Scientists using a computer model of the typical European summer with no human influences (without global warming) have compared it to the same model with global warming included. They have concluded that human influence makes extremely warm summers (i.e. summers as warm as the 2003 heatwave) more than twice as likely.
Commentators have suggested that in the future this may open the way for lawsuits against heavy polluters by persons whose livelihoods have been affected by adverse climate change. However, the report stesses that though it is in fact impossible to pin any specific extreme climate event on either global warming or natural climate variation, human activities increase the frequency of such events.
The study is based upon climate activity during the last 50 years, which can only be replicated with computer models which include human forcing (CO2 and other greenhouse gases (GHGs)). However, even within a warmer world the summer of 2003 was anomalously warm in Europe. Most simulations of 2003 climate produce cooler weather in Europe than what actually happened.  It is not yet known if flaws in the models cause this difference, if the warmth of 2003 was part of a trend, or 2003 was merely unusually hot. If there will be a warming trend, a model predicts that such summers could become an event of 1-in-2 probability only by 2050.
In the Americas, a series of sudden weather changes has been more frequent in the last 15 years since the discovery of the phenomenon knows as El Niño, in which vast regions of North and South America suffered weather extremes ranging from high tides to extreme hail, rain and snowstorms during summer and winter seasons following extremely hot summers. There's still no scientific evidence that El Niño is directly influenced by human factors.
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Humans may have contributed to 2003 European heatwave