Did Humans Cause the Midwest Flooding?

James WangThis post is by James Wang, Ph.D., a climate scientist at Environmental Defense.

The record floodwaters in Iowa and other parts of the Midwest are claiming lives, destroying homes and crops, contaminating drinking water, and – as the AP puts it – spreading "a noxious brew of sewage, farm chemicals, and fuel that could sicken anyone who wades in." The cost in human anguish is incalculable.

But why is it happening? Is it just a freak of nature? One causal element, as reported in today’s Washington Post, may be human reengineering of the landscape. Mary Kelly, who heads up EDF’s rivers and deltas program, gives a good overview of these issues.

Another element may be global warming, which increases the probability of extreme weather events like torrential rain.

Global Warming and Heavy Rainfall

Global warming intensifies the "hydrological cycle" – the process in which water evaporates into the air, forms clouds, and then rains back down on the Earth.

Higher temperatures cause evaporation to occur more quickly. This can cause very dry conditions on land, even drought. But there’s another side to it. The greater amount of water vapor that a warm atmosphere can hold causes wetter clouds to form, so the rain, when it comes, can be unusually heavy – heavy enough to cause flooding. This intensification of the hydrological cycle causes some seasons to be very wet while others are very dry.

We can’t say for sure that global warming caused the unusually heavy rain in the Midwest – or any specific weather event. But we can say that the probability of torrential rainfall is increased due to global warming.

The IPCC’s 2007 report [PDF] says:

  • "The frequency of heavy precipitation events has increased over most land areas, consistent with warming and observed increases of atmospheric water vapour."
  • "It is very likely that hot extremes, heat waves and heavy precipitation events will continue to become more frequent."

Global warming doesn’t fully explain the catastrophe in the Midwest, but it likely plays a role. The sooner we can bring emissions under control, the better.

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  1. mikes
    Posted June 19, 2008 at 3:27 pm | Permalink

    I have been checking your site waiting for the inevitable posting linking GW to the Midwest floods.

    You say, “Global warming intensifies the ‘hydrological cycle’ – the process in which water evaporates into the air, forms clouds, and then rains back down on the Earth.”

    I say, “What warming?” If a warm earth causes Midwest flooding, why didn’t massive flooding occur in 1998, the year of peak temperatures in the modern era? The opposite is more likely true.

    The earth, by all measures, is much cooler than ten years ago. See: http://wattsupwiththat.files.wordpress.com/2008/06/rss_may_081.png . Temperatures have cooled rapidly the past year.

    Now, go back and look for another year with rapid cooling and focus on months 150-160. See the cooling? Turns out that was right after Mt. Pinatubo exploded causing…cooling! And, record floods in the Midwest shortly thereafter.

    Why? Don’t know for certain but the following is likely: Since the tropical oceans lose their heat more slowly than land, during a period of rapid cooling a more intense than average thermal gradient sets up over the central U.S. causing more intense storms with heavy rains, more tornadoes, etc.

    Linking the current flooding in the Midwest to global warming completely unjustified by the facts and scientific circumstances.


  2. Posted June 19, 2008 at 4:18 pm | Permalink

    Mike, if you don’t believe our scientists or the scientists in the IPCC, then maybe you’ll believe the scientists who work for the U.S. government? They just released a new report today. Here’s an A.P. story about it:

    Extreme weather to increase with climate change

    It talks about an increase in heavy downpours, and how these changes – caused by global warming – “can lead to the type of events that we are seeing in the Midwest.”

    I don’t expect a practitioner of truthiness like yourself to be convinced by this, but others reading the blog may be interested. We may post more about this new study next week.

  3. mikes
    Posted June 19, 2008 at 4:36 pm | Permalink


    “Science” isn’t words, it is theories that can be tested and falsified. I linked to the scientific data showing cooling temperatures. I note you don’t comment on that.

    It is obviously false that the earth is currently warming. Even Dr. Hansen’s inconvenient data shows the recent cooling: http://data.giss.nasa.gov/gistemp/graphs/Fig.C.lrg.gif .

    It is bad science to link the current Midwest flooding to “warming.”

    I call ’em as I see ’em.


  4. Posted June 20, 2008 at 11:14 am | Permalink


    Even though there was some cooling at the beginning of this year, this past winter was still warmer than most of the winters prior to 2001. (See an earlier blog post of ours: http://blogs.edf.org/climate411/2008/03/24/cooling_from_la_nina/)

    In other words, the cooling earlier this year was far from enough to offset the warming we’ve seen over recent decades.

    Global warming refers to long-term trends over many decades, so it’s inappropriate and misleading when people show only the last ten years of Hansen’s data starting from the exceptionally warm El Nino year of 1998. As to whether or not flooding occurred in 1998, I know that there were heavy, El Nino-linked rains in the U.S. that year, especially in the Southwest.

    Note that we never linked global warming to flooding specifically in the Midwest. We just said that wherever and whenever it rains, it will tend to be more extreme because of global warming.

    Of course, there are further details in climatic changes besides just a general intensification of the hydrological cycle. There may also be an overall poleward shift in the storm track, changes in thermal gradients, strengthening of pressure systems, etc. All of these will interact to produce the weather experienced in a particular area.

