ScienceDaily
In 50 years, summers across most of the globe could regularly be hotter than any summer experienced so far by people alive today, according to a study by scientists at the National Center for Atmospheric Research (NCAR).
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| If greenhouse gas emissions remain unabated. virtually every summer between 2061-2080 could be hotter than any in the historical record. Credit: © Sunny Forest / Fotolia
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If climate change continues on its current trajectory, the
probability that any summer between 2061 and 2080 will be warmer than
the hottest on record is 80 percent across the world's land areas,
excluding Antarctica, which was not studied.
If greenhouse gas emissions are reduced, however, that probability drops to 41 percent, according to the study.
"Extremely hot summers always pose a challenge to society," said NCAR
scientist Flavio Lehner, lead author of the study. "They can increase
the risk for health issues, but can also damage crops and deepen
droughts. Such summers are a true test of our adaptability to rising
temperatures."
The study, which is
available online, is part of an upcoming special
issue of the journal Climatic Change that will focus on quantifying the
benefits of reducing greenhouse gas emissions. The research was funded
by the U.S. National Science Foundation (NSF) and the Swiss National
Science Foundation.
Simulating a range of summers
The research team, which includes NCAR scientists Clara Deser and
Benjamin Sanderson, used two existing sets of model simulations to
investigate what future summers might look like. Both had been created
by running the NCAR-based Community Earth System Model 15 times, with
one assuming that greenhouse gas emissions remain unabated and the other
assuming that society reduces emissions.
The Community Earth System Model is funded by NSF and the U.S.
Department of Energy. The simulations were run on the Yellowstone system
at the NCAR-Wyoming Supercomputing Center.
By using simulations that were created by running the same model
multiple times, with only tiny differences in the initial starting
conditions, the scientists could examine the range of summertime
temperatures we might expect in the future for the "business-as-usual"
and reduced-emissions scenarios.
"This is the first time that the risk of record summer heat and its
dependence on the rate of greenhouse gas emissions has been so
comprehensively evaluated from a large set of simulations with a single
state-of-the-art climate model," Deser said.
The scientists compared the results to summertime temperatures
recorded between 1920 and 2014 as well as to 15 sets of simulated
summertime temperatures for the same historic period. By simulating past
summers -- instead of relying solely on observations -- the scientists
established a large range of temperatures that could have occurred
naturally under the same conditions, including greenhouse gas
concentrations and volcanic eruptions.
"Instead of just comparing the future to 95 summers from the past,
the models give us the opportunity to create more than 1,400 possible
past summers," Lehner said. "The result is a more comprehensive and
robust look at what should be considered natural variability and what
can be attributed to climate change."
Emissions cuts could yield big benefits
The scientists found that between 2061 and 2080, summers in large
parts of North and South America, central Europe, Asia, and Africa have a
greater than 90 percent chance of being warmer than any summer in the
historic record if emissions continue unabated. This means that
virtually every summer would be as warm as the hottest to date.
In some regions, the likelihood of summers being warmer than any in
the historical record remained less than 50 percent, but in those places
-- including Alaska, the central U.S., Scandinavia, Siberia, and
continental Australia -- summer temperatures naturally vary a great
deal, making it more difficult to detect the impact of climate change.
Reducing emissions would lower the global probability that future
summers will be hotter than any in the past, but the benefits would not
be spread uniformly. In some regions, including the U.S. East Coast and
large parts of the tropics, the probability would remain above 90
percent, even if emissions were reduced.
But it would be a sizable boon for other regions of the world. Parts
of Brazil, central Europe, and eastern China would see a reduction of
more than 50 percentage points in the chance that future summers would
be hotter than the historic range. Since these areas are densely
inhabited, a large part of the global population would benefit
significantly from climate change mitigation.
"We've thought of climate change as 'global warming'; among what
matters is how this overall warming affects conditions that hit people
where they live," said Eric DeWeaver, program director in NSF's Division
of Atmospheric and Geospace Sciences, which funds NCAR. "Extreme
temperatures pose risks to people around the globe. These scientists
show the power of ensembles of simulations for understanding how these
risks depend on the level of greenhouse gas emissions."
Lehner recently published another study looking at the overlay of
population on warming projections. "It's often overlooked that the
majority of the world's population lives in regions that will see a
comparably fast rise in temperatures," he said.
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