy by 2100 to match RCP 4.5?  Renewable energy has grown quite rapidly over the last decade or so, but is still only a very small percentage of total energy.  Growth to date has not been enough to make a detectable difference in total Co2.  Just the same as growth in Antarctic ice loss has not been enough to make a detectable difference in global sea levels so far.  But in both cases future growth is likely to make a significant difference.

Do you believe that economic growth will continue without slow down right through to 2100 despite the consequences of climate change?

[AbruptSLR]

Here are two plots showing that we are well above the RCP 8.5 80% CL (for GHG) pathway.  Who knows what the future will hold.

So you believe that the recent trends in total CO2 in the atmosphere is the best judge of the future Co2 pathways then? 

First, my post says: "Who knows what the future holds", but the second figure shows that following the current CoP21 pledges we will be emitting about 54 Gtons/yr of CO2-e though 2030. 

Second, the linked Kevin Anderson article notes that to avoid the 2C limit our remaining CO2 budget might be about 450 Gt.

http://www.skepticalscience.com/Anderson.html

Extract: "Therefore, instead of a 1000 Gt CO2 budget, we might only have 450 Gt available for fossil-fuel energy emissions.”

Therefore, at 53 to 54 Gtons Co2-e per year, Anderson is indicating that we might only have 8.5 more years (plus an effective lag time of say 5.5 to 7.5 years for the emission period from 2016 to 2030) until 2030 to 2032 when we exceed the 2C limit (assuming an AR5 climate sensitivity, and assuming a GWP100 for methane of 34, while GWP20 for methane is 105).

Edit: Further, I note that during the late Eemian (that DeConto & Pollard 2016 also modeled) the GMST was between 2 to 3C above pre-industrial

[Michael Hauber]

I find it interesting that their model actually shows net negative current SLR contributions from Antarctica and only positive contributions starting in 2050.  I guess the model is looking at a 3C rise in Antarctica Deep Water current starting sometime in 2035 or so.  Since recent satellite analysis shows net mass loss this seems to be a significant under-estimation.

The last paragraph in the Future Simulations section explains that the CCSM4 simulations that they use underestimate the penetration of warm water underneath the ice sheets.  A correction for this factor yields improved match to current observations and increases sea levels in 2100 by 9 cm.  Their simulations find that warm water is important for early ice sheet melt, but in future decades melting from above becomes the most important factor as the globe warms up enough for significant areas of surface melting. 

[Michael Hauber]

Second, the linked Kevin Anderson article notes that to avoid the 2C limit our remaining CO2 budget might be about 450 Gt.

http://www.skepticalscience.com/Anderson.html

Extract: "Therefore, instead of a 1000 Gt CO2 budget, we might only have 450 Gt available for fossil-fuel energy emissions.”

Therefore, at 53 to 54 Gtons Co2-e per year, Anderson is indicating that we might only have 8.5 more years (plus an effective lag time of say 5.5 to 7.5 years for the emission period from 2016 to 2030) until 2030 to 2032 when we exceed the 2C limit (assuming an AR5 climate sensitivity, and assuming a GWP100 for methane of 34, while GWP20 for methane is 105).

And that is relevant to whether 4.5 or 8.5 is the more likely future how?

I think its obvious that 4.5 is far more likely, and the projection in the DeConto and Pollard paper is for 40cm (due to Antarctica only) by 2100 under that scenario.  While it is indeed a preliminary modelling result which may go up or down in the future, it looks far more detailed and realistic than anything else I've seen on this topic.  It makes Hansen's paper look like a joke in comparison.

[AbruptSLR]

And that is relevant to whether 4.5 or 8.5 is the more likely future how?

