PEW Center on Global Climate Change
Scientists state unequivocally that the earth is warming. Climate change is happening, it is caused in large part by human activity, and it will have many serious and potentially damaging effects in the decades ahead. Greenhouse gas emissions from cars, power plants, and other human activitiesrather than natural variations in climateare the primary cause of contemporary global warming. Due largely to the combustion of fossil fuels, atmospheric concentrations of carbon dioxide (CO2), the principal greenhouse gas, are at a level unequaled for at least 800,000 years.