Carbon offseting is the practice of purchasing or building carbon sinks which match or exceed an individual or corporation's carbon dioxide emmissions. Carbon offset activities could include buying stocks in a "green" power company, using solar power, and the like. The goal is to keep an individual or corporation's net carbon emissions at zero, in order to stave off the theoretical warming due to the greenhouse effect.
Critics point out that, while carbon dioxide has the largest volume of controllable contributions to theorized atmospheric warming, it has the lowest effect on a per molecule basis of any of the greenhouse gases.
It is important to note that, while planting trees is percieved to be a "green" activity, a simple balance over the life and death of the tree will show that this activity does not actually decrease carbon dioxide in the atmosphere. While carbon is consumed by the tree during growth, the exact same amount of carbon is produced during decomposition. The net carbon gain, therefore, is zero over the entire lifecycle of the tree.
Some politicians and activists, particularly environmental activists such as Al Gore use carbon offsetting as a status symbol within their community.