More evidence that global warming is man-made

From The Economist, February 24, 2005:

SOME people do not believe global warming is happening; some believe it is happening, but that it is the result of natural variation; and some believe it is being caused by human activity. A paper presented to the AAAS by Tim Barnett, of the Scripps Institution of Oceanography in La Jolla, California, provides further evidence that the third camp is right.

Most published research on climate change looks at the atmosphere. That is partly because the records are good and partly because it is in the atmosphere that the human-induced changes that might be causing it are happening. One of these changes, which would promote global warming, is a rise in the level of so-called greenhouse gases (particularly carbon dioxide) which trap heat from the sun and thus warm the air. Another, which would oppose warming, is a rise in the quantity of sulphate-based aerosols, which encourage cloud formation and thus cool the air by reflecting sunlight back into space. Dr Barnett, however, thinks that the air is the wrong place to look. He would rather look in the sea. Water has a far higher capacity to retain heat than air, so most of any heat that was causing global warming would be expected to end up in the oceans.

And that was what he found. In a follow-up to a preliminary study published four years ago, he looked at ocean-temperature surveys made over the past 65 years. He confirmed that the sea has got warmer since the 1940s, and particularly since the 1960s. Furthermore, it has done so from the top down. At a depth of 700 metres, things are almost unchanged. But surface temperatures in all six of the ocean basins he examined (the north and south Atlantic, Indian and Pacific oceans) have increased by about half a degree Celsius.

So the Earth has, indeed, warmed up over the past few decades, as most climatologists already believed. But the actual pattern of temperature change in each of the six ocean basins is different, and that diversity allowed Dr Barnett to test the idea that people, rather than natural phenomena, are the reason for the warming. He took two widely respected models of the world's climate (which couple events in the atmosphere with events in the sea, and take account of both greenhouse gases and aerosols) and played with their variables in different ways. He tried mimicking the effects of the natural variability caused by feedback loops within the climate, and also the effects of small changes in the sun's output and the consequences of volcanic eruptions, both of which affect the climate. But the only changes that produced patterns of heating which matched reality were the man-made ones. And the match was good in all six basins. Which is confirmation, in Dr Barnett's eyes at least, that the guilty party in global warming is industrial man.