If climate sceptics are right, it is time to worry

By Paul Klemperer

Published: February 28 2008 18:38 | Last updated: February 28 2008 18:38

Al Gore says the science on global warming is clear and there is a major problem. Vaclav Klaus, Czech president, contends that climate change forecasts are speculative and unreliable. Whose claims are scarier?

Of course, Mr Klaus exaggerates (he is a politician) but if he is partly right, we should be more concerned, not less. Consider an analogy. If, like many of my neighbours in Oxford, you believe that new building exacerbates flooding, how would you feel if models that predicted bad news were discredited?

It depends. If the original models were biased, your best guess of the height of future floods is now lower. But if the models merely underestimated the uncertainty, the range of plausible outcomes is now greater, so flood defences would need to be higher for us to feel safe.

Likewise, if our understanding of climate systems is flawed, our best guess about the dangers we face may be less pessimistic, but extreme outcomes are more likely.

Mr Klaus is probably right that there are fewer certainties than many claim. Even commentators who support the conclusions of the Intergovernmental Panel on Climate Change point to methodological weaknesses in its economics. A UK High Court judge recently required that a list of “scientific errors” be sent to schools that show Mr Gore’s remarkable polemic, An Inconvenient Truth – confirming the impression that the film goes some way beyond established facts (Mr Gore is also a politician).

But we hardly need Mr Klaus to teach us that experts’ models can be incomplete and a strong consensus can be badly flawed. Financial Times readers do not need reminding that, only last summer, hedge fund managers found their stock market models’ predictions were, in their own words, “25 standard deviations” from the outcomes; that, less than two years ago, a new drug nearly killed six human volunteers in tests in London, even though the dose was 1-500th of the amount administered to animals; or that it was a complete “out of model” surprise to biologists that feeding bonemeal to cattle would cause an epidemic of mad cow disease.

How confident can we be about the way a system as complex as earth will respond to conditions it has never encountered before? Although greater uncertainty means climate change might be less bad than we fear – for example, an “iris” effect means increases in cloud cover may slow global warming – it also means it might be much worse. While the central predictions of climate change models are arguably not so much worse than many other difficult problems the world faces, the worst possibilities are far, far nastier.

Consider the “clathrate gun hypothesis” that warming seas could lead to clathrates (the frozen chunks of methane at the bottom of the sea) exploding into the air, which is what might have caused mass extinction at the end of the Permian era. Or the concern that the carbon dioxide could cause hydrogen sulphide gas to build up first in the oceans then in the atmosphere, exterminating most of life (and potentially also attacking the ozone layer, permitting the sun’s ultraviolet radiation to kill remaining life) – this, too, has been blamed for previous mass extinctions.

I am not losing any sleep about these specific scenarios. In part that is because they seem so improbable (in spite of Mr Klaus’s eloquent expositions of how little we really know). But it is also because the fact that we have already thought of these risks means that, if it becomes necessary, we probably have time to organise a last-ditch geoengineering solution (seeding the ocean with an antidote, for example) that would at least mitigate the very worst consequences.

But what of completely unanticipated possibilities? Even Donald Rumsfeld, former US defence secretary, understood that it is the “unknown unknowns” that should really worry us. Serious scientists worry that feedback effects such as release of methane from the Siberian permafrost (or those underwater clathrates), or reductions in the earth’s reflectivity due to polar ice loss, could cause runaway greenhouse warming, with unforeseeable outcomes that would look like bad science fiction from today’s perspective.

The continuing scientific uncertainty about the pace of climate change should make us more concerned, not less. And it is those who doubt the climatologists’ models who should be the most frightened.

The writer is Oxford university’s Edgeworth professor of economics