[Vision2020] Catastrophic Floods in Nashville etc.: EPA Data and MIT Theory on Extreme Precipitation Events Linked to Anthropogenic Warming

[Ted Moffett]

It is well known that anthropogenic climate warming is predicted to cause an
increase in extreme precipitation events, including snowfall, such as
possibly the past winter's record setting snowfall in the US Northeast,
"Snowmageddon," as President Obama phrased it.  The recent flooding in
Tennessee, Kentucky and Mississippi (I won't describe the destruction and
loss of life given the widespread media coverage), which some sources
indicate is a 500 year flood, which has devastated Nashville, might be
linked to anthropogenic climate warming.  However, individual extreme
precipitation events are difficult to link directly to a warming planet,
given these events could happen even assuming a planet without rapid human
impacts on climate.  Data however reveals that extreme precipitation events
are increasing in the US, presented below.  And a recent (well, July 2009)
scientific study from MIT, also referenced below, reveals that there are
regional differences in predicting extreme precipitation increases from
climate warming, between the tropics and extra-tropical zones. The science
involved indicates extra-tropical zones will experience increasing extreme
precipitation events, though not proportional to the level of increase in
atmospheric water vapor, as some claim, as climate warms.

>From page 29 of the 76 page Environmental Protection Agency pdf "Climate
Change Indicators in the United States" released April 2010, a superb
presentation to render the complex science regarding anthropogenic climate
warming comprehensible to the general public:

*Heavy Precipitation*. In recent years, a higher percentage of precipitation
in the United States has come in the form of intense single-day events.
Eight of the top 10 years for extreme one-day precipitation events have
occurred since 1990. The occurrence of abnormally high annual precipitation
totals has also increased.
   *<http://www.wwfblogs.org/climate/sites/default/files/US_global_precipitation_3.png>
*
*Rate of Precipitation Change in the United States, 1901–2008.* This figure
shows how the amount of precipitation has changed in different parts of the
United States since the early 20th century (since 1901 for the lower 48
states; since 1905 for Hawaii). Alaska is not shown because of limited data
coverage.* [For more information on this indicator see "U.S. and Global
Precipitation," p 28-29 in Climate Change Indicators in the United
States<http://www.epa.gov/climatechange/indicators/pdfs/ClimateIndicators_full.pdf>[PDF],
April 2010]
*
------------------------------------
http://web.mit.edu/press/2009/rain-081709.html
 MIT study sees heavier rainstorms ahead
Analysis shows climate change to yield more extreme rainfall
David Chandler, MIT News Office
August 17, 2009
*Contact*
Jen Hirsch
MIT News Office
jfhirsch at mit.edu
617-253-1682
 *EMBARGOED FOR RELEASE*
MONDAY, AUG. 17, 2009, 5:00 P.M. ET

Contact: Jen Hirsch, MIT News Office
E: jfhirsch at mit.edu, T: 617-253-1682

Written by David Chandler, MIT News Office

CAMBRIDGE, Mass. -- Heavier rainstorms lie in our future. That’s the clear
conclusion of a new MIT and Caltech study on the impact that global climate
change will have on precipitation patterns.


But the increase in extreme downpours is not uniformly spread around the
world, the analysis shows. While the pattern is clear and consistent outside
of the tropics, climate models give conflicting results within the tropics
and more research will be needed to determine the likely outcomes in
tropical regions.

Overall, previous studies have shown that average annual precipitation will
increase in both the deep tropics and in temperate zones, but will decrease
in the subtropics. However, it’s important to know how the frequency and
magnitude of extreme precipitation events will be affected, as these heavy
downpours can lead to increased flooding and soil erosion.


It is the frequency of these extreme events that was the subject of this new
research, which will appear online in the Proceedings of the National
Academy of Sciences during the week of Aug. 17. The report was written by
Paul O’Gorman, assistant professor in the Department of Earth, Atmospheric
and Planetary Sciences at MIT, and Tapio Schneider, professor of
environmental science and engineering at Caltech.


Model simulations used in the study suggest that precipitation in extreme
events will go up by about 6 percent for every one degree Celsius increase
in temperature. Separate projections published earlier this year by MIT’s
Joint Program on the Science and Policy of Global Change indicate that
without rapid and massive policy changes, there is a median probability of
global surface warming of 5.2 degrees Celsius by 2100, with a 90 percent
probability range of 3.5 to 7.4 degrees.

