[asa] AAAS update since IPCC 2007

This morning I attended a symposium on "What is new and surprising since the IPCC fourth assessment?". You may have seen the press announcement that Chris Field made at noon today. He was the first speaker of the symposium.

Chris Field, Carnegie Institution for Science, Stanford, CA, "Carbon-Climate System and the Terrestrial Biosphere"
  The AR4 assessments were the input assumptions used in the IPCC Fourth assessment. The carbon emission levels projected for the future were based on various possible (not probable) scenarios extrapolating from past measurements. In the 1990's the rate of increase of carbon emissions was 0.9%/year. This was analyzed by breaking down the emission into three factors: (PgC/GDP)*(GDP/P)*P where P = population growth, GDP = global gross domestic product, and PgC = petagrams of carbon emitted. The trend has been for P and GDP/P to increase steadily at fairly stable and predictable rates. PgC/GDP has been gradually dropping and was predicted to continue dropping.
 The new news in the last 3-4 years (2005 was the cutoff date for data for papers taken into account by IPCC fourth assessment) is that PgC/GDP has unexpectedly been increasing for reasons that are still not understood. The result has been that PgC has gone up substantially more than in any of the entire range of scenarios considered in AR4. In fact, the increase from 2000 to 2007 averaged 3.5%/year, a major unforeseen increase.
 With the current economic crisis one might think that GDP/P would drop a bit but that is likely to be offset by an even further increase in PgC/GDP as the economy entices the use of cheaper fuel which is more carbon intensive. The impact is that due to this effect alone, global carbon emissions need to be reduced by another 50 PgC beyond the IPCC estimate.

 Then Chris talked about two specific feedback mechanisms which had not been included in AR4 since they were not understood sufficiently at that time. The first one was the effect of the forest area feedback. This could theoretically have gone either way. As global warming encourages more rapid growth of trees, it could lead to more rapid absorption of carbon and hence a negative feedback loop. But when total land use is taken into account with deforestation and fires (which are more frequent as warming trends make it more flammable) then it could be a positive feedback. Now it has been determined that the effect is a positive feedback and the additional required reduction in carbon emission is 100-510 PgC.

 The second positive feedback not considered is the permafrost. It is now known that 24% of the northern hemisphere is permafrost which has significant amounts of methane equivalent to 960 PgC. As warming occurs, the permafrost melts and releases the methane, causing another positive feedback. Calculations now indicate that up to 90% of the permafrost could be lost and this leads to an increased required reduction of 100-750 PgC which is still a large range of uncertainty.

  In sum, just these effects which were not in IPCC give a total range of 250-1300 PgC of additional required reduction in carbon emission. That is not impossible but makes the job tougher to accomplish.

Peter Lemke, Alfred Wegener Institute for Polar and Marine Research, Potsdam, Germany "Climate Change in the Arctic"
 Sea ice in the Arctic region has historically been decreasing in area at the rate of -2.7%/decade in summer and very little in winter. More recently the reduction has been -7.4%/decade in summer and -2.8%/decade in winter. The last few years, especially 2007 and 2008 have seen a dramatic reduction so that the rate of loss is -11.8%/decade in summer. This is well above the AR4 assumption.
 The average ice thickness has been reduced from 2.5 to 1 meter. An elaborate submerged buoy system has measured temperature and flow rate of north Atlantic ocean water into the Arctic region. The water temperature at a depth of 250 meter was a record warm temperature in 2007 and a little cooler in 2008. This warmer water and a stable high pressure area that brought wind into the region are most likely responsible for the recent record loss of ice.
 Measured sea ice is already below the predictions of the scenarios in the IPCC report. Warming in the arctic region is about 2X that of the global warming rate. The loss of albedo as sea ice is lost is another postive feedback loop.
Net: the Arctic has warmed and freshened at a greater rate than envisioned.

I missed the remaining talks since I had to go to the meeting on an AAAS Coaltion of Affiliations on Science and Religion. But I talked with other attendees about the presentation on sea level rise. I'm still trying to confirm the quantitative results but this is what I learned. The historical rate of sea level is around 0.1mm/yr but has recently been 1.9mm/yr. About half of this was found to be due to thermal expansion as the water warms and the other half due to ice melt. The latest data indicate that the rise now is 3.5mm/yr and that 80% is due to ice melt and only 20% due to thermal expansion. This makes a significant difference. The IPCC based its projections on future sea level rise only on the influence of warming on thermal expansion but not on an increase in ice melt. This new data will force a significant increase in projected sea level rise.

Net: things aren't getting better.

Randy

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Received on Sun Feb 15 01:11:34 2009