August 21, 2003
Global Warming Will Make Grasslands Wetter

Higher temperatures unexpectedly make soil more moist.

Grassland ecosystems could become wetter as a result of global warming, according to a new study by researchers from Stanford University and the Carnegie Institution of Washington. This surprising result, published in the Proceedings of the National Academy of Sciences (PNAS), contradicts numerous climate models predicting that higher temperatures could dry out natural landscapes, including grasslands.

The study, to be posted the week of Aug. 4 on PNAS Online, is based on a two-year field experiment conducted in the grassy foothills above Stanford's main campus. Instead of causing the soil to dry up, the experiment revealed that higher temperatures actually increased soil moisture by as much as 10 percent.

"Warming accelerates evaporation, so we expected warmer to mean drier," said lead author Erika S. Zavaleta, a former Stanford doctoral student now on the faculty of the University of California-Santa Cruz. "We were surprised to find that warming actually increased moisture in our grassland plots during those critical weeks in late spring at the end of the growing season, when moisture shapes which plant species prevail. We traced this unexpected moisture increase to the plants themselves."


The discovery that higher temperatures can significantly dampen soil is at odds with several climate models that predict that global warming will make grassland ecosystems drier, not wetter. Those models are based on the assumption that higher temperatures will increase the amount of water that evaporates from the soil and the surface of living plants -- a process called "evapotranspiration."

At the Jasper Ridge site, most soil moisture evaporates through plants. But during the course of the experiment, researchers discovered that warming caused the early demise of numerous grasses and wildflowers. In fact, some experimental plots that were exposed to higher temperatures suffered the premature loss of 17 percent of their green vegetation. Since evapotranspiration only occurs through living plants, the fact that so many died early could explain the unexpected rise in soil moisture, the authors noted.

"In California grasslands, plants control most of the water exiting the system by transpiring water through their leaves until they die," Zavaleta said. "Simulated global warming accelerated the death of the dominant grasses in our plots, leaving slightly more water in the soil for other species like oaks and summer wildflowers to use. This doesn't mean climate change is good for California grasslands, but it reinforces the importance of paying attention to how plants and animals could modify its effects."

If higher temperatures kill off some of the plants but leave more moisture for the other plants then what are the effects on the remaining plants? Do they stay healthy longer during dry periods? It is not clear what the net effect is of the higher temperatures. One effect might be to cause a change in the ratios of species of plants in any given area to select for more heat-resistant plants.

Note that independent of the heat effect the plots that were exposed only to higher CO2 also had more moisture in their soil. That result, while not unexpected due to previous research, is also interesting in light of another report previous posted here: Rising Carbon Dioxide Causing Forests To Expand Into Deserts. A hotter planet with more CO2 in the atmosphere may turn out to be a much greener planet overall.

Share |      Randall Parker, 2003 August 21 02:13 PM  Climate Trends

Paul Banks said at August 22, 2003 8:37 PM:

I have a quick question and a quick comment as well; first the question: how does this finding square with previous models of Earth's changing climates (for example, the flourishing, now desert choked Anatolian Pen.)? Is it possible that temps. change constantly with the effects noted, perhaps making California into more and more a desert and perhaps the Midwest more and more like, say, Florida?

Now the comment: while I was in an intoductory geo-physics class, my teacher mentioned that, potentially, we could be headed towards another ice age, the exact opposite of the greenhouse theory. I mention this because, as shown here, moisture is increased in certain areas because of this process; if this is true on a larger global scale, is not, as my teacher pondered, a key criteria for an ice age met, namely, that more percipitation occurs than ice melt?

Would love to get some comments (and I apologize for the crude presentation of my prof.'s case, it was years ago and I basically only remember the precip. v melt thesis with regards to ice ages, not the details).

Paul Banks
Washington University in St. Louis Law School

Randall Parker said at August 22, 2003 10:19 PM:

As for whether the Earth is really going to warm or cool in the next hundred years: We don't know for the simple reason that we don't know all the variables involved and there are variables we can not predict. For instance, will Solar output increase or decrease or stay the same on average? Will cosmic ray concentrations hitting the atmosphere go up or down? There was an existing warming trend that predates the Industrial Revolution that may continue independent of CO2 levels. Or the long term cycle may start to shift back toward the cooler level that the Earth has experienced for most of its history. Based on long term cycles we ought to start back toward cooling within 1000 years or so and quite possibly much sooner. That is probably what your prof was referring to.

Check out my Trends Climate Archive for previous posts on the subject. Also see my Environmental Engineering Archive for other relevant posts.

Then there is the question what a warming trend caused by CO2 would really mean. If it causes the deserts to retreat then that could make more of the Earth arable land.

Randall Parker said at August 22, 2003 10:27 PM:

BTW, to answer your specific question wrt California and the Midwest: it depends very importantly on how rain patterns shift. A warming that shifted rains toward California could make it extremely lush with vegetation.

People want answers to these questions in order to set energy policy. But we simply don't know the answers. Given another decade or two we will have much better climate models and also ecological models.

Since we don't know whether or how much warming will happen or what will be its cause if it does happen or what the effects will be in all the locales around the world it doesn't seem prudent to foist huge costs on the economy to try to reduce CO2 emissions prematurely. It would make more sense to spend considerable money on research and development to develop new energy technologies that would be cheaper than fossil fuels. Then we could reduce CO2 emissions at very little cost.

Of course, if it turned out that the climate was going to cool we might want to keep burning coal in order to warm the planet.

Sydney Phillips said at May 20, 2004 3:08 PM:

I think that if global warming does start to affect the grasslands that they will turn into forests. Do you have a comment on that?

Global Warming Team said at September 20, 2004 12:26 PM:

Scientists are convinced the accumulation in the atmosphere of carbon dioxide and other greenhouse gases is heating up the earth, but they cannot say exactly how much warming will occur. Global warming is likely to produce much more erratic weather because a warmer atmosphere means the evaporation of more water from the oceans, leading to greater precipitation. It also means the exchange of more energy, leading to greater atmospheric violence. Which means some areas might be hit by droughts, while others could suffer more frequent and violent storms. Thus, it would hurt developing economies.

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