Earlier this week the National Aeronautics and Space Administration (NASA)’s Goddard Institute of Space Studies published a new statistical analysis which has found that the Earth’s land areas are more likely to experience extreme summer heat now than they were in the middle of the 20th century.
They looked at average (mean average) summer temperatures since 1951 and found that the odds have increased in recent decades for what they define as “hot”, “very hot” and “extremely hot” summers.
Between 1951 and 1980 less than 1% of the Earth’s land area experienced “extremely hot” mean summer temperatures. Since 2006 about 10% of land area across the Northern Hemisphere has experienced those temperatures.
As the weather is governed by natural ups and downs or variability, lead author, NASA climatologist Dr James Hansen and colleagues were concerned that very large variations might be disguising a trend. They turned to Statistics to help them work things out. NASA used global temperature anomaly data (data on how much warming or cooling regions of the world have experienced) collected betweeen 1951 and 1980 as a base period with which they could compare subsequent surface temperature data from 1981 to the present. In this way, they were able to better understand the relatively stable climate bewteen 1951 and 1980 and to compare this with the increasing frequency of extreme heat events in the following 30 years.
They developed a bell or normal curve*, a much used type of graph, to describe how those anomalies are changing. They found that summer temperature anomalies during the base period of 1951 to 1980 when the climate was relatively stable, fitted well with the normal curve.
With mean average temperature at the top of the curve, decreasing in frequency to the left of centre with “cold” “very cold” and “extremely cold” events. Decreasing in frequency to the right of centre are “hot”, “very hot” and then “extremely hot” events.
When plotting bell curves for the 1980s, 1990s and 2000s, the curve shifted to the right, meaning that more hot events have become normal. The curve also flattened and widened, including a wider range of variability. An average 75% of land area across Earth experienced summers in the “hot” category during the past decade, compared with only 33% in 1951-1980. Widening the curve also led to a new category of “extremely hot” events being developed….events which were all but non-existent between 1951 and 1980.
Dr Hansen says summer 2012 in the US looks set to fall into the extreme category. Global maps of temperature anomalies show that heatwaves in Texas, Oklahoma and Mexico in 2011, and in the Middle East, Western Asia and Eastern Europe in 2010 also fall into the new “extremely hot” category.
NASA colleagues are now settled on bell curves as the statistical tool which most easily communicates the change in temperature anomalies, particularly at the extremes.
However, in the past NASA has used other tools to communicate variability and the growing frequency of extreme temperature events. In the 1980s, Dr Hansen introduced the analogy of loaded dice On one of the six-sided dice, he painted two sides blue, two white and two red to represent the chance of a cold, average, or warm summer season. On the other – loaded – die, he painted one side blue, one white and four red to represent how climate models suggested the dice would roll by the first decade of the 21st century.
Looking back, the dice analogy seem to be a fair reflection of how many sides would now be red as opposed to blue to represent today’s climate. Of course, Dr Hansen would now have to replace one of the red sides of the dice with an entrely new colour – brown, to represent ‘extremely hot events’ !