18 February 2014 Last updated at 11:29

Article written by David Shukman Science editor

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Barrage over climate change link to floods

 

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As the barrage of bad weather eases, another kind of turbulence is brewing over one of the potential causes.

Listen to some environmental campaigners and you might think that there is total certainty that global warming led to the recent rain; listen to some climate sceptics and there is absolutely no connection at all.

Viewers have berated me either for failing to explicitly blame climate change in my reporting of the floods - or for suggesting that the rain may conceivably have been made more likely by the rising presence of manmade greenhouse gases.

For anyone coping with clearing up a flooded home, this question will not exactly be the highest priority.

However, political figures have raised its profile, making the connection rather more forcefully than many scientists.

The Prime Minister, David Cameron, set the tone by telling the Commons that he "very much" suspects that climate change is involved. And the Labour leader, Ed Miliband, warned that "we are sleepwalking into a national security crisis on climate change".

Of course not every politician agrees. Lord Lawson, on the Today programme, dismissed any link to the weather, saying, "the question is whether global warming has marginally exacerbated it. Nobody knows that".

Different takes

If we stand back from the Westminster hothouse, what do the scientists actually say?

The fact is that attributing a human influence to individual weather events is an emerging area of research and is acknowledged by those involved to be extremely challenging because so many factors are at work.

One leading figure in climate science, Professor Sir Brian Hoskins, summed it up bluntly: "There's no simple link - we can't say 'yes' or 'no' this is climate change."

Instead, he and others point to a range of factors which would make intense downpours more likely.

The key one is a basic physical relationship: since warmer air can hold more moisture, it makes sense that our warming atmosphere would produce more intense rain.

But how much rain? And where? The computer models used to explore scenarios for the impacts of different levels of greenhouse gases are recognised to be weaker on rainfall than on temperature.

Surely, you might think, the UN's Intergovernmental Panel on Climate Change (IPCC), the consensus assessment of the latest science, might clear this up? As so often, you can read its documents in different ways.

If you think global warming is overplayed, you focus on this conclusion in the most recent IPCC report:

"There continues to be a lack of evidence and thus low confidence regarding the sign of trend in the magnitude and/or frequency of floods on a global scale."

Translated, that means we're not seeing more floods, story over.

However, if you do think climate change is serious, your eye may fall, first, on the line that "the frequency or intensity of heavy precipitation events has likely increased in North America and Europe."

Second, the IPCC predicts that "extreme precipitation events" over the mid-latitudes (which includes Europe) will very likely become more intense and more frequent. Doesn't this explain the recent British weather? Is this the smoking gun? No, because this scenario will unfold "by the end of this century" rather than right now.

Looking for answers

Another take comes in a report by the European Academies Science Advisory Council published in November last year.

It suggests a future in northern Europe in which "high intensity and extreme precipitation become more frequent…" and that "future projections suggest increases in flood risk over a wide area of Europe…"

So bad news on the way, clearly, but none of this categorically nails the question we began with - exactly how much manmade greenhouse gases are involved in the current weather.

A study by the Met Office and Centre for Ecology and Hydrology concluded that "it is not possible, yet, to give a definitive answer on whether climate change has been a contributor or not."

Their report points to the sea level rising and an increase in storminess in the North Atlantic as factors consistent with climate change. But it also highlights what is not properly understood, including the path of the jet stream, which has acted as a conveyor belt, delivering storm after storm.

At the launch of the report, the Met Office chief scientist, Dame Julia Slingo, seemed to go a bit beyond what appeared in print.

She said: "All the evidence suggests there is a link to climate change." Not some of the evidence, but all of it.

So what about that unexplained path of the jet stream? The Mail on Sunday quoted one Met Office scientist, Professor Mat Collins, as saying that "there is no evidence that global warming can cause the jet stream to get stuck in the way it has this winter."

The Met Office scrambled to produce a statement to assert that there was no disagreement. It also confirmed the "uncertainty" about the storm track in the North Atlantic but did not address whether the chief scientist had gone beyond the conclusions of their own report.

Does this leave us any wiser? No. In my experience scientists always disagree - that's how research advances.

Dr Tim Osborn of the University of East Anglia is among climate researchers concerned about the science of extreme weather being portrayed as a little more certain than it might appear.

"You've got a lot of natural variability superimposed on the long term trend - in the next 20 years, the frequency of weather like this winter's could drop below the trend or rise above it. We're not expecting a year on year change."

The only way to detect a human fingerprint on weather is to run simulations of the event as it actually happened - and then to repeat them having stripped out the greenhouse gas component in the models.

Previous studies of this kind, for example into the 2000 floods in England, have found that the storms were made more likely because of manmade climate change - likely but not certain.

The answer is framed as an increased probability. A categoric answer may never be possible.

As the country copes with the floods and starts repairs and thinks about making things safer for the next one, people will look up at the skies and want certainty about whether wild winters will become normal. And at the moment, the science cannot provide that.