NOW there is indisputable evidence that humans are causing global warming, claims a team that studied the extent and pattern of the rise in ocean temperatures. The oceans eventually absorb 84 per cent of the Earth鈥檚 extra heat, and the distribution of that heat closely matches what climate models predict would be the effect of human activity. Natural causes are ruled out, the team says.
鈥淭he evidence is so strong that it should put an end to any debate about whether humanity is causing global warming,鈥� says Tim Barnett of the Scripps Institution of Oceanography in La Jolla, California.
Barnett鈥檚 team analysed records of ocean temperatures from the past 40 years. They found that oceans have warmed by around 0.5 掳C at the surface, and the warming extends to a depth of several hundred metres. They then looked at the potential causes of the warming, such as increased solar radiation, changes in the amount of sulphate particles from volcanic emissions in the air, natural fluctuations and human activity. The human component included both the warming effect of greenhouse gases and the shading effect of the aerosols in urban smog and forest fires, which prevent radiation from reaching the Earth鈥檚 surface.
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Computer simulations of the three possible natural causes showed effects many times weaker than the warming actually observed, says Barnett. But simulations of human effects agreed with observations 鈥渨ith a confidence exceeding 95 per cent鈥�.
鈥淣atural causes are ruled out. This should put an end to any debate about whether humanity is causing global warming鈥�
The human fingerprint also shows in the pattern of warming. For instance, warming has been greatest in the southern oceans, where man-made aerosols are at a minimum. In the northern Indian Ocean there is a cool layer of water above a warmer one. The upper layer is almost certainly a result of the thick 鈥渂rown haze鈥� of pollutants over southern Asia, which has largely cancelled out atmospheric warming in the region, says Barnett (Science, DOI: 10.1126/science.1112418).
The accuracy of the model is so striking that it should bring confidence that science can predict the pace and pattern of climate change in the coming decades, he adds.
Jonathan Gregory of the University of Reading in the UK assessed Barnett鈥檚 results, and agrees that computer models, which have already accurately predicted atmospheric heating due to human activity, are getting better. 鈥淥ur confidence is increased when we reach the same conclusion [on man-made climate change] by examining both the atmosphere and the ocean,鈥� he says.