The point is that once you get to late Eemian temperatures (2 to 3C above pre-industrial) the WAIS surface ice starts to melt so hydrofracturing starts.  Your opinion that RCP 4.5 is most likely, is just that, your opinion.  My point is that cliff failures and hydrofracturing could begin in key coastal regions of the WAIS as early at 2030- 2035, and that DeConto & Pollard 2016 are not final ice mass loss estimates as they do not include many different considerations including:

- Positive ice-climate feedback (ala Hansen et al 2016)
- Use of a much finer mesh particularly in the Amundsen Sea Embayment area (which has increased ice mass loss in other models).
- Their paleo ice cliff and hydrofracturing calibrations most likely (appropriately) err on the side of least drama, ESLD, as: (a) the bed topology is smoother now than in paleo times; and (b) with the modern radiative forcing increasing at a rate over 10-times that of the PETM, the net positive feedbacks could accelerate faster than calibrated.
- The Byrd Subglacial Basin has an unusually high geothermal basal heat value that could accelerate ice mass loss.
- Their model spins-up from 1950 with negative ice mass contribution from the Antarctic Ice Sheet, AIS, to SLR (due to snowfall); while in actuality the AIS is currently contributing to SLR.
- They focus on ice mass loss from land based ice, while currently the ice mass loss from Antarctic ice shelves is dominate (& does not contribute to SLR) and is already triggering the ice-climate feedback (ala Hansen et al 2016).
- We are currently in a 20 to 30-year climate phase with a positive PDO which should make El Ninos more intense and more frequent than for the average climate state.  In general terms El Ninos telecommunicate more energy to the Bellingshausen & Amundsen Sea marine glaciers via both the ocean & the atmosphere).
- They ignore the recent evidence that ECS may be between 4 and 4.5C.
- I suspect that they ignore the influence of the extant ozone hole over Antarctica.
- They do not capture the consideration that as the Antarctic sea ice extant increases (with increased ice mass loss discharge into the ocean), more snow will fall on the sea ice rather than on the land.
 

[sidd]

"It makes Hansen's paper look like a joke ..."

I differ. Hansen is no joke, and to me, DeConto strengthens the case for Hansen. As I have pointed out in the WAIS collapse thread and elsewhere, DeConto is merely 50 yrs behind Hansen for his 8.5 scenario. DeConto cannot capture present mass waste without imposing a 3C warming of ASE with his ocean model, and i submit that the model is deficient in the way Hansen points out, that the mixed layer is too deep and stratification due to melt is not well represented.  DeConto has no melt feedback yet, although his melt flux is comparable.

Hansen I think has a more realistic ocean and DeConto has the ice sheet model, which Hansen lacks. I think when they put these together, the result will be closer to Hansen.

We shall see whether ocean forcing will tip the balance before the Mercer 0C midsummer isotherm arrives but I fear so.

[Sleepy]

Here are two plots showing that we are well above the RCP 8.5 80% CL (for GHG) pathway.  Who knows what the future will hold.

So you believe that the recent trends in total CO2 in the atmosphere is the best judge of the future Co2 pathways then?  In that case why don't you believe that current total SLR is the best judge of future SLR pathways?  Current SLR is just over 3mm a year, and has not sped up significantly over the last few decades (but has sped up over longer time frames).

Do you believe that we cannot get to the 40% or so level of renewable energy by 2100 to match RCP 4.5?  Renewable energy has grown quite rapidly over the last decade or so, but is still only a very small percentage of total energy.  Growth to date has not been enough to make a detectable difference in total Co2.  Just the same as growth in Antarctic ice loss has not been enough to make a detectable difference in global sea levels so far.  But in both cases future growth is likely to make a significant difference.

Do you believe that economic growth will continue without slow down right through to 2100 despite the consequences of climate change?

Why not look at right now and the most successful nation within the EU? Sweden now has 52.1% renewables in energy consumption and we met our 2020 target back in 2013.
http://ec.europa.eu/eurostat/documents/2995521/6734513/8-10032015-AP-EN.pdf/3a8c018d-3d9f-4f1d-95ad-832ed3a20a6b

But our total emissions per capita is still increasing, because we travel more in our cars, we buy more of them and our new ecofuel is diesel, we fly more, we eat more meat, we use more energy and all of that is projected to increase. When it comes to heating our homes we are trending downwards though, since we are helped by warmer winters...

