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Weather extremes shed light on prairie’s past and environment’s future

18.08.2003


Highway travelers view much of the Midwest as little more than barren flatlands. The formation of the region and its rich soils, especially tall grass areas that seemingly should support diverse forests, however, have long fascinated scientists. Newly available, long-term climate data now say the area is the product of weather extremes.



Compared with adjacent regions, the tall-grass area of the plains endures more frequent periods of severe drought, more lightning strikes and subsequent fires from frequent winter thunderstorms, dryer cold weather and more rapid plant and soil moisture evaporation, a team of researchers from the Illinois State Water Survey and University of Illinois at Urbana-Champaign says in the current issue of the journal Physical Geography.

"Beyond the 100 years of scientific curiosity is that these extremes of weather and their frequency or their non-frequency that we have found to be critical factors for the plains are actually very important issues as we face global climate change," said Stanley A. Changnon, a water survey scientist and professor of geography. "The long-term data we’ve gathered and are analyzing can provide us with very useful guidance as we talk about potential changes to our agricultural systems and to the way we as people live in general."


Changnon and his State Water Survey colleagues have digitized national climate data going back to 1890. Information from before 1948, when the federal government began a formal record-keeping procedure on computer punch cards, was taken from records left by volunteer weather observers. Once they interpreted and entered the information into digital records, the researchers began analyzing individual weather factors and running comparisons.

The triangular-shaped tall grass area scrutinized in the study stretches from Tulsa, Okla., to Fargo, N.D., to Indianapolis. European explorers entering what they called "an inland sea" found a humid area with grasses up to 6 feet tall.

Long before, the plains were bulldozed flat during four major glacial advances and retreats that left behind sands, nutrient-rich soils and rocks. Wide flat rivers drained the melting ice finally about 11,500 years ago, at which time the tall grasses arose. Shorter domesticated grasses and farmers’ fields of corn, wheat and soybeans have since replaced the tall grasses.

To the north and south of the tall-grass region, there emerged extensive forests. A long-debated scientific question, the researchers noted, is why the tall-grass prairie only supported grass when the soil easily could have sprouted diverse forests. Numerous ecological and climate-related theories have been raised, disputed and discarded.

The newly acquired data -- housed at the Midwestern Regional Climate Center at the State Water Survey -- are providing scientifically strong details, Changnon said.

One of the factors that emerged in the study was not new. The role of fire in sustaining the prairie and preventing the growth of trees was first established in the early 1950s, based on pioneer descriptions of deliberate actions taken by Indians in pursuit of buffalo. The impact and frequency of fire, however, are now strengthened by the new data on thunderstorms. The plains average 60 to 80 storms and more than 10,000 minutes of storm activity each year. Only Florida and the Gulf Coast experience more. The tall grass region also faces a three-to-five-times greater risk of fire from lightning.

In addition to the heat and searing of fires, severe droughts struck the region in 15 percent of the almost 105 years covered by the study; adjacent forested regions to the east and west had droughts in less than 10 percent of the years, and areas to the north and south were struck by drought even less frequently.

While rainfall was of similar frequency during summers, cold-weather precipitation in the tall-grass plains has been dramatically less. In 71 of the years covered, the tall-grass prairie region received less than 15 inches of precipitation during the winters; adjacent forested lands to the south had only eight dry seasons.

The periods of dry winters also contributed to dry vegetation, making the grasses more susceptible to fires set by Indians or caused by lightning.

Another contributing factor to sustaining the grassland was the frequency and amount of warm-season evapotranspiration -- a process in which moisture evaporates from the soil and transpires from plants.

Most of the tall-grass triangle had precipitation/evaporation ratios of .75 or higher, a number that suggests unusually wet warm-season conditions, in 70 percent of the years studied; adjacent regions in the high plains had much lower ratios. The researchers theorize that evapotranspiration rates played a major role in the formation and maintenance of the western boundary (Tulsa to Fargo) separating the short grasses of the plains and the tall grass triangle. The tall grasses needed enough warm-weather precipitation to produce the higher evapotranspiration rate, the researchers found.

The experience of the Midwest’s prairie is one of extremes, including some of the very factors that could be more widespread as a result of global warming, said Changnon, chief emeritus of the State Water Survey.

"What this shows is that the whole environment of the Midwest has been very sensitive to certain extreme weather events," he said. "Having long-term data lets us talk more intelligently about potential changes in global climate. Most climate modeling generates average changes, not the frequency of extreme events. Talk of the occurrence of a 100-year flood really hasn’t been based on 100 years of data; it may be extrapolated from just 40 years of records, so scientists must say that a 100-year flood will happen at least once, not necessarily only once, in 100 years."


Co-authors of the study, funded by the National Oceanic and Atmospheric Administration, were Changnon, Kenneth E. Kunkel, head of the atmospheric environment section of the State Water Survey and professor of atmospheric sciences at Illinois, and Derek Winstanley, chief of the State Water Survey and professor of geography at Illinois.

Jim Barlow | UIUC
Further information:
http://www.uiuc.edu/

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