Crank of the Week - October 20, 2008 - Tom M. L. Wigley

A few weeks ago we gave the Crank of the Week award to John Latham for suggesting the creation of a fleet of 1500 robot sailing ships to combat global warming. This week the award goes to Tom M. L. Wigley, Senior Scientist at the National Center for Atmospheric Research (NCAR) and an aspiring geoengineer, for suggesting that global warming be curbed by releasing sulfur dioxide (SO2) into the stratosphere. There it would react with oxygen and water to form minute sulfate droplets made up of water, sulfuric acid (H2SO4) and other particles thus increasing Earth's cloud cover. Increased cloud cover should make Earth’s surface cooler.

Geoengineering schemes fall into two general categories: controlling how much sunlight reaches Earth’s surface and controlling how much heat escapes back into space. The later type of scheme depends on how much CO2 is in the atmosphere and hence concentrate on ideas for removing CO2. Fertilizing the oceans with iron or processing air through growing mats of algae would attempt to reduce the amount of carbon dioxide in the atmosphere. Unfortunately, such actions would take decades to have much of an effect. Alternatively, reducing incoming sunlight by increasing cloud cover could stop global warming immediately (in theory).

The idea to inject several million tons of SO2 into the stratosphere a year was first proposed by the late Russian physicist Mikhail I. Budyko, in 1974. Since then the idea has been taken up by Dutch atmospheric chemist Paul J. Crutzen and our crank nominate, Dr. Wigley. Few scientists would suggest that blocking the sun is a substitute for stopping the rise of atmospheric CO2 or that geoengineering can fix the CO2 problem by itself. The supporters of such schemes argue instead that it might buy us time to convert the world to carbon-neutral energy sources. “The reason I think geoengineering should be considered,” says Wigley, “is I don’t think we are going to save the planet with the emissions-reductions approaches that are on the table. No one is taking the magnitude of the technological challenge seriously.”

Will this idea work? In 1991 the volcanic eruption of Mount Pinatubo in the Philippines put 20 million tons of SO2 into the stratosphere. Scientists attribute that injection with cooling Earth by one degree Fahrenheit for about a year. Budyko’s original idea had been to send planes into the stratosphere burning high-sulfur fuel; Crutzen proposed delivering the SO2 with balloons. Estimates vary as to just how much SO2 would be needed to counteract, say, a doubling of CO2 over preindustrial levels. Wigley put the number at five million tons a year; Crutzen and Philip J. Rasch of NCAR have calculated that 1.5 million tons would do the job. Crutzen estimated that his scheme would cost between $25 billion and $50 billion a year—between $25 and $50 for each citizen of the developed countries.

But SO2 and CO2 behave differently in the atmosphere. CO2 causes warming everywhere, particularly at the higher latitudes and polar regions. SO2 induced cooling would be strongest in tropical regions around the equator. How the two effects would interact is not clear and the few model simulations done so far suggest the effects of a sulfate sunshade are not simple to predict.

SO2 would not just affect the planet’s temperature, the sulfate sunshade could have serious drawbacks on other grounds. It could change winds and precipitation in ways that are not yet understood. As less sunlight reached the earth’s surface, there would be less evaporation, particularly in the tropics, which could make rain and freshwater scarcer than they are today. A further concern is that pumping tons of sulfuric acid into the stratosphere could cause acid rain while not actually reducing CO2 will accelerate acidification of the world's ocean waters.

Finally, some scientists worry that the protective ozone layer would be adversely affected. The likelihood and extent of this negative outcome is uncertain, like just every other aspect of sulfate geoengineering. Raymond T. Pierrehumbert, a climate modeler at the University of Chicago, worries that putting efforts into a sunshade scheme will distract from the real work of reducing greenhouse gas emissions—in effect hiding the problem, not fixing it. In short, geoengineering cannot solve the CO2 problem and may even make matters worse. When most of your fellow climate scientists think you are crazy, you really deserve a Crank of the Week award, so this one is for you Tom. But will someone try to build the sunshade? To quote Pierrehumbert: “We are quite capable of doing stupid things.”