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Saturday, August 11, 2012

Hansen, Sato & Ruedy: Increasing Climate Extremes and the New Climate Dice

Increasing Climate Extremes and the New Climate Dice

10 August 2012

James Hansen, Makiko Sato, and Reto Ruedy


We address questions raised about our study "The New Climate Dice" by using longer base periods that include the 1930s. We show that the 2012 summer heat wave in the United States (June-July data) exceeds any that occurred in the 1930s. We reconfirm our conclusion that the increasing extremity of heat waves and the area covered by extreme events is caused by global warming. The location and timing of weather extremes depends on many factors and to a large degree is a matter of chance. Changing climate can be described, usefully and realistically, by the combination of "climate dice" and a shifting, broadening "bell curve", an approach that we believe can be appreciated by the general public.

Our recent paper [1], popularly described as "The New Climate Dice" although the publishers eliminated that phrase from the paper's title, showed that rapid global warming during the past three decades is driving a large increase in extreme heat waves with important consequences. Our paper was greeted with enthusiasm by many scientists. A perceptive review and discussion by Karl and Katz of the changing "bell curve," which we use to quantitatively describe local temperature anomalies, will appear soon in the journal that published our paper [2].

In addition, a few media outlets included strong criticisms of our study. First, it was stated that our study goes back only to the 1950s ignoring heat waves of the 1930s "which dwarf what we have now" [3]. Second, it was stated that we offer no proof of our conclusion that the extreme heat waves are a consequence of climate change [4]. Both statements are wrong. The support critics offered for their assertions was that the 1951-1980 base period that we used to define climate anomalies was biased. 

Here we repeat our analysis with alternative base periods, reconfirming and strengthening our conclusions, and we add further information.

1. Base period for analysis and standard deviation

Studies of climate change generally use some base period to define an average climate and calculate "anomalies" relative to that average, i.e., climate anomalies are the deviations from that average climate. In our papers [1, 5, 6] we used 1951-1980 as the base period.

Global temperature change over the past century (Fig. 1) helps us discuss possible effects of the choice of base period. Our choice of 1951-1980 as a base period has several merits:

(1) The period 1951-1980 is prior to the large warming of the past few decades. If we wish to examine the effect of that global warming on climate, we must compare with the climate that existed prior to that warming.

(2) The 1951-1980 period has the best global data coverage and can best characterize climate variability. Spatial coverage of data was poorer at earlier times.

(3) 1951-1980 was the base period used by the National Weather Service and other researchers when we made our first analyses of observations and climate simulations [5]. For comparison with these early analyses and climate simulations we should use the same base period.

(4) Many of today's adults, baby-boomers, grew up during 1951-1980, so it is recent enough for many people to remember what the climate was like.

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