A recent statistical analysis conducted by NASA concludes that Earth's land areas have become much more likely to experience an extreme summer heat wave than they were in the middle of the 20th century.
According to James Hansen of NASA's Goddard Institute for Space Studies (GISS) in New York, the study also indicates that the recent bouts of extremely warm summers - including the intense heat wave afflicting the US Midwest this year - are very likely the consequence of global warming.
"This summer people are seeing extreme heat and agricultural impacts," Hansen explained. "We're asserting this is causally connected to global warming and we present the scientific evidence for that."
Indeed, Hansen and colleagues analyzed mean summer temperatures since 1951 - demonstrating that the odds have increased in recent decades for what they define as "hot," "very hot" and "extremely hot" summers.
"Extremely hot" is defined as a mean summer temperature experienced by less than one percent of Earth's land area between 1951 and 1980. However, approximately 10 percent of land area across the Northern Hemisphere has experienced these temperatures each summer since 2006.
Way back in 1988, Hansen warned that global warming would eventually reach a point in future decades when the connection to extreme events would become more apparent. While some warming should coincide with a noticeable boost in extreme events, natural variability in climate and weather could effectively disguise the trend.
To distinguish the trend from natural variability, Hansen and colleagues Makiko Sato and Reto Ruedy analyzed surface temperature data to establish the growing frequency of extreme heat events in the past 30 years - a period in which the temperature data show an overall warming trend.
The researchers found that a bell curve was a good fit to analyze summertime temperature anomalies for the base period of relatively stable climate from 1951 to 1980. While plotting curves for the 1980s, 1990s, and 2000s, the team noticed the entire curve shifted to the right, meaning that more hot events are the new normal. The curve also flattened and widened, indicating a wider range of variability.
Specifically, an average of 75 percent of land area across Earth experienced summers in the "hot" category during the past decade, compared to only 33 percent during the 1951 to 1980 base period. Widening of the curve also led to the designation of the new category of outlier events labeled "extremely hot," which were almost nonexistent in the base period.
Hansen confirmed this summer (2012) is shaping up to fall into the new extreme category.
"Such anomalies were infrequent in the climate prior to the warming of the past 30 years, so statistics let us say with a high degree of confidence that we would not have had such an extreme anomaly this summer in the absence of global warming," he noted.
Other regions around the world also have felt the heat of global warming, including Texas, Oklahoma, Mexico, the Middle East, Western Asia and Eastern Europe.