<div>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.</div>
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<div>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:</div>
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<div><strong>Heavy Precipitation</strong>. 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.</div>
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<div class="rtecenter"><strong><a href="http://www.wwfblogs.org/climate/sites/default/files/US_global_precipitation_3.png"><img alt="" src="http://www.wwfblogs.org/climate/sites/default/files/US_global_precipitation_3.jpg" width="415" height="296"></a></strong></div>
<div><strong>Rate of Precipitation Change in the United States, 1901–2008.</strong> 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.<em> [For more information on this indicator see "U.S. and Global Precipitation," p 28-29 in <a href="http://www.epa.gov/climatechange/indicators/pdfs/ClimateIndicators_full.pdf">Climate Change Indicators in the United States</a> [PDF], April 2010]</em></div>
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<div><a href="http://web.mit.edu/press/2009/rain-081709.html">http://web.mit.edu/press/2009/rain-081709.html</a></div>
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<h1>MIT study sees heavier rainstorms ahead </h1>
<div class="dek"><span style="LINE-HEIGHT: normal; FONT-FAMILY: Helvetica, Helvetica, Arial, sans-serif; COLOR: rgb(0,0,0)" class="Apple-style-span">Analysis shows climate change to yield more extreme rainfall</span><br>
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<div class="byline">David Chandler, MIT News Office
<div class="date">August 17, 2009 </div></div>
<div><strong>Contact</strong> </div>
<div class="contact">Jen Hirsch<br>MIT News Office<br><a href="mailto:jfhirsch@mit.edu">jfhirsch@mit.edu</a><br>617-253-1682<br></div>
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<div style="MARGIN: 0px; FONT: 12px Helvetica; font-size-adjust: none; font-stretch: normal; -x-system-font: none"><span style="LINE-HEIGHT: normal; FONT-FAMILY: Helvetica; COLOR: rgb(0,0,0)" class="Apple-style-span"><b>EMBARGOED FOR RELEASE</b></span> </div>
<div style="MARGIN: 0px; FONT: 12px Helvetica; font-size-adjust: none; font-stretch: normal; -x-system-font: none"><span style="LINE-HEIGHT: normal; FONT-FAMILY: Helvetica; COLOR: rgb(0,0,0)" class="Apple-style-span">MONDAY, AUG. 17, 2009, 5:00 P.M. ET</span> </div>
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<div style="MARGIN: 0px; FONT: 12px Helvetica; font-size-adjust: none; font-stretch: normal; -x-system-font: none"><span style="LINE-HEIGHT: normal; FONT-FAMILY: Helvetica; COLOR: rgb(0,0,0)" class="Apple-style-span">Contact: Jen Hirsch, MIT News Office</span> </div>
<div style="MARGIN: 0px; FONT: 12px Helvetica; font-size-adjust: none; font-stretch: normal; -x-system-font: none"><span style="LINE-HEIGHT: normal; FONT-FAMILY: Helvetica; COLOR: rgb(0,0,0)" class="Apple-style-span">E: <a href="mailto:jfhirsch@mit.edu">jfhirsch@mit.edu</a>, T: 617-253-1682</span> </div>
<div style="MARGIN: 0px; FONT: 12px Helvetica; font-size-adjust: none; font-stretch: normal; -x-system-font: none"><span style="LINE-HEIGHT: normal; FONT-FAMILY: Helvetica; COLOR: rgb(0,0,0)" class="Apple-style-span"><br>
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<div style="MARGIN: 0px; FONT: 12px Helvetica; font-size-adjust: none; font-stretch: normal; -x-system-font: none"><span style="LINE-HEIGHT: normal; FONT-FAMILY: Helvetica; COLOR: rgb(0,0,0)" class="Apple-style-span"><span style="FONT-STYLE: italic; FONT-FAMILY: Helvetica" class="Apple-style-span">Written by David Chandler, MIT News Office</span></span> </div>
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<div style="MARGIN: 0px; FONT: 12px Arial; font-size-adjust: none; font-stretch: normal; -x-system-font: none">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. </div>
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<div style="MARGIN: 0px; FONT: 12px Arial; font-size-adjust: none; font-stretch: normal; -x-system-font: none">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. </div>
