Image: United States Geological Survey
BOULDER, USA, 19 September 2006: Creating fake volcanic eruptions could help combat global warming, according to a new U.S. study.
“This can significantly offset future warming and provide additional time to reduce dependence on fossil fuels,” said Tom Wigley of the National Centre for Atmospheric Research in Boulder, Colorado, author of the study.
He proposed a two-pronged approach to stabilising climate, with injections of climate-cooling sulfates into the atmosphere - like those produced in volcanic eruptions - combined with cuts in greenhouse gas emissions. While his paper did not address the many technical and political challenges involved in potential such an effort, Wigley suggested that geoengineering could buy some time in the fight against global warming.
The study, published in the U.S. journal Science, calculated the climatic impact of injecting sulfate particles equivalent to those produced by the eruption of Mount Pinatubo in 1991, which had a short-term cooling effect on the climate.
If found to be environmentally and technologically viable, such injections could provide a "grace period" of up to 20 years before major cutbacks in greenhouse gas emissions would be required, according to Wigley.
If climate change were addressed only through emission reductions, then massive cuts in emissions would be needed to keep temperatures from rising more than 2.0 degrees celsius over present levels. This amount of warming has often been cited as a benchmark of dangerous climate change.
Given the difficulties of making such massive cuts, scientists have begun to reexamine a variety of schemes proposed over the last few decades to reduce the impact of climate change through global-scale technological fixes, often referred to as 'geoengineering'.
One strategy first proposed in the 1970s is to inject large amounts of sun-blocking sulfate particles into the stratosphere via aircraft or other means. The idea would be to cool the climate for a year or more with each injection, much as the largest volcanic eruptions do.
Using computer models to track sunlight and other energy flows in the Earth system, Wigley examined two scenarios that project the impact of emissions on climate from now to the year 2400.
In one scenario, total emissions would have to start decreasing immediately, and would need to be cut by around 50 per cent in the next 50 years to keep global climate from warming by more than the 2˚C benchmark. An alternative scenario, the "overshoot" approach, allows a period of increasing total emissions, extending to the 2030s, before stringent cutbacks begin.
To see how geoengineering might change this picture, Wigley took the overshoot scenario and added three frequencies of Pinatubo-scale injections of sulfates into the stratosphere - the layer of atmosphere between about 10 km and 50 km above the Earth’s surface.
The frequencies tested were equivalent to an eruption every year, every two years, and every four years. In all three cases, global temperature stayed approximately constant for the next 40 to 50 years. After 2050, the cumulative effect of greenhouse gases produced a slow temperature rise, though it was muted by the injections.
"Geoengineering could provide additional time to address the economic and technological challenges faced by a mitigation-only approach," said Wigley. He noted, however, that it's not a panacea.
Carbon dioxide from fossil fuel burning has, for example, led to acidification of Earth's oceans. Even if geoengineering could help limit global warming, the oceans would continue to acidify as greenhouse-gas emissions climb, threatening the health of marine ecosystems.
Mitigation alone can potentially solve both the warming and ocean acidification problems, but it has its own set of difficulties, said Wigley. The rapid emissions reductions required to keep below the 2˚C warming threshold would be costly, perhaps unacceptably so, and would pose severe technological challenges.
"A relatively modest geoengineering investment could reduce the economic and technological burden on mitigation by deferring the need for immediate or near-future cuts in carbon dioxide emissions," Wigley said.