Setting the Record Straight on Global Warming

From the archives:

(23 April 2006) The media is by its very nature sensational, and on the issue of global warming this can swing both ways. Therefore, there was a big fuss over a study in Nature this past Thursday that seemed to lay out a more conservative estimate for the expected increase in temperature due to global warming. Although some coverage was more damning than others, the quantity of press on the subject was extensive.

As an article the next day in Science explained, though, all the Nature paper did was narrow down the range of possible scenarios to what was already the widely accepted range. The paper had found that the earth's climate sensitivity (the number of degrees Celsius that the earth's temperature is expected to increase following a doubling of the amount of carbon dioxide in the atmosphere) falls more comfortably within the range of 1.5°C to 6.2°C:

Here we demonstrate that such observational estimates of climate sensitivity can be tightened if reconstructions of Northern Hemisphere temperature over the past several centuries are considered. We use large-ensemble energy balance modelling and simulate the temperature response to past solar, volcanic and greenhouse gas forcing to determine which climate sensitivities yield simulations that are in agreement with proxy reconstructions. After accounting for the uncertainty in reconstructions and estimates of past external forcing, we find an independent estimate of climate sensitivity that is very similar to those from instrumental data. If the latter are combined with the result from all proxy reconstructions, then the 5-95 per cent range shrinks to 1.5-6.2 K, thus substantially reducing the probability of very high climate sensitivity.

In short, by combining various diverse sources of data, the researchers were able to make a more specific prediction of future temperature increases. A figure from the Science article illustrates this point:

Previous analyses (left) indicated that there was a real chance that the earth's temperature could increase anywhere from less than 1°C to 10°C or more. The new analysis (right) narrows this range, and although it decreased the upper estimate, it also increased the lower one. According to the Science article, the expected warming may not be outrageous as some had predicted, but it is still significant:

Now two new studies that combine independent lines of evidence agree that climate sensitivity is at least moderately strong--moderate enough so that a really scorching warming appears unlikely. Even with the most conservative assumptions, says climate researcher Chris E. Forest of the Massachusetts Institute of Technology in Cambridge, the studies cool the maximum warming. And the reinforced low end of the range, he says, means continued emissions will fuel a substantial warming in this century....

The two new studies rein in those soaring upper limits for climate sensitivity while reinforcing the substantial lower limit. Climate modeler Gabriele Hegerl of Duke University in Durham, North Carolina, and colleagues started with Northern Hemisphere temperatures between 1270 and 1850 extracted from records such as tree rings. In those preindustrial times, volcanoes, the waxing and waning of the sun, and natural variations in greenhouse gases were changing temperature. Hegerl and her colleagues then combined the preindustrial temperature response to those climate forcings with the global response in the 20th century to volcanoes, rising greenhouse gases, and thickening pollutant hazes. In this week's issue of Nature, they report a 5% probability that climate sensitivity is less than 1.5°C and a 95% chance that it's less than 6.2°C. That's still pretty high, but a far cry from 9°C or 11°C.

Although the ideas of rapid heating and catastrophic sea level increases make for better headlines, the fact of the matter is that changing weather patterns, loss of arctic ice, and threats to ecosystems and endangered species have always been much more likely outcomes of global warming and are just as threatening in their subtlety. These events can occur (and already have begun to occur) with only moderate increases in temperature well within the predicted values of the latest studies.