Turning down the Earth's temperature won't be fast, simple or inexpensive
In case you haven't heard, the Earth is getting hotter, and most scientists blame mankind. The average global temperature has risen 1.3 degrees in the last century, and the future might be quite a bit warmer. As international climate negotiators convene in Cancun, Mexico, this week, the Intergovernmental Panel on Climate Change is projecting that the temperature could rise as many as 10 degrees more by the year 2100 if we do nothing to stop it.
Is there any way to get ourselves back to those 19th-century temperatures in a hurry?
Not without extraordinary risk. Even if we immediately stopped burning fossil fuels and all became vegans, average temperatures probably wouldn't drop much for about 500 years, according to Ken Caldeira, a climatologist at the Carnegie Institution and Stanford University.
There are two reasons for that. First, carbon is really persistent. For every four molecules we release into the atmosphere, one will still be there five millennia later. Second, the oceans have been absorbing extra heat for a long time, and it would take more than a century for all of it to radiate through the atmosphere and into space.
The 1991 eruption of Mount Pinatubo in the Philippines, which released 20 million tons of sulfur and cooled the Earth by about a degree for a year, proved that shading can affect the temperature rather quickly. Some models suggest that, if we were really aggressive and everything worked out just right, shading could return us to 19th-century temperatures.
Nevertheless, shading is a last resort because it could have terrifying side effects, according to Damon Matthews, a climate scientist at Concordia University in Montreal. The shade might diminish or stop the monsoons that water South Asia's crops. It would undermine the solar panels that help cut our fossil fuel use. And who knows how it would affect plants and the animals that rely on them, including us?
There might be geopolitical consequences as well. Some countries, such as Russia, stand to gain a relative advantage from a little global warming. They might not be happy if another country unilaterally dimmed the sun. (The United Nations has issued a moratorium on geo-engineering.)
There are also less drastic proposals to manage solar radiation. For example, some have suggested sending ships to the equatorial seas to continuously foam up the water. The white surface would absorb less sunlight than the great blue expanse. Such plans are in their infancy, though, says Jay Gulledge of the Pew Center on Global Climate Change, and no one knows how much they would affect the temperature.
Another option is to filter excess CO2 out of the atmosphere. Researchers have developed synthetic trees that might do the job. They're capable of extracting carbon from the air 1,000 times as fast as regular trees, according to Columbia University's Klaus Lackner, one of the scientists behind the technology. But it'll be a while before we build a synthetic forest.
Removing carbon from the open air is far more challenging than just catching it from the business end of a smokestack, a process that could slow, but not reverse, climate change. It would be expensive as well. Right now, it would cost about $1 trillion to filter out 10 billion tons of carbon using the smokestack method; climate scientists are estimating about $100 per ton using current technology. To capture carbon from free air probably would cost two or three times that much. And scientists aren't even certain how much the scheme would lower temperatures. It's unlikely we could safely scrub out enough carbon to cool down the planet within the next century.
Palmer, a freelance writer based in New York, is a regular contributor to Slate's Explainer column.