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 fear that new building is exacerbating 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 friendly commentators who support the conclusions of the Intergovernmental Panel on Climate Change (the organisation which shared the Nobel Peace Prize with Gore) point to methodological weaknesses in its economics. It is also notable that the IPCC’s recently published synthesis report is more uncertain about outcomes than its predecessor – “because of the broader range of models available”. Furthermore, a UK High Court judge's recent requirement that a list of "scientific errors" be sent to schools that show Gore's remarkable polemic, An Inconvenient Truth, supports critics' contentions that the movie is, in truth, an "inconvenient half-truth"; certainly, the film conveys impressions that go some way beyond established facts (Gore is also a politician).
But we hardly need the likes of Klaus to teach us that experts’ models can be incomplete and a strong consensus can be badly flawed. Vox 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, just as the Nobel prize-winning economists who advised LTCM believed the probability that the fund would lose more than half its money was way below a billionth (until, that is, they lost almost all their money).
And these are people with their own money on the line. Al Gore ridicules the oil-company lobbyists by quoting Upton Sinclair's aphorism that "it is difficult to get a man to understand something when his salary depends upon his not understanding it", but it could equally apply to some who have jumped on the climate-change bandwagon.
In medicine, we know theoretical predictions are not enough - tests are necessary, and those tests sometimes go wrong: a new drug nearly killed six human volunteers in London less than two years ago, even though the dose was 1-500th of the amount found safe in animals. Similarly, it came as a complete "out of model" surprise to biologists that feeding bonemeal to cattle would cause an epidemic of mad-cow disease.
Small-scale environmental experiments are replete with unintended consequences, from Hawaiian cane toads (that failed to do their intended job of eradicating beetle infestations in sugarcane crops) running riot across Australia, to melaleuca trees that were introduced to suck up the Everglades' “excess” water but were instead hugely destructive to Florida's endangered species.
So 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 as might have caused mass extinction at the end of the Permian era. Or the concern that the CO2 could cause hydrogen sulphide gas to build up first in the oceans, and 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 highly 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 (e.g., seeding the ocean with an antidote) that would at least mitigate the very worst consequences.
But what of completely unanticipated possibilities? (Even Donald Rumsfeld 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 - beyond some unknown “tipping point” - 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.
Paul Klemperer’s VoxEU analysis of the key issues in climate change is at http://www.voxeu.org/index.php?q=node/440, and his policy prescription for climate change is also in VoxEU at http://www.voxeu.org/index.php?q=node/803. A shorter version of this article was published in the Financial Times, February 29, 2008.