Fund a tree planting via Offset Earth. Offset Earth fund Eden Reforestation Projects, particularly in Madagascar where the project creates jobs for local people planting and protecting forests in an attempt to recover from their extreme (over 90% of their rainforests) deforestation.
The Mangrove trees being planted capture up to four times as much carbon per acre than tropical rainforests. Three quarters of tropical fish rely on their root systems for nurseries and shelter.
Planting trees captures CO2 from the atmosphere and is thought to be the most effective defence against global warming.
If you'd like to contribute yourself I would encourage you to check out their homepage and surprisingly affordable plans that appear to make a large impact.