On Friday of this week, the Intergovernmental Panel on Climate Change (IPCC) releases the first part of its fourth report. Like the previous IPCC reports in 1990, 1996 and 2001, this document will attempt to assimilate and distill all the climate-change research done over the previous 5 years or so, and to produce a number of 'headline' statements intended to influence international political policy-makers.
Of particular interest is the predicted effect of CO2 emissions upon global temperatures by 2100. The IPCC obtains a predicted temperature range by taking the output from a number of different Global Climate Model (GCM) supercomputer simulations, conducted by various climate research institutions across the world. Whilst the 2001 report predicted a global temperature increase of 1.4-5.8°C by 2100, the New Scientist article suggests that the new predicted temperature increase will be 2.0-4.5°C. However, I am very interested to find out the specifics which underlie this prediction.
The range of the temperature uncertainty here comes both from the range of different GCMs, and the range of different emission scenarios which are input into the models. The global temperature profile over the 21st century which a GCM outputs, depends upon the 'scenarios' input into the simulation, and the fidelity of the simulation model itself. The future CO2 emission profile, (and that of the other greenhouse gases), and the sulphate emission profile, largely define a scenario. The simulation scenarios which obtained a temperature increase above 2.5°C in the 2001 IPCC report tended to assume (i) that CO2 emissions would rise by 1% each year, and that (ii) SO2 (sulphate) emissions would drop off significantly beyond 2030. See, for example, the SRES A2 scenario.
A number of critical points were made about these scenarios in Bjorn Lomborg's book, 'The Skeptical Environmentalist', and these same points raise questions about the 2007 report:
Sulphates are thought to have a cooling effect on the Earth, but because they are associated with pollution, it is assumed that legislation will remove this cooling effect beyond 2030. If it is now assumed in the 2007 scenarios that sulphate emissions will be reduced earlier, then the GCMs will predict higher temperature increases for this reason.
Actual CO2 emissions have been rising by about 0.4% throughout the 1980s and 1990s, so if this rate of increase were to continue, it follows that the 2001 IPCC scenarios greatly over-estimated the amount of global warming. However, I read now that CO2 emissions have increased by 3% over the past 5 years, which is approximately a 0.6% annual increase. This, perhaps, is a consequence of Chinese economic growth. I would like to know what CO2 increases have been assumed in the current IPCC report, and whether those assumptions can be justified by global economic projections.
The price of solar power is decreasing by 50% per decade, and, if this continues, solar power will be a competitive alternative to fossil fuels by 2030-2040. Whilst the IPCC scenario referred to as the A1T scenario does incorporate a transition to renewable energy sources circa 2040, the IPCC scenarios which generate the highest predicted temperature increases fail to take this into account.
It was noticeable in the 2001 IPCC reports that the effect of changing from one simulation model to another had a greater effect upon the projected temperature increase than the effect of changing the input scenario for a fixed simulation model. As Lomborg put it, the 'noise' created by the various simulations is greater than the strength of the signal. This casts significant doubt upon the reliability of the global climate models which underpin the IPCC reports.
The IPCC chairman, R.K. Pachauri says "I hope this report will shock people and governments into taking more serious action." My worry, however, is that the scientific understanding and data is being manipulated to meet this pre-conceived requirement. It may well be necessary to exaggerate the severity of anthropogenic global warming in order to galvanise governments into action, but the long-term consequence may be that the credibility of scientists becomes eroded in the public conscience.