Researchers have said that the recent heat waves and extreme summers were likely caused by global warming.
James Hansen and colleagues examined the role of global warming in recent high profile heat waves, such as those in Texas and Oklahoma in the summer of 2011 and in Moscow in 2010.
Using historical data and an empirical approach that did not require knowledge of the cause of the observed climate change, the researchers compared recent June, July, and August surface temperature anomalies relative to the base period of 1951-1980.
Their findings revealed that extremely hot summers-those with temperatures three standard deviations greater than the mean temperature in the base period-occurred much more frequently in the past several years than during the base period, when they were practically absent.
These extremely hot summers have affected an estimated 10 percent of global land area in recent years, compared with less than 1 percent of the Earth's surface during the base period.
The study concluded that the recent extreme summer climate anomalies would likely not have occurred in the absence of global warming.
Continued warming could potentially make extremely hot summers the norm and possibly contribute to extreme droughts and floods, the researchers said.