[Vision2020] 8-6-2012: NASA GISS in PNAS: Research Links Extreme Summer Heat Events to Global Warming

[Ted Moffett]

http://www.pnas.org/content/early/2012/07/30/1205276109.abstract

Perception of climate change

   1. James Hansen</search?author1=James+Hansen&sortspec=date&submit=Submit>
   a <#aff-1>,1 <#corresp-1>,
   2. Makiko Sato
</search?author1=Makiko+Sato&sortspec=date&submit=Submit>a<#aff-1>,
   and
   3. Reto Ruedy
</search?author1=Reto+Ruedy&sortspec=date&submit=Submit>b<#aff-2>

Abstract

"Climate dice," describing the chance of unusually warm or cool seasons,
have become more and more "loaded" in the past 30 y, coincident with rapid
global warming. The distribution of seasonal mean temperature anomalies has
shifted toward higher temperatures and the range of anomalies has
increased. An important change is the emergence of a category of summertime
extremely hot outliers, more than three standard deviations (3σ) warmer
than the climatology of the 1951-1980 base period. This hot extreme, which
covered much less than 1% of Earth's surface during the base period, now
typically covers about 10% of the land area. It follows that we can state,
with a high degree of confidence, that extreme anomalies such as those in
Texas and Oklahoma in 2011 and Moscow in 2010 were a consequence of global
warming because their likelihood in the absence of global warming was
exceedingly small. We discuss practical implications of this substantial,
growing, climate change.

-----------------------------------

http://www.giss.nasa.gov/research/news/20120806/
Research Links Extreme Summer Heat Events to Global Warming

Aug. 6, 21012

A new statistical analysis by NASA scientists has found that Earth's land
areas have become much more likely to experience an extreme summer heat
wave than they were in the middle of the 20th century. The research was
published today in the journal Proceedings of the National Academy of
Sciences.

Earth's Northern Hemisphere over the past 30 years has seen more "hot"
(orange), "very hot" (red) and "extremely hot" (brown) summers, compared to
a base period defined in this study from 1951 to 1980. This visualization
shows how the area experiencing "extremely hot" summers grows from nearly
nonexistent during the base period to cover 12 percent of land in the
Northern Hemisphere by 2011. Watch for the 2010 heat waves in Texas,
Oklahoma and Mexico, or the 2011 heat waves the Middle East, Western Asia
and Eastern Europe. Credit: NASA/Goddard Space Flight Center Scientific
Visualization Studio
+ Download hi-res
visualization<http://svs.gsfc.nasa.gov/vis/a000000/a003900/a003970/>

The statistics show that the recent bouts of extremely warm summers,
including the intense heat wave afflicting the U.S. Midwest this year, very
likely are the consequence of global warming, according to lead author
James Hansen of NASA's Goddard Institute for Space Studies (GISS) in New
York.

"This summer people are seeing extreme heat and agricultural impacts,"
Hansen says. "We're asserting that this is causally connected to global
warming, and in this paper we present the scientific evidence for that."

Hansen and colleagues analyzed mean summer temperatures since 1951 and
showed that the odds have increased in recent decades for what they define
as "hot," "very hot" and "extremely hot" summers.

The researchers detailed how "extremely hot" summers are becoming far more
routine. "Extremely hot" is defined as a mean summer temperature
experienced by less than one percent of Earth's land area between 1951 and
1980, the base period for this study. But since 2006, about 10 percent of
land area across the Northern Hemisphere has experienced these temperatures
each summer.

James Hansen and colleagues use the bell curve to show the growing
frequency of extreme summer temperatures in the Northern Hemisphere,
compared to the 1951 to 1980 base period. The mean temperature for the base
period is centered at the top of the green curve, while hotter than normal
temperatures (red) are plotted to theright and colder than normal (blue) to
the left. By 1981, the curve begins to shift noticeably to the right,
showing how hotter summers are the new normal. The curve also widens, due
to more frequent hot events. Credit: NASA/Goddard Space Flight Center
Scientific Visualization Studio
+ Download hi-res
visualization<http://svs.gsfc.nasa.gov/vis/a000000/a003900/a003975/>
In 1988, Hansen first asserted that global warming would reach a point in
the coming decades when the connection to extreme events would become more
apparent. While some warming should coincide with a noticeable boost in
extreme events, the natural variability in climate and weather can be so
large as to disguise the trend.