The social cost of carbon will be very high, since that seems to be the typical cost of happiness?

Unsurprisingly, the disturbing picture that emerges from the graph is that a high Ecological Footprint is the typical cost of happiness. In this year’s ranking, Denmark was No. 1, followed by Switzerland, Iceland, Norway, Finland, Canada, the Netherlands, New Zealand, Australia and Sweden. Most have strong social safety nets and high Ecological Footprints.
http://www.footprintnetwork.org/en/index.php/blog/af/imagine_happiness_treading_lightly_on_the_earth

We are nothing but Shiny Happy People. What we are doing right now using Sweden as an example, can't get us to RCP4.5. But I would love to see some facts and figures that changes my opinion. I really do.

[AbruptSLR]

The linked article emphasizes that the DeConto & Pollard (2016) is only a preliminary and partial scenario for likely future SLR, as for instance it does not consider the worst case dynamic SLR contribution from Greenland's ice sheet; which according to Eric Rignot might bring the total SLR by 2100 up to about 10-ft (3m).  Also, I note that such large changes in ice sheet mass also induce a "finger print" effect on relative SLR, which could make local RSLR still higher:

http://thinkprogress.org/climate/2016/03/31/3765266/coastal-cites-carbon-pollution/

Extract: "To be clear, though, the five to six feet of sea level rise is not the worst-case scenario. For instance, it doesn’t include a more dynamic modeling of what will happen to Greenland’s ice sheet.
For a “worse-case scenario,” NASA scientist Eric Rignot directs us to the study he coauthored with James Hansen and others. That study posits 10 feet of sea level rise by 2100 is possible. But, again, it doesn’t provide a physical mechanism for how that much ice-melt could occur that fast, so some scientists tend to view it as unrealistic."

Edit: Also, for what it is worth, ice mass loss from the GIS & AIS can interact creating a positive feedback via the ocean (there is a thread in the Antarctic folder on this topic).

[jai mitchell]

Let us remember what RCP 4.5 actually means.

RCP 4.5 is the emissions pathway that, if the IPCC estimates of GHG atmospheric contribution to radiative forcing are correct, will produce 4.5 watts per meter squared of radiative forcing by 2100.

Currently the IPCC holds that, in 2011, the global GHG radiative forcing was 2.83 watts per meter squared. 

It is very likely that we are above 3.0 now even by their calculations - with about 1.5 watts per meter squared currently being offset by anthropogenic aerosols (with some studies showing values as high as 2.37 watts per meter squared).  This means that we are likely already at a net 4.5 watts per meter squared.

Since they did not include permafrost decomposition in their calculation,

since they did not project widescale destruction of tropical and boreal forests by fire in their estimation-leading to significantly increased emissions,

since they expect a future Atmospheric Fraction (AF) of residual emissions (emissions less any GHGs captured by the biosphere and removed from the atmosphere by natural sources) - even though many future AF models have as much variance in outcomes as the entirety of the emissions contained in the RCP 8.5 scenario,

since they DO include massive amounts of carbon capture and sequestration in their estimates of future emissions pathways,

Since we are still building coal power plants and (even more so) shifting to shale natural gas as a 'bridge fuel' even though it has a higher warming potential than Coal on a 50-year timeline,

since they do not include the albedo effects of an ice-free arctic september in 2025 - leading to much more rapid frozen soils emissions and (though not within the definition of radiative forcing) an additional 3.0 watts per meter squared of albedo effect (globally averaged) by 2050.

Then RCP 4.5 is technically impossible. 

even looking strictly at per unit anthropogenic emissions reductions, the reliance on a CCS miracle makes these equally technically impossible, whatever the actual anthropogenic impacts on the earths radiative budget (on a per unit scale) are.

