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Geoengineering: should we pump sulfur into the stratosphere?

By: Dr. Jeff Masters, 3:43 PM GMT on February 13, 2008

Professor Paul Crutzen, winner of the 1995 Nobel Prize for his work on the Antarctic ozone hole, has proposed an emergency geoengineering solution to cool off the planet: dump huge quantities of sulfur particles into the stratosphere to reflect sunlight. His paper, "Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma?" was published in the August 2006 issue of the journal Climatic Change. A recent editorial in the New York Times by Ken Caldeira called for more research into geoengineering schemes like this to cool the planet, proposing that 1% of the $3 billion federal Climate Change Technology Program should be spent thusly.

Dr. Crutzen proposes that balloons or artillery guns could propel burning sulfur into the stratosphere, where chemical reactions would convert the sulfur to highly reflective sulfate aerosol particles. This is the same process that occurs when a major volcanic eruption throws sulfur high into the atmosphere, cooling the planet. The 1992 eruption of Mt. Pinatubo in the Philippines cooled the Earth by about 0.5° C the following year. Crutzen estimates that a lesser amount of sulfur would be required to compensate for a doubling of carbon dioxide, and that the cost of lofting the required sulfur into the stratosphere would be about $132 billion. These costs would be per year, since the sulfur only stays in the stratosphere about a year.

Could it work? Sure it could. Volcanos periodically pump huge quantities of sulfur into the stratosphere, cooling the planet. Wunderblogger Dr. Ricky Rood shows a nice plot in his blog this week showing how three major volcanic eruptions in the past 50 years have cooled off the planet. Are there problems with the scheme? Yes, many:

1) The climate might undergo substantial and disruptive changes. Evaporation from the oceans would lessen, changing precipitation patterns. The sulfate aerosols would warm the upper troposphere and lower stratosphere, changing the stability of the atmosphere. This would affect thunderstorm activity and large-scale weather patterns. Increased warming of Europe and Asia in winter has been noted after volcanic eruptions, for example. A 2005 study tied an increase in greenhouse gases and sulfur particles to drought in the Sahel region of Africa. Increasing greenhouse gases and sulfur particles even further might intensify drought conditions there.

2) The small sulfur particles might settle into the upper troposphere, where they might act as condensation nuclei for the formation of cirrus clouds. An increase in these high cirrus clouds may warm the planet, since they keep heat from escaping to space.

3)Stratospheric sulfur causes destruction of the protective ozone layer. The 1982 eruption of the El Chichon volcano reduced ozone by 16% at 20 km altitude at mid-latitudes. Decreased ozone would result in an increase in ultraviolet light at the surface, potentially increasing skin cancer rates.

4) Acid rain would increase.

5) The scheme would do nothing to reduce CO2, and the oceans would continue to acidify. The rate of acidification of the Earth's oceans is causing concern that regional collapses of the food chain may occur later this century.

6) A sudden collapse of the effort to keep firing sulfur into the stratosphere, due to the lack of political will to continue to fund this expensive effort, would result in a sudden transition of the climate to a radically warmer state. The resulting shock to the world's weather might cause dramatic changes that would be difficult to adapt to.

7) What do you do if the scheme causes serious climate problems in a country that then threatens war unless the effort is stopped?

As climate scientist Ray Pierrehumbert wrote in a 2007 blog on RealClimate.org, "It's not really insurance. It's more like building a lifeboat, but a lifeboat based on a design that has never been used before which has to work more or less perfectly the first time the panicked passengers are loaded into it." Pierrehumbert thought that the proposal to spend $30 million of the annual $3 billion climate change research budget was far too much money.

I'm not a big fan of geoengineering schemes. It makes far more sense to spend this kind of money of reducing carbon emissions, since the cure may be worse than the disease. Still, research into geoengineering should continue. We need to keep all of options open for the very uncertain future of our climate. When you're team's down two touchdowns late in the game, sometime you have to take risks you ordinarily would not take. But how much money should be spent on geoengineering research? If you're a wunderground member, take the wunderpoll at the right.

Jeff Masters

Climate Change

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.