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Tipping the balance

William Laurance

20th June, 2008

Are they environmental doom-mongering, journalistic hype or the straw that breaks the camel's back? William Laurance examines the complexities of tipping points - those small changes in a natural system that can sometimes provoke sudden and irrevocable collapse

Tipping points abound in nature – you can hardly go anywhere without tripping over them. Nature is dynamic. Nature is often nonlinear. Nature is complex and interconnected. All of these features can create tipping points.

In many cases tipping points are trivial. Warm a single ice cube by just a tiny amount, from just below freezing (say, -0.01ºC) to just above it (0.1ºC) and presto! Your ice cube becomes a little puddle. In this context the melt is inconsequential, but imagine crossing the same tiny temperature threshold in Siberia, where some 11 million square kilometres of land – an area the size of France and Germany – are locked up in permafrost.

The Siberian permafrost contains hundreds of billions of tonnes of carbon, which has slowly accumulated as frozen peat during many millennia. The peat hasn’t decomposed because there is no freestanding water, which is crucial for soil microbes.

When the permafrost melts, however, the microbes leap into action, consuming the peat and emitting as waste products methane and carbon dioxide – both potent greenhouse gases. Suddenly, many billions of tonnes of carbon emissions flood into the atmosphere. Global temperatures shoot up, causing even more permafrost to melt and spew out even more carbon emissions. All we did was nudge the temperature up a tiny fraction of a degree – but we’ve created a runaway monster.

Tipping points can arise in at least three situations. The first is a runaway chain reaction. These usually happen when elements in a system are tightly coupled to one another. Tip over that first domino and a thousand more may quickly fall in sequence. Compress enriched plutonium and you provoke nuclear fission; as each plutonium atom splits in two it shoots out a bullet-like proton that then splits another atom, which in turn fires off another atom-splitting proton, and so forth. If the reaction escalates quickly enough, you get a nuclear explosion.

In the natural world, epidemics are like runaway chain reactions. A sick person infects a few other people, each of whom infects a few others – and suddenly you have a global pandemic. This is especially so with our ultra-mobile modern society, where no pathogen on the planet is ever more than a plane ride away. New pathogens cause deadly epidemics because many people in the population lack immunity and therefore are similarly vulnerable – in effect their fates are tightly coupled, like dominoes ready to fall.

For humans, major epidemics are scary and could potentially kill millions, but epidemics can be even more devastating for wildlife, such as the hundreds of different amphibian species now being driven to extinction by a mysterious chytrid fungus. According to a prevailing theory, this virulent pathogen, which decimates healthy frog populations in just days, might have originated in Africa and humans could be inadvertently transporting its spores around the world.

A second, related cause of tipping points is an abrupt threshold. This is illustrated in the US by the disaster of Florida Bay, the triangular stretch of water enclosed by the south coast of the mainland and the Florida Keys. In the early 1990s it hit a tipping point, changing abruptly from a clear-water system that supported sea grasses and manatees to a murky dead zone overwhelmed by plankton blooms. Nobody saw this coming because nothing seemed to be changing. The bay received a steady stream of pollution from septic systems, but one day hit a critical threshold that turned everything topsy-turvy.

Oceans, too, have scary thresholds. As global temperatures rise, some scientists fear a sudden collapse of the Atlantic thermohaline circulation, a current that brings warm surface water to northern Europe and returns cold, deep-ocean water south. This climatic conveyor belt seems to be slowing (evidently because it is being diluted by freshwater from melting Arctic and Greenland ice). If it collapses altogether it could lock northern Europe into a deep freeze while destabilising the north-Atlantic climate. The last time this conveyor belt failed, some 8,200 years ago, the land temperature of Greenland fell by more than 5°C.

Still another threshold involves the acidity of the oceans. As carbon dioxide accumulates in the atmosphere, some of it dissolves in the oceans, elevating acidity and changing water chemistry. Ocean acidity has already risen by more than 25 per cent above pre-industrial levels. Greater acidity makes it harder for many marine organisms, such as corals, sea urchins, starfish and winged snails, to form their crucial skeletons or shells. In just decades, some scientists believe, the oceans will hit an acidity threshold that will kill many cold-water species, with acidic conditions spreading soon afterward into warmer waters.

The final cause of tipping points is positive feedbacks. These self-reinforcing feedbacks are reminiscent of a Russian slapping contest, where the two opponents take turns smacking each other, always hitting their foe a little harder than they were slapped. Self-amplifying situations such as this can quickly spiral out of control.

Positive, negative

The Amazon rainforest appears vulnerable to an alarming positive feedback. The rainforest generates much of its own rainfall – which is crucial for the forest’s survival – because the dense vegetation quickly recycles moisture and returns it to the atmosphere. As deforestation proceeds, however, less water vapour is recycled, so clouds and rainfall decline. As the forest dries out, wildfires increase and destroy yet more forest, further depressing rainfall. No one knows how far the Amazon can be pushed before it collapses in a rage of droughts and forest fires.

Tropical forests might be susceptible to another kind of positive feedback, according to a prominent but controversial theory. Long-term research in Costa Rica suggests that plant growth in undisturbed forests declines in warmer years. This could be because the metabolism of plants rises with higher temperature, much in the same way as cold-blooded lizards speed up in warm weather.

