Why cleaner air could speed global warming Aerosol pollution, which is now on the downswing, has helped keep the planet cool by blocking sunlight.

Why cleaner air could speed global warming
Aerosol pollution, which is now on the downswing, has helped keep the planet cool by blocking sunlight. Tackling another pollutant, soot, might buy Earth some time.
Opinion

You're likely to hear a chorus of dire warnings as we approach Earth Day, but there's a serious shortage few pundits are talking about: air pollution. That's right, the world is running short on air pollution, and if we continue to cut back on smoke pouring forth from industrial smokestacks, the increase in global warming could be profound.

Cleaner air, one of the signature achievements of the U.S. environmental movement, is certainly worth celebrating. Scientists estimate that the U.S. Clean Air Act has cut a major air pollutant called sulfate aerosols, for example, by 30% to 50% since the 1980s, helping greatly reduce cases of asthma and other respiratory problems.
But even as industrialized and developing nations alike steadily reduce aerosol pollution -- caused primarily by burning coal -- climate scientists are beginning to understand just how much these tiny particles have helped keep the planet cool. A silent benefit of sulfates, in fact, is that they've been helpfully blocking sunlight from striking the Earth for many decades, by brightening clouds and expanding their coverage. Emerging science suggests that their underappreciated impact has been incredible.

Researchers believe greenhouse gases such as CO2 have committed the Earth to an eventual warming of roughly 4 degrees Fahrenheit, a quarter of which the planet has already experienced. Thanks to cooling by aerosols starting in the 1940s, however, the planet has only felt a portion of that greenhouse warming. In the 1980s, sulfate pollution dropped as Western nations enhanced pollution controls, and as a result, global warming accelerated.

There's hot debate over the size of what amounts to a cooling mask, but there's no question that it will diminish as industries continue to clean traditional pollutants from their smokestacks. Unlike CO2, which persists in the atmosphere for centuries, aerosols last for a week at most in the air. So cutting them would probably accelerate global warming rapidly.

In a recent paper in the journal Climate Dynamics, modelers forecast what would happen if nations instituted all existing pollution controls on industrial sources and vehicles by 2030. They found the current rate of warming -- roughly 0.4 degrees Fahrenheit per decade -- doubled worldwide, and nearly tripled in North America.

Despite intransigence on carbon emissions, even China is taking aggressive steps to cut sulfate pollution, and temperatures have risen as a result.

But surely the answer can't be to slow our drive to clean our air. One way to buy time might be to tackle another air pollutant that warms the planet: soot. In 2008, scientists estimated that so-called black carbon, soot's prime component, is responsible for 60% more global warming above that caused by greenhouse gases. Cleaner-burning diesel engines in the West and more efficient cookstoves in the developing world are the answer. But on both scores, "relatively little has been done to address the problem," says the Boston-based Clean Air Task Force.

The Heat Over Bubbling Arctic Methane

MARCH 5, 2010, 12:57 PM

The Heat Over Bubbling Arctic Methane

By ANDREW C. REVKIN

One of the great challenges in assessing the meaning of changes in Arctic climate and other environmental conditions is putting today’s observations in long-term context. This is as true for the bubbling emissions of methane from the frozen, but warming, sea bed as for sea ice around the North Pole. Two recent studies of methane emissions from frozen sea-bed sediments, including one published in Science and described in The Times today, found substantial bubbling flows of methane, a potent greenhouse gas, were reaching the atmosphere. In its news release, the National Science Foundation, which helped underwrite the research, described the emissions as taking place “at an alarming rate.”

But are these emissions new, or simply newly observed? Does this mean that the Arctic system is coming unglued, and that a great outpouring of this heat-trapping gas is about to upend the global climate system?

Despite portentous headlines — including one on the news release from the University of Alaska, Fairbanks, saying “Arctic seabed methane stores destabilizing…” — there is no evidence (yet) that what is happening is fundamentally new or destabilizing, at least according to some of the scientists most closely tracking levels and sources of this gas from the poles to the tropics. Here’s how Ed Dlugokencky, one of the top federal scientists tracking methane trends, summarized the situation in an e-mail message:

Are these emissions new, brought on by increasing temperature of bottom waters, or have they been there unnoticed for decades or longer? Based on our atmospheric observations, I suspect they have been there. We saw an increase in CH4 growth rate in 2007 in the Arctic (likely from very warm temperatures in wetland regions increasing microbial CH4 production), but it did not increase in 2008.

Also, the difference in annually averaged CH4 between Arctic and Antarctic latitudes is a sensitive indicator of changing CH4 emissions at high northern latitudes. The only persistent large change in this difference we’ve observed was from 1991 to 1992 when the economy of former Soviet Union collapsed. The difference has varied since then, but has not recovered.

Dr. Dlugokencky has told me previously that, for the moment, it appears that methane releases from warming Arctic soils and other sources constitute a potentially amplifying warming influence, in which warming releases more gases that contribute to further warming. Such a “positive feedback” adds to the logic for working to limit human-driven warming, many climate scientists say.

But Dr. Dlugokencky, like quite a few other scientists assessing Arctic warming, sees no evidence for a “tipping point” beyond which this cascades uncontrollably. That doesn’t mean this is impossible, just that there’s no evidence pointing to such a prospect.

Martin Heimann, who wrote an accompanying analysis in Science and is a researcher at Germany’s Max Planck Institute for Biogeochemistry, sent this cautionary note:

Indeed, at this point, it is impossible to tell whether these Arctic emissions are directly caused by recent Arctic warming or whether they have been persistent over at least much of the Holocene. This can only be answered from longer time series; complemented, maybe by borehole measurements in this shelf permafrost. Therefore, these new emission estimates do not allow yet a quantification of the permafrost methane-climate feedback.

Personally, I do believe that this feedback exists, but it doubt very much that it is “catastrophic” with large emissions over relatively short time scales (20-50 years) as implied by the “tipping point” metaphor. Even under strong warming the melting of permafrost takes time and the release of greenhouse gases will be quite gradual and will manifest itself as increased leakages.

Can society grasp the basics here without having to be pummeled with messages overstating the case? Given the seductive power of the front-page thought, scientists, their institutions and grant providers can — wittingly or unwittingly — drive overheated media coverage that, in the end, can kick back and undercut credibility.

One thing that would help clarify Arctic trends and their causes is more monitoring of methane on the ground in and around the Arctic, Dr. Dlugokencky and other researchers say.

Euan Nisbet, a University of London scientist from a team that found methane bubbling elsewhere in the Arctic, said this in an e-mail message:

The Arctic is a good example of the need for “in situ” monitoring. Satellites use reflected sunlight, and so cannot see the Arctic in winter. In situ work is relatively cheap, but unloved. We need both in situ and satellite monitoring to assess greenhouse gases. The U.S. deserves a lot of credit for the superb work it does supporting in situ monitoring by NOAA, Scripps and AGAGE.

He added that the European Union had canceled a project monitoring methane that was particularly valuable because it could distinguish between emissions from terrestrial bogs, gas fields and the like and ocean sources.

Source: http://j.mp/GWARM