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<div style="MARGIN: 0px; FONT: 12px Arial; font-size-adjust: none; font-stretch: normal; -x-system-font: none">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. </div>
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<div style="MARGIN: 0px; FONT: 12px Arial; font-size-adjust: none; font-stretch: normal; -x-system-font: none">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. </div>
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<div style="MARGIN: 0px; FONT: 12px Arial; font-size-adjust: none; font-stretch: normal; -x-system-font: none">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. </div>
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<div style="MARGIN: 0px; FONT: 12px Arial; font-size-adjust: none; font-stretch: normal; -x-system-font: none">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. </div>
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<div style="MARGIN: 0px; FONT: 12px Arial; font-size-adjust: none; font-stretch: normal; -x-system-font: none">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. </div>
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<div style="MARGIN: 0px; FONT: 12px Arial; font-size-adjust: none; font-stretch: normal; -x-system-font: none">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. </div>
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<div>MIT study referenced on extreme precipitation increases from anthropogenic climate warming, from The Proceedings of the National Academy of Sciences:</div>
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<div><a href="http://www.pnas.org/content/early/2009/08/18/0907610106.abstract?sid=6024442c-0068-431b-aa9f-5f5e690e8452">http://www.pnas.org/content/early/2009/08/18/0907610106.abstract?sid=6024442c-0068-431b-aa9f-5f5e690e8452</a></div>
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<h1 id="article-title-1">The physical basis for increases in precipitation extremes in simulations of 21st-century climate change</h1>
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<ol id="contrib-group-1" class="contributor-list">
<li id="contrib-1"><span class="name"><a class="name-search" href="http://www.pnas.org/search?author1=Paul+A.++O'Gorman&sortspec=date&submit=Submit">Paul A. O'Gorman</a></span><a id="xref-aff-1-1" class="xref-aff" href="http://www.pnas.org/content/early/2009/08/18/0907610106.abstract?sid=6024442c-0068-431b-aa9f-5f5e690e8452#aff-1"><sup>a</sup></a><span class="xref-sep">,</span><a id="xref-corresp-1-1" class="xref-corresp" href="http://www.pnas.org/content/early/2009/08/18/0907610106.abstract?sid=6024442c-0068-431b-aa9f-5f5e690e8452#corresp-1"><sup>1</sup></a> and </li>
<li id="contrib-2"><span class="name"><a class="name-search" href="http://www.pnas.org/search?author1=Tapio++Schneider&sortspec=date&submit=Submit">Tapio Schneider</a></span><a id="xref-aff-2-1" class="xref-aff" href="http://www.pnas.org/content/early/2009/08/18/0907610106.abstract?sid=6024442c-0068-431b-aa9f-5f5e690e8452#aff-2"><sup>b</sup></a></li>
</ol>
<p class="affiliation-list-reveal"><a class="view-more" href="http://www.pnas.org/content/early/2009/08/18/0907610106.abstract?sid=6024442c-0068-431b-aa9f-5f5e690e8452#">+</a> Author Affiliations</p>
<ol class="affiliation-list hideaffil">
<li class="aff"><a id="aff-1" name="aff-1"></a>
<address><sup>a</sup>Massachusetts Institute of Technology, Cambridge, MA 02139; and </address></li>
<li class="aff"><a id="aff-2" name="aff-2"></a>
<address><sup>b</sup>California Institute of Technology, Pasadena, CA 91125 </address></li></ol>
<ol class="fn-track">
<li id="fn-2" class="fn-com">
<p id="p-2">Communicated by Kerry A. Emanuel, Massachusetts Institute of Technology, Cambridge, MA, July 14, 2009 (received for review March 24, 2009) </p></li></ol></div>
<div id="abstract-1" class="section abstract">
<h2>Abstract</h2>
<p id="p-3">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. </p>
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<p>Vision2020 Post: Ted Moffett</p></div></div>