The evidence that global temperatures are increasing, ice caps melting and sea levels rising is "unequivocal", according to a major report issued by the IPCC on 2 February. The publication is a scientific summary for policymakers and the first part of the IPCC’s long-awaited fourth assessment report.
The 600-plus scientists behind it say they are more than 90% sure the changes are the result of human activity and warn that heat waves, droughts and rainstorms will become more common. The panel’s third report, released in 2001, put the likelihood of a causal link at 66-90%.
The estimates of temperature rises by 2100 have also tightened since the last report. Six years ago, the IPCC quoted a possible range of 1.4-5.8°C, with the exact figure depending on which scenario future emissions followed (ENDS Report 312, p 5 ).
It now offers a "best estimate" of average temperatures under each scenario. These range from 1.8°C in a service-industry dominated world where clean and more efficient energy becomes the norm to 4°C where rapid, fossil fuel-intensive economic growth continues until 2050.
The increased certainty reflected in these figures is the result of more sophisticated computer models and greater consideration of carbon cycle processes such as absorption by vegetation. But these processes can have a feedback effect, increasing uncertainty. This is reflected in the overall range of "likely" warming, which is now estimated to be 1.1-6.4°C (ENDS Report 381, pp 34-36 ).
A string of 12 exceptionally warm years - 11 of which were the warmest on record - also pushed up the IPCC’s estimate of human-induced warming to date from 0.6°C in 2001 to 0.74°C. The effect would have been even greater if particulate and aerosol pollution had not reflected some sunlight back into space.
Environment Secretary David Miliband branded the IPCC’s report "another nail in the coffin of the climate deniers" and said it confirmed the urgent need for "international political commitment".
The Bush administration’s response briefly mentioned that the report would be "valuable" to policymakers and that it confirmed mankind’s responsibility for the existing warming. But most of the 600-word statement was devoted to the workings of the IPCC and praise for the US’s climate research programme. Not surprisingly it failed to mention recent allegations about the gagging of leading climate scientists.
The Cato Institute in the US, a leading critic of the IPCC’s findings, said its own models suggest the warming rate is at the lowest end of the IPCC’s spectrum and is not increasing. The best policy, it argues, is to wait for economic growth to provide capital for investment in clean technologies.
The IPCC warns that about 0.4°C of warming can be expected over the next 20 years, irrespective of which scenario emissions follow. This is because the world’s oceans take a long time to warm, creating a time lag. Even if greenhouse gas emissions ceased today there would still be 0.2°C of warming by 2027.
Predictions of total sea level rise by the end of the century have narrowed from 9-88cm to 28-58cm in the latest report. But there are uncertainties about the rate of ice sheet melt and the report notes that the last time temperatures were 3-5°C warmer, sea levels eventually reached a level four to six metres higher than today.
The panel will offer more detailed recommendations on climate change mitigation and adaptation, and an analysis of the broader impacts of climate change in two more reports to be released later this year. The full version of the scientific report is due this summer.