The policies discussed to date in the global warming conferences have addressed only half the arena for remedial action-the inside of the "greenhouse." They seek to end the accumulation of greenhouse gases (principally carbon dioxide and methane) within the atmosphere, where these gasses entrap infrared radiation rising from the Earth-surface and lower atmosphere. This entrapment is akin to the glass cover of a greenhouse that keeps the planted vegetables warmer than if they were left exposed to the open air, by admitting the shorter wavelengths of sunlight while reflecting back into the greenhouse the longer wavelengths of thermal infrared radiation and thus preventing the loss of "trapped" heat.

The other arena for action is outside of the "greenhouse." It offers opportunities for reducing global warming by increasing the fraction of incoming sunlight that is reflected outward by the upper atmosphere back into space. Expressed in the metaphoric language of the "greenhouse effect", this type of climate geoengineering would put a parasol over the greenhouse to scatter away roughly 2 percent of incoming sunlight, instead of letting this small fraction impinge on our planet's biosphere through the "greenhouse roof" (which in fact is the Earth's lower atmosphere). In the language of climate science, such geoengineering would increase by a few percent the Earth's albedo-the ratio of incoming sunlight reflected back into space relative to the total inbound from the Sun.⁴

Nature routinely varies the local values of the Earth's albedo by substantial amounts, with clouds being the most familiar and quantitatively most important reflective entities, and ice- and snow-covered regions the next most significant. Episodically, large regions of our planet have been cooled for several years by major volcanic eruptions, which inject millions of tons of fine particulate material-mostly sulfate aerosols-into the stratosphere, where they increase the albedo until they're slowly removed by natural processes. For instance, the Mount Pinatubo eruption in 1991 caused a cooling of most of the Earth for a few years, of a magnitude which was roughly equivalent to reversing half of the total global warming that occurred during the entire twentieth century.

The idea of artificially increasing the Earth's albedo is not new. In 1992, a report by the National Academy of Sciences found the prospect of lower stratospheric-based albedo enhancement to be "feasible, economical, and capable. . . ." And it doesn't necessarily have unpleasant side effects. Professor Paul Crutzen, who received the Nobel Prize for his work on atmospheric ozone, wrote in 2005 that climate geoengineering with sulfate aerosols sufficient to offset the global warming caused by a doubling of the atmospheric carbon dioxide content (which might occur by 2100) would probably do less damage to the upper stratospheric ozone layer than did the Mount Pinatubo volcanic eruption in 1991.

Injecting sulfate aerosols into the stratosphere certainly is not the only climate geoengineering option. It's simply the one that occurs most often naturally. For example, the huge eruption of Mount Tambora in November 1815 caused "the year without a summer" in 1816, with frosts in July in New England and famines from crop failures that reportedly led to the deaths of several hundred thousand people in Europe.

In the 1990s, Professor Edward Teller and his colleagues analyzed some of the most promising advanced approaches to climate geoengineering, those aimed at minimizing the amount of materials to be deployed in the upper atmosphere by a careful use of materials better-suited than sulfate aerosols, while also enhancing beneficial side effects, such as reductions in the ultraviolet component of sunlight. They found that various approaches might involve deploying as much as a million tons per year to offset global warming levels anticipated for the later 21^st century, with estimated annual operating costs of less than a billion dollars. More recent proposals from Professors Stephen Salter and John Latham offer a remarkably near-term, low cost, almost instantly reversible climate engineering option. It involves the spraying of fine mists of seawater into low-lying maritime cloud-layers to enhance their albedo for post-injection intervals of a week or so. But deploying mirrors or lenses in space to enhance the planetary albedo or to modify the incident intensity of sunlight-an idea that has been mentioned in newspaper stories-will not possibly be attractive options until space-launch costs become far lower.

 

RECENT COMPUTER modeling studies led by Stanford's Ken Caldeira suggest that inexpensive, readily implemented geoengineering measures could swiftly and lastingly stabilize at present levels the climate of the Arctic, or of the entire Earth, and could even stabilize these regional or global climates at the cooler levels that prevailed decades ago.

