[MCN] NCAR/UCAR: Update on expectations for increasing heat

[Lance Olsen]

SEARING HEAT WAVES DETAILED IN STUDY OF FUTURE CLIMATE
https://www2.ucar.edu/atmosnews/news/19559/searing-heat-waves-detailed-study-future-climate
February 23, 2016

Sweltering heat waves that typically strike once 
every 20 years could become yearly events across 
60 percent of Earth's land surface by 2075, if 
human-produced greenhouse gas emissions continue 
unchecked. 

If stringent emissions-reduction measures are put 
in place, however, these extreme heat events 
could be reduced significantly. Even so, 18 
percent of global land areas would still be 
subjected yearly to these intense heat waves, 
defined as three exceptionally hot days in a row.

These are among the findings of a new study by 
Claudia Tebaldi of the National Center for 
Atmospheric Research and Michael Wehner of the 
Lawrence Berkeley National Laboratory. The study, 
funded by the U.S. Department of Energy and 
published in the journal Climatic Change, 
quantifies the benefits society would reap, in 
terms of avoiding extreme heat events, if action 
is taken now to mitigate climate change.

"The study shows that aggressive cuts in 
greenhouse gas emissions will translate into 
sizable benefits, starting in the middle of the 
century, for both the number and intensity of 
extreme heat events," Tebaldi said. "Even though 
heat waves are on the rise, we still have time to 
avoid a large portion of the impacts."

MORE FREQUENT, MORE SEVERE
Tebaldi and Wehner used data generated by the 
NCAR-based Community Earth System Model to study 
20-year extreme heat events-those intense enough 
to have just a 1-in-20 chance of occurring in any 
given year. The model was developed with support 
from the Department of Energy and the National 
Science Foundation, NCAR's sponsor.

The researchers looked at two things: how 
frequently today's typical 20-year heat wave may 
occur in the future, as well as how much more 
intense future 20-year heat waves will be.

Besides finding that today's 20-year heat waves 
could become annual occurrences across more than 
half of the world's land areas by 2075, the study 
also concluded that heat waves with a 1-in-20 
chance of occurring during a future year will be 
much more extreme than heat waves with the same 
probability of occurring today.

For example, if emissions remain unabated, a heat 
wave with a 1-in-20 chance of occurring in 2050 
would be at least 3 degrees Celsius (5.4 degrees 
Fahrenheit) hotter for 60 percent of the world's 
land areas. For 10 percent of land areas, a 
20-year heat wave in 2050 would be at least 5 
degrees C (9 degrees F) hotter.

A few degrees may not seem like much on a mild 
day, but during extreme heat events, they can 
mean the difference between life and death for 
vulnerable populations, Wehner said.

"It's the extreme weather that impacts human 
health; this week could be 2 degrees 
Celsius hotter than last week, and that doesn't 
matter," he said. "Now, imagine the hottest day 
that you can remember and instead of 42 degrees C 
(107.6 degrees F) it's now 45 degrees C (113 
degrees F). That's going to have a dangerous 
impact on the poor, the old and the very young, 
who are typically the ones dying in heat waves."

By 2075, the situation is likely to become much 
more dire if greenhouse gas emissions-produced 
largely by the burning of fossil fuels-are not 
reduced. The percent of land areas subject to 
20-year events that are at least 5 degrees C 
hotter swells from 10 to 54 percent.

However, if emissions are aggressively cut, the 
severity of these 20-year events could be 
significantly reduced over the majority of the 
world's land areas, though portions of the Earth 
would still face dangerous heat extremes. For 
example, in 2075, almost a quarter-instead of 
more than a half-of land areas could experience 
20-year heat waves that are at least 5 degrees C 
hotter than today's. "But even with such dramatic 
reductions in carbon dioxide emissions, future 
heat waves will be far more dangerous than they 
are now," Wehner said.

The researchers also looked at single-day extreme 
heat events, as well as single-day and three-day 
blocks when the overnight low temperature 
remained exceptionally warm. Past research has 
shown that human health is especially endangered 
when temperatures do not cool off significantly 
at night. All of these events had similar 
increases in frequency and intensity.

A TOOL FOR COST-BENEFIT ANALYSIS
The fact that extreme heat events are expected to 
increase in the future as the climate changes-and 
the fact that emission reductions could 
ameliorate that increase-is not a surprise, 
Tebaldi said. But this study is important because 
it puts hard numbers to the problem.

"There is a cost attached to reducing emissions," 
Tebaldi said. "Decision makers are interested in 
being able to quantify the expected benefits of 
reductions so they can do a cost-benefit 
analysis."
Tebaldi and Wehner's paper is part of a larger 
project based at NCAR called the Benefits of 
Reduced Anthropogenic Climate Change, or BRACE. 
For the project, researchers from across NCAR and 
partner organizations are working to quantify how 
emission reductions may affect health, 
agriculture, hurricanes, sea level rise, and 
drought.

About the article
Title: Benefits of mitigation for future heat 
extremes under RCP4.5 compared to RCP8.5
Authors: Claudia Tebaldi and Michael F. Wehner
Publication: Climatic Change

Writer:
Laura Snider, Senior Science Writer and Public Information Officer


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The University Corporation for Atmospheric 
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Atmospheric Research under sponsorship by the 
National Science Foundation. Any opinions, 
findings and conclusions, or recommendations 
expressed in this publication are those of the 
author(s) and do not necessarily reflect the 
views of the National Science Foundation.

-- 
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"  Š organisms have a physiological response to 
temperature, and these responses have important 
consequences Š. biological rates and times (e.g. 
metabolic rate, growth, reproduction, mortality 
and activity) vary with temperature.

Anthony I. Dell, Samraat Pawar and Van M. Savage. 
Temperature dependence of trophic interactions 
are driven by asymmetry of species responses and 
foraging strategy. Journal of Animal Ecology 2013
===========================================================
"All organisms live within a limited range of 
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and kinetic coordination of molecular, cellular, 
and systemic processes. Functional constraints 
result at temperature extremes. Increasing 
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molecules, and for animals and plants than for 
unicellular organisms (8). Direct effects of 
climatic warming can be understood through fatal 
decrements in an organism's performance in 
growth, reproduction, foraging, immune 
competence, behaviors and competitiveness. 
Performance in animals is supported by aerobic 
scope, the increase in oxygen consumption rate 
from resting to maximal (9). Performance falls 
below its optimum during cooling and warming. At 
both upper and lower pejus temperatures, 
performance decrements result as the limiting 
capacity for oxygen supply causes hypoxemia (4, 
8) (see the figure, left). Beyond low and high 
critical temperatures, only a passive, anaerobic 
existence is possible."

Hans O. Pörtner and Anthony P. Farrell. 
Physiology and Climate Change. SCIENCE 31 OCTOBER 
2008	VOL 322
===============================================================
"Between 1C and 2C increases in global mean 
temperatures most species, ecosystems and 
landscapes will be impacted and adaptive capacity 
will become limited. With the already ongoing 
high rate of climate change, the decline in 
biodiversity will therefore accelerate and 
simultaneously many ecosystem services will 
become less abundant."

Rik Leemans and Bas Eickhout. Another reason for 
concern: regional and global impacts on 
ecosystems for different levels of climate 
change. Global Environmental Change 14 (2004) 
219-228
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