Climate change in black and white

Climate change in black and white

When air pollution hurts people’s health and heats up the climate it makes sense to do something about it. But what about pollution that cools the planet?

Feb 17th 2011 | from PRINT EDITION


AN IDEAL fossil-fuel power-plant would produce power, carbon dioxide and nothing more. Less-than-ideal ones—not to mention other devices for the combustion of carbon, from diesel generators to brick kilns and stoves burning dung—also emit various gases and gunk. These often cause local environmental problems, damaging lungs, hurting crops and shortening lives. And some of the gunk, notably soot or “black carbon”, can warm the planet, too.

Next week ministers attending the governing council of the United Nations Environment Programme (UNEP) in Nairobi will be presented with the summary of a new report on how fighting air pollution can help the global climate (the report itself is due to follow a couple of months later). The summary makes a powerful case for acting on two short-lived climate “forcings”, factors that change the amount of energy the atmosphere absorbs, as carbon dioxide does, but stay in it only briefly. One is black carbon and the other is ozone, which is vital for blocking ultraviolet rays in the stratosphere but hazardous in the bits of the atmosphere where plants live and people have to breathe.

According to the UNEP report, implementing measures known to be effective against these two pollutants over the next 20 years would have “immediate and multiple” benefits, including temperatures between 0.2°C and 0.7°C lower than they would otherwise be by 2050 and the saving of between 0.7m and 4.6m lives with improved air quality. For black carbon the measures are largely in the form of more efficient ways of burning things; for ozone they mostly involve reducing emissions of methane, which encourages reactions in the atmosphere that make ozone. The black-carbon measures save a lot more lives than ozone control, but are trickier to assess in terms of climate.

Clearing the air

UNEP’s interest in black carbon dates back to a plane journey taken a decade ago. Veerabhadran Ramanathan, of the Scripps Institute of Oceanography in La Jolla, California, and Paul Crutzen, a Dutch climate scientist who was one of the first to theorise about “nuclear winter”, wanted to find out what aerosols (particles small enough to float in the atmosphere) were doing to the climate. Their campaign, which brought together 150 scientists and a plethora of research aircraft and satellites, revealed the hitherto unappreciated extent of an “Asian brown cloud” thousands of kilometres across and fed by fires, diesel fumes and all manner of other things.

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Want to Fight Global Warming? Don't Just Focus on CO₂

by Eli Kintisch on 17 February 2011, 6:25 PM

WASHINGTON, D.C.—Carbon dioxide is the elephant in the room for any discussion of how to stem global warming. But a new report suggests that tackling emissions of two other short-lasting pollutants—methane and the black component of soot—could slow expected warming by a full 0.5˚C beyond what targeting CO₂ alone could accomplish by 2070.

The report, which will be discussed here Sunday at the http://news.sciencemag.org/sciencenow/aaas_2011/?ref=topst">annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW), includes a lot of uncertainty. But it fits with what scientists have learned about these pollutants in recent years. Methane is a more potent warming agent than CO₂, although its duration in the atmosphere is measured in decades rather than centuries. Methane also contributes to asthma-causing pollution. Black carbon, the product of burning wood or other carbon-based fuels, heats the air directly, accelerates the melting of any snow it lands on, and creates so-called brown clouds that warm the sky. Although harmful to the hundreds of millions who breathe it each day, black carbon settles out of the atmosphere in a few weeks. So reducing emissions of it would have a nearly immediate impact on global temperatures.

In the study, an international team of researchers first examined 2000 different pollution-control measures for the two pollutants and chose 16. The measures include stemming methane leaks from coal mines or landfills and stopping black carbon pollution from primitive stoves and diesel construction equipment. The scientists then ran two separate http://www.mpimet.mpg.de/en/wissenschaft/modelle/echam.html">climate models to learn how the rate of global warming might change if the 16 measures were deployed, with and without carbon dioxide controls.

Under control runs without any pollution controls, the global temperature rose by 2.5˚C—plus or minus about 0.7˚C—by 2070. Stemming CO₂ (to an atmospheric level of http://www.imaginaryplanet.net/weblogs/idiotprogrammer/2010/11/achieving-450-ppm-requires-that-us-peak-in-ghg-by-2015/">450 ppm) reduced that warming by about 0.5°C. Deploying the 16 controls reduced the warming by an additional 0.5˚C, again with big error bars. (Since 1850, scientists estimate Earth's temperatures have risen by 0.7˚C.)

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