How climate change is blowing hot and cold

A rainy day in July does not falsify climate change

In dark moments, I sometimes catch myself listing reasons why the chance of humanity successfully tackling climate change is not very great. There's the time lag (act now to stop disaster in 50 years), the diffusion of responsibility (what about all the Chinese?) and the expense - not to mention the entrenched interests of the fossil-fuel merchants.

What's more, we can't easily sense climate change. Planetary warming may seem real during a heatwave or a snowless winter, but what about a rainy day in July or a January freeze? Where's your global warming then? Understanding why short-term fluctuations don't tell us much about long-term trends requires a bit of scientific knowledge. Take the "global cooling" saga: great story, but very little basis in science, and what basis there is has been widely misunderstood.

The suggestion that global warming might stop for a few years gained much prominence after a paper (innocently entitled "Advancing Decadal-Scale Climate Prediction in the North Atlantic Sector") was published in the journal Nature last year. The last line of the abstract set the shadowy corners of the blogosphere haunted by climate-change deniers (we call it the "denialosphere" for short) buzzing. "Our results suggest
that global surface temperature may not increase over the next decade," the scientists wrote, "as natural climate variations in the North Atlantic and tropical Pacific temporarily offset the projected anthropogenic warming" - though somehow the first part of the sentence generated more interest than the second.

No one who knows their climate onions ever expected global warming to be a linear trend of year-on-year temperature rises, and the continuing role of natural variability - in particular cycles in the world's oceans, which store vastly more heat than the atmosphere - is a perfectly legitimate area of research. This is actually all rather technical and arcane: it's about how best to tune the models that give us an insight into the globe's likely climate future. But that's fine - and it doesn't falsify global warming: unless, that is, you're a "climate sceptic", desperate to add a tiny scrap of false scientific credibility to your ideological position.

Here is the shocking truth: global warming has "stopped" many times already - but it is still getting warmer. Two US-based climatologists recently published a fascinating paper in the journal Geophysical Research Letters simply entitled: "Is the climate warming or cooling?" They concluded that although it is true that if you draw a straight line from 1998 to 2008 "there is no real trend", the same would go for the periods 1977-85 and 1981-89; even though, overall (between 1975 and 2008), there was "substantial warming". How confusing.

And expect this confounding real-world complexity to keep on happening. Scientists at the Met Office's Hadley Centre recently ran their model forward until the end of the century (ten times in total) and counted plenty of short periods (of a decade or less) when global temperatures fell or held steady. But overall, the model warmed by a couple of degrees by 2100 - and that's what counts.

So no, a rainy day in July - or even a decade of little or no observed warming - does not falsify climate change. To understand why not, however, you need to comprehend basic-level statistics rather than rely on common sense. And that, I suppose sadly, is another one to add to my list.

Mark Lynas's column will appear fortnightly.

Mark Lynas has is an environmental activist and a climate change specialist. His books on the subject include High Tide: News from a warming world and Six Degree: Our future on a hotter planet.

This article first appeared in the 05 October 2009 issue of the New Statesman, The tories/the people