Tuesday, January 3, 2012

Hansen Still Argues 5m 21st C Sea Level Rise Possible


This is interesting - here is the latest paper from James Hansen and coauthor Miki Sato Paleoclimate Implications for Human-Made Climate Change.  If you are up to reading climate science papers it's highly recommended (I'm a little slow in getting to it - the press release was Dec 8th 2011 but I just got to reading it yesterday and today).

A little background is in order - one of the serious scientific debates in the climate science community over the last decade has been the implications of the unexpectedly large acceleration of glacier discharge in Greenland and Antarctica and in particular a discovery by Zwally et al in 2002 that surface melt water can get down to the base of a glacier and lubricate its motion.  Prior to the early 2000s it was assumed that ice sheets would decay mainly by melting on the surface and climate models all assumed that they would decay only very slowly in a warmer world - it was a surprise to realize that the most important breakdown mode was actually basal lubrication and sliding down into the ocean.

Hansen in particular became the leading spokesman for the view that the ice sheets on Greenland and parts of Antarctica would prove quite unstable under Anthropocene conditions and might break down in a rapid non-linear manner and cause very large levels of twenty-first century sea level rise.  See for example this essay from 2005 in which he says:
Consider the situation during past ice sheet disintegrations. In melt-water pulse 1A, about 14,000 years ago, sea level rose about 20 m in approximately 400 years (Kienast et al., 2003). That is an average of 1 m of sea level rise every 20 years. The nature of glacier disintegration required for delivery of that much water from the ice sheets to the ocean would be spectacular (5 cm of sea level, the mean annual change, is about 15,000 cubic kilometers of water). “Explosively” would be an apt description, if future ice sheet disintegration were to occur at a substantial fraction of the melt-water pulse 1A rate.

Are we on a slippery slope now? Can human-made global warming cause ice sheet melting measured in meters of sea level rise, not centimeters, and can this occur in centuries, not millennia? Can the very inertia of the ice sheets, which protects us from rapid sea level change now, become our bete noire as portions of the ice sheet begin to accelerate, making it practically impossible to avoid disaster for coastal regions?
This kind of nigh-apocalyptic rhetoric from a very senior and respected climate scientist provoked a flurry of papers in response seeking to analyze the situation.  Most of these suggested various reasons why 21st century sea level rise, while likely worse than previously projected (for example in the 3rd IPCC report in 2001), would not be as bad as the worst fears of Hansen.  Hansen and Sato's own description of this new literature seems fair to me:
Rahmstorf (2007) made an important contribution to the sea level discussion by pointing
out that even a linear relation between global temperature and the rate of sea level rise, calibrated with 20th century data, implies a 21st sea level rise of about a meter, given expected global warming for BAU greenhouse gas emissions.  Vermeer and Rahmstorf (2009) extended Rahmstorf's semi-empirical approach by adding a rapid response term, projecting sea level rise by 2100 of 0.75-1.9 m for the full range of IPCC climate scenarios. Grinsted et al. (2010) fit a 4-parameter linear response equation to temperature and sea level data for the past 2000 years, projecting a sea level rise of 0.9-1.3 m by 2100 for a middle IPCC scenario (A1B).  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 these approximated as linear in his graph.  The 5 m estimate is what Hansen (2007) suggested was possible under IPCC's BAU climate forcing.  Such a graph is comforting – not only does the 5-meter sea level rise disagree with all other projections, but its half-meter sea level rise this decade is clearly 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, and that IPCC's BAU forcing is so huge that it is difficult to see how ice shelves would survive.  As warming increases, the number of ice streams contributing to mass loss will increase, contributing to a nonlinear response that should be approximated better by an exponential than by a linear fit.  Hansen (2007) suggested that a 10-year doubling time was plausible, and pointed out that such a doubling time, from a 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.

