# What's the Right Temperature for the Earth?

January 29, 2015

By Andy Parker and David Keith

And what happens when countries disagree about what it should be?

Imagine being able to control the temperature of the Earth like a home thermostat, turning it down a few notches to reduce the effects of global warming. That’s the goal of solar geoengineering. By spraying aerosols into the stratosphere, we could block a fraction of inbound sunlight and temporarily cool the Earth.

But just as home thermostats are notorious for setting off domestic squabbles — she bumps it up to 72, he ratchets it down to 64 — solar geoengineering could spark serious conflicts, ranging from sanctions to war between world powers.

The question is: How should we approach technology with such lifesaving potential when it could also disrupt the international order on a scale not seen since the advent of the atom bomb?

Long treated as an illegitimate child of the climate-science community and rarely mentioned in polite company, solar geoengineering is now coming of age. The Royal Society, the oldest scientific academy in the world, mainstreamed the issue with the publication of the seminal report “Geoengineering the Climate” in 2009. Many respected institutions have published their own major reports since then, and the U.S. National Academy of Sciences is scheduled to release one in February. Meanwhile, the first small-scale, real-world experiments are taking shape and, if they can secure funding, could begin within two years.

This more serious consideration is due in part to the realization that reducing carbon emissions won’t solve our climate problems; it can only stop things from getting worse. Put bluntly, if we miraculously stopped all CO2 emissions immediately, the Earth would keep warming for decades, and much of the CO2 emitted since the Industrial Revolution would remain in the atmosphere, altering the climate, for millennia. Even the so-called breakthrough climate agreements between the United States and China and at a global conference in Lima, Peru, last year commit the world to massive new quantities of greenhouse gases in the decades ahead, which will accelerate climate change.

We mislead ourselves if we assume that we can easily adapt to the rising sea levels, desertification and intensifying storms that will accompany this change. Hurricane Sandy hit one of the richest areas in the wealthiest, most technologically advanced country the planet has ever known, and it still caused dozens of deaths and more than $60 billion in damage. And so attention is turning to solar geoengineering, also known as solar radiation management. Although the concept of injecting sulfur dioxide into the stratosphere has so far been tested only using computer simulations, there’s high confidence that it would work to cool the Earth because it would mimic the well-understood cooling effect of large volcanic eruptions. A gram of aerosol in the stratosphere, delivered perhaps by high-flying jets, could offset the warming effect of a ton of carbon dioxide, a factor of 1 million to 1. The tiny sulfate aerosols would stay up there, reflecting away a small amount of sunlight, for a year or two, so the material would need to be continually renewed for as long as the cooling effect was needed. A consistent and growing body of evidence indicates that this technology would be fast-acting — reducing global temperatures immediately after deployment — and relatively cheap, costing an average of$1 billion a year over the next half-century to cut the rate of warming in half.

It wouldn’t eliminate the need to cut emissions, as it would only mask the symptoms of climate change. It would create an approximate and artificial balance between the warming effect of greenhouse gases trapping heat in the lower atmosphere and the cooling effect of aerosols reflecting away solar energy in the upper atmosphere.

We don’t yet have a full understanding of what the side effects would be — whether this technique would result in ozone loss, for example, or changed weather patterns. But early evidence from climate modeling overwhelmingly indicates that it would make the planet more livable for people and ecosystems.

The biggest concern should be the politics of it. In the political arena, solar geoengineering could be a hot mess.