Blowing Hot and Cold Climate change may be slow and uncertain, but that is no excuse for inaction. One reason why uncertainty over climate change looks to be with us for a long time is that the oceans, which absorb carbon from the atmosphere, act as a time-delay mechanism. Their massive thermal inertia means that the climate system responds only very slowly, to changes in the composition of the atmosphere. Another complication arises from the relationship between carbon dioxide (CO2), the principal greenhouse gas (GHG), and sulphur dioxide (SO2), and a common pollutant. Efforts to reduce man-made emissions of GHGs by cutting down on fossil-fuel use will reduce emissions of both the gases. The reduction in CO2 will cut warming, but the concurrent (同时发生的) SO2 cut may mask that effect by contributing to the warming. There are so many such fuzzy (模糊的) factors—ranging from aerosol particles to clouds to cosmic radiation—that we are likely to see disruptions of familiar climate patterns for many years without knowing why they are happening or what to do about them. Tom Wigley, a leading climate scientist and member of the UN's Intergovernmental Panel on Climate Change (IPCC), goes further. He argues in an excellent book published by the Aspen Institute, 'US Policies on Climate Change: What Next?', that whatever policy changes governments pursue, scientific uncertainties will 'make it difficult to detect the effects of such changes, probably for many decades.' As evidence, he points to the negligible short-to medium-term difference in temperature resulting from an array of emission 'pathways' on which the world could choose to embark if it decided to tackle climate change. He plots various strategies for reducing GHGs that will lead in the next century to the stabilization of atmospheric concentrations of CO2 at 550 parts per million (ppm). That is roughly double the level which prevailed in pre-industrial times, and is often suggested by climate scientists as a reasonable target. But even by 2040, the temperature differences between the various options will still be tiny—and certainly within the magnitude of natural climatic variance. In short, in another four decades we will probably still not know if we have over-or under-shot. Ignorance is not bliss However, that does not mean we know nothing. We do know, for a start, that the 'greenhouse effect' is real: without the heat-trapping effect of water vapor, CO2, methane and other naturally occurring GHGs, our planet would be a lifeless 30℃ or so colder. Some of these GHG emissions are captured and stored by 'sinks', such as the oceans, forests and agricultural land, as part of nature's carbon cycle. We also know that since the industrial revolution began, mankind's actions have contributed significantly to that greenhouse effect. Atmospheric concentrations of GHGs have risen from around 280ppm two centuries ago to around 370ppm today, thanks chiefly to mankind's use of fossil fuels and, to a lesser degree, to deforestation and other land-use changes. Both surface temperatures and sea levels have been rising for some time. There are good reasons to think temperatures will continue rising. The IPCC has estimated a likely range for that increase of 1.4℃-5.8℃ over the next century, although the lower end of that range is more likely. Since what matters is not just the absolute temperature level but the rate of change as well, it makes sense to try to slow down the increase. The worry is that a rapid rise in temperatures would lead to climate changes that could be devastating for many (though not all) parts of the world. Central America, most of Africa, much of south Asia and northern China could all be hit by droughts, storms and floods and otherwise made miserable. The colder parts of the world may benefit from warming, but they too face danger. One is the conceivab