Nonlinear ice sheet disintegration can be slowed by negative feedbacks.  Pfeffer et al.
(2008) argue that kinematic constraints make sea level rise of more than 2 m this century
physically untenable, and they contend that such a magnitude could occur only if all variables quickly accelerate to extremely high limits.  They conclude that more plausible but still accelerated conditions could lead to sea level rise of 80 cm by 2100
I had been following this debate and reading the papers in question and had been somewhat reassured that 21st century sea level rise would be not too problematic for civilization at large (though it clearly would be very painful for coastal property owners and jurisdictions).

(Before we go on it's worth emphasizing the important aside - hardly any climate scientists doubt that huge quantities of the Greenland and Antarctic ice sheets would eventually melt and cause tens of meters of sea level rise as a result of human climate modifications - the debate is solely about how much of the consequences of our actions we will experience in the 21st century).

However, Hansen is not reassured by these new papers and is doubling down:
The kinematic constraint may have relevance to the Greenland ice sheet, although the assumptions of Pfeffer at al. (2008) are questionable even for Greenland. They assume that ice streams this century will disgorge ice no faster than the fastest rate observed in recent decades. That assumption is dubious, given the huge climate change that will occur under BAU scenarios, which have a positive (warming) climate forcing that is increasing at a rate dwarfing any known natural forcing. BAU scenarios lead to CO2 levels higher than any since 32 My ago, when Antarctica glaciated. 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.

The main flaw with the kinematic constraint concept is the geology of Antarctica, where large portions of the ice sheet are buttressed by ice shelves that are unlikely to survive BAU climate scenarios. West Antarctica's Pine Island Glacier (PIG) illustrates nonlinear processes already coming into play. The floating ice shelf at PIG's terminus has been thinning in the past two decades as the ocean around Antarctica warms (Shepherd et al., 2004; Jenkins et al., 2010). Thus the grounding line of the glacier has moved inland by 30 km into deeper water, allowing potentially unstable ice sheet retreat. PIG's rate of mass loss has accelerated almost continuously for the past decade (Wingham et al., 2009) and may account for about half of the mass loss of the West Antarctic ice sheet, which is of the order of 100 km^3 per year (Sasgen et al., 2010).

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 the West Antarctic ice sheet, with at least 5 m of sea level, and about a third of the East Antarctic ice sheet, 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).
However, probably their main point is that the data we have on the Antarctic/Greenland meltdown is relatively short and is consistent with the idea that it's doubling with a relatively short (decade or less) timescale and if you extrapolate that out over the 21st century you get to very large values of sea level rise (a point I made in a blog post back in 2006).  This leads them to include this figure (which I take to be a conceptual sketch rather than an exact forecast):


The picture that emerges is a relatively slow manageable sea level rise in the first part of the century followed by increasingly catastrophic levels of change in the latter part of the century as the rapid breakdown of the ice sheets overwhelms everything else.

I take Hansen's opinions very seriously.  It's certainly true that there isn't enough data to rule out this scenario yet (though another decade of data should help a lot).  Obviously at this point he hasn't succeeded in persuading most of his colleagues, but neither have they persuaded him.  Only more data is likely to resolve the situation.

8 comments:

ColdNorth said...

James Hansen certainly makes interesting reading - I am a third of the way through his 2009 "Storms of my Grandchildren" - an interesting mix of science and perspectives by a very experienced and successful scientist on the failures of making effective policy.

The Pine Island Glacier appears to be calving a glacier at about the same point as it did in 2001 (see the images from 2011 and 2001 at http://earthsky.org/earth/crack-in-pine-island-glacier-and-birth-of-an-iceberg). This conflicts with Hansen's comment that the terminus of the glacier is retreating. It is possible that the velocity of flow of this glacier is increasing.

I have been wondering what happened to the Wilkins ice sheet. Back in 2008 there was a big focus on the collapse of this ice bridge and the animations were dramatic. However, I have not seen subsequent information suggesting that the ice sheet has in-fact diappeared. Anyone have more recent evidence that the disappearance of the ice bridge did in-fact lead the larger ice-shelf to disappear?

Chris R said...

