James Hansen - Regional Climate Change and National Responsibilities

06 July 2020 James Hansen and Makiko Sato Global warming of about 1.6°F (0.9°C) over the past half century “loads the climate dice.”  Fig. 1 updates the “bell curve” analysis of our 2012 and 2016 papers for Northern Hemisphere land, which showed that extreme hot summers now occur noticeably more often than they did 50 years ago.  Our 2016 paper showed 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 about 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 one practical implication of Fig. 2 is that the public in the U.S. is more likely to notice the warming in the summer.  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 occurs with frequency about 7% in the U.S. and 19% in China. Warming in Europe (see paper) is moderately larger than in the U.S.  In China (Fig. 2) warming is now almost two standard deviations in summer and more than 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. The Middle East and Mediterranean summer bell curve shifts are almost 3 standard deviations (Fig. 2).  Every summer is now warmer than average 1951-1980 climate, and the period with “summer” climate is longer.  Given that summers were already 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 hot, are now two to three standard deviations and extreme hot spells can occur in multiple seasons. Fig. 1.  Frequency of occurrence of local temperature anomalies (relative to 1951-80 mean) divided by local standard deviation (horizontal axis) for Northern Hemisphere land. See our 2012 and 2016 papers. Continue Reading