Climate is complex. Greenhouse gases, living systems, the circulation of the oceans and the planet's orbit all influence the earth's climate. But can the study of past climate change allow us to predict future trends?by Philip Ball / February 20, 2000 / Leave a comment
Published in February 2000 issue of Prospect Magazine
The last year of the last millennium was its hottest. There was no global drought; no summer heatwave swept across the planet. Yet enough months were, quietly, that little bit above average to add up to a record breaker, for Britain at least. What conclusions should we draw? The most popular one is that this is the result of a human-induced greenhouse effect. That is almost certainly true, but the connection is not as obvious as it seems. Why scientists have been hedging their bets on the matter, when there is evidence of global warming by about half a degree centigrade over the past century, seems puzzling-until you appreciate the full complexity of the earth’s climate.
Climate has no big idea. Evolution has Darwinian natural selection; cosmology has the big bang; genetics has DNA. Climate is just “one damned thing after another”; worse, it is lots of damned things at the same time. This makes for messy history. For example, it is not true that climate was steady and comfortable until we started pouring greenhouse gases into the atmosphere. Consider how global temperatures have fluctuated over the past 1m years: the graph looks like the jagged profile of the Dolomites. Focus in on the past millennium or even the past century and you see the same pattern: a series of peaks too fine to have been discernible in the million-year record.
We can make only one sweeping statement about the climate system: the average temperature of the earth’s surface depends on a balance between how much heat it receives from the sun and how much it radiates back into space. Changes in global mean temperature are ultimately caused by changes in the amount of heat into or out of the planet.
The complexity arises from the fact that so many phenomena induce such changes. One is the greenhouse effect. The earth’s atmosphere is roughly four fifths nitrogen and one fifth oxygen; but 1 per cent or so is made up of a m?nge of other gaseous compounds, many in such small quantities that they were detected only over the past few decades. Most of this is argon, and the rest is primarily water vapour and carbon dioxide. Yet these tiny proportions exert a huge effect. Water vapour, carbon dioxide and other greenhouse gases such as methane absorb some of the heat radiated by the warm surface of the earth. So “greenhouse gases” retain some of the sun’s energy which would otherwise be absorbed and then re-radiated to space. This warms up the lower atmosphere, and so surface temperatures rise.