The two most important reasons why Rainforests are important in preventing global warming are:
 
1. Tropical rainforests are one of the world’s primary carbon sinks as they draw down atmospheric carbon dioxide (CO2), through photosynthesis and store it in biomass and other carbon stocks. In other words, they absorb and store massive amounts of CO2, the most important greenhouse gas responsible for global warming.

2. Deforestation is considered the second largest source of greenhouse gas emissions (approx. 20%) after the burning of fossil fuels. This is more than the emissions from the entire global transport sector and larger than the annual emissions of the USA or China (IPCC, 2007).
 
Tropical deforestation has a particularly strong effect on emissions because trees in tropical forests typically hold about 50% more carbon per hectare than trees outside the tropics.  Tropical forests store 120-400 tonnes of carbon per hectare therefore burning them contributes huge quantities of CO2 emissions.
 
In aggregate, there is more carbon stored in rainforests than there is in the atmosphere. Just one day of tropical forest emissions from deforestation is equal to 12.5 million people flying from London to New York (Forest NOW - GCP, 2008)
 
As a result, tackling the issue of tropical deforestation will be essential if the world is to achieve the goal of limiting global warming to below 2º degrees Celsius this century and avoiding catastrophic climate change as concluded by IPCC.