Global Warnings

Scientists are on a quest to predict sudden shifts in ecosystems, before we reach the point of no return.

The Sahara is probably the most iconic desert on Earth, evoking visions of desiccated landscapes dominated by scraggly shrubs and naked soil.

But it wasn’t always that way. Just 6,000 years ago, the Sahara was lushly vegetated and nourished by frequent rainfall. Within the course of a few centuries (or a few decades, depending on the study), everything changed. The rains dried up, plants withered and the modern Sahara emerged.

For most of the 20th century, scientists had no idea how to explain this. Evidence of a radical shift—provided by fossils, sediment deposits and abandoned villages—was overwhelming. According to standard ecological theory, however, such a massive change shouldn’t have happened so quickly.

The most popular explanation was that slow and subtle shifts in Earth’s orbit changed solar radiation levels, causing a drop in rainfall patterns. But computer simulations of the shift failed to replicate the speed at which this had taken place.

Meanwhile, researchers were baffled by a more contemporary puzzle in the Sahel, a band of arid grasslands that cross Africa beneath the Sahara. Starting in 1969, the Sahel experienced a devastating drought, one that continues today. Whereas most other droughts around the world last for just a few months, this one has lasted for three decades, repeatedly reaching new lows in rainfall.

In the past, many scientists attributed this to changing sea surface temperatures and tropical circulation patterns. But changing temperatures alone couldn’t account for the drought’s unprecedented severity and duration.

It was only when ecologists developed new models describing interlocking relationships among land, vegetation, atmosphere and oceans that things started to make sense. Suddenly they could reproduce the Sahara’s rapid desertification and the Sahel’s record dry spell.

Out of this work—along with research on polluted ponds and damaged coral reefs—came a new theory of ecosystem change: Transition doesn’t need to be linear and gradual. Rather, it can take place rapidly and unpredictably. Ecosystems can exist in “alternative stable states,” with only a nudge needed to flip them from one to the other.

Today, scientists believe these so-called critical transitions can take place in many different ecosystems. But even if they happened only in arid and semi-arid lands, there would be plenty of reason to pay attention. Such lands cover some 40 percent of the world’s surface and support a billion people. As ever more water is diverted to grow crops and more animals are put out to pasture, the nudges are many and strong. And greenhouse gas pollution and climate change are only adding to the pressure.

“We don’t know where the thresholds are,” says Marten Scheffer, an ecologist at the Netherlands’ Wageningen University. “But we know they’re there and that we cross them.”

From Theory to Practice

Today, scientists are trying to figure out how to handle these critical transitions. One approach acknowledges the possibility of thresholds to guide land management strategies. If strategies needed to be calculated from scratch for each region, the task might be impossible. But a set of generic models, called typologies, might sketch the parameters of any arid system.

“We try to get lots of data from where thresholds have clearly been crossed, and understand where the threshold and the system were when that happened,” says Brian Walker, an ecologist at Australia’s Commonwealth Scientific and Industrial Research Organization. “Then you can say, ‘I’m in a semi-arid rangeland in Africa, and this is what I should be watching out for.’”

Many of the world’s semi-arid rangelands behave in much the same way, with their states determined by feedback loops among water, vegetation, soil and climate.

The findings of Walker and his colleagues are now used to guide grazing and fire prevention patterns at test sites in South Africa and Southeast Australia. Although those tests may prove successful, applying the approach elsewhere may prove tricky.

“When you get to the field, you have to deal with particular ecological patterns. You have to deal with the technology and methodology that’s available for collecting data. You have to deal with the reality of sampling, and figure out how you’re going to use those samples and analyze them,” explains Craig Allen, a research ecologist with the U.S. Geological Survey who’s studied transitions in the American southwest.

In addition to typologies, scientists need early warning signs: something to tell them if their plans aren’t working or if a system is in danger of tipping. Theoretically, this is possible. According to models and real-world testing, a system that’s approaching a critical transition should lose its equilibrium in mathematically verifiable ways.

When approaching a critical transition, a slightly unusual season can be followed by dramatic changes in plant cover. At one time, these changes would have reverted back to normal, but instead they linger. Lines on graphs that chart the system become jagged. The center no longer holds.

“Up until about five years ago, we thought that regime shifts were essentially unpredictable. They were like accidents waiting to happen, and would catastrophically come out of nowhere. There was no possibility of predicting them or dealing with them in advance,” says Steve Carpenter, a professor and early warning sign expert at the University of Wisconsin-Madison.

Carpenter says the availability of early warning indicators could provide enough advance notice to take action and prevent crossing the threshold. The best examples come from systems that have been studied for decades, where researchers can compare new readings against historical baselines. But such thorough datasets are the exception, not the rule.

“For many circumstances, the time needed to detect variance is too long,” says Walker. “You cross the threshold before you’ve detected it.”

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Ecosystems on the Edge

To many people, the term “tipping point” suggests The Day After Tomorrow-style ice storms, or Malcolm Gladwell’s examples of Hush Puppy sales and crime in New York City. But there are tipping points, or “critical transitions,” in ecosystems, too.

The dynamics of critical transitions are very different from those of traditional ecological theory, in which change is supposed to happen more or less gradually, in a straightforward and fairly predictable way. Sometimes that’s true, but not always.

Scientists first noticed these transitions in lakes and ponds, where nutrient pollution quickly turned clear waters that once supported a rich ecosystem into oxygen-starved, algae-dominated soup. Other critical transitions are visible in coral reefs (where overfishing can rearrange the food chain in ways that leave reefs vulnerable to disease), as well as on land in tundra, grasslands and jungles.

Many ecologists now think that critical transitions can be found almost anywhere. The science is still maturing, but certain basic patterns appear to be universal. Shifts follow a period in which parts of an ecosystem, such as water and nutrient availability, vegetation patterns or animal populations, are altered. On the surface, the system doesn’t seem to change, but what was formerly a stable arrangement becomes internally unbalanced.

New feedback loops—between, for example, a new plant species and local climate—kick in. Suddenly a relatively small impact, like a wildfire or a few seasons of heavy grazing, can produce long-lasting changes. The ecosystem seems to be drawn toward some other configuration.

When it arrives, this new configuration is as stable as the old one. And if the transition in question involves an ecosystem important to us, such as farmland that turns to desert, that one-way trip is bad news.

Scientists are now trying to flesh out their models with real-world details, applying numbers to concepts and mapping the boundaries between states.

This is hard work, says Marten Scheffer, a Wageningen University ecologist and regime shift pioneer. “Do we understand what’s happening? In theory, yes, but in practice, there’s a problem: We don’t know the signals very well yet,” he explains.

“If you see it too late, your system can shift in a way that’s difficult to recover.”