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Planetary Watch: Drier Summers (Even With More Rain)?

by Bill Chameides | May 27th, 2008
posted by Erica Rowell (Editor)

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Does this make sense to you? It’s been raining more but summers have been getting dryer.

A new study by Pavel Groisman and Richard Knight published in the Journal of Climate sheds some light on this conundrum. A careful analysis of rainfall recordsshows that for much of the United States summers have gotten drier over the past four decades even as rainfall has increased.

Specifically their research found that over the last 40 years while temperatures and rainfall have increased:

==> the warm season got longer especially in the Southwest,
==> the average duration of dry spells (of 30 days or longer) got longer (by as many as 15 days in California and Nevada), and
==> the dry spells occurred more frequently.

For example, the authors found that four decades ago dry spells of 30 days or more used to occur about every 15 years. Now the eastern United States experiences them every 6 to 7.

It should be noted that these findings did not hold for the entire country. Parts of the upper Great Lakes and Pacific Northwest regions didn’t show these trends.

So how can it be that we are experiencing drier summers while average precipitation is increasing? Here’s how.

Scientists have predicted for some time that a warming planet will deliver more rainfall as well as more storms that dump more rain over a shorter period. The reason for this is that warmer temperatures mean more evaporation, so the clouds are fuller. When they let go, lots more water comes out — hence heavier storms. This trend has already been confirmed in parts of the United States and other areas around the globe. More intense storms have been occurring even in areas that are not seeing much change in total precipitation.

Recent studies have shown that the increase in total rainfall can be explained almost entirely by an increase in the intensity of storms not by an increase in the number of storms. In fact, the number of storms or frequency of rain events has actually decreased. In other words the time between storms has been getting longer, while the storms have been getting stronger.

This spells trouble for the summertime, the season when most of our crops are grown, because longer stretches of time between storms can lead to a double whammy. When temperatures are high, water evaporates faster. Without any rain, soils get drier faster, and water levels in lakes and reservoirs decline faster. Increasing temperatures from global warming exacerbate the problem by further accelerating water evaporation. Now, combine that with extended periods between storms and you have a vicious cycle that leads to summers with longer and more intense periods of dry, drought-like conditions even as average annual rainfall amounts remain flat or even increase.

Here’s a case in point. I moved to North Carolina last June. At the beginning of the summer, the Southeast was in the midst of a drought, but shortages were not extreme. Reservoir levels in the Research Triangle area, where Duke University is located, were within manageable levels. Then, in late summer, the area got hit with an extended rainless period that coincided with a record-breaking heat wave. The drought situation went from moderate to exceptional in no time flat. Government officials seemed like deer in the headlights trying to figure out what to do. Solutions ranged from conservation to prayer vigils.

Of course dry spells don’t usually last forever. Eventually it rains and the dry period ends. However, that doesn’t mean the problems are over, because when the rains do come they tend to be more intense. The past three years in north Texas, Oklahoma, and Arkansas provide a disturbing example. The region experienced a severe drought during the summers of 2005 and 2006. Crop failures and wildfires abounded. The drought finally broke in 2007, but brought record floods that summer.

So what’s the takeaway message? Here’s what I think: we may call it “global warming,” but it’s really climate disruption. And these kinds of disruptions can be profound and even counterintuitive — along with increasing rainfall and more brutal storms we should expect longer, more intense stretches of drought. Not a great prospect.

filed under: climate change, drought, faculty, global warming, Planetary Watch, rainfall
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