Mother Nature's Cleanser May Help Study of Global Warming

Scientists have long known about the atmosphere's ability to cleanse itself of nasty pollutants such as benzene and methane. But they haven't always understood how nature's cleanser worked.

Now, thanks to a new study by the National Oceanic and Atmospheric Administration, they do. The cleanser, a chemical substance known as hydroxyl radical, is much more reliable than they thought. That means scientists will be better able to predict future pollution levels in the Earth's atmosphere of various greenhouse gases, including methane.

That, in turn, could assist research on global warming by adding certainty to atmospheric forecasts.

"There are important connections to global warming," said Stephen Montzka, the study's lead author and a chemist at NOAA's Global Monitoring Division in Boulder, Colo. "For these gases, now we will have a better picture for how the earth should respond."

Jos Lelieveld, a German atmospheric chemist who worked on the study, characterizes the findings this way: "In the climate debate, there is also a large discussion about uncertainty. Uncertainties sometimes paralyze people from doing something. This study takes away some of the uncertainties."

NOAA's findings are detailed in a report published today in Science.

To be sure, the impact of the new study on climate change should not be overstated.

Hydroxyl radical, for example, does not cleanse carbon dioxide, the leading greenhouse gas driving global warming, from the atmosphere.

"This is a sub field of a sub field," said Gavin Schmidt, a climatologist at NASA's Goddard Institute for Space Studies in New York.

Still, Schmidt said, the study is important for validating atmospheric chemistry data that is used in global warming modeling.

"This is very careful work," he said. "They're trying to work out exactly what's going on. It's like forensic science. Think of it as 'CSI' for Planet Earth."

Hydroxyl radical is formed when one atom of oxygen links with one atom of hydrogen after sunlight reacts with atmospheric gases. But because the reaction last less than a second, Montzka said, "it is very tricky to measure."

Measurements are taken indirectly by examining air samples collected from remote sites
around the world.

"We have people go out and wait until the wind is blowing from a clean sector," Montzka said. "They fill the cans with air, and send those cans to us in Boulder. We analyze the composition of the air in those cans, week in, week out, year in, year out. We have done it for the past 15, 20 years. This is a story about the power of long-term measurements."

But the measurements of hydroxyl radical varied so widely that they raised questions about the accuracy of projections for the amount of pollutants in the atmosphere. Montzka wanted to solve the riddle of the variations.

Did they mean that the estimates of pollutants emitted into the air were wrong? Or did they mean that the levels of hydroxyl radical fluctuated more widely than originally believed?

In 2007, Montzka teamed up with Lelieveld and several other German and Dutch scientists to get to the bottom of it.

By coincidence, the banning of the industrial pollutant methyl chloroform helped provide the answer. After the chemical was banned in 1996, new emissions all but disappeared. So scientists no longer had to sort out new emissions from the degrading methyl chloroform left in the atmosphere.

And they found that nature's cleanser is fairly consistent in its work to clean up fouled air.

"A much clearer picture of how hydroxyl radical behaved emerged," Montzka said. "Having a good understanding of how hydroxyl radical changes from year to year allows us to provide better projections of methane and its climate influence."