Whether one is fat or thin, part of the rich in the developed or of the poor in the developing world, or the other way around, what counts is the obvious fact that there is only one atmosphere. Therefore, the argument runs, greenhouse gas emissions saved by one person or in one country are just as valuable as savings by someone else: the atmosphere will still benefit. So if someone wants to emit more than they want to or are allowed to, why not simply pay to help reducing emissions elsewhere? If you want to make a trans-Atlantic flight, for example, then counteract the climate damage you are doing by paying for a specific number of trees to be planted to soak up the carbon you generate. This is the system known as carbon offsetting.
One mystery that puzzles many people considering offsets is working out how much carbon reduction they will really achieve, particularly for projects which sequester carbon. And a question often asked about forestry projects is how permanent their effects will be. For example, during the life-cycle of a tree it will absorb a certain amount of carbon. But if it is burnt or rots away, some of this stored carbon will be released into the atmosphere again. New trees planted will always absorb CO2 as they grow, but if the land was cleared of a natural forest, or another carbon sink, the net effect may be much lower, or even negative.There is also scientific debate over the usefulness of tree planting as a remedy for climate change anyway, with evidence that it may well work at lower latitudes but in temperate regions may even have a warming effect, because the tree canopy absorbs sunlight rather than reflecting it.
There can be other problems with tree-based offsets too. Restoring natural forests may be good. But creating plantations of a single species will produce few benefits for people or wildlife. Non-native fast-growing (and commercially attractive) species like thirsty eucalyptus can cause havoc to local ecosystems.
But trees still have a lot to offer. For a start they are a cheap way of removing CO2 from the atmosphere: US$90 will pay for 900 trees, enough to remove as much carbon annually as the average American generates each year from fossil fuels. They can be a source of fuel wood and therefore slow deforestation. They also help wildlife to thrive, slow down soil erosion, provide timber, fruit and other products – and they are potent symbols of environmental health which most of us recognize. But each time a tree is lost it should be replaced. In short, planting a tree is almost always a good thing. Not all trees planted, however, can be considered as offsets.
To put it simply, carbon offsets aim to neutralize the amount of your GHG contribution by taking your money to fund projects which should cause an