What is to be blamed for global warming?

Since the 1980s, emissions of greenhouse gases caused by human activity have been designated the principal culprit, especially carbon dioxide emitted from burning coal and petroleum products. Many measures have been proposed to reduce these emissions. And because climate change does not stop at national borders, European governments, the United Nations, and more recently the United States have tried to establish worldwide emission goals by organizing a cavalcade of international conferences—from the 1997 Kyoto conference to the December 2007 convention in Bali.

Yet no binding agreement has been reached on reducing global carbon dioxide levels, let alone the means to ensure compliance. Decades into this debate, we have neither widely agreed-upon limits on future greenhouse gas emissions nor the administrative capability to carry out such limits. Moreover, climate scientists warn that emission controls alone may not stabilize the climate.

Can anything be done?

Yes. We can develop ways to increase the amount of sunlight that is reflected back into space by the upper atmosphere, an approach rarely discussed at global warming conferences. This strategy falls under an approach called climate geoengineering. Within the metaphor of the greenhouse effect, the solar strategy would work like this: scientists would put up a “parasol” over the greenhouse to deflect 1 to 2 percent of the sunlight that now reaches the earth. We would increase by a few percent the earth’s albedo—the ratio of incoming sunlight reflected back into space relative to the total that arrives from the sun. Scattering this small fraction into space would reduce global warming.

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We know it would work because it happens naturally all the time. Clouds routinely deflect sunlight and thereby cool the earth. Volcanoes— when they erupt and inject millions of tons of fine particulate material into the stratosphere (mostly sulfate aerosols)—have also cooled large regions of the globe. Mount Pinatubo in the Philippines erupted in 1991 and cooled most of the earth for a few years, erasing for a short time roughly half of the global warming that took place during the entire twentieth century.

We know reducing sunlight would work to cool the earth because it happens naturally all the time—thanks to volcanoes and clouds.

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 stratospheric albedo enhancement “feasible, economical, and capable.” And we have a great many geoengineering options apart from adding sulfate aerosols to the stratosphere.

But climate geoengineering is anathema to committed enemies of fossil fuels. Although several geoengineering options appear to be highly cost-effective, ideological opposition to them is often fierce. Bloggers raise fears about who might gain from deliberate weather manipulation or what might go wrong, or claim that geoengineering is an evasion of global warming responsibility rather than a response to it. Thanks to this intimidating opposition, no serious geoengineering research programs have been undertaken. But without small-scale tests, no one will understand the benefits, costs, or possible harmful side effects of such approaches.

Many of these concerns could be addressed at the cost of a tiny fraction of the funds that have been allocated to climate studies focused on greenhouse gas emissions. Opponents of climate geoengineering should understand that none of the suggested options is meant to be a free-standing, long-term solution to global warming. If the greenhouse effect continued to increase, the geoengineering measures would have to be not only maintained indefinitely but also gradually augmented to keep pace. Moreover, we would still have to address the accumulating carbon dioxide in the atmosphere over the next few centuries, which would make the oceans harmfully acidic.

Without small-scale tests, no one will understand the benefits, costs, or possible harmful side effects of sunlight-deflecting approaches.

Clearly, we need both adequately explored geoengineering options for contingent climate stabilization and truly effective, practical measures to reduce emissions of greenhouse gases.

Our energy system evolved during the twentieth century as an offspring of the Industrial Revolution. It may take almost as long to replace this system with the new energy sources and distribution networks that future generations will need. Developing and testing geoengineering options will greatly help this transition, providing a safe breathing space while staving off a massive global warming crisis.

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