Public Perception of Climate Change and the New Climate Dice

by James Hansen^a, Makiko Sato^a, Reto Ruedy^b

Summary. Should the public be able to recognize that climate is changing, despite the notorious variability of weather and climate from day to day and year to year? We investigate how the probability of unusually warm seasons has changed in recent decades, with emphasis on summer, when changes are likely to have the greatest practical effects. We show that the odds of an unusually warm season have increased greatly over the past three decades, but also the shape of the frequency distribution has changed so as to enhance the likelihood of extreme events. A new category of hot summertime outliers, more than three standard deviations (3σ) warmer than climatology, has emerged, with the occurrence of these outliers having increased 1-2 orders of magnitude in the past three decades. Thus we can state with a high degree of confidence that extreme summers, such as those in Texas and Oklahoma in 2011 and Moscow in 2010, are a consequence of global warming, because global warming has dramatically increased their likelihood of occurrence.

We illustrate observed variability of seasonal mean surface air temperature anomalies in units of standard deviations, including comparison with the normal distribution ("bell curve") that the lay public may appreciate. We take 1951-1980 as an appropriate base period, because temperatures then were within the Holocene range to which humanity and other planetary life are adapted. In contrast, we infer that global temperature is now above the Holocene range, as evidenced by the fact that the ice sheets in both hemispheres are shedding mass (1) and sea level is rising at a rate [more than 3 mm/year or 3 m/millennium (2)] that is much higher than the rate of sea level change during the past several millennia.

The frequency of occurrence of local summer-mean temperature anomalies was close to the normal distribution in the 1950s, 1960s and 1970s in both hemispheres (Fig. P1A, B). However, in each subsequent decade the distribution shifted toward more positive anomalies, with the positive tail (hot outliers) of the distribution shifting more than the negative tail. The temporal change of the anomaly distribution for the contiguous United States (Fig. P1C) is similar to the global change, but much noisier because the contiguous U.S. covers only ~1.5% of the globe.

Winter warming exceeds that in summer, but the standard deviation of seasonal mean temperature at middle and high latitudes is much larger in winter (typically 2-4°C) than in summer (typically ~1°C). Thus the shift of the anomaly distribution, in the unit of standard deviations, is less in winter than in summer (Fig. P1D).

A concept of "climate dice" was suggested (3) to describe the stochastic variability of local seasonal mean temperature, with the implication that the public should recognize the existence of global warming once the dice become sufficiently "loaded" (biased). Specifically, the 10 warmest summers (Jun-Jul-Aug in the Northern Hemisphere) in the 30-year period of climatology (1951-1980) define the "hot" category, the 10 coolest the "cold" category, and the middle 10 the "average" summer. Thus it was imagined that two sides of a six-sided die were colored red, blue and white for these respective categories. The divisions between "hot" and "average" and between "average" and "cold" occur at +0.43σ and -0.43σ for a normal distribution of variability.

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^aNASA Goddard Institute for Space Studies and Columbia University Earth Institute,
^bTrinnovim LLC, New York, NY 1002