--------------
2.37 watts per meter squared aerosol study (high end estimation) https://www.researchgate.net/profile/Hua_Zhang35/publication/291949505_The_updated_effective_radiative_forcing_of_major_anthropogenic_aerosols_and_their_effects_on_global_climate_at_present_and_in_the_future/links/56aeb89408ae28588c61e660.pdf

[AbruptSLR]

I agree with jai that without major application of NET (negative emissions technology) that achieving RCP 4.5 is impracticable.  Furthermore, the first & second attached images indicate that without future assumed ratcheting (which assume such large scale use of NET) the current CoP21 INDCs will result in about 3.7C increase using AR5 assumptions; which is close to the RCP 6.0 response.

While I sincerely hope that CoP21 works, I think that it is important to consider the case that RCP 8.5 (8.5 watts/sq m of radiative forcing by 2100) might actually be the pathway we follow no matter what our good intensions are (I note that INDCs are voluntary; that the GOP has promised to cancel US contributions to CoP21 if they get the White House; that current international trade agreements ban the use of subsidies for renewable energy technology; that tracking GHG emissions is difficult & people cheat; and that the future assumed ratcheting may never be agreed to).  Therefore, I offer the following reasons (in addition to jai's) that we might reach 8.5 watts per sq m of radiative forcing by 2100:

1. The third image shows that the largest reason for continued BAU emissions is growth of per capita worldwide.  Thus even if we build more renewables it will likely me that we just consume more, rather than that we will cut GHG emissions.
2. By 2100 we might have 12 Billion people on Earth and food production to feed these people is already driving current methane and nitrous oxide emissions to record high levels.
3. ECS may already be 4.3C (as indicated by recent paleo-evidence).
4. Accelerating net positive feedback mechanisms could make the effective ECS as high as 5C by 2100.
5. The Hansen et al 2016 identified ice-climate positive feedback could temporarily raise the effective ECS well above 6C by 2100.
6. As discussed in the "Conservative Scientists and its Consequences" thread many of the authors of the RCP pathways now acknowledge that they are 10 to 15% too low.
7. The ocean heat content is currently abnormally high due to 20 years of negative PDO.
8. If the WAIS collapses a lot of methane hydrates would be destabilized.
9. The Southern Ocean is current absorbing high levels of CO2 but with continued warming and ocean acidification it could both vent CO2, and experience a plankton die-off that would both further reduce CO2 absorption & would reduce DMS emissions that currently cause negative aerosol forcing.
10. Currently, high CO2 concentrations are driving a bloom of plant growth, but with more warming this trend could reverse (due to plants sensitivity to heat & slow ability to adapt).
11. The tropical rainforests around the world are all near tipping points.
12. The RCP scenarios use a GWP for methane that is too low.
13. Wildfires & pests could destroy large areas of boreal forests and their associated BVOCs & SOAs.
14. TCR is dependent on GHG emission rates and if the combination of anthropogenic and natural (say due to increase soil microbe activity) emission rates increase the TCR will increase (as we are currently at a low point for TCR.

While none of these points prove that we will continue to follow a RCP 8.5 pathway (which we are currently following), they do indicate that we may need to be lucky to avoid following this pathway.

[oren]

ASLR and all the resident experts - I assume that the IPCC not  only created charts for the future emissions pathways, but also has somewhere a chart of the expected concentrations of each GHG that should result from each emissions scenario.
As quantifying actual emissions is difficult and approximate, but measuring actual concentrations in the atmosphere is relatively straightforward, I would like to see what the IPCC expected for these concentrations and compare it to reality. This should uncover not just discrepancies between their assumed emission pathways and actual emissions, as ASLR showed a few posts ago, but should also uncover discrepancies between their estimate of other emission sources and sinks, and actual reality.
So can anyone post such a chart, assuming it exists?

