The reason is carbon dioxide (CO2).

According to the Intergovernmental Panel on Climate Change, CO₂ is the most important driver of global warming. At approximately 400 parts per million, atmospheric carbon dioxide is now at its highest level in at least the past 800,000 years. The burning of fossil fuels and other human activities are currently adding nearly 40 billion tons of carbon dioxide to the atmosphere each year, producing an unprecedented buildup.

FMI: check http://science.nasa.gov/science-news/science-at-nasa/2014/24jun_oco2/


Reply to comment: please check my blog .... I have been trying to find facts and figures to your question. After reading all the stuff it looks like they are all covering it up in jargon and what they gonna do. So there.