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Winter/Spring 1997

Science Bits

University of Wisconsin-Madison, NCAR
Feedbacks to a greener, wetter, early Sahara

Scientists have known for decades that the bone-dry Sahara and Sahel regions of Africa were once considerably more lush. Fossil pollen from plants that couldn't survive today's drought, dried lake beds with ancient fishing villages on their shores, and other evidence testify to a more temperate rainfall regime during the early and middle Holocene period, 12,000 to 5,000 years ago. Researchers have attributed the increased aridity to a slight shift in the earth's orbit, which dried up the monsoons that used to sweep the region.

Now, John Kutzbach (University of Wisconsin-Madison), Gordon Bonan (NCAR), and two other colleagues have found that the orbital shift is only part of the picture. Using a modified version of NCAR's Community Climate Model 2 (CCM2) and a land surface model, they simulated both the climate changes that resulted from the orbital shift and the changes in vegetation and soil that followed the drier climate.

The simulations showed that changes in vegetation and soil fed back to increase the magnitude of the climate change. When they modeled only the change from the earth's present orbit to the earlier one, summer rainfall increased by 12%. Adding the shift from today's barrren earth to the grasslands of the past increased precipitation another 6%, and going from desert soil to more loamy soil raised the summer rainfall another 10%. Thus the feedbacks accounted for more than half of the modeled rainfall increase of 28% for both the Sahara and the Sahel. The size of the drier Sahara region also shrank, dwindling by 11% as a result of the orbital shift alone and by 20% when the vegetation and soil changes were added in. These larger changes are in closer agreement with fossil and archaeological records than those obtained by modeling the orbital shift alone.

"All of this points to the fact that you can't ignore the role of vegetation in forcing its own changes in climate," Kutzbach says. When it comes to predicting future climate change, "One cannot simply look at global warming and the resulting impact on rainfall and temperatures. We must also consider what's happening on the ground--the changes in soil and vegetation--that could add to the climate change."

For further information, contact Kutzbach (608-262-2839 or jek@facstaff.wisc.edu).

Georgia Institute of Technology
Laser wind sensor conquers crosswinds

A prototype optical sensor has proven more useful than mechanical anemometers at measuring average winds over a wide area, offering potential savings over large arrays of traditional sensors. The device was developed by a team at the Georgia Tech Research Institute (GTRI).

The new instrument, a single-ended, long-path laser wind sensor, can measure air movements too faint for standard anemometers. It was initially developed for use outside chemical plants to help monitor the rates at which pollutants enter the atmosphere.

The sensor uses a helium-neon laser (the type used in grocery store checkout lanes) mounted on a large telescope. The laser shoots a beam of light at a target about 30 meters away, which is made of retro-reflective material. The light is reflected off the target back to the telescope, which channels it through a series of optics to a pair of horizontally separated detectors. Each detector is focused on a spot on the target that is inside the area where the laser beam strikes. As the air between telescope and target moves, wave patterns or fringes flow across the laser light, like the ripples of light seen on the bottom of a pool. By measuring the times at which each of the two detectors senses the movement of a single wave, a computer can establish the average velocity of the air crossing the laser beam.

"Even though air may be flowing erratically--some going one direction at one end of the beam and some going exactly the opposite direction--you can get net flow across the laser beam with this method," says David Roberts, a scientist at GTRI. Thus a single sensor indicates overall wind movement in a larger area than could be monitored by one anemometer.

In tests with the target 100 feet (30 meters) away from the telescope, the sensor correlated extremely well with anemometer readings at higher wind speeds. The sensor works at night as well as in the day; but, as expected, it does not perform well in rain or fog.

Its developers believe that the sensor may have uses in aviation and meteorology, particularly in areas where erratic winds are the norm. For further information, contact Mikhail Belen'kii (404-894-0140 or mikhail.belenkii@gtri.gatech.edu), Gary Gimmestad (404-894-3419 or gary.gimmestad@gtri.gatech.edu), or Roberts (404-894-3493 or david.roberts@gtri.gatech.edu).

University of Wisconsin-Madison
Ancient shells offer clues to past climate

Sonya Skoog of the University of Wisconsin-Madison has found that the ancient remains of ostracodes--microscopic clamlike animals related to today's shrimp--offer a new way to study past climates.

Using a collection of 57 sediment cores that were retrieved from the deep Arctic Ocean in the 1960s and 1970s, Skoog sorted, counted, and dated the ostracode remains. "The abundance of these animals can tell us something about climate changes," she says, "and if we see immigrant species from the Atlantic Ocean or the Pacific Ocean, we can begin to say something about ocean circulation."

Skoog has cataloged more than 20 different species of the ostracodes. "I don't think one particular species will tell us a lot about climate," she explains. "What will be important will be looking at when and how many species are coming in" to the Arctic Ocean.

Although her findings are preliminary, Skoog believes she has some definite clues to very ancient climates. "Between 1.5 and 2 million years ago," she says, "you see an increase in ostracodes, which is probably related to increased circulation in the Arctic Ocean. It reflects a change in ocean currents." Also, this was an interglacial period, with a climate similar to ours, which allowed the ostracode population to boom. Besides this surge, Skoog has also identified peak glacial episodes. At these times, "you expect to find low abundances and coarse sediment deposits, and that seems to hold up in our record."

For more information, contact Skoog (608-262-8960 or sskoog@geology.wisc.edu).

Do you have a Science Bit?

The UCAR Quarterly is looking for short science stories from UCAR member universities to publish in upcoming issues. Please contact Carol Rasmussen (303-497-8611 or carolr@ucar.edu) with suggestions.

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