[S.Pansa]

Hi Oren,

perhaps the RC-Database is what you are looking for?
You can select regions, scenarios and in point (3) also GHG emissions and concentrations. The projected concentrations can be found under "Climate indicators".
For 2020 for instance the projected CO2 numbers are:
RCP 2.6 - 412
RCP 4.5 - 411
RCP 6.0 - 409
RCP 8.5 - 416

More interesting perhaps the CO2-eq projections for 2020:

RCP 2.6 - 471
RCP 4.5 - 472
RCP 6.0 - 469
RCP 8.5 - 483

The actual CO2-eq concentration for 2014 was 481

[Laurent]

CO2e link see the table at the end
http://www.esrl.noaa.gov/gmd/aggi/aggi.html

[jai mitchell]

ASLR

Hansen et al 2016 identified ice-climate positive feedback could temporarily raise the effective ECS well above 6C by 2100

Was he the lead on this? I can't find the paper.  link?

[Lennart van der Linde]

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

[AbruptSLR]

Hi Oren,

perhaps the RC-Database is what you are looking for?

Alternately (with a different interface), you can find the same RCP data at the following Potsdam link:

http://www.pik-potsdam.de/~mmalte/rcps/

[AbruptSLR]

ASLR

Hansen et al 2016 identified ice-climate positive feedback could temporarily raise the effective ECS well above 6C by 2100

Was he the lead on this? I can't find the paper.  link?

While Lennart provided the correct link to Hansen et al (2016); Hansen has discussed this concept for several years now as indicate by the attached image (Fig 7 of Hansen & Sato 2012) & the following references:

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.

[AbruptSLR]

The linked article by Kolbert confirms that we only need to stay on a CO₂-e BAU pathway for a few more decades and the WAIS might contribute about 1m to SLR by 2100:

http://www.newyorker.com/news/daily-comment/climate-catastrophe-coming-even-sooner

Extract: "When DeConto and Pollard revised their model to account for this possibility, the results, as the Times put it, were “striking.” The revised model could account for earlier sea-level rises. More significantly, it suggested that what had happened then could easily happen again. The researchers concluded that just a few more decades of “unabated” carbon emissions could result in more than three feet of sea-level rise from WAIS by the end of this century. (The over-all rise would be much greater, as ice would also be lost from Greenland and from mountain glaciers.) Over the longer term, melt from Antarctica could raise sea levels by fifty feet."

[AbruptSLR]

ASLR and all the resident experts - I assume that the IPCC not  only created charts for the future emissions pathways, but also has somewhere a chart of the expected concentrations of each GHG that should result from each emissions scenario.
As quantifying actual emissions is difficult and approximate, but measuring actual concentrations in the atmosphere is relatively straightforward, I would like to see what the IPCC expected for these concentrations and compare it to reality. This should uncover not just discrepancies between their assumed emission pathways and actual emissions, as ASLR showed a few posts ago, but should also uncover discrepancies between their estimate of other emission sources and sinks, and actual reality.
So can anyone post such a chart, assuming it exists?

While both S.Pansa and I have provided links to RCP databases, the linked article confirms what I have already said, that the RCP pathways underestimate radiative forcing scenarios (& and should be revised for AR6):

The linked, open access, reference indicates that if the AR5 global mean surface temperature, GMST, projections had not adopted procedures (w.r.t. carbon cycles) that err on the side of least drama, they would have projected higher values of GMST, with wider ranges of uncertainty, as illustrated by the attached plot with the caption cited below:

Bodman, R. W., Rayner, P. J. and Jones, R. N. (2016), "How do carbon cycle uncertainties affect IPCC temperature projections?",  Atmosph. Sci. Lett., doi: 10.1002/asl.648

http://onlinelibrary.wiley.com/doi/10.1002/asl.648/abstract

Abstract: "Carbon cycle uncertainties associated with the Intergovernmental Panel on Climate Change temperature-change projections were treated differently between the Fourth and Fifth Assessment Reports as the latter focused on concentration- rather than emission-driven experiments. Carbon cycle feedbacks then relate to the emissions consistent with a particular concentration. A valuable alternative is to include all uncertainties in a single step from emissions to temperatures. We use a simple climate model with an observationally constrained parameter distribution to explore the carbon cycle and temperature-change projections, simulating the emission-driven Representative Concentration Pathways. The resulting range of uncertainty is a somewhat wider and asymmetric likely range (biased high)."

