Nick Shepherd via Getty Images
03/02/2016 11:43 am ET | Updated Mar 02, 2016
- Dr. James Hansen Climatologist and Adjunct Professor, Columbia University Earth Institute
Global warming of about 1°F (0.6°C) over the past several decades now “loads the climate dice.” Fig. 1 updates the “bell curve” analysis of our 2012 paper1 for Northern Hemisphere land, which showed that extreme hot summers now occur noticeably more often than they did 50 years ago. Our new paper2 shows that there are strong regional variations in this bell curve shift, and that the largest effects occur in nations least responsible for causing climate change.
In the United States the bell curve shift is just over one standard deviation in summer and less than half a standard deviation in winter (Fig. 2). Measured in units of °F (or °C) the warming is similar in summer and winter in the U.S., but the practical implication of Fig. 2 is that the public in the U.S. should notice that summers are becoming hotter but is less likely to notice the change in winter. Summers cooler than the average 1951-1980 summer still occur, but only ~19% of the time. Extreme summer heat, defined as 3 standard deviations or more warmer than 1951-1980 average, which almost never occurred 50 years ago, now occur with frequency about 7%.
Warming in Europe (see paper) is modestly larger than in the U.S. In China (Fig. 2) warming is now almost 1½ standard deviations in summer and one standard deviation in winter, a climate change that should be noticeable to people old enough to remember the climate of 50 years ago. Bell curve shifts in India (see paper) are slightly larger than in China.
In the Mediterranean and Middle East the bell curve shift in summer is almost 2½ standard deviations (Fig. 2). Every summer is now warmer than average 1951-1980 climate, and the period with “summer” climate is now considerably longer. Given that summers were already very hot in this region, the change affects livability and productivity as noted below. Bell curve shifts in the tropics, including central Africa (see paper) and Southeast Asia (Fig. 2), which also was already quite hot, are about two standard deviations and occur all year round.