BioEd Online

New research suggests summer thunderstorms across the United States inject water vapour far higher into the atmosphere than previously believed, enabling a cascade of chemical reactions that could pose a new threat to the Earth’s protective ozone layer as the climate warms.

A team led by Harvard University atmospheric scientist James Anderson made the discovery while investigating the origins of high-altitude cirrus clouds, the thin and wispy clouds that frequently blanket the sky – and trap heat, contributing the greenhouse effect.

Working aboard two aircraft as part of a larger NASA investigation, Anderson’s team expected to see average summer storms pump water vapour into the upper troposphere to supply cirrus clouds at altitudes of around 14 kilometres. Instead, they reported today in Science, about half of the storms injected vapour all the way into the stratosphere, reaching altitudes of up to 20 kilometres.

“We were shocked,” he says. “Standard run-of-the-mill Midwestern thunderstorms are far more capable of injecting water vapour into the stratosphere than we once thought.”

The implications for stratospheric ozone, which shields the Earth from ultraviolet radiation, are significant given that the chlorine reactions that destroy ozone are governed primarily by temperature and water vapour temperature. If storm activity increases as expected due to global warming, Anderson says, the destruction of stratospheric ozone – and consequent increase in ultraviolet radiation levels – could be significant.

Given that those same reactions also depend on sulphate aerosols, the research also raises questions about potential geoengineering schemes that involve pumping sulphate aerosols into the stratosphere to reflect sunlight back into space. This has prompted initial discussions about a series of atmospheric experiments as well as an erroneous press report of a possible geoengineering experiment.

Andrew Dessler, an atmospheric scientist at Texas A&M University in College Station, who was not involved in the current study, says the paper represents a schematic assessment that must now be addressed with further observations. “I’m not at all surprised that this happens, but I think a challenge is really going to be quantifying all aspects of the problem,” he says, including the stratospheric reactions and the frequency of the water vapour injections.

The affected area of the stratosphere – located between 15 and 20 kilometres in altitude – contains about 20 percent of the total stratospheric ozone. Anderson says he sees the same conditions for rapid ozone destruction that scientists have investigated for decades in Antarctica and more recently in the Arctic. Assuming cases in which water vapour content increases by 150 and 190 percent, Anderson’s team suggests that between 25 and 35 percent of the ozone in that area could be destroyed within a week under the right conditions.

Kerry Emanuel, a climatologist at the Massachusetts Institute of Technology in Cambridge said the study puts a spotlight on a question that his profession has yet to pin down: How will these storms respond to climate change? “We really don’t know the answer to that question,” he says. “This is a surprisingly under-developed field.”

Geoengineering risk

The study also raises questions about geoengineering, because these chemical reactions depend on the presence of sulphate aerosols; artificially increasing the sulphate aerosol concentrations would increase the surface area where the reactions take place. “The worst cocktail you can think of is to inject a combination of sulphur and water into the stratosphere, and that is exactly what would be happening,” Anderson says. “Nature would be injecting the water, and humans would be injecting the sulphates.”

David Keith, a leading geoengineering expert and colleague of Anderson’s at Harvard, says the study speaks to the need for further analysis but does not necessarily rule out geoengineering. The increased risk from water vapour and sulphates must be weighed against the fact that chlorine levels in the atmosphere are dropping due to successful implementation of the Montreal Protocol on ozone-depleting substances, Keith says. Initial calculations, he adds, suggest that the two effects could roughly cancel each other out.

Anderson and Keith are now discussing ways to conduct a series of atmospheric experiments – via balloon or aircraft – to test the risks to ozone, including the release of both water vapour and sulphates into the stratosphere. The latter possibility sparked a brief media frenzy last week when a news report surfaced suggesting that Anderson and Keith were planning to conduct a geoengineering experiment within the year. Both say the idea is still in the discussion and modelling phase and would need to go through the full suite of public funding and regulatory approvals before it moves forward.

Although releasing sulphates into the stratosphere would clearly qualify as a geoengineering experiment, Keith says it would be justified in order to understand the basic chemistry that is under way at present in the stratosphere today. And although they have not determined how much sulphur would be necessary, Keith points out that even an injection of one tonne would barely register given that global sulphur emissions currently add up to around 50 million tonnes annually.

“In my view we must simply know more, and knowing more means knowing the risks,” Keith says. “And you cannot know the risks just by computer modelling.”