Alas, environmental extremists fiercely oppose any proposal for any type of climate geoengineering. They regard the advocates of geoengineering as doubly guilty. First, because these advocates allege that their proposed measures avert global warming while doing no discernible harm to our planet (a false assertion according to the extremist position); and second, because such geoengineering would reduce political pressure to restrict the burning of fossil fuels. Thus, while several climate geoengineering options promise to be eminently cost-effective, readily available and innately reversible, ideological opposition to them is intensely emotional and remarkably obstinate. Because of this ferocious resistance, no serious experimental research programs have been started by Congress or the always timorous bureaucracies in charge of funding research.

Such willful ignorance is plainly irresponsible. Without some small-scale testing in the "real" atmosphere, not enough data will become available to assess the benefits, costs and side effects of conceptually attractive climate geoengineering options. Much could be learned about the other half of the global warming story for a tiny fraction of the funds that have been allocated to climate change studies focused on greenhouse gas emissions; that is to say, global warming problems as viewed exclusively from inside the "greenhouse."

The opponents of geoengineering should understand that none of the suggested options is meant to be a free-standing, long-term solution to global warming issues. To keep pace with a growing greenhouse effect associated with ever-increasing levels of greenhouse gases, such geoengineering measures would not only have to be maintained indefinitely but also gradually augmented, while the accumulating carbon dioxide in the atmosphere would make the oceans harmfully acidic on multi-century timescales. Most certainly then, we should be intellectually prepared for both-near-term geoengineering and longer-term reductions in carbon emissions. We should continue to pursue all reasonable measures for reducing emissions of greenhouse gases so that longer-term atmospheric levels of these gases are kept within tolerable bounds. And we should more vigorously explore geoengineering options for climate stabilization in the near term and for use in the event of a global warming emergency situation.

It is prudent to prepare for such an emergency, in light of the significant risk that our current or near-term policies for addressing carbon emissions might prove insufficient. At the IPCC's most recent meeting in November, warnings were voiced that greenhouse gas emissions were substantially outpacing previous estimates, while the Earth's natural "sinks" for CO₂-forests and oceans, for example-were becoming less effective than previously projected, so that atmospheric CO₂ levels have been rising more rapidly over the past half-decade than forecast even quite recently. The measures actually implemented to reduce CO₂ emissions have accomplished little, while the world's total burning of fossil fuels keeps increasing. This should not be surprising. The world population is still growing, mainly in poorer countries, where the people aspire to the lifestyles of the citizenry of wealthy nations. In order to remain in power, governments in developing nations of all ideological persuasions will seek to service these basic human yearnings-a fundamental point about which the Chinese government is charmingly candid.

 

SO THE task ahead is clear. Programs should be funded to conduct serious research in climate geoengineering and to carefully evaluate the most promising options, while international efforts to curtail greenhouse gas emissions continue. The United Nations' IPCC could be a forum for commenting on specific geoengineering options and helping to monitor field tests. But maneuverings under varied international umbrellas should not prevent a coalition of like-minded nations from moving ahead with such research. When the research and test results warrant, nations technically capable of implementing specific climate geoengineering options, and willing to provide the necessary financing, might aptly form an ad hoc group to carry out the chosen options.

Such a coalition of the capable might include China, India, the Gulf States, the United States and other nations that could suffer significant damage from higher temperatures, rising sea levels or other warming effects. Fossil fuel-exporting nations may also be interested in climate geoengineering as it might temper the current near-frantic character of moves to restrict the burning of fossil fuels. Some nations, however, might oppose any initiative for climate geoengineering unless it has been blessed by the United Nations. Some other nations might object because they surmise that their own benefits from warming will outweigh the probable damage for them.

 

LET US admit that even the most successful geoengineering initiatives would leave us with unfinished business. As already noted, we likely need both: effective climate geoengineering and long-term measures for keeping atmospheric levels of greenhouse gases within bounds. The long-term management of greenhouse gas emissions will require a gradual transformation of the worldwide system for meeting mankind's energy needs.

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