Specialists in the field called the new report by O’Gorman and Schneider a
significant advance. Richard Allan, a senior research fellow at the
Environmental Systems Science Centre at Reading University in Britain, says,
“O’Gorman's analysis is an important step in understanding the physical
basis for future increases in the most intense rainfall projected by climate
models.” He adds, however, that “more work is required in reconciling these
simulations with observed changes in extreme rainfall events.”   The basic
underlying reason for the projected increase in precipitation is that warmer
air can hold more water vapor. So as the climate heats up, “there will be
more vapor in the atmosphere, which will lead to an increase in
precipitation extremes,” O’Gorman says.

However, contrary to what might be expected, extremes events do not increase
at the same rate as the moisture capacity of the atmosphere. The extremes do
go up, but not by as much as the total water vapor, he says. That is because
water condenses out as rising air cools, but the rate of cooling for the
rising air is less in a warmer climate, and this moderates the increase in
precipitation, he says.

The reason the climate models are less consistent about what will happen to
precipitation extremes in the tropics, O’Gorman explains, is that typical
weather systems there fall below the size limitations of the models. While
high and low pressure areas in temperate zones may span 1,000 kilometers,
typical storm circulations in the tropics are too small for models to
account for directly. To address that problem, O’Gorman and others are
trying to run much smaller-scale, higher-resolution models for tropical
areas.
-----------------------------------------
MIT study referenced on extreme precipitation increases from anthropogenic
climate warming, from The Proceedings of the National Academy of Sciences:

http://www.pnas.org/content/early/2009/08/18/0907610106.abstract?sid=6024442c-0068-431b-aa9f-5f5e690e8452

 The physical basis for increases in precipitation extremes in simulations
of 21st-century climate change

   1. Paul A. O'Gorman<http://www.pnas.org/search?author1=Paul+A.++O'Gorman&sortspec=date&submit=Submit>
   a<http://www.pnas.org/content/early/2009/08/18/0907610106.abstract?sid=6024442c-0068-431b-aa9f-5f5e690e8452#aff-1>
   ,1<http://www.pnas.org/content/early/2009/08/18/0907610106.abstract?sid=6024442c-0068-431b-aa9f-5f5e690e8452#corresp-1>and
   2. Tapio Schneider<http://www.pnas.org/search?author1=Tapio++Schneider&sortspec=date&submit=Submit>
   b<http://www.pnas.org/content/early/2009/08/18/0907610106.abstract?sid=6024442c-0068-431b-aa9f-5f5e690e8452#aff-2>

+<http://www.pnas.org/content/early/2009/08/18/0907610106.abstract?sid=6024442c-0068-431b-aa9f-5f5e690e8452#>Author
Affiliations

   1. aMassachusetts Institute of Technology, Cambridge, MA 02139; and
   2. bCalifornia Institute of Technology, Pasadena, CA 91125


   1.

   Communicated by Kerry A. Emanuel, Massachusetts Institute of Technology,
   Cambridge, MA, July 14, 2009 (received for review March 24, 2009)

 Abstract

Global warming is expected to lead to a large increase in atmospheric water
vapor content and to changes in the hydrological cycle, which include an
intensification of precipitation extremes. The intensity of precipitation
extremes is widely held to increase proportionately to the increase in
atmospheric water vapor content. Here, we show that this is not the case in
21st-century climate change scenarios simulated with climate models. In the
tropics, precipitation extremes are not simulated reliably and do not change
consistently among climate models; in the extratropics, they consistently
increase more slowly than atmospheric water vapor content. We give a
physical basis for how precipitation extremes change with climate and show
that their changes depend on changes in the moist-adiabatic temperature
lapse rate, in the upward velocity, and in the temperature when
precipitation extremes occur. For the tropics, the theory suggests that
improving the simulation of upward velocities in climate models is essential
for improving predictions of precipitation extremes; for the extratropics,
agreement with theory and the consistency among climate models increase
confidence in the robustness of predictions of precipitation extremes under
climate change.

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Vision2020 Post: Ted Moffett
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