FOUR years ago, a vast part of the Larsen B ice shelf broke away from the coast of the Antarctic peninsula exposing sediments that had been covered in ice for 10,000 years. Elsewhere, most of the world鈥檚 glaciers are shrinking. The extent of sea ice in the Arctic has fallen by a fifth in less than 30 years. This week we have yet more examples of ecosystems shifting to keep up with changing temperatures: the tree line in Siberia is moving north, and marine mammals are being pushed northwards as warmth-loving fish invade their feeding grounds in the northern Bering Sea (see 鈥淲hales move north as oceans warm鈥�).
You don鈥檛 have to consult a thermometer to see that the world is warming. Why is it happening? It cannot be explained by increases in the amount of sunlight reaching Earth or a change in the planet鈥檚 orbit, or by any other natural factors that influence major climatic events such as ice ages. But since pre-industrial times there has been a 35 per cent increase in concentrations of carbon dioxide in the atmosphere. Carbon dioxide traps heat. Hence, it is the most likely explanation we have for the rising temperature.
It is the job of scientists to try to find flaws in this theory. That is how science progresses: exposing errors and finding solutions to them leads to a better theory 鈥� or a new one altogether. Indeed, there have been discrepancies in the anthropogenic theory of global warming that could have seriously undermined it, such as measurements suggesting the temperature of the atmosphere closest to Earth鈥檚 surface was falling, not rising. Last summer, though, that anomaly was explained by simple measurement errors (快猫短视频, 20 August 2005, p 10).
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What about that contentious hockey stick graph? The graph is a reconstruction of temperatures in the northern hemisphere over the past 1000 years based on clues such as tree rings and ice cores. Its shape shows temperatures are higher now than in the last 1000 years. Some climate change sceptics argue that the graph is plain wrong, an artefact of unsound statistics. Flaws in the original graph have been corrected, but there is still plenty to argue about, given the uncertainties inherent in reconstructing past climate. Still, the hockey stick has proved remarkably robust in the face of numerous attempts to demolish it (see 鈥淕rudge match鈥�).
Inevitably, climate sceptics have used the supposed flaws in the graph to attack the whole notion of global warming. This weapon has been wielded most notably by influential Republican politicians in the US, such as Texas representative Joe Barton. It would be an achievement indeed if they were to drop their criticisms now and acknowledge that the science of climate change has moved on.
But even if they had been right about the hockey stick, it makes no difference to the basic theory of global warming: it is still happening, and we are still almost certainly to blame. Ice shelves are collapsing, glaciers are retreating. For politicians, the message from the science is clearer than ever: global warming is real and it鈥檚 changing the world. Now deal with it.