To increase its metabolism, a plant has to burn more energy, and so less energy is available for growth. Hence, as the Earth heats up, rainforest plants may grow ever more slowly. If these observations are correct, rainforests will actually shrink in the future, progressively becoming dominated by smaller, slower-growing trees as global temperatures rise.

Why is this important? Tropical forests store vast quantities of carbon in their dense vegetation – more than any other forest type on earth. If the rainforests are literally shrinking, they could emit billions of tons of carbon into the atmosphere. This, in turn, would accelerate global warming, which would make rainforests shrink even faster – a positive feedback. In a hotter world, even protecting a rainforest within a national park might not ensure that its carbon is safely locked away – though this is surely better than no park at all, because unprotected rainforests are often razed to the ground.

Into the unknown

As we’ve seen, at least three situations in nature can create tipping points. First, uncontrolled chain reactions can be unleashed when the elements in a system are tightly linked, like a line of falling dominoes. Second, nonlinearities can generate abrupt thresholds – the proverbial straw that breaks the camel’s back. Finally, runaway positive feedbacks, like a pair of slap-happy Russians, can easily ensue when two or more phenomena amplify each other.

But perhaps the scariest thing about tipping points is that they are devilishly hard to predict. Donald Rumsfeld, the former US Secretary of Defence, must have had tipping points in mind when he famously distinguished between ‘known unknowns’ and ‘unknown unknowns’. Known unknowns are like the impacts of global warming on cold-adapted species: we know they’re happening, even if we can’t predict their ultimate effects. Many tipping points are unknown unknowns, however; they can come at us completely out of the blue, with no warning at all.

Because of their inherent unpredictability, tipping points are a daunting challenge for decision-makers. The influential Intergovernmental Panel on Climate Change (IPCC) has downplayed the influence of some tipping points, such as the possibility that the Arctic and Greenland ice sheets might quickly collapse, or that global warming will magnify El Niños and thereby increase droughts in many parts of the world.

Some leading researchers, however, such as Tim Lenton of the University of East Anglia and his colleagues, have challenged this view, arguing that several climatic tipping points are more likely than suggested by the IPCC. Likewise, a respected team headed by Peter Cox of the British Met Office in Hadley has asserted that the Amazon is more vulnerable to future droughts than the IPCC proposes. Much blood is being spilled on the floor as IPCC scientists and governmental representatives debate how to deal with tipping points in their technical reports. But because they are subjected to multiple reviews, in which controversial issues are mercilessly axed by dissenting nations or scientists, IPCC reports are inherently conservative, especially when it comes to tipping points.

Risk-management experts and insurers are also struggling with tipping points, given that destructive storms, floods and crop-killing droughts might abruptly increase in the future. They are further worried because many different phenomena, which affect and are affected by the environment, can also have tipping points.

For example, one reason stock markets are so volatile is that stock traders have a herd mentality – buying and selling in unison. In effect, they’re like nervous wildebeest, ready to panic and stampede when bad news arrives. Under these conditions, even small disturbances, such as a spike in energy or food prices, can sometimes generate major economic shocks that reverberate across the planet. In an effort better to predict such rare, shattering events, some risk-management experts are even exploring new fields of mathematics – but the challenge is daunting.

Expect the unexpected

Despite our best efforts, there is only one real certainty with tipping points: expect the unexpected. Nevertheless, all is not hopeless. While it can be fiendishly difficult to predict specific tipping points, we now understand some key factors that make them more likely.

One key risk factor is increasing globalisation, which creates complicated economic and environmental linkages across the planet. Rising demand for furniture in the US and Europe is driving an expansion of Chinese furniture manufacturing, for example, which in turn is causing a spike in illegal logging in developing nations in Asia and Africa that export their logs to China. In a complex, interconnected world, yanking on a string in one location can cause painful jolts in far-flung and unpredictable places.

Globalisation is risky for another reason. It promotes centralisation and eliminates ‘redundancy’, because those able to produce a product or service most cheaply tend to eliminate less-efficient producers. While this has certain economic benefits, it also magnifies the chances a small disturbance will quickly snowball into a bigger crisis. In 2003, for instance, North America and Europe were slammed by major power blackouts that arose when small nodes in the electrical grids failed. Had their electrical systems been less centralised with more redundancy, they would have been far more resilient in the face of small, local disturbances.

Tipping points also become far more likely when a system is stressed, and the Earth today is a poster-child for stress. With our burgeoning population and escalating demands for food, energy and goods, and with seismic changes in our increasingly globalised economy, we are stressing our planet in countless alarming ways.

Environmental and economic shocks are not just a possibility in the next few decades – they are practically guaranteed.

Though hard to predict, new catastrophes are certainly coming, so we must plan and save accordingly. If we wish to survive future tipping points we need to prepare seriously for them, by keeping some of our resources in reserve. We also need to reign in globalisation, especially with the environment potentially at risk. A far broader public debate is needed on globalisation, because so few economists are trained to understand its real perils. Finally, we must take our foot off the economic accelerator, to reduce the intense and varied stresses on our global environment.

With tipping points we really are in fearsome, uncharted waters. Let’s be careful that we don’t sink.

William Laurance is a senior scientist at the Smithsonian Tropical Research Institute in Panama, and former president of the Association for Tropical Biology and Conservation. He is the author of several books as well as nearly 300 scientific articles

This article first appeared in the Ecologist August 2008


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