Alan Robock: Want to halt global warming? Here are your choices…

LET'S be clear: man-made global warming is real. As a result of all the carbon dioxide, methane, soot and other substances that human beings pump into the atmosphere every year, average global temperatures over the past half-century have risen.

While some northern countries relish the prospect of extracting minerals from an ice-free Arctic Ocean and using the Northwest Passage, global warming is not good for most of the planet. After all, it means rising sea levels, stronger storms and more frequent flooding, plus drier and longer-lasting droughts, ocean acidification (destroying corals and other sea life), and the migration of pests, such as malarial mosquitoes and pine beetles. Moreover, fundamental threats to the food and water supply – especially food in the tropics and water in the subtropics – are coming if we continue business as usual.

Unlike the questions surrounding climate change and its consequences, all of which can be answered by scientists, what we want to do about it depends on values – that is, what is important to us. The choices are: (1) nothing; (2) mitigation (ie, reducing greenhouse gas emissions); (3) attempted adaptation to the changes; (4) geoengineering.

If you are a big oil or coal company, your choice would most likely be nothing. You would choose to continue to make as much money as possible, while the consumers of your products use the atmosphere as a sewer, with no sewerage charges.

But if you are like me and want to minimise the damage, then choose mitigation and, where necessary, adaptation.

As for geoengineering, we do not have enough information to decide, but preliminary studies show that it presents more problems than it solves.

There are basically two types of actions that have been labelled "geoengineering". One, carbon dioxide reduction, involves removing from the atmosphere the main gas that is causing global warming. This is a good idea, but so far it seems expensive, and nobody has yet designed a system to keep the carbon out of the atmosphere.

Carbon dioxide reduction proposals include artificial trees that use chemicals to capture the , then pump it underground or under the ocean; planting fast-growing trees and then burying them; and fertilising the ocean with iron to make plankton grow faster, hoping they will fall to the ocean bottom. This last idea has not been shown to work and may severely damage the entire ocean ecosystem.

The other geoengineering idea, called solar radiation management, envisions cooling the Earth by putting mirrors in space; pumping salt spray into clouds to make them brighter; or filling the stratosphere with a sulphuric acid cloud, just as volcanic eruptions occasionally do. But volcanic eruptions teach us that, while a cloud in the stratosphere would indeed cool the planet and stop ice from melting and the sea from rising, it would also destroy ozone and produce regional droughts.

There are other potential problems with solar radiation management – for example, it would not stop ocean acidification. More importantly, there is no way for us to decide what temperature the Earth should be.

All this underscores the need for further study. We have to quantify the benefits, risks and costs of geoengineering and compare them with the benefits, risks and costs of our other options, so that we can make an informed decision. Solar radiation management might be needed in a planetary emergency – say, if continued warming rapidly accelerates ice-sheet melting and sea-level rise, or if it accelerates methane and emissions from thawing tundra, which would then accelerate the warming itself.

But geoengineering is not a magic solution – right now, it looks dangerous. So we must redouble our efforts to shift our economies to a post-carbon world, while allowing the billions without adequate food, water and education to improve their lives.

This offers tremendous economic opportunities, if we can put a price on the dangers to the planet of our current carbon emissions. So long as governments begin to push the world in this direction, we have a good chance to save our planet without geoengineering.

• Alan Robock is associate director of the Centre for Environmental Prediction in the Department of Environmental Sciences at Rutgers University, New Jersey.