Like many scientists, Myles Allen did not initially believe in man-made climate change: “I signed up to what many people believed in the early 1990s, which was that most of what we saw was generated by chaos in the atmospheric system. We were relatively sceptical about the thesis that it was man-made.”

As a scientist he found the debate frustrating: “It was obvious that neither party to the argument had a way of objectively defending their forecasts. It broke down into a slanging match.”

So in 1999, at a conference, the Oxford University physics lecturer made a proposal: “I suggested that we run a simulation of climate many, many times, as they do with weather.”

If predictions are carried out thousands of times, with minute variables in each instance, distinct patterns might emerge and scientific uncertainty would be reduced.

His proposal was deemed impossible: “At the time people were proud to have run a model four times, not 4,000 times. These are incredibly complicated processes. The next speaker at the conference got a big laugh by making lots of jokes about my idea.”

Nobody is laughing now. As principal investigator behind the ClimatePrediction.net project, Allen deserves credit for launching the most detailed climate projection ever undertaken.

Allen set up a website asking if anyone would be prepared to leave a PC on for six months to run his climate model. “People were used to the idea that you can only run climate models on a supercomputer,” he explains. “But the modern PC can do what a supercomputer could do only a few years ago.”

Twenty thousand people signed up in a couple of days. Then the BBC got involved and each climate model downloaded by viewers was unique. Comprising 1m lines of code, it represented a global picture of climate, including atmosphere, oceans and landmass, and took several months — working in the background on the PCs’ “spare” processing capacity — to model the period 1920-2080.

Using mathematical equations that govern the real climate, it simulated everything you see in weather forecasts: wind, rainfall, air pressure, ocean currents, cloud cover and more, in fine detail.

Now the results have been compared and clear patterns have emerged. Using each viewer’s model to produce a “hindcast” for 1920-2000, and comparing the spread of forecasts with observations by the Met Office and others of what actually happened in that period, the scientists ranked the models and then examined what those high-ranking models predicted for the future. “The surprise was how tight the results are,” says Allen. “The uncertainties are not so much ‘whether’ but ‘when’.”

This evening Sir David Attenborough will explain what that means, showing in detail the alarming consequences for Britain — how climate change will affect us all. Specifically, Attenborough presents three snapshots, from 2020, 2050 and 2080. One key point is the increased likelihood of heatwaves. The murderously hot summer of 2003 is 25 times more likely to recur by 2020.

It will be regarded as a normal summer by 2050 — and might even seem cool by 2080.

Another area of impact is changes in rainfall: water shortages in summer and floods in winter. Some floods will be caused by lots of rain over a long period, as happened in Glasgow in 2002. Others will be caused by short but heavy downpours, as affected Boscastle in 2004.

“There are going to be winners and losers,” says Allen. The programme shows a farmer in Devon making a cunning investment in climate change by planting olive trees. “But it’s going to be a postcode lottery. At the high end of a street you might comfortably enjoy warmer summers, but at the lower end you will be flooded every winter. People need to do is assess their own vulnerability. Living in a changing climate will be expensive.” ()

It could be worse even than Allen’s model suggests. One scientist on the programme is Professor Peter Cox, of Exeter University, an authority on the world’s natural “carbon sinks” — the plants and oceans that soak up as much as half of the emissions caused by humans. Cox is watching out for signs that the sinks might fail, causing the quantities of atmospheric carbon to increase rapidly. The signs might include sudden changes in ocean circulation, the death of the Amazon and other rainforests, or the release of vast quantities of methane from permafrost that has already started melting.

The UK Climate Impacts Programme is funded by government to help decision makers to understand climate change and adapt to it. Dr Chris West, its director, was one of the outsiders who vetted Attenborough’s script: “Increasingly people are accepting that as well as trying to reduce emissions we have to deal with the changes that are, for the next 20 years or so, inevitable.”

Take railways: rails snap in cold weather or buckle when it is hot. To avoid problems, operators increasingly need to introduce speed limits, to the immense frustration of travellers. As the world heats up, they may need to reset every rail.

Sewers, already struggling to cope with rainfall, will need to be enlarged and houses might need stronger and broader guttering. “Gutters are designed to take a certain amount of water,” West says. “If there’s more than that amount once or twice, that’s okay. But if it’s frequent the exterior of the house will be damaged.” Building standards today were designed to deal with cold winters. Many homes are badly suited to hot summers. Fitting air conditioning would be costly — and increase emissions.

The British seaside could become more popular with tourists than the too-hot Mediterranean. At the moment, West says, heatwaves are for only a few days every few years. In future it may be the whole of August every year. Some lines on London’s Underground, too narrow for air conditioning, could be rendered unusable for weeks on end: “And we may have to stop people working outdoors in the middle of the day.”

But he is not all doom. “Some of the things that need to be done are very simple. For instance, someone had the smart idea of painting London buses white on the roof. It didn’t cost anything, but it reduced the heat burden on passengers. This is an issue that is global in scale and it’s going to last for a long time. When people say you can help by changing a light bulb, the difference in scale between the problem and the solution makes some people disbelieve and switch off. But the fact is that a lot of people doing a little bit really is effective.”

West believes that his paymasters in government are finally getting to the point of asking, before introducing any new policy: “Is this robust under climate change?” That may be so, but the programme shows the folly of recent decisions, such as John Prescott’s plan to build hundreds of thousands of homes on the Thames Gateway. These will use up scarce fresh water and are also at grave risk of flooding. Building a new Thames barrier further out will cost an estimated £ 20 billion. Smaller communities elsewhere, unlikely to get similar protection, will protest fiercely.

To be fair, the decisions facing government are difficult. If you want to find out how difficult, try playing the interactive game Climate Challenge where as “president of Europe” players choose policies to reduce emissions over the 21st century, while making sure there is enough electricity, water and food for the people and managing spending to remain popular with voters. It is a green version of Sim City.

Like the climate modelling underlying Attenborough’s programme, Climate Challenge would not have come about without Allen, who happened to be in a pub one day with a colleague and her husband, Gobion Rowlands. “When I started looking into climate change,” says Rowlands, managing director of an Oxford-based games company,“I was frankly rather depressed. But as we looked into it we realised there’s a lot we can do.”

Allen encouraged him to create the game and provided technical input.

In trials, participants reported that they felt more positively about their own role in tackling climate change as a result of playing the game. Nearly half said it gave them a better understanding of climate change.

Trying the game myself, I introduced policies that “emitted a very low level of carbon . . . well done!” (as the game told me). Better still, the economy grew continuously under my charge. But I was soon voted out of office: “You were a deeply unpopular leader who cared nothing for the happiness of the population.”

My least popular policies were discouraging flying and raising the retirement age to 70.

Keep trying, says Rowlands, who based the policies on actual government policy documents: “It’s a good game for replaying.”

Does Allen think we are doomed? “In the very long term I’m optimistic, simply because our children won’t tolerate what they see around them. But I’m less optimistic that we will solve it in the most economic manner as the Stern report recommended. Past performance on other problems suggests we may leave it too late.”







To play the game, log on to www.climateprediction.net.



Climate Change: Britain Under Threat is on BBC1 tonight at 8pm