Several studies of the consensus have been undertaken.[1] Among the most-cited is a 2013 study of nearly 12,000 abstracts of peer-reviewed papers on climate science published since 1990, of which just over 4,000 papers expressed an opinion on the cause of recent global warming. Of these, 97% agree, explicitly or implicitly, that global warming is happening and is human-caused.[2][3] It is "extremely likely"[4] that this warming arises from "human activities, especially emissions of greenhouse gases"[4] in the atmosphere.[5] Natural change alone would have had a slight cooling effect rather than a warming effect.[6][7][8][9]