5 October 2009

The idea for this blog came from two sources. The first source was a comment on an earlier story posted in the Earth, Science, and Technology section (to see the story click here).

In the comment section, donnielex wrote:

 "By the way, the article above does not inform you that global warming does not cause droughts or dry weather. Global warming would mean MORE RAIN. I would defer to Mr. Gandy on that though. Am I correct Mr. Gandy, wouldnt global warming mean more evaporation of surface water and snow and more evapotranspiration from plants, causing more rain?"

This seemed intuitive, more water vapor, more rain.  Just over a year ago I was at a panel discussion in Denver, Colorado, where Dr. Kevin Trenberth mentioned that water vapor had increased about 7% globally.  Dr. Trenberth was a lead author of the 2001 and 2007 IPCC Scientific Assessment of Climate Change and serves on the Scientific Steering Group for the Climate Variability and Predictability program.  He currently heads the Climate Analysis Section at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.  This information has since been confirmed in several papers.

The second source came from a recent study (citation below) published 5 September 2009, in the Geophysical Research Letters (GRL).  It has changed my thinking after a careful review of the paper.

First of all, I think donnielex sums up a common misconception even among some meteorologists.  In this panel discussion it was mentioned that global warming has resulted in a poleward expansion of the Hadley cell of about 4 degrees.  The Hadley cell is a vertical circulation pattern in the tropical region roughly from 30 degrees north latitude to 30 degrees south.  It consists of rising air at the equator with the air moving poleward 10-15 kilometers above the surface, sinking in the subtropics, and flowing equatorward at the surface.  It is associated with trade winds, tropical rain belts, subtropical desserts, and jet streams.  Thus, global warming is changing weather patterns which in turn can cause droughts or dry weather patterns.  Indeed, there is concern that the expansion of the Hadley cell northward could seriously impact the southern U.S. causing more and severe droughts.  This is especially true for the Southwest.

The study in GRL examined data from 1979-2007 from the Global Precipitation Climatology Project (GPCP).  The intensity of precipitation was divided into 10 percent bins (or divisions).  The findings were:

1) The global average precipitation intensity increases by about 23% for each degree of Celsius.  This is substantially more than the 7% increase in water vapor or the atmospheric water-holding capacity estimated by the Clausius-Clapeyron equation (an equation used in thermodynamics, see Wikipedia for more information).

2) The top 10% bin of precipitation intensity increases about 95%/C.

3) The 30-60% bins of precipitation intensity decreases about 20%/C.

Thus, the study finds that there is an increase in heavy or extreme precipitation events and a decrease in light and moderate precipitation intensity as the global mean temperatures rises.   The scientists in the study state in their introduction that: "Long-term changes in precipitation extremes are of great importance to the welfare of human beings as well as the entire ecosystem. Increases in heavy precipitation can lead to more and worse floods, while persistent chronic decreases of light and moderate precipitation pose a serious threat to the drought problem because light and moderate precipitation are a critical source of water for the replenishment and retention of soil moisture."  I could not have said it better myself.

I know that this presents a complicated picture of precipitation and global warming, but there is a connection between precipitation intensity and temperature change.

However, it does not imply that more rain will fall with warmer temperatures.  This may be true for some regions, but in the Southeast this is not being observed.  For example, Scott Ryan and I have found that the mean annual precipitation for Columbia, South Carolina, is lower for the 30-year period 1979-2008 (45.04 inches) than for the previous 30-year period 1949-1978 (48.82).  Yet, we have noticed in increase in extreme rain events recently.

Bottom-line, less rain is falling, but when it does rain there are more heavy rain events.  This is a flood and drought cycle.  The weather patterns associated with this have been discussed by Stu Ostro, senior meteorologist at the Weather Channel (see Stu's blog).

When you compare the results from the observations to that from the latest coupled computer models there is a surprising result.  An ensemble of 17 computer models predicts that the increase in precipitation intensity is a mere 2%/C (Sun et al., 2007).  This is an order of magnitude lower that the 23%/C found from the observations.  Clearly the models are not handling the precipitation response well.  The authors note: "They raise a serious concern that the risk of extreme precipitation events due to global warming, including floods as well as droughts, is substantially greater than that estimated by the ensemble of climate models. The societal and ecological impacts of the increased risk would be enormous."

A friend of mine, who is a soil engineer, recently told me that the current flood plains are inadequate measures.  He said that the current use of 100-year flood is insufficient.  It is time to think in terms of the 500-year flood as currently defined.  One is likely to find that it occurs much more frequently that once in 500 years.

Think about it.  The current ensemble of computer models is forecasting a 3 degree C rise in the global mean temperature this century.  The results from the observations in the study would imply a 640% increase in the top 10% of precipitation intensity.  Now think of a tropical cyclone that produces 10 inches of rain in one day.  If this represents the top 10% of precipitation intensity now, then one would expect a similar event to produce 74 inches of rain in one day by the end of the century.

Is this possible?  Yes.  Consider some of the 24-hour precipitation records in the U.S.  One comes to mind in the Houston, Texas area in the town of Alvin.  Tropical Storm Claudette dumped 42 inches of rain in a 24-hour period in 1979.  I believe that this is still the U.S. 24-hour rainfall record.  Maybe it will not be the record for long.

The implications of this are numerous, not to mention the effect on agriculture.  Studies indicate that losses to crops could be up to $3 billion dollars on average by the 2030s (Rosenzweig et al., 2002).  This is just from water-logged fields.  Keep in mind that there will be additional losses from droughts and extreme rain events.

It would be easy to dismiss the finding from Liu et al. if they were computer projections.  However, this study is from the analysis of observational data.  The change is already occurring, not some projection in the future.  They are simply showing a correlation between precipitation intensity and temperature change.  They are not forecasting what the temperature change in the future will be.

So, this changes my thinking about precipitation and global warming.  It is not a simple matter of more warming, more rain.  It is far more complicated in ways I have not thought about.  We will likely see more extremes in precipitation going forward in both directions, hence more floods and droughts.

References:

Liu, S. C., C. Fu, C.-J. Shiu, J.-P. Chen, and F. Wu (2009), Temperature dependence of global precipitation extremes, Geophys. Res. Lett., 36, L17702, doi:10.1029/ 2009GL040218.

Rosenzweig, C.E., F. Tubiello, R. Goldberg, E. Mills, and J. Bloomfield (2002), Increased crop damage in the U.S. from excess precipitation under climate change, Global Environ. Change, 12, 197-202, doi:10.1016/S0959-3780(02)00008-0.

Sun, Y., S. Solomon, A. Dai, and R. W. Portmann (2007), How often will it rain?, J. Clim., 20, 4801- 4818, doi:10.1175/JCLI4263.1.