Global Warnings, continued
A Critical Timesaver
Given that detecting thresholds could take decades, researchers are looking for a shortcut—namely, indicators that can be applied to any arid region and require the ecological equivalent of a thermometer under the tongue.
The most promising of these is changing vegetation patterns. The beginning of a grassland’s transition to desert is marked by localized outbreaks of relatively sparse shrubs. Where soil once held by the grass’ roots had acted like a sponge, water no longer penetrates. Wind blows faster over bare ground, piling eroding earth at the base of shrubs, which require more of the system’s water.
As the shrubs spread, the desertification accelerates. Patches of grass shrink and become even more vulnerable to local change. They suffer “micro-extinction events.” Bare soil between bushes is dark, where the grassland soil had been beige. Ground-level temperatures rise, making water evaporate even faster. Soil is blown into the air. If enough ultra-fine dust particles enter the local atmosphere, they disrupt rain formation, which requires water molecules clustering around particles of larger size.
Scientists believe this phenomenon is responsible for the unusually long Sahel drought.
According to studies in the Sahel, Africa’s Kalahari Desert and rangelands around the Mediterranean, all this produces telltale patterns of vegetation patch size and shape. In a healthy system, these can be plotted on an orderly curve. As the likelihood of transition increases, data points fall off the line. Aberrations can be seen by the naked eye.
“The spatial indicators we’re working at are the only class that requires a snapshot in time, meaning that spatial patterns suffice to know that one is approaching a regime shift. That’s what makes it so appealing,” says Max Rietkerk, a geoscientist at the Netherlands’ Utrecht University.
The most advanced test of spatial indicators is taking place in the U.S. southwest, where scientists with the Long Term Ecological Research Network are studying the boundary between Great Plains grasses and shrub-dominated desert. So far, the results are mixed: Aerial imagery has failed to match patterns to desertification, but this might be a shortcoming of low-resolution images rather than the approach itself.
Some LTER researchers are taking a similar approach but on a smaller scale, comparing hand-measured variation in hectare-sized plots. “If we start to see bare patches forming, and they get big and we see erosion out of those patches, that’s a good indicator that we’re losing resilience,” says Brandon Bestelmeyer, a research ecologist with the U.S. Department of Agriculture. “We don’t know how close we are, but we’re closer than before.”
Researchers have tentatively observed similar spatial patterns in other ecosystems, from bogs to tundra and coral reefs. It’s not certain yet whether they indicate critical transition as clearly as they appear to in grasslands. But it’s at least possible that the patterns could someday serve as a universal indicator of tipping points.
“I hope there are universal early indicators. If we have to figure this out for every system, then we’re up a creek without a paddle,” says Aaron Ellison, a forest ecologist at Harvard University. “If we have to spend 30 years on a system that we want to manage in some way, they’ll all be gone before we have a chance.”
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BRANDON KEIM is a Brooklyn, N.Y.-based freelance journalist specializing in science, technology and culture. He’s a frequent contributor to Wired.com’s award-winning Wired Science blog. His work has also appeared in the The Christian Science Monitor, USA Today, Seed and many other publications.
The Mongolia Experiment
According to scientists, the Mongolian grasslands can handle the grazing pressure of 18 million cattle. According to the Mongolian government, the correct number is 100 million.
In that range of numbers may hang the fate of Mongolia’s plains, which directly support more than one-third of the nation’s 2.4 million people. Communism eroded the cultural rules that once guided land use; its decline unleashed a rangeland free-for-all.
“They’ve seen degradation. What they need to do is quantify its nature, where it’s happening, and at what point it becomes irreversible,” says USDA ecologist Brandon Bestelmeyer.
He’s helped set up a network of more than 1,000 grassland monitoring stations and trained people to make some of the same on-the-ground measurements currently used in New Mexico.
Best case scenario, Mongolian farmers and the government will use that data to safely manage their herds. Bestelmeyer hopes the system will be ready in five years.
“In the United States, we’re in maintenance and recovery mode. Over there, they’re at the precipice,” he says. “They’re looking over the cliff, and we don’t know what’s going on.”
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Last modified on January 23, 2012