Our role in global warming

It is not really an issue whether our climate has changed. It has. One sign is the variation of surface temperature averaged over the globe since the middle of the last century: because instruments used to measure temperature have changed during this period and the fraction of the planet for which we have observations has varied, there are formidable problems in reconstructing this variation. Nevertheless, after these factors have been taken into account, it seems clear that the earth's surface has warmed by a few tenths of a degree.

A much bigger issue is whether we can explain that warming; in particular, can we attribute any part of it to human activity? Traditionally, many climate scientists have hedged their bets, using the double negative that warming is "not inconsistent" with the effects of human activity.

Many factors can influence climate. Natural factors include variations in the sun's output and the effects of volcanic eruptions. Until the early Eighties the dominant cause of human-induced climate change was believed to be increased atmospheric carbon dioxide, due to burning of coal, oil and gas and deforestation.

In the past decade, a plethora of other mechanisms have been identified; increased concentrations of gases, such as methane, add substantially to carbon dioxide's warming effect. But human activity has also led to increased atmospheric concentrations of tiny particles. These are believed to reflect the sun's energy and tend to cool the planet. And changes in ozone, both near the surface and in the stratosphere, add pieces to the climate change "jigsaw puzzle". Unfortunately, we have yet to quantify the size of some of these factors with much confidence.

The Intergovernmental Panel on Climate Change at its Madrid meeting this week has gone one step beyond "not inconsistent" to state that "the balance of evidence suggests a discernible human influence on global climate". Although, as one of the IPCC authors, I accept that this statement represents the majority view, I remain nervous about it. The computer simulations that were used as its basis have not included many of the pieces of the jigsaw puzzle. These pieces need to be added, but we may never be able to unambiguously identify the human influence on the climate during the past century.

It seems clear that on a global scale, over the century as a whole, the human-induced factors are substantive compared to the natural factors that cause climate to vary. Some of these, in particular carbon dioxide, will continue to grow bigger in the coming decades. But in terms of characterising changes at the sub-continental scale, we have a long way to go. Ultimately, it is these regional changes that will affect our lives.

The writer is a reader in the department of meteorology at the University of Reading and has been a leading author of the 1994 and 1995 scientific assessments of the Intergovernmental Panel on Climate Change.