As delegates from around the world meet in Montreal to discuss climate change, what science should inform their deliberations?
THE climate changes. It always has done and it always will. In the past 2m years the temperature has gone up and down like a yo-yo as ice ages have alternated with warmer interglacial periods. Reflecting this on a smaller scale, the 10,000 years or so since the glaciers last went into full-scale retreat have seen periods of relative cooling and warmth lasting from decades to centuries. Against such a noisy background, it is hard to detect the signal from any changes caused by humanity's increased economic activity, and consequent release of atmosphere-warming greenhouse gases such as carbon dioxide.