Warming Brings Poisonous Ozone Down to Earth

Sept. 9, 2009 -- Climate change could bring a cloud of poisonous ozone gas tumbling down from high in the atmosphere, according to a new study.

Aloft in the stratosphere, an enormous blanket ozone gas (O3) regulates the amount of ultraviolet radiation that hits Earth's surface. It's so essential to life that in 1989, most of the developed world banded together to sign the Montreal Protocol, which effectively ended the emission of any gases that damage the protective layer.

Scientists predict high-level ozone could return to normal levels by mid-century.

However, climate change is likely to interfere, altering global winds such that they will drag millions of tons of ozone down toward the planet's surface, where it becomes a greenhouse gas that is toxic to people and plants.

Globally, as much as 23 percent more ozone -- 151 million tons -- will be pouring its way down each year by the end of the century.

Global warming will also have an impact on the amount of ultraviolet radiation that reaches Earth's surface, according to the study's authors.

"Climate change leads to an increase in the seasonally averaged ultraviolet index of up to 20 percent," in the lower reaches of the Southern Hemisphere, Michaela Hegglin and Theodor Sheppard of the University of Toronto write in an paper published this week in Nature Geoscience.

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Most regions will get off easier than the southern hemisphere. The tropics are expected to witness just a 4 percent uptick in radiation. Winds in the northern hemisphere will pile up ozone, reducing the amount of UV light reaching Earth surface by 9 percent.

Still, as ozone flows down to lower altitudes people and plants will have to contend with a gas that is toxic when inhaled, and a potent driver of greenhouse warming.

Today, car exhaust and emissions from power plants mingle with sunlight to produce most of the surface-level ozone we breathe. Crops suffer when exposed to ozone, and so do people -- exposure to the gas damages lungs and aggravates respiratory diseases. Worldwide it claims tens of thousands of lives each year.

It remains to be seen whether the authors' prediction will come true. But the finding points out how many unintended consequences of human-induced climate change we have yet to understand, let alone plan for.

David Stevenson of the University of Edinburgh in the United Kingdom takes the findings as a call to action.

"If the influx of stratospheric ozone to the troposphere does increase with climate change, and background tropospheric ozone levels rise," said Stevenson, "then increasingly stringent ozone pollution control policies will be needed to attain the air quality standards required to protect the biosphere."

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