IT TOOK three years to write and contains six years' worth of research. The full report, to be published later this year, will contain 11 chapters. One chapter alone, seen by New Scientist, runs to 150 pages and includes more than 850 references. The authors say it will resolve many critical questions about climate change and support the unequivocal language of the summary published last week.
Warming is now an "incontrovertible" fact for two reasons, the scientists say. First, because doubts raised by satellite data - which suggested that recent warming has been far less than surface thermometers indicate - have now been resolved. In short, the thermometers have been shown to be right (New Scientist, 20 August 2005, p 10). In any case, nothing else but global warming can explain the rapid melting of ice round the world.
The declaration that warming is "very likely" to be due to human activity is justified by an increasing agreement between measurements from the real world and the detailed predictions of statistical models of warming. "It is a very rigorous statistical analysis, comparing measurements and models in space and time in a more detailed way than ever before," says Susan Solomon, head of the US group that led the assessment. Key to this has been the observed greater warming over land masses compared with the oceans, and the combination of warming in the lower atmosphere with cooling in the stratosphere.
Researchers also claim to have a better idea of how much warming from greenhouse gases is being masked by dust and smoke aerosols put into the air by human activity. This again improves the match between models and the real world.
The team is also much more confident now about the unique nature of recent warming. The IPCC's 2001 summary report was heavily criticised for including a graph - known as the "hockey stick" - which purported to show that the world is now warmer than for at least the past 1000 years. The claim was based on sporadic proxy data such as tree rings, and was widely attacked. Now a huge amount of extra data collected since 2001 "all supports the interpretation that warming in the past half-century is unusual in at least the last 1300 years", the summary says.
Where does this leave the prognosis for the planet? The report sets the likely range of average temperature changes for a doubling of carbon dioxide concentrations - expected around the end of the century - at between 2 and 4.5 °C. This is roughly in line with previous reports, though this time it adds, with a nod to possible positive feedbacks, "values substantially higher than 4.5 °C cannot be excluded".
This amount of warming will likely deliver an ice-free Arctic and a 30 per cent drop in rainfall in many subtropical regions, including a huge area from the Mediterranean and North Africa through the Middle East to central Asia, and another across southern Africa. Meanwhile, higher latitudes will get wetter as the air warms and storm tracks move, and hurricanes will become more intense.
Global warming, the report says, contains a deadly time lag. That's because 80 per cent of the extra heat currently being trapped by man-made greenhouse gases is being drawn into the oceans. As the oceans warm, more of that heat will remain in the air. Even if emissions of greenhouse gases were sharply reduced, the world would continue to warm by 0.1 °C per decade for some time.
The solar effect
It is one of the few areas where the sceptics' argument has had some force. What role has the sun played in recent climate change? As if to underline the controversy, last week's debate on this issue lasted some 10 hours.
The IPCC scientists wanted to halve their previous estimate of the maximum possible solar influence on warming over the past 250 years, from 40 per cent to 20 per cent. Government delegations from China and Saudi Arabia refused to accept that, based on new ideas about cosmic rays from outer space.
Cosmic rays ionise the atmosphere, which could, the theory goes, create clouds. Thus, anything that reduces the amount of cosmic rays could diminish cloud cover and so warm the Earth's surface. An increase in solar activity would do just that - by deflecting cosmic rays away from Earth. China and Saudi Arabia were buoyed by claims that small changes in radiation from the sun could be amplified by their potential effect on clouds. Thus, they said, the sun could have a greater effect than the scientists claimed.
Most climate scientists are unconvinced. "Right now there is no evidence," says IPCC author Piers Forster of the University of Leeds, UK. In any case, IPCC scientists believe, most of today's warming can be explained by man-made influences
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