A
1-page description
of the bell curves and climate dice is available as the author summary
of our PNAS paper[3] on the topic. The variability of summer mean
temperature in the base period (1951-1980, prior to the period of rapid
global warming), is described by the bell curve on the left (Fig. 1). By
definition, the one-third of summers closest to average temperature are
the white, warmer summers are red and colder are blue, thus each color
covering 2 of the 6 sides of the dice. By the last decade of the 20
th
century, red conditions were occurring 63% of the time on Northern
Hemisphere land, which is almost the 67% needed to cover 4 sides. Twenty
years later, the red area is 87.6%, five sides of the dice. So, the
answer to Andy’s question is: yes, one more side is red.
The more important point is the dark red area on the bell curve, the
portion that exceeds three standard deviations. These are extremely hot
summers that seldom occurred in the base period (less than 1% of the
time). Chance of occurrence now is more than 20%, thus covering more
than one side of the dice (one side is 16.7%). Increase of such climate
extremes has the greatest practical importance. Moreover, the moisture
extremes are more important than the temperature extremes.
In
testimony to Congress in 1988 and 1989,
and in a paper attached to the 1989 testimony, we noted that global
warming causes wet places to get wetter and dry places to get drier. In
places where the average precipitation changes little, we showed that
wet times get wetter and dry times get drier. We also found that storms
become stronger in a warmer world. We found that the “fuel” for storms,
something called “moist static energy” – the sum of sensible heat,
latent heat, and geopotential energy – increases in a warmer world.
Moist static energy increases especially near the surface, so a warmer
climate is prone to more powerful vertical convection that reaches
greater altitude. In our climate model, doubled CO
2 caused
the height of convective cumulus storm cells to increase several hundred
meters. Higher absolute humidity and deeper penetration of moist
convection cause a larger portion of the rainfall to occur in intense
thunderstorms, as opposed to the gentler rainfall from large-scale
stratiform clouds. Increased storm strength is not limited to
thunderstorms. Kerry Emanuel of MIT used sea surface temperature
increases from our 2×CO
2 experiment to estimate the effect on
maximum intensity of tropical cyclones. Minimum sustained surface
pressure fell from 880 mb to 800 mb and maximum potential wind speed
increased from 175 to 220 miles per hour.
A new climate frontier? Suspicion that we are headed
into new climate territory, not seen in the past million years,[4] is
fueled by the present extraordinarily large Earth’s energy imbalance
(EEI). EEI (Fig. 3) is the proximate cause of global warming: as long as
more energy is coming in than going out, we must expect global warming
to continue. Given the acceleration of EEI in the past several years
(Fig. 3), we anticipate an acceleration of global warming. Acceleration
is difficult to measure because of the large interannual variability of
global temperature associated with tropical El Nino/La Nina
oscillations. If the newborn El Nino proves to be comparable to the
prior two strong El Ninos (1997-98 and 2015-16), the 2023-24 global
temperature will provide a measuring stick that helps determine whether
the global warming rate is accelerating.
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