In light of the new U.S. administration's decision to cut back on environmental protection and cultivate the coal industry, carbon emissions are unlikely to go down over the next four years.

So scientists are considering a scheme to shade the atmosphere from the sun and cool Earth to compensate for global warming. It's a risky plan.

The concept is called Solar Geoengineering. One of the ways it could work, scientists say, is by injecting tiny particles high into the atmosphere, where together they would act as a sun shield, reflecting sunlight back into space and cooling the planet.

It would be like turning down the thermostat on the enormous heat engine that drives the weather systems in our atmosphere.

The idea is not as wild as it sounds. Nature does this all the time with volcanoes.

When Mt. Pinatubo erupted in 1991, 20 million metric tonnes of sulphur dioxide was blown into the stratosphere. There the molecules reacted with water vapour to form tiny particles that were carried on high altitude winds, producing a global haze. The average temperature of the Earth dropped by 0.5 C for more than a year after the eruption.

The Sunshade Option

The effect could be similar to the way some volcanic eruptions have caused temporary cooling by spewing ash into the atmosphere. (Channi Anand/Associated Press)

The geoengineering project would do the same thing on a much smaller scale, using a fleet of aircraft to spray 250,000 metric tonnes of sulphur dioxide, or some other material such as calcite into the lower stratosphere.

From an astronomical perspective, the particles change the albedo of the planet, or how bright it is. White snow and ice do the same thing by reflecting sunlight and keeping arctic temperatures low.

That's one of the reasons why scientists are concerned about the loss of ice in the north. As the ice melts away, it exposes dark seawater, which absorbs sunlight rather than reflecting it, raising the temperature.

Scientists estimate that by brightening the atmosphere with these particles, they could reflect one percent of sunlight back into space and provide enough cooling to balance the warming effect of the carbon emissions coming from industry.

Harvard Professor David Keith estimates the project would have to be an international effort and cost about $1 billion to $10 billion per year. That sounds like a lot, but it pales compared to the U.S. military budget, for example, which is expected to increase to $639 billion dollars in 2017.

The scientists also admit that this is a Band-Aid solution to a problem that should be fixed at the source. Of course, the best way to mitigate climate change is to reduce emissions. But since that doesn't seem to be happening — at least in the U.S., one of the world's largest emitters — we need to look at other options.

Testing the concept

The first step is to do a small scale, proof-of-concept experiment with a high-altitude balloon called a StratoCruiser. At an altitude of 20 kilometres, where the air is stable, a small propeller will drive the balloon forward as it releases a plume of particles that would stretch to about one kilometre in length.

The balloon will then turn around and fly back and forth through the plume to study how well the particles disperse over time. A follow-up experiment will examine any chemical reactions that take place between the particles and gases in the atmosphere. There is evidence that sulphates can destroy ozone, so care will be needed when choosing which type of compound to use.

The scientists are anxious to get the experiment underway. Many tests have been done in laboratories to prove this concept, but very little has been done in the atmosphere itself.

The risks

Not everyone agrees that solar geoengineering is a good idea. The Earth's atmosphere is a hugely complex system that can react in unpredictable ways. After all, we can't even predict the weather accurately beyond a week or two.

For the last approximately 250 years, we have been conducting an uncontrolled experiment by dumping greenhouse gasses into the air, affecting everything from monsoons to ocean currents and glaciers. So the idea of using what is effectively another pollutant to counter the problem seems like the wrong way to go.

The Sunshade Option

Solar geoengineering is a last resort in a warming world. (Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center/Associated Press)

It would be like standing in a crowded elevator where one person lights a huge cigar, filling the small space with smoke. But instead of asking the person to put it out, someone else sprays air freshener. It might make the elevator smell a little better, but it doesn't solve the problem

If the atmosphere can be sprayed and cooled artificially, that might become a licence for the big cigars around the world spewing carbon into the air to continue business as usual. That means carbon levels in the atmosphere will continue to rise, even though the planet is not getting warmer. And that is not good for ocean acidification and the bleaching of coral reefs around the world.

Solar geoengineering is a last resort in a warming world. Scientists hope that it will not be needed. But they are also say that the research needs to be done now, so that if we do find ourselves in a desperate situation, we will know whether the air freshener approach will even work.