April 3, 2013

Unsettled Science

These two charts from the March 30, 2013 Economist show that global mean temperatures have flattened over the past 10 to 15 years and are at the low end of projections. The Economist has hitherto been a stout proponent of mainstream climate science, and so its expression of doubts about the settled character of the science is quite notable. The major anomaly is that while temperatures have flattened, emissions have continued their meteoric rise. "The world added roughly 100 billion tonnes of carbon to the atmosphere between 2000 and 2010," about a quarter of all the carbon dioxide released by human activity since 1750. And yet, concedes climate hawk James Hansen, "the five-year mean temperature has been flat for a decade." Also stalling out is the long term rise in surface seawater temperatures, as another chart from the article illustrates:

How to explain these anomalies? What follows is the first half of the Economist's explanation:

The mismatch might mean that—for some unexplained reason—there has been a temporary lag between more carbon dioxide and higher temperatures in 2000-10. Or it might be that the 1990s, when temperatures were rising fast, was the anomalous period. Or, as an increasing body of research is suggesting, it may be that the climate is responding to higher concentrations of carbon dioxide in ways that had not been properly understood before. This possibility, if true, could have profound significance both for climate science and for environmental and social policy.

The term scientists use to describe the way the climate reacts to changes in carbon-dioxide levels is “climate sensitivity”. This is usually defined as how much hotter the Earth will get for each doubling of CO concentrations. So-called equilibrium sensitivity, the commonest measure, refers to the temperature rise after allowing all feedback mechanisms to work (but without accounting for changes in vegetation and ice sheets).

Carbon dioxide itself absorbs infra-red at a consistent rate. For each doubling of CO levels you get roughly 1°C of warming. A rise in concentrations from preindustrial levels of 280 parts per million (ppm) to 560ppm would thus warm the Earth by 1°C. If that were all there was to worry about, there would, as it were, be nothing to worry about. A 1°C rise could be shrugged off. But things are not that simple, for two reasons. One is that rising CO levels directly influence phenomena such as the amount of water vapour (also a greenhouse gas) and clouds that amplify or diminish the temperature rise. This affects equilibrium sensitivity directly, meaning doubling carbon concentrations would produce more than a 1°C rise in temperature. The second is that other things, such as adding soot and other aerosols to the atmosphere, add to or subtract from the effect of CO. All serious climate scientists agree on these two lines of reasoning. But they disagree on the size of the change that is predicted.

The Intergovernmental Panel on Climate Change (IPCC), which embodies the mainstream of climate science, reckons the answer is about 3°C, plus or minus a degree or so. In its most recent assessment (in 2007), it wrote that “the equilibrium climate sensitivity…is likely to be in the range 2°C to 4.5°C with a best estimate of about 3°C and is very unlikely to be less than 1.5°C. Values higher than 4.5°C cannot be excluded.” The IPCC’s next assessment is due in September. A draft version was recently leaked. It gave the same range of likely outcomes and added an upper limit of sensitivity of 6°C to 7°C.

A rise of around 3°C could be extremely damaging. The IPCC’s earlier assessment said such a rise could mean that more areas would be affected by drought; that up to 30% of species could be at greater risk of extinction; that most corals would face significant biodiversity losses; and that there would be likely increases of intense tropical cyclones and much higher sea levels.

Other recent studies, though, paint a different picture. An unpublished report by the Research Council of Norway, a government-funded body, which was compiled by a team led by Terje Berntsen of the University of Oslo, uses a different method from the IPCC’s. It concludes there is a 90% probability that doubling CO emissions will increase temperatures by only 1.2-2.9°C, with the most likely figure being 1.9°C. The top of the study’s range is well below the IPCC’s upper estimates of likely sensitivity.

This study has not been peer-reviewed; it may be unreliable. But its projections are not unique. Work by Julia Hargreaves of the Research Institute for Global Change in Yokohama, which was published in 2012, suggests a 90% chance of the actual change being in the range of 0.5-4.0°C, with a mean of 2.3°C. This is based on the way the climate behaved about 20,000 years ago, at the peak of the last ice age, a period when carbon-dioxide concentrations leapt. Nic Lewis, an independent climate scientist, got an even lower range in a study accepted for publication: 1.0-3.0°C, with a mean of 1.6°C. His calculations reanalysed work cited by the IPCC and took account of more recent temperature data. In all these calculations, the chances of climate sensitivity above 4.5°C become vanishingly small.

If such estimates were right, they would require revisions to the science of climate change and, possibly, to public policies. If, as conventional wisdom has it, global temperatures could rise by 3°C or more in response to a doubling of emissions, then the correct response would be the one to which most of the world pays lip service: rein in the warming and the greenhouse gases causing it. This is called “mitigation”, in the jargon. Moreover, if there were an outside possibility of something catastrophic, such as a 6°C rise, that could justify drastic interventions. This would be similar to taking out disaster insurance. It may seem an unnecessary expense when you are forking out for the premiums, but when you need it, you really need it. Many economists, including William Nordhaus of Yale University, have made this case.

If, however, temperatures are likely to rise by only 2°C in response to a doubling of carbon emissions (and if the likelihood of a 6°C increase is trivial), the calculation might change. Perhaps the world should seek to adjust to (rather than stop) the greenhouse-gas splurge. There is no point buying earthquake insurance if you do not live in an earthquake zone. In this case more adaptation rather than more mitigation might be the right policy at the margin. But that would be good advice only if these new estimates really were more reliable than the old ones. And different results come from different models.
The issue brief goes on to describe the differing climate models extant. Neither the "general circulation models" nor the "energy balance models" are without serious limitations. So far as the science is concerned, the general conclusion is that a "a small reduction in estimates of climate sensitivity would seem to be justified: a downwards nudge on various best estimates from 3°C to 2.5°C, perhaps; a lower ceiling (around 4.5°C), certainly. If climate scientists were credit-rating agencies, climate sensitivity would be on negative watch. But it would not yet be downgraded."

The accompanying leader from the Economist ("Apocalypse Perhaps a Little Later") notes that the recent flattening of temperatures is not a good reason to stop worrying. Though the risk of extreme warming is arguably diminished, not-so-extreme warming still represents a real risk. Other bits of evidence, like the death spiral of Arctic sea ice, continue to point in a more alarmist direction. Moreover, the world has not really begun to take measures to counteract the risk of warming: though "climate rhetoric has been based on fears of high sensitivity, climate policy has not been."
If climate policy continues to be this impotent, then carbon-dioxide levels could easily rise so far that even a low-sensitivity planet will risk seeing changes that people would sorely regret. There is no plausible scenario in which carbon emissions continue unchecked and the climate does not warm above today’s temperatures.

Bad climate policies, such as backing renewable energy with no thought for the cost, or insisting on biofuels despite the damage they do, are bad whatever the climate’s sensitivity to greenhouse gases. Good policies—strategies for adapting to higher sea levels and changing weather patterns, investment in agricultural resilience, research into fossil-fuel-free ways of generating and storing energy—are wise precautions even in a world where sensitivity is low. So is putting a price on carbon and ensuring that, slowly but surely, it gets ratcheted up for decades to come.
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"Climate Science: A Sensitive Matter,"  The Economist, March 30, 2013

Climate hawks dispute the significance of the flattening of the upper ocean heat content anomaly: about 90% of global warming goes into heating the oceans, they argue, and much of the "missing heat" has been found in the deep oceans below 700 meters.

 April 17, 2013

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