  5. mikes
    Posted June 20, 2008 at 11:52 am | Permalink


    Thank for your post.

    However, your comment, “we never linked global warming to flooding specifically in the Midwest” is disingenuous when the headline says, “Did Humans Cause the Midwest Flooding?” and you go on to say, “another element may be global warming.”

    You correctly state, “global warming refers to long-term trends over many decades,” which is another way of saying “climate.” But it is NOT “inappropriate” to link current temperatures to WEATHER. The atmosphere, when it creates ‘weather,’ is responding to current atmospheric conditions. So, it is entirely accurate to show a graph of current temperatures when attempting to determine factors affecting the Midwest floods. And, recent world atmospheric temperatures are not ‘warming.’

    You are correct that it was extremely wet in the Southwest in 1998 (see: http://www.cpc.noaa.gov/products/analysis_monitoring/regional_monitoring/palmer/1998/06-20-1998.gif for conditions the week of June 20, 1998). It was only mildly wet — with no significant flooding — in Iowa that same week. If Iowa weather responds to warming temperatures, as you contend, then there would have been massive floods during that period. There were not, which falsifies the linking of the current Midwest floods to global warming.

    As I said in my original comment, it is far more likely that Midwest weather has responded to the rapid cooling and increased temperature gradient across the region.

    Using the human tragedy of the Midwest floods to try to score public relations points for your global warming cause — especially when the science does not in any way justify it — is sleazy. I would recommend taking down this post.


  6. cdietrick
    Posted June 23, 2008 at 2:26 pm | Permalink

    I agree with Mike. Tired of hearing the “global warmers” blame every tornado, hurricane and downpour on global warning. Floods, tornadoes and other bad weather have been around for a while. Has anyone charted the number of hurricanes, tornadoes, over the past few decades?


  7. mikes
    Posted June 23, 2008 at 4:24 pm | Permalink


    Sheryl links to the AP story about the Report rather than the Report itself.

    As is usually the case in global warming ‘science,’ the actual Report doesn’t read anything like the press release about the report. You can judge for yourself here: http://sciencepolicy.colorado.edu/prometheus/archives/climate_change/001462what_the_ccsp_extrem.html

    As the actual report says, there has been no increase in tornadoes. Yes, damage is way up but that is because of inflation and because we are foolishly putting more and more “stuff” in harm’s way.


  8. Posted June 23, 2008 at 4:47 pm | Permalink


    It’s true that extreme weather has always been around. However, the concern is that global warming is changing the norm and making what used to be extreme and rare more common.

    We have a web page summarizing the latest research on hurricanes and climate (up through early this year): http://www.edf.org/page.cfm?tagid=654

    There is evidence that the intensity of hurricanes around the world has been correlated with ocean temperatures, although scientists are still trying to understand how exactly global warming may have contributed to the recent increase in hurricane intensity.

    As for tornadoes, scientists have not made any linkages between recent changes in tornado activity and warming. Small-scale, local phenomena like tornadoes are difficult to monitor comprehensively. Even with regular records on tornadoes going back to the 1950s in the U.S., many tornadoes are missed and the way their intensity is estimated has changed over time. That’s not to say that global warming won’t affect tornado activity. It’s just that it will be difficult to confirm any model predictions on tornadoes, in part because of difficulties in monitoring them.

  9. mikes
    Posted June 23, 2008 at 6:11 pm | Permalink


    You need to delete the first item on your “hurricanes and climate: list. Kerry Emanuel has “repealed” his conclusions regarding hurricanes and “global warming.” Here is his newer peer-reviewed paper: http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2FBAMS-89-3-347 .


  10. Posted June 24, 2008 at 7:51 am | Permalink

    The link to the full report is in this post from Lisa Moore, posted yesterday.

  11. dlt1580
    Posted June 27, 2008 at 8:39 pm | Permalink

    This is simply moronic junk science. I suggest you read some Fred Singer and get an education. The climate on all the planets in the solar system is going through allthe same stuff. Duh…this means that man has NOTHING to do with the climate change. Carbon footprints by man is less than 2% of all of it. Less than 2%.

    There was a man working at nasa that skewered the data to make it look like 1998 was the hotest. They had to recind that. Why do you constantly lie to try and prove something that does not even exist.

    In the last year alone, they not only recaptured all of the so called melted sea ice, they gained 1/3 they didnt even have.

    The truth of the matter is you are out of a grant(job) if you can not fabricate something. Yet when real climate scientists, the tops in the field say man has nothing to do with the climate, you spin more lies. James you are spinning even in your responce when you mention last winter. I hate to tell you this, but it was brutal in America in the North last winter.

    You are trying to make your point in not even a full cycle. That is intellectually dishonest. I have come to expect nothing less than half truths and lies from the so called environmentalist. 17000 scientists agree man has nothing to do with the climate. Your group has been embarrassed for the last 45 years in your ridiculous onslaught of what you have turned into a religion and not a science.

    Of the original 2400 scientist who originally signed up to the farse of Global Warming, more than half have now signed off on it.

  12. Posted January 21, 2010 at 9:51 am | Permalink

    Thank you so much, Great information… You keep writing and I’ll keep reading.