In today's New York Times, Thomas Friedman makes the point that annual GHG emissions from the clearing of tropical forests exceed the total emissions from the entire global transportation sector. It is well-documented that tropical forest clearing accounts for 18-20% of anthropogenic carbon emissions and destroys globally significant carbon sinks (Gibbs, et al; Meridian Institute). As the Meridian report points out, forests will contribute to climate change mitigation only if their value increases to a level that makes protecting forests consistent with viable development strategies. After visiting a Brazilian community that lives in and off the tropical forests, Friedman reaches the same conclusion: To save an ecosystem of nature, you need an ecosystem of markets and governance.
In a way, this is really a large, low-hanging fruit as far as cutting GHG emissions immediately. But it is contingent on developing sustainable livelihoods for people that depend on tropical forests for food, materials and land. It is impossible to separate economic sustainability from environmental sustainability (my 2004 article on this).