Caption: "Plume plots for ΔGMT change projections 2000–2100, ∘C relative to 1986–2005. MAGICC results with carbon cycle temperature feedbacks on (CC-on) and switched off (CC-off) (a) RCP2.6, (b) RCP4.5, (c) RCP6.0 and (d) RCP8.5. Shaded regions indicate the 67% confidence interval for CC-on (green) and CC-off (blue), with median results as solid green and dashed blue lines, respectively."

Edit: See also:

Joeri Rogelj, Michiel Schaeffer, Pierre Friedlingstein, Nathan P. Gillett, Detlef P. van Vuuren, Keywan Riahi, Myles Allen & Reto Knutti (2016), "Differences between carbon budget estimates unraveled", Nature Climate Change, Volume: 6, Pages: 245–252, DOI: 10.1038/NCLIMATE2868

http://www.nature.com/nclimate/journal/v6/n3/full/nclimate2868.html

[jai mitchell]

Math error alert

It is very likely that we are above 3.0 now even by their calculations - with about 1.5 watts per meter squared currently being offset by anthropogenic aerosols (with some studies showing values as high as 2.37 watts per meter squared).  This means that we are likely already at a net 4.5 watts per meter squared.

Correction to the above,

As measured from pre-industrial abundances of GHGs in the earths atmosphere the radiative forcing value of energy re-radiated down to planet earth is indeed about 3 watts per meter squared by the IPCC adopted methodology.  The aerosol value subtracts from this value and is not added to provide a total radiative forcing value of 4.5 Watts per meter squared.  The GHG forcing value would still be only 3 watts per meter squared.

However, since the earth has warmed since pre-industrial times the amount of blackbody radiation emitted has also increased and this would together (with aerosols, changes in albedo and GHG radiative forcing-including lapse rate) produce the net top of atmosphere radiation imbalance (TOA).  This value is empirically determined by the study of gains in Ocean Heat Content using the ARGO buoy network and has shown to be somewhere greater than 1 watt per meter squared  (first quarter global ocean heat content increases in 2016 are pending release from NODC)  over the last 2 years, (~1.2 Watts per meter squared).

So, this tells me that the 3 Watts per meter squared for GHG is underestimated and, if the 2.73 watts per meter squared of aerosol forcing is correct, then the GHG forcing amount is significantly underestimated by the IPCC.

-------------------

in looking for ASLRs james hansen paper I came across this one:
https://www.researchgate.net/profile/Karina_Schuckmann/publication/292188056_An_imperative_to_monitor_Earth's_energy_imbalance/links/56b8724a08ae44bb330d223b.pdf

An imperative to monitor Earth’s energy imbalance
K. von Schuckmann1,2*, M.D. Palmer3 , K.E. Trenberth4, A. Cazenave5,6, D. Chambers7 , N. Champollion6, J. Hansen8, S.A. Josey9 , N. Loeb10, P.-P. Mathieu11, B. Meyssignac5 and M. Wild12

abstract:
The current Earth’s energy imbalance (EEI) is mostly caused by human activity, and is driving global warming. The absolute value of EEI represents the most fundamental metric defining the status of global climate change, and will be more useful than using global surface temperature. EEI can best be estimated from changes in ocean heat content, complemented by radiation measurements from space. Sustained observations from the Argo array of autonomous profiling floats and further development of the ocean observing system to sample the deep ocean, marginal seas and sea ice regions are crucial to refining future estimates of EEI. Combining multiple measurements in an optimal way holds considerable promise for estimating EEI and thus assessing the status of global climate change, improving climate syntheses and models, and testing the effectiveness of mitigation actions. Progress can be achieved with a concerted international effort.

the following graphic (from the paper) shows clear correlation to ARGO 0-2000 meter depth ocean heat content to TOA