August 7, 2012

Extreme Summer Heat Events Linked to Global Warming

April Flowers for - Your Universe Online

Earth's land areas have become more likely to experience an extreme summer heat wave according to a new statistical analysis by NASA scientists.

The study, published August 6, 2012, in Proceedings of the National Academy of Sciences, show that the recent  bouts of extremely warm summers, including the intense heat wave afflicting the U.S. Midwest this year, are likely the consequence of global warming.

According to James Hansen of NASA's Goddard Institute for Space Studies (GISS), "This summer people are seeing extreme heat and agricultural impacts. We're asserting that this is causally connected to global warming, and in this paper we present the scientific evidence for that."

The research team, led by Hansen, analyzed mean, or average, summer temperatures since 1951 and show that the odds of what they define as "hot," "very hot," and "extremely hot" summers have increased in recent decades.  In fact, the study found that "extremely hot" summers are becoming far more routine.

What is an extremely hot summer? The team used the years of 1951 to 1980 as a base period for the study. They averaged the temperatures and defined "extremely hot" as a mean summer temperature experienced by less than one percent of Earth's land area in that time period. But since 2006, about 10% of land area across the Northern Hemisphere has experienced these temperatures each summer.

Hansen has been collecting data on this trend for a long time. In 1988, he asserted that global warming would reach a point in the coming decades when the connection to extreme events would become more apparent. While some warming should coincide with a noticeable boost in extreme events, the natural variability of weather and climates can be so large as to disguise the trend.

Hansen and his team turned to statistics to distinguish the trend from natural variability. The GISS researchers did not focus on the causes of temperature change, instead they analyzed surface temperature data to establish the growing frequency of extreme heat events in the past 30 years. The significance of this time period is that temperature data shows an overall warming trend.

NASA climatologists have collected data for a long time on global temperature anomalies, which describe how much warming or cooling regions of the world experience when compared with the base period of 1951 to 1980. The team used a bell curve to illustrate how those anomalies are becoming the norm.

We all remember the dreaded bell curve from school. Teachers used it to designate the mean grade as  C, the top of the bell. The curve falls off equally to both sides, showing that fewer students receive B and D grades and even fewer receive A and F grades.

As much as the bell curve was disliked in school, Hansen and team found that it was a good fit to summertime temperature anomalies for the base period. The relatively stable climate of 1951 to 1980, when plotted onto the curve, showed the mean temperature centered at the top of the bell, with "cold," "very cold," and "extremely cold" decreasing in frequency on the left side. "Hot," "very hot," and "extremely hot" decreased in frequency down the right side of the curve.

The researchers then plotted curves for the 1980's, 1990's, and 2000's. These curves showed a marked shift to the right for all three decades, meaning that more hot events were becoming the new normal. The curve also flattened and widened, indicating a wider range of variability.

Specifically, an average of 75% of land across Earth experienced summers in the "hot" category during the past decade, compared to only 33% in the base period. Widening of the curve also led to the designation of outlier events labeled "extremely hot." Such events were almost nonexistent in the base period. In statistics, an outlier is an observation that appears to deviate markedly from other members of the sample in which it occurs.

Hansen says this summer 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 says. “¨“¨Other regions around the world also have felt the heat of global warming, according to the study. Global maps of temperature anomalies show that heat waves in Texas, Oklahoma and Mexico in 2011, and in the Middle East, Western Asia and Eastern Europe in 2010 fall into the new "extremely hot" category.