ON NOVEMBER 2ND the Intergovernmental Panel on Climate Change (IPCC), which represents mainstream scientific opinion, said that it was extremely likely that climate change is the product of human activity. Extremely likely in IPCC speak means having a probability of over 95%. The claim forms part of its fifth assessment on the state of the global climate. In its first assessment, in 1990, the IPCC had said that "the observed increase [in air temperatures] could be largely due to natural variability." Why have climate scientists become so much more certain that climate change is man-made, not natural?

Many factors influence the climate but perhaps the single most important is carbon dioxide (CO2). CO2 absorbs infra-red heat at a constant rate and at a higher rate than nitrogen and oxygen?the main constituent parts of the atmosphere?so the more CO2 in the air, the more the atmosphere will tend to warm up. Scientists attribute climate change to human activity mainly because people have been responsible for large increases in CO2. At the start of the industrial revolution, in about 1800, there were 280 parts per million (ppm) of CO2 in the atmosphere. That had been the level for most of human history. This year, however, concentrations exceeded 400 ppm, the first time it had reached that level for a million years.

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