Scientists are becoming even more certain that humans are a key contributor to global warming, and some are even contending that human causes have been the most significant factor leading to warming in recent decades. A landmark 1995 report by the U.N. Intergovernmental Panel on Climate Change found a “discernable human influence” on climate change, but it made no judgement as to whether that influence was big or small. Today’s New York Times surveys several recent studies that conclude that the human impact on climate change is very significant; it also quotes scientists who caution that the degree of human impact is still not clear.