Stuart,

I think that the point of the graphic you present is that with an exponential series the initial rate of increase may seem small (compared to linear) but can grow to a large value. So relying on linear reasoning based on current rates of ice-loss / SLR is unsound.

Hansen has correctly pointed out the mass loss from Greenland (and I see from the pre-print figure 8 Antarctica) can be taken as indicative of a doubling in the decade of mass loss assessments. Time will tell whether this is the case.

Rigid sceptical scientific reasoning can give us conservative estimates of the outcome of this process during this century. What Hansen is doing is trying to stitch together disparate evidence from a variety of sources to form a less formal estimate. I last read this paper back in July when it was available as a pre-print. At the time I recall finding it horribly persuasive.

I think the concentration on 2100 is short-sighted. What I find troubling is the paleo-climate evidence regards long term SLR, e.g. from the pre-print: "Global warming of 2°C would make Earth much warmer than in the Eemian, when sea level was 4-6 meters higher than today." As Armour & Roe and Archer show; if we overshoot 2 degrees we can commit the future to such sea-level rises.

For most of this century I find Hansen's recent white paper "Climate Variability and Climate Change: The New Climate Dice" more alarming. http://dosbat.blogspot.com/2011/11/hansens-climate-dice.html

Stuart Staniford said...

Chris R:

Thanks - I hadn't seen the other Hansen paper you mention. Figure 4 is definitely an "Oh shit" moment isn't it? I'll try to read the whole thing in the morning.

Greg said...

I'm glad you got to this, Stuart.

The thing that bothers me about Hansen's scenario is that exponentials show up when the rate of change of something is a fixed proportion of the level of the something.

Exponentials tend to appear in simple, unconstrained systems - populations of bacteria on a petri dish, or the human population after the discovery of fertilizer and the microbe theory of disease, say.

I can't find any plausible candidate, in terms of the physics, of what the "something" might be in the case of the ice sheets. I don't know that ice sheet dynamics can be described as 'simple' in the same way as unconstrained populations. There are many physical mechanisms at work, and no single one of them dominates, as I understand it.

On the other hand, complex systems are rarely linear. Often they can change from one state to another, and during the transition they can appear effectively unconstrained, so exponential-like behaviour can show up for a short time. (For the ice sheets, a few centuries might be considered a short time.) As well as more data, we need better models.

sunbeam said...

Looking at the link posted above,

http://dosbat.blogspot.com/2011/11/hansens-climate-dice.html

What does that drought severity global index map actually mean?

Are Arkansas and Louisiana going to have the rainfall patterns of present day Nevada?

I know these maps aren't good news, but what exactly are we talking about here?

Mississippi with a climate like current day Greece? Algeria? The Sahara?

Stuart Staniford said...

Sunbeam - that's a PDSI map - you can get a lot of background on the PDSI from my "Future of Drought" series up in the "Recent Favorites" at top right. However, the PDSI doesn't facilitate the comparison you want to make because it's basically comparing current drought conditions to the historical distribution for the same locality.

You may find the map of "Number of days over 100F" in
http://earlywarn.blogspot.com/2010/03/us-in-high-emissions-scenario.html

helpful for your purpose

biffvernon said...

>Only more data is likely to resolve the situation.

By which time it will be too late to do anything about it. Bother. Nice idea, the precautionary principle.

Chris R said...

Biffvernon,

Another decade of sticking to the exponential would be enough for me. As it is the intense warm events and drought are more of a concern this century.

Stuart, Sunbeam,

I've just remembered that I crossed out the 'Palmer' in the legend. That was because strictly speaking the index is a development of the PDSI.

The source of that is "Drought under global warming: a review." By Aiguo Dai. You'll find a good plain speaking page here:
http://www2.ucar.edu/news/2904/climate-change-drought-may-threaten-much-globe-within-decades

The graphics on there may help you Sunbeam because you can compare how things are now to the model projections.

As the Arctic transitions to a seasonally sea-ice state there will be substantial impacts on NH climate - these are being identified and pinned down at present. What this may do to the drought pattern is an unknown.