Global warming is a fact whatever its deniers - encouraged by a cool year - have to say
The deniers of global warming are about to latch on to a new argument. The world is cooling. And they are right - well, slightly.
Globally, this year is likely to be the coolest for some time - back to the average of the early 90s, according to some unpublished forecasts. This is no refutation of man-made global warming. It is the inevitable consequence of one of nature's climatic cycles. The La Niña, the cold phase of the El Niño cycle in the Pacific, has sent average global temperatures plunging this year.
And there is more. Longer term climate cycles that play out over a decade or so will also be working to cool us in the coming decade. In particular, changes in the currents of the north Atlantic - which have caused Europe to warm more than anywhere else in the past decade and helped melt all that Arctic ice - are about to go into reverse.
A Germany study published earlier this month predicts the world will cool over the coming decade. British climate modellers at the Met Office don't go so far. They think nature's cooling will be more than counterbalanced by the warming effect of man-made carbon dioxide in the atmosphere.
But nobody is sure. In any case, we can expect the deniers to make the most of this opportunity to pour cold water on the whole climate change narrative. No year has yet been hotter than 1998, they will say. True: it was a huge El Niño year. Now we are on the way back down, they will say. Nonsense. The underlying trend remains upwards; and as every decade passes, natural cycles can do less and less to counter the growing human influence on temperature.
By late next decade, natural warming will once again combine with man-made warming to push temperature rise into overdrive. The surge that we saw through the 1980s and 1990s will resume with a vengeance. That could be the moment that climate change passes a point of no return, when ice sheets start to collapse and parched rainforests and soils dump their carbon into the air, accelerating warming.
Now, a sceptic might say that if the modellers are only just learning about the importance of natural cycles to climate forecasts, why should we believe their predictions at all? Fair point. In their desire to persuade us about the big picture of global warming, scientists have sometimes got cocky about colouring in the detail.
Recently I attended a conference in Reading where some of the world's top experts discussed their failings. How their much-vaunted models of the world's climate system can't reproduce El Niños, or the "blocking highs" that bring heatwaves to Europe - or even the ice ages. How their statistical mimics of tropical climate are "laughable", in the words of the official report.
This sudden humility was not unconnected with their end-of-conference call for the world to spend a billion dollars on a global centre for climate modelling. A "Manhattan project for the 21st century", as someone put it.
Even so, scientists are concerned that many of their predictions about how climate change will play out in different parts of the world are little better than guesses. But whatever the local wrinkles and whatever natural cycles may intervene, man-made global warming is real, current and matters a great deal.
Physicists have known for 200 years about greenhouse gases. They first calculated the likely global effect 100 years ago. They have been measuring the accumulation of these gases for 60 years. The world has been warming strongly for 30 years, and nobody has come up with a half-way plausible explanation other than the most obvious. It's the greenhouse gases, stupid.
· Fred Pearce is the author of The Last Generation