From the ArsTechnica article above.
The problem is, all those headlines describe a study, and that study doesn’t predict anything. It certainly doesn’t predict 10 feet of sea level rise by 2100 (or even 2050) as a number of stories have claimed.
Sure, but to regular people this is just playing with words. And who needs to understand where we are heading? Regular people, non scientists.
On another blog, I selected the exact same qoute as A-team above from the essay on Hansens webpage on the 27th, and another part. Why? I do believe normal people who don't care to read papers are more affected by Hansens personal opinions. Even (especially) deniers...
Discussing science is one thing, communicating where we are heading it is another. I think Hansen is doing the right thing here. I like it.
I concur with you, and I have always found your posts to be well grounded in common sense.
That said, many/most policymakers will only accept a projection from a computer model as adequate political cover for taking any action, and they will not accept paleo-analogies from the late-Eemian, nor estimates (say 10-year doubling time) of probable ice mass loss from the WAIS no matter how intelligent or experienced the researcher (say Hansen). Unfortunately for the BSB/Thwaites case, most existing ice models leave the "ice plug" effectively blocking the basin threshold for one to two hundred years before the main phase collapse begins; however, as Rignot notes (& in my opinion no one speaks with more authority on this topic) these existing models are incomplete and at best represent a lower bound. This leaves policymakers free to do as little as possible on this matter, and as indicated by the following clip when Alley provided heavily hedged (milk-toast) testimony to a Republican congressman who wanted to enact a carbon pricing plan, Alley did not give his fellow Republican adequate ammunition so the whole measure failed (leaving the world at risk):https://www.youtube.com/watch?v=4tMeqjbA94I
Now Alley feels that he was protecting scientific integrity; however, in the following clip he is well aware of the risks of a BSB/Thwaites collapse, but instead of communicating them holistically as Hansen does he prefers to focus on getting more research dollars to support his graduate students by focusing on incremental improvements to old ice models; while poo-pooing significant risk factors that would be difficult for his team to model for the BSB/Thwaites threshold area (I do acknowledge his many valuable scientific contributions including input to the hydrofracturing and cliff failure mechanisms).https://www.youtube.com/watch?v=yCunWFmvUfo
For example in Alley's April INSTAAR talk (linked above) on possible WAIS collapse, he admits that he has a proposal to make a more detailed model of the Thwaites threshold area; however, as far as I can tell from looking at the clip Alley missed: 1. Hydrodynamic water pressure pulse from cliff failure calving event; 2. influence of increasing Southern Ocean cyclones & associated storm surge on flushing-out Thwaites mélange; 3. local ASE sea level has not changed due to GIS contribution & magma inflow to keep gravitational attraction of seawater surface; 3. influence of ENSO (we are entering a period of increased EL Nino intensity & frequency), & IPO (we are existing a two decade long negative IPO phase) on CDW inflow into the ASE; 4. projected changes in local ocean currents, including the influence of advective flow out of the Pine Glacier on the current flow to the Thwaites grounding line (see the first two attached image); 5. The influence of the outflow of buoyant basal water from beneath Thwaites (see the third attached image) on both surging of the Thwaites Ice Tongue and on cutting channels on the underside of both the Thwaites Ice Tongue and Ice Shelf; and 6. The probable early loss of the pinning points on the Thwaites Ice Shelf and Ice Tongue due to accelerated calving and break-up around these pinning points (see the fourth image).
Furthermore, I suspect that Alley will use computer generated estimates of the warm CDW flow into the ASE; which as indicated by the following underestimates observations of the CDW in the Southern Ocean during the recent period of negative IPO:
Sunke Schmidtko, Karen J. Heywood, Andrew F. Thompson, Shigeru Aoki (2014), "Multidecadal warming of Antarctic waters", Science 5 December 2014: Vol. 346 no. 6214 pp. 1227-1231 DOI: 10.1126/science.1256117 http://www.sciencemag.org/content/346/6214/1227
Abstract: "Decadal trends in the properties of seawater adjacent to Antarctica are poorly known, and the mechanisms responsible for such changes are uncertain. Antarctic ice sheet mass loss is largely driven by ice shelf basal melt, which is influenced by ocean-ice interactions and has been correlated with Antarctic Continental Shelf Bottom Water (ASBW) temperature. We document the spatial distribution of long-term large-scale trends in temperature, salinity, and core depth over the Antarctic continental shelf and slope. Warming at the seabed in the Bellingshausen and Amundsen seas is linked to increased heat content and to a shoaling of the mid-depth temperature maximum over the continental slope, allowing warmer, saltier water greater access to the shelf in recent years. Regions of ASBW warming are those exhibiting increased ice shelf melt."https://robertscribbler.wordpress.com/tag/circumpolar-deep-water/
Extract: "For a study this week confirmed that Antarctica is now seeing a yearly loss of ice equal to one half the volume of Mt Everest every single year. A rate of loss triple that seen just ten years ago. An acceleration that, should it continue, means a much more immediate threat to coastal regions from sea level rise than current IPCC projections now estimate.
