Since the turn of the century the planet has been heating up at only half the rate claimed by scientists in the 2007 report of the Intergovernmental Panel on Climate Change. Sceptics see this as an "inconvenient truth", a term usually associated with evidence of global warming. The slowdown is unexpected after rapidly rising greenhouse gas emissions from big developing nations led by China. The latest IPCC report explains it away by pointing out that 15-year hiatus periods are common in historical climate records.
In any case, the slowdown has not shaken scientists' belief in climate change and the consequences unless governments take action, and that human activities are to blame. The report says the latter probability has risen to 95 per cent from 90 per cent in 2007. As a result, greenhouse gas concentrations have risen to levels unprecedented in at least 800,000 years; sea levels have risen faster than in 2,000 years; and climate extremes have become more severe.
The IPCC reports form the scientific basis of United Nations negotiations on a successor to the Kyoto Protocol, the sole legally binding plan to halt growing greenhouse gas emissions. Kyoto applies only to rich, developed nations - except for the US, which never ratified it. Negotiations on a new treaty to take effect from 2020, under which all countries would tackle global warming, brings to a head the question of how to divide responsibility between the rich world, which is historically to blame for most emissions, and developing nations. This issue is so vexed that one idea under discussion is voluntary reductions to be made subject to legally binding rules.
Meanwhile, emissions targets are being missed and extreme weather events such as heat waves and freezes, flood and drought are forming a pattern. Climate-change negotiators have until 2015 to find how nations can work together effectively to cut emissions. It is up to China and the US, to which the developing and developed worlds respectively look for leadership, to show the way forward.