Part 1 is a rewrite of a blog post that I published in 2009. It is as important today as it was then. Part 2 will be an update of the observed rainfall extremes in the Southeast and U.S. Part 3 will look at drought and new research as to why drought is becoming more common across the southern U.S.
From 2009:
The idea for
this blog came from two sources. The first source was a comment on an earlier
story.
In the
comment section, a reader 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, wouldn't global warming mean more evaporation of surface water and snow and more evapotranspiration from plants, causing more rain?”
I would defer to Mr. Gandy on that though. Am I correct Mr. Gandy, wouldn't 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 top 10 daily rainfall totals for Columbia, SC. Click on the image for a larger version. Image Credit: Climate Central. |
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, the commenter 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 temperature 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 an 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.
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 64 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 projects. 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.
