LIKE Sherlock Holmes鈥檚 dog that didn鈥檛 bark, it is the warming that hasn鈥檛 happened that has finally convinced climatologists that human activity is probably to blame for global warming.
Worldwide average temperatures have risen by half a degree in the past century 鈥� the biggest warming since the end of the last ice age. But the intensity of warming has been patchy. Nonetheless, last week the scientific working group of the UN鈥檚 Intergovernmental Panel on Climate Change, in its first full report for five years, threw aside past statements that the warming was within the range of natural variability.
Instead, for the first time, it argues that current warming 鈥渋s unlikely to be entirely natural in origin鈥� and that 鈥渢he balance of evidence suggests a discernible human influence on global climate鈥�. The reason for their new confidence is that they now believe they can explain the hitherto perplexing patchiness of the warming.
Advertisement
The chief human influence on climate is the general warming effect of the buildup of carbon dioxide and other greenhouse gases in the atmosphere -largely from burning fossil fuels. But the IPCC鈥檚 full 400-page report, to be published in April, argues that the patchiness of global warming is caused by a second human influence. In many parts of the world, including much of Europe and North America, warming has been partly masked by another form of pollution from burning fossil fuel 鈥� aerosols of sulphates and soot that form a thin haze which reduces solar heating.
The IPCC concludes that, globally, aerosols have a cooling effect of around 0.5 watts per square metre, compared with the global warming from greenhouse gases of 2.5 watts. But, because aerosols only remain in the atmosphere for a few days, their cooling effect is concentrated over industrial regions. Crucially, those are the places where warming has failed to happen.
By combining the impacts of greenhouse gases and sulphate aerosols, the IPCC says that its models of the world鈥檚 climate can now reproduce a pattern of temperature change that is 鈥渋ncreasingly evident in the observed surface air temperature data鈥� (see Diagram).
In a key chapter of the report, Ben Santer of the Lawrence Livermore National Laboratory in California and Tom Wigley of the US National Center for Atmospheric Research in Colorado, say that the pattern can be distinguished from natural variability 鈥渨ith a high level of statistical confidence鈥�.
During last week鈥檚 meeting, which was also open to governments and industry lobbyists, the scientists defeated attempts by producers and users of fossil fuels to remove this interpretation from the summary document for policy makers 鈥� due to be submitted to an IPCC governmental meeting in Rome next week. The loudest objections came from Saudi Arabia, Kuwait, and Dow Chemicals.
An earlier draft summary circulated privately in July said that 鈥渁 pattern of climate response to human activities is identifiable鈥�, but this was criticised by some IPCC scientists for going too far (This Week, 11 November). The final wording reads: 鈥淭he balance of evidence suggests a discernible human influence.鈥�
鈥淏y far the bulk of our meeting was spent going over these few words,鈥� said the chairman of the working group, John Houghton, former director of Britain鈥檚 Meteorological Office. 鈥淏ut in the end, everyone was happy with that.鈥�
The latest report predicts that a doubling of carbon dioxide levels in the atmosphere, predicted for the end of the next century, would push temperatures up worldwide by between 1 and 3.5 掳C. This is half a degree less than the IPCC鈥檚 1990 estimate, mainly, says Houghton, because estimates about increasing amounts of sulphate aerosols in the atmosphere have been included for the first time.
The continuing wide band of error in the prediction, he says, is largely because scientists do not know whether the world will become cloudier as it warms 鈥� and how those clouds will affect the climate.
However, the calculations about aerosols introduce a major new uncertainty into assessments of how global warming will influence climate in individual countries. The report suggests that efforts to reduce urban smog and the effects of acid rain in Europe and North America will reduce aerosols over those regions, exposing them to the full force of global warming for the first time.
But in countries that are industrialising now, including India and the countries of Southeast Asia, some models project an overall cooling beneath a shroud of local pollution by 2050. Climate models suggest that if global warming is considered alone, the south Asian monsoon will become more intense, causing floods. But, according to the IPCC report, if the models include the impact of aerosols, it shows that less rain will fall during the monsoon, triggering droughts.
The IPCC warns that aerosol cooling 鈥渋s not a simple offset of the warming effect of greenhouse gases鈥�. The phenomena are very different. While carbon dioxide has a lifetime in the atmosphere of decades or centuries, aerosols usually last only a few days. So while greenhouse gases constantly build up in the atmosphere, aerosols do not. Aerosols鈥� ability to mask warming in the future is much less predictable than the warming effect itself, but is likely to diminish.
And if all emissions from fossil fuel burning were to cease overnight, the cooling from aerosols would have ended within a week, while the greenhouse effect of carbon dioxide would persist for centuries.
As the world鈥檚 climate evolves beyond the boundaries of recent experience, there is a greater chance of 鈥渟urprises and unanticipated rapid changes鈥�, says the IPCC. Among the most likely, it speculates, is a radical disruption in the deep ocean circulation. This is driven by the formation of ice in the North Atlantic and, among other things, sustains the Gulf Stream, which keeps Western Europe several degrees warmer than it would be otherwise.
Some models predict a shift in the circulation, which could 鈥渃ause parts of Europe to become cooler as the rest of the world warms鈥�, says Houghton. Is it likely? Ocean circulation has been very different in the past, reports the IPCC, with changes occurring suddenly, triggering alterations in average temperatures in the North Atlantic region of about 5 掳C over a few decades.