To distinguish the trend from natural variability, Hansen and colleagues
turned to statistics. In this study, the GISS team including Makiko Sato
and Reto Ruedy did not focus on the causes of temperature change. Instead
the researchers analyzed surface temperature data to establish the growing
frequency of extreme heat events in the past 30 years, a period in which
the temperature data show an overall warming trend.

NASA climatologists have long collected data on global temperature
anomalies, which describe how much warming or cooling regions of the world
have experienced when compared with the 1951 to 1980 base period. In this
study, the researchers employ a bell curve to illustrate how those
anomalies are changing.

A bell curve is a tool frequently used by statisticians and society. School
teachers who grade "on the curve" use a bell curve to designate the mean
score as a C, the top of the bell. The curve falls off equally to both
sides, showing that fewer students receive B and D grades and even fewer
receive A and F grades.

Hansen and colleagues found that a bell curve was a good fit to summertime
temperature anomalies for the base period of relatively stable climate from
1951 to 1980. Mean temperature is centered at the top of the bell curve.
Decreasing in frequency to the left of center are "cold," "very cold" and
"extremely cold" events. Decreasing in frequency to the right of center are
"hot," "very hot" and "extremely hot" events.

Plotting bell curves for the 1980s, 1990s, and 2000s, the team noticed the
entire curve shifted to the right, meaning that more hot events are the new
normal. The curve also flattened and widened, indicating a wider range of
variability. Specifically, an average of 75 percent of land area across
Earth experienced summers in the "hot" category during the past decade,
compared to only 33 percent during the 1951 to 1980 base period. Widening
of the curve also led to the designation of the new category of outlier
events labeled "extremely hot," which were almost nonexistent in the base
period.

Hansen says this summer is shaping up to fall into the new extreme
category. "Such anomalies were infrequent in the climate prior to the
warming of the past 30 years, so statistics let us say with a high degree
of confidence that we would not have had such an extreme anomaly this
summer in the absence of global warming," he says.

Other regions around the world also have felt the heat of global warming,
according to the study. Global maps of temperature anomalies show that heat
waves in Texas, Oklahoma and Mexico in 2011, and in the Middle East,
Western Asia and Eastern Europe in 2010 fall into the new "extremely hot"
category.
Related Links:

+ GISS Science Brief:The New Climate Dice: Public Perception of Climate
Change <http://www.giss.nasa.gov/research/briefs/hansen_17/>

+ NASA What on Earth blog:The New Climate
Dice<http://blogs.nasa.gov/cm/blog/whatonearth/posts/post_1344022702866.html>

+ GISS Science Brief:A Common Sense Climate Index: Is Climate Changing
Noticeably? <http://www.giss.nasa.gov/research/briefs/hansen_04/>

+ GISS Surface Air Temperature Analysis <http://data.giss.nasa.gov/gistemp/>
Reference

Hansen, J., Mki. Sato, and R. Ruedy, 2012a:Perception of climate
change<http://pubs.giss.nasa.gov/abs/ha00610m.html>
.Proc. Natl. Acad. Sci., doi:10.1073/pnas.1205276109. Early draft posted as
"Public perception of climate change and the new climate dice",
arXiv.org:1204.1286 <http://arxiv.org/abs/1204.1286>.
Media Contacts

Leslie McCarthy, NASA Goddard Institute for Space Studies, New York, N.Y.,
212-678-5507, leslie.m.mccarthy at nasa.gov
Media Contacts

This article was originally prepared by Kathryn Hansen as a NASA
portalEarth news
feature<http://www.nasa.gov/topics/earth/features/warming-links.html>
.

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Vision2020 Post: Ted Moffett
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