Stanford climatologist Ken Caldeira examines a simulation of atmospheric alterations aimed at combating global warming. New research indicates that hacking the atmosphere – pumping microscopic particles into the stratosphere or clouds to block sunlight and offset global warming caused by greenhouse gases – is imminently possible. The problem is we could never, ever stop doing it.

Climate scientists Damon Matthews of Concordia University and Ken Caldeira of Stanford ran the numbers on atmospheric geo-engineering through a climate simulation and found that while cranking out carbon dioxide at business-as-usual rates we can geo-engineer our way back toward pre-industrial temperatures in short order, reaching 1900 levels in about five years. Not only that, it would be fairly cheap and easy to do.

Pumping 20 to 25 liters of aerosols per second to keep enough particles in the stratosphere would cool temperatures, causing the planet's carbon sinks to suck more carbon dioxide out of the atmosphere.

"That kind of flow rate can be handled by a single fire hose," said Caldeira. "For something like $100 million a year you could probably keep a hose in the stratosphere suspended by an array of balloons with pumps along the way."

The problem is what happens if we stop short or screw it up.

Bring the geo-engineering process to a halt, and those sun-warmed carbon sinks spit the carbon dioxide right back into the atmosphere. The rebound warming, to temperatures that would have been reached without the geo-engineering, would be 10 to 20 times the pace of today's global warming. The rapid warming, up to 7 degrees Fahrenheit per decade, would wreak havoc on the planet and threaten civilization.

To prevent disaster, the geo-engineering process would have to continue as long as carbon-dioxide levels were elevated. A quarter of the carbon dioxide that comes out of your car's tailpipe is still in the atmosphere a thousand years later, Caldeira said.

"We've never had systems work for a thousand years without failure," he added.

Even without technical failures, other circumstances including a world war, economic collapse or a simple change of political will could turn off the hose.

Matthews and Caldeira's study underscores the need for any country that attempts geo-engineering to make a durable and long-term commitment to the effort, said David Victor, an energy and sustainable development specialist at Stanford Law School. "It's a bit like riding a bicycle down a steep hill without brakes," he said.

The idea of injecting particles into the stratosphere to block sunlight has hovered just beyond the bounds of scientific respectability since Soviet climatologist Mikhail Budyko first suggested it in the '70s. That changed last year when Nobel laureate Paul Crutzen published an essay calling for scientists to seriously consider the possibility. (See "This Global Warming Fix Stinks.")

But studying geo-engineering is not the same as doing it, and Crutzen said that this latest research shows the significant risks involved in counteracting global warming induced by carbon dioxide. "Far better to reduce the emissions of CO2," he said.

For Caldeira the danger is not in geo-engineering itself but in the risk that politicians will turn to it to avoid the hard work of transforming our energy and transportation systems. "If somebody is driving an SUV, I would be against deploying geo-engineering. If there is a coal-fired power plant still spewing carbon dioxide into the atmosphere, I would be against geo-engineering."

We should think of geo-engineering only as a parachute, said Caldeira. It's something you desperately hope you never need.

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