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Whatever happened to global warming?

Paul Hudson | 12:28 UK time, Friday, 9 October 2009

The title of this may be a surprise. So might the fact that the warmest year recorded globally was not last year, or 2007, but 1998. For the last decade we have not observed any increase in global temperatures. What's more, climate models did not forecast it even though man-made carbon dioxide, the gas thought to be responsible for warming our planet, has continued to rise. So what on earth is going on?

Climate change sceptics, who passionately and consistently argue that man's influence on our climate is overstated, say they saw it coming. They argue there are natural climate cycles over which we have no control which dictate how warm the planet is. But what is the evidence for this?

During the last few decades of the 20th century, our planet did warm quickly. Sceptics argue that the warming we observed was down to the energy from the sun increasing; 98% of the earth's warmth comes from the sun. But research published by the Royal Society two years ago seemed to rule out solar influences. Its approach was to look at solar output and cosmic ray intensity over the last 30-40 years, and compare them with changing global temperatures. The results were clear. "Warming in the last 20 to 40 years can't have been caused by solar activity," said Dr Piers Forster from Leeds University, a leading contributor to this year's Intergovernmental Panel on Climate Change (IPCC).

One solar scientist, however, Piers Corbyn, from the long range forecasters Weather Action, disagrees. He claims that solar charged particles impact us far more than is currently accepted, so much so he says, that they are almost entirely responsible for what happens to global temperatures.

Then there are oceans; the earth's great heat stores. And according to research conducted by Professor Don Easterbrook from Western Washington University last November, the temperatures of the oceans and the planet are related. He says the oceans have a cycle in which they warm and cool cyclically. The most significant is called Pacific Decadal Oscillation (PDO). For much of the 1980s and 90s, it was in a positive cycle- ie warmer than average. Global temperatures were warm too. But in the last few years it's started to cool down. In the past such cycles have lasted for nearly 30 years.

So could global temperatures follow? The global cooling from 1945 to 1977 coincided with one of these cold Pacific cycles. Professor Easterbrook continues 'The PDO cool mode has replaced the warm mode in the Pacific Ocean, virtually assuring us of about 30 years of global cooling'

So what does it all mean? Climate change sceptics argue this is evidence they have been right all along. They say there are so many other natural causes for warming and cooling, that even if man is warming the planet, it's insignificant compared to nature.

But those scientists who are equally passionate about man's influence on global warming argue their science is solid. The Met Office's Hadley Centre, responsible for future climate predictions, incorporates factors like solar variation and ocean cycles, along with man-made greenhouse gases, into its climate models. It says temperatures have never gone up in a straight line, and there will always be periods of slower warming, or even temporary cooling. What's crucial is the long-term trend in global temperatures. And that, according to the Met Office, is upward.

To confuse the issue even further, last month, another member of the IPCC, Dr Mojib Latif of the Leibniz Institute of Marine Sciences at Kiel University , said that we may indeed be in a period of cooling worldwide temperatures that could last another 10-20 years. But he makes it clear he has not become a sceptic; he believes that this cooling will be temporary, before the overwhelming force of man made global warming reasserts itself.

So what can we expect in the next few years? Both sides have very different forecasts. The Met Office says warming is set to resume quickly and strongly. Indeed, it predicts that from 2010 to 2015 at least half the years will be hotter than the current hottest year on record (1998). Sceptics say it's unlikely temperatures will reach the heights of 1998 until 2030 at the earliest; indeed it's possible that because of ocean and solar cycles a period of global cooling is more likely.

One thing's for sure. The debate about what's causing global warming is far from over. Some would say it's hotting up.

A version of this article also appears in the Science & Environment section of the BBC News website.


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