Why cleaner air could speed global warming

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.

In the face of severe climate risks, credible scientists are beginning to study geo-engineering -- tinkering with global systems to reduce warming directly. One scheme is to spew sulfates or other sun-blocking particles miles high in the stratosphere. If it worked, it would mimic the natural cooling effect of volcanoes, replacing the near-surface sulfate mask with a much higher one. But the possible side effects could be dire, including damage to the ozone layer. The potential geopolitical implications, like wars over the thermostat, could be devastating as well.

We might need geo-engineering to stave off the worst effects of the warming. But most climate scientists think we’re not there yet. And so the most important thing we can do now is to train our sights on both the unexpectedly helpful sulfates and the unexpectedly pernicious carbon. We can’t continue to only focus on traditional pollutants without reducing greenhouse emissions. We simply have to find a way to clean our air of both.

Eli Kintisch is the author of the just-published “Hack the Planet: Science’s Best Hope -- or Worst Nightmare -- for Averting Climate Catastrophe.”