A recent scientific US study http://www.nap.edu/ 
"In the past, a typical climate scientist’s response to questions about climate change’s role in any given extreme weather event was, “We cannot attribute any single event to climate change.” The science has advanced to the point that this is no longer true as an unqualified blanket statement. In many cases, it is now often possible to make and defend quantitative statements about the extent to which human-induced climate change (or another causal factor, such as a specific mode of natural variability) has influenced either the magnitude or the probability of occurrence of specific types of events or event classes."