Hansen 5 metres by 2100...

From Hansen today in addition to my previous post:

Its worth reading the paper in full, please:


Hansen says:

Fig. 7. Five-meter sea level change in 21st century under assumption of linear change (Alley, 2010) and exponential change (Hansen, 2007), the latter with a 10-year doubling time. 

...These projections are typically a factor of 3-4 larger than the IPCC (2007) estimates, and thus they altered perceptions about the potential magnitude of human-caused sea level change. 

   Alley (2010) reviewed projections of sea level rise by 2100, showing several clustered around 1 m and one outlier at 5 m, all of which he approximated as linear. The 5 m estimate is what Hansen (2007) suggested was possible, given the assumption of a typical IPCC's BAU climate forcing scenario. Alley's graph is comforting, making the suggestion of a possible 5 m sea level rise seem to be an improbable outlier, because, in addition to disagreeing with all other projections, a half-meter sea level rise in the next 10 years is preposterous. 
   However, the fundamental issue is linearity versus non-linearity. Hansen (2005, 2007) argues that amplifying feedbacks make ice sheet disintegration necessarily highly non-linear. In a non-linear problem, the most relevant number for projecting sea level rise is the doubling time for the rate of mass loss. Hansen (2007) suggested that a 10-year doubling time was plausible pointing out that such a doubling time from a base of 1 mm per year ice sheet contribution to sea level in the decade 2005-2015 would lead to a cumulative 5 m sea level rise by 2095.  


By mid-century most of Greenland would be experiencing summer melting in a longer melt season. Also some Greenland ice stream outlets are in valleys with bedrock below sea level. As the terminus of an ice stream retreats inland, glacier sidewalls can collapse, creating a wider pathway for disgorging ice. 
   PIG and neighboring glaciers in the Amundsen Sea sector of West Antarctica, which are also accelerating, contain enough ice to contribute 1-2 m to sea level. Most of West Antarctica, with at least 5 m of sea level, and about a third of East Antarctica, with another 15-20 m of sea level, are grounded below sea level. This more vulnerable ice may have been the source of the 25 ± 10 m sea level rise of the Pliocene (Dowsett et al., 1990, 1994). If human-made global warming reaches Pliocene levels this century, as expected under BAU scenarios, these greater volumes of ice will surely begin to contribute to sea level change. Indeed, satellite gravity and radar interferometry data reveal that the Totten Glacier of East Antarctica, which fronts a large ice mass grounded below sea level, is already beginning to lose mass (Rignot et al., 2008). 
   These data records are too short to provide a reliable evaluation of the doubling time, but, such as they are, they yield a best fit doubling time for annual mass loss of 5-6 years for both Greenland and Antarctica., consistent with the approximate doubling of annual mass loss in the period 2003-2008. There is substantial variation among alternative analyses of the gravity field data (Sorensen and Forsberg, 2010), but all analyses have an increasing mass loss with time, providing at least a tentative indication that long-term ice loss mass will be non-linear. 
   We conclude that available data for the ice sheet mass change are consistent with our expectation of a non-linear response, but the data record is too short and uncertain to allow quantitative assessment. The opportunity for assessment will rapidly improve in coming years if high-precision gravity measurements are continued. 
   Finally, we note the existence of a strong negative feedback described by Hansen (2009) that comes into play when the rate of sea level rise approaches the order of a meter per decade. Such an iceberg discharge rate temporarily overwhelms greenhouse warming, cooling high latitude atmosphere and ocean mixed layer below current levels. Ice sheet mass loss may slow in response to this cooling, but, as described qualitatively by Hansen (2009), it will be no consolation to humans. Stronger storms driven by increased latitudinal temperature gradients, combined with multi-meter sea level rise, will produce global havoc. 

   However, we must expect ice sheet mass balance changes will occur simultaneously in both hemispheres. Why? Because ice sheets in both hemispheres were in near-equilibrium with Holocene temperatures. That is probably why both Greenland and Antarctica began to shed ice in the past decade or so, because global temperature is just rising above the Holocene level. 
   Ice sheet disintegration in Antarctica depends on melting the underside of ice shelves as the ocean warms, a process well underway at the Pine Island glacier (Scott et al., 2009). The glacier's grounding line has retreated inland by tens of kilometers (Jenkins et al., 2010) and thinning of the ice sheet has spread inland hundreds of kilometers (Wingham et al., 2009). 

d. Scenarios and predictions 
   Predictions of future sea level change are inherently difficult because, we assert, ice sheet disintegration is fundamentally a non-linear process. However, in addition, the climate forcing scenario is uncertain. When predictions are made, or statements that can be construed as predictions, it is important to be clear what climate forcing scenario is being considered. 
   IPCC BAU (business-as-usual) scenarios assume that greenhouse gas emissions will continue to increase, with the nations of the world burning most of the fossil fuels including unconventional fossil fuels such as tar sands. 
   An alternative extreme, one that places a substantial rising price on carbon emissions, would have CO2 emissions beginning to decrease within less than a decade, as the world moves on energy systems beyond fossil fuels, leaving most of the remaining coal and unconventional fossil fuels in the ground. In this extreme scenario, let's call it fossil fuel phase-out (FFPO), CO2 would rise above 400 ppm but begin a long decline by mid-century (Hansen et al., 2008). 
   The European Union 2°C scenario, call it EU2C, falls in between these two extremes. 
   BAU scenarios result in global warming of the order of 3-6°C. It is this scenario for which we assert that multi-meter sea level rise on the century time scale are not only possible, but almost dead certain. Such a huge rapidly increasing climate forcing dwarfs anything in the peleoclimate record. Antarctic ice shelves would disappear and the lower reaches of the Antarctic ice sheets would experience summer melt comparable to that on Greenland today. 
   The other extreme scenario, FFPO, does not eliminate the possibility of multi-meter sea level rise, but it leaves the time scale for ice sheet disintegration very uncertain, possibly very long. If the time scale is several centuries, then it may be possible to avoid large sea level rise by decreasing emissions fast enough to cause atmospheric greenhouse gases to decline in amount. 
   What about the intermediate scenario, EU2C? We have presented evidence in this paper that prior interglacial periods were less than 1°C warmer than the Holocene maximum. If we are correct in that conclusion, the EU2C scenario implies a sea level rise of many meters. It is difficult to predict a time scale for the sea level rise, but it would be dangerous and foolish to take such a global warming scenario as a goal. 


And from me in 2009, taking an empirical view...

Nigel Williams' estimate of sea level rise.  Note the exponential curve through 5 metres rise at 2100, 

As noted in earlier postings, once ice sheet disintegration gets under way, unless there is some drastic (and currently not anticipated) reversal in climate forcings the process will simply continue until the bulk of the ice sheets are gone.

We're on our way to a very wet and wild time!

No comments:

Post a Comment