Last lull before the storm?

Global temperatures have not risen since 1998 but, rather than being an indicator that global warming has stopped, recent studies suggest that this stability might be setting the stage for more severe climate change in the future.

New research suggests that there may not be much more time for mankind to take effective action against global warming.

Climate change is the most controversial scientific issue of our time - and small wonder, given its potential impact on everything from geography to geopolitics. Health experts commissioned by the leading medical journal The Lancet have just declared climate change to be "the biggest global health threat of the 21st century". Yet while concern about the impact of global warming shows no sign of ending, the phenomenon itself seems to have done so. Or at least, that is what sceptics of climate change have been saying for some time now - to the intense irritation of many scientists. And the sceptics do seem to have a point: measurements suggest the warming trend ended as long ago as 1998, with global temperatures being more or less constant ever since.

Scientists have insisted that the fallacy in the claim lies in portraying global warming as like heating water on a stove. The response of the Earth's atmosphere to increasing levels of greenhouse gases - the driving force of global warming - is so complex that there are bound to be blips in the overall trend. Indeed, many such blips appear in the otherwise upward trend in temperature over the years - between 1983 and 1985, for example, or between 1990 and 1995.

Only the most determined climate change denier would seize on just a couple of years of stasis to claim global warming has ended. But that still leaves the question of how long is long enough? Is the current decade-long pause just another blip, or the sign of something more significant? Measurements of global temperature over the past century do little to shed light on the question. Over that time, emissions of carbon dioxide - claimed to be the prime source of man-made global warming - have grown inexorably and exponentially. The response of the Earth's climate, in contrast, has been all over the place. Global temperatures actually fell in the first decade of the 20th century, then increased during the 1920s and 1930s, fell again from the 1940s to the mid 1970s, then soared until the late 1990s, since when they have been static.

For sceptics, all this is clear evidence that there's no real link between man-made greenhouse gases and global warming. Climatologists, meanwhile, point to the 1°C increase over the 20th century as proof that the link exists, albeit in complex form - and is quite capable of producing periods of stasis or even cooling. So who is right? If the complexity argument has any merit, computer models of the supposed link between greenhouse gases and global temperatures should be able to reproduce the bizarre variability seen in the real-life data. Now Dr David Easterling, of the US National Climatic Data Center, North Carolina, and Dr Michael Wehner, of the Lawrence Berkeley National Laboratory, California, have put this to the test.

The results, recently published in Geophysical Research Letters, show that steadily increasing greenhouse gas levels can indeed lead to periods of stasis or even cooling of the planet lasting a decade or more. Or, to be precise, they can do so under 20th century conditions. For the researchers went further, examining the link between greenhouse gas levels and global temperature during the 21st century. They found that as the levels increase, periods of static temperatures or cooling become progressively unlikely. So it seems that the current controversy about the role of man-made global warming may soon be put beyond doubt: if climatologists are right, we will soon start to see global temperature records being broken every few years.

It is already clear, however, that it would be foolhardy to take comfort even from a decade of static global temperatures. It may well prove to be merely a period of respite before temperatures soar again. How long we have before they kick in depends on those complex interactions that muddy the link between greenhouse gases and global temperatures. Understanding these is clearly crucial, and some important insights have now been uncovered by professors Kyle Swanson and Anastasios Tsonis of the University of Wisconsin, Milwaukee.

They have been focusing on patterns of atmospheric and oceanic conditions blamed for dramatic weather events worldwide. The best known is the notorious El Nino effect, in which a vast region of the tropical Pacific warms up, triggering torrential rain thousands of kilometres away. Climatologists have identified other such patterns, such as the North Atlantic Oscillation (NAO), an atmospheric effect which influences Europe's winters: cold and dry when the NAO is in one state, mild and wet when in another.

Despite their importance for weather forecasting, predicting these patterns remains a major challenge. They are not random, but nor are they as regular as clockwork: the El Nino effect takes place at anything from three- to eight-year intervals. But what is clear is that every so often the patterns can come into synch with one another - with global consequences. By analysing past climate data, professors Swanson and Tsonis have discovered that El Nino, the NAO and two other patterns came into synch four times in the past century, in periods centred around 1915, 1941, 1958 and 1979. Intriguingly, all but the third of these dates marked major changes in global climate, with cooling turning into warming or vice versa.

According to Swanson and Tsonis, this suggests that changes in the Earth's climate lasting a decade or more may be presaged by these patterns coming into synchrony. And as it happens, the last time they did that was around 2001 - just when the warming trend started to level out. Thus this new research backs the findings of Easterling and Wehner, which suggest the current stasis in global temperatures is only temporary.

Reporting their findings in Geophysical Research Letters, Swanson and Tsonis warn of the dangers of extrapolating from events in the 20th century: since then, greenhouse gas levels have increased dramatically, and could alter the link between these patterns and future climate. This, too, is in line with the work of Easterling and Wehner, who found that the future climate is much less likely to offer any respite from warming.

Thus we may be living through the final lull before global warming really takes off. When this window of opportunity for action will end is anyone's guess, but these new findings suggest we are already a decade into it - and shouldn't count getting any more. Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England