According to a new study led by Sunke Schmidtko, this deep water current has been warming at a rate of 0.1 degrees Celsius per decade since 1975. Even before this period of more rapid deep water warming, the current was already warmer than the continental shelf waters near Antarctica’s great glaciers. With the added warming, the Circumpolar Deep Water boasts temperatures in the range of 33 to 35 degrees Fahrenheit — enough heat to melt any glacier it contacts quite rapidly.
Out in the deep ocean waters beyond the continental shelf zone surrounding Antarctica, the now warmer waters of this current can do little to effect the great ice sheets. Here Sunke’s study identifies the crux of the problem — the waters of the Circumpolar Deep Water are surging up over the continental shelf margins to contact Antarctica’s sea fronting glaciers and ice shelves with increasing frequency.
In some cases, these warm waters have risen by more than 300 feet up the continental shelf margins and come into direct contact with Antarctic ice — causing it to rapidly melt. This process is most visible in the Amundsen Sea where an entire flank of West Antarctica is now found to be undergoing irreversible collapse. The great Pine Island Glacier, the Thwaites Glacier and many of its tributaries altogether composing enough ice to raise sea levels by 4 feet are now at the start of their last days. All due to an encroachment of warm water rising up from the abyss."
Furthermore, the following Weber et al 2014 reference makes it clear, by studying ice-rafted debris from the 26,000–19,000 years ago, how important the ocean temperature is on determining ice mass loss:
M. E. Weber, P. U. Clark, G. Kuhn, A. Timmermann, D. Sprenk, R. Gladstone, X. Zhang, G. Lohmann, L. Menviel, M. O. Chikamoto, T. Friedrich, & C. Ohlwein (05 June 2014), "Millennial-scale variability in Antarctic ice-sheet discharge during the last deglaciation", Nature, Volume: 510, Pages: 134–138, doi:10.1038/nature13397http://www.nature.com/nature/journal/v510/n7503/abs/nature13397.html
Abstract: "Our understanding of the deglacial evolution of the Antarctic Ice Sheet (AIS) following the Last Glacial Maximum (26,000–19,000 years ago) is based largely on a few well-dated but temporally and geographically restricted terrestrial and shallow-marine sequences. This sparseness limits our understanding of the dominant feedbacks between the AIS, Southern Hemisphere climate and global sea level. Marine records of iceberg-rafted debris (IBRD) provide a nearly continuous signal of ice-sheet dynamics and variability. IBRD records from the North Atlantic Ocean have been widely used to reconstruct variability in Northern Hemisphere ice sheets, but comparable records from the Southern Ocean of the AIS are lacking because of the low resolution and large dating uncertainties in existing sediment cores. Here we present two well-dated, high-resolution IBRD records that capture a spatially integrated signal of AIS variability during the last deglaciation. We document eight events of increased iceberg flux from various parts of the AIS between 20,000 and 9,000 years ago, in marked contrast to previous scenarios which identified the main AIS retreat as occurring after meltwater pulse 1A and continuing into the late Holocene epoch. The highest IBRD flux occurred 14,600 years ago, providing the first direct evidence for an Antarctic contribution to meltwater pulse 1A. Climate model simulations with AIS freshwater forcing identify a positive feedback between poleward transport of Circumpolar Deep Water, subsurface warming and AIS melt, suggesting that small perturbations to the ice sheet can be substantially enhanced, providing a possible mechanism for rapid sea-level rise."http://www.climatecentral.org/news/warm-water-invasion-fueling-striking-antarctic-ice-melt-18401
Extract: "What the team found was that the waters circulating around the Antarctic continental shelf have warmed considerably in some areas. In the Bellingshausen and Amundsen Seas, the waters had warmed by about 0.2 to 0.5°F per decade since the 1990s, which meshed with the melting trends seen in both areas.
In fact, the other new study found that the melt rate of the glaciers of the Amundsen Sea Embayment had tripled in just the past decade.
"The mass loss of these glaciers is increasing at an amazing rate," study co-author Isabella Velicogna, also of UC Irvine and JPL, said in a statement.
That study compared data of ice loss from the glaciers from four different sources and found that they all told a similar story.
“It’s sort of a confirmation because it’s using multiple techniques and the signal is so big you can’t miss it,” Rignot, who was not involved with either new study, said."
Therefore, due to difficulties with modeling the true risks, I suspect that policymakers will continue to take inadequate action no matter how masterfully Hansen et al explain the over all risk from ASLR.