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How Scientists Measure the Sensitivity of Our Climate.

 

Wednesday, July 11, 2007.

 

By Jeremy Hsu

 

As greenhouse gases have built up in the Earth’s atmosphere, scientists have struggled to come up with a credible projection of how much warmer the planet will be when the amount of carbon dioxide reaches twice its pre-industrial level – a dubious milestone that many experts think will occur before the year 2100.

 

Now, many of the world’s climatologists are closer to agreeing on the impact that greenhouse gases will have on the earth’s temperature. With a doubling of atmospheric carbon dioxide, they predict a temperature increase of between 2 and 4.5 degrees Centigrade, with a most likely estimate of three degrees Centigrade, or 5.4 degrees Fahrenheit.

 

This new estimate for so-called “climate sensitivity” is prominently mentioned in the newly released report of the Intergovernmental Panel on Climate Change, and many researchers are already incorporating it into computer models to project future climate scenarios.

 

Climate sensitivity is a “critical measure,” said William Collins, a scientist at the US National Center for Atmospheric Research in Boulder, Colo., and a lead author on the section of the IPCC report dealing with future projections. Thanks to the sensitivity measure, he said, “we can estimate how much the planet will warm if you tell me how much greenhouse gases and aerosols are pumped into the atmosphere.”

 

Regardless of what the actual carbon dioxide level will be by 2100, scientists can now use climate sensitivity to show policymakers how much warmer the Earth might become. It’s something like being able to look at the temperature settings on an oven; even if you don’t actually turn the dial all the way up, you know how warm the oven will get at that point.

 

Since no one can predict exactly how many emissions the world will be producing in the future, the IPCC report explores a range of scenarios, each with different emission levels.

 

A world with cleaner and more efficient technologies, as well as global efforts to improve sustainability, might only see an increase of 3.2 degrees Fahrenheit by 2100, while a world that continues to use fossil fuels heavily may heat up by 7.2 degrees.

 

Better climate models and new studies on the paleoclimate record of Earth’s past have helped significantly in determining climate sensitivity. There are particularly good climate records after the Industrial Revolution of the 18th century, thanks to direct temperature measurements and, more recently, satellite data. Those measurements can be used to improve climate models by testing their accuracy.

 

“We have very reliable estimates of how much greenhouse gases changed over that time period, and we can attempt to estimate how much the ocean has warmed over the last few centuries,” said Collins. “If you can observe how much the greenhouse gases and aerosols have increased, and you know how much the oceans have warmed, you can estimate the climate’s sensitivity to changes in the atmospheric composition.”

 

The new improved models and better climate data explain why the latest IPCC assessment was able to arrive at a more specific estimate for sensitivity than the previous IPCC effort, which included a likely climate sensitivity range of 2.7 to 8.1 degrees Fahrenheit. The newest climate sensitivity range is stated as likely to be from 3.6 to 8.1 degrees Fahrenheit, and very unlikely to be less than 2.7 degrees Fahrenheit.

 

The fourth assessment report has “considerably narrowed the uncertainties of the previous report,” declared Michel Jarraud, Secretary-General of the World Meteorological Organization, at the report’s unveiling in Paris this February.

 

However, Collins also noted continuing uncertainties in both climate models and the paleoclimate record. Natural variation, caused by changes in the sun’s brightness and volcanic eruptions spewing light-reflecting particles into the air, is one source of uncertainty. Scientists still struggle to understand the total effect of aerosols on climate change, since aerosols can have both warming and cooling effects.

 

That uncertainty haunts scientists who are trying to recreate the sensitivity of the climate in models. Like an allergy sufferer who is more sensitive to cat hair than to pollen, the climate also has different sensitivities.

“For solar forcing, the climate might be a bit more sensitive than for carbon dioxide,” said Judith Lean of the Naval Research Laboratory’s Space Science Division in Washington, D.C., and a lead author on the IPCC report.

 

She observed that differences in climate sensitivity depend on the different forcing factors such as solar brightness or volcanic eruptions, and that many current models do not take this into account.

 

As a result, Collins called climate sensitivity an “emergent quantity,” contrasting it with well-defined scientific measures such as the speed of light. Yet he pointed out that scientists are embracing uncertainty by exploring a wide range of relevant models.

 

“It is a slowly evolving number,” said Collins.

 

Jeremy Hsu is with New York University's School of Journalism Scienceline.

 

Main Picture: Jerry Riley at www.jerryriley.com

 

 

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