Cast an eye over the many forests' worth of scientific literature on global warming and it quickly becomes clear that working out what a temperature rise of a few degrees will mean for life anywhere on the planet is far from straightforward. Vast ice sheets may melt, sea levels will rise, and faced with a new climate, species must adapt, move or perish. Yet the precise details of how any of it will happen are, frankly, unknown.
Now it seems the future has become even more uncertain. Climate scientists say they have identified a dozen weak links around the world, regions where global warming could bring about the sudden, catastrophic collapse of vital ecosystems. The consequences will be felt far and wide. An abrupt halt in one ocean current could devastate Antarctic fish stocks, while disruption to another could make temperatures in Britain and elsewhere plunge. When rains return to the Sahara, disease and crop damage from pests could soar. Meanwhile, a drier Amazon will trigger huge dieback of the forests, threatening many species with extinction. Losing the forests will itself exacerbate global warming.
Earlier this week, scientists reported that we may have less time to combat global warming than we realized. Measurements of carbon dioxide, a greenhouse gas, taken from the Mauna Loa Observatory, 12,000 feet up a mountain in Hawaii, suggest atmospheric carbon dioxide levels have risen sharply and inexplicably in the past two years, prompting fears of runaway global warming. Though it is too early to confirm that it is a definite upward trend, the results came as an unwelcome surprise to climatologists.
Over the span of the coming century, even the most extreme global warming scenario predicted by the Intergovernmental Panel on Climate Change -- an increase of 5.8 degrees Celsius -- seems gentle. Surely civilization will have enough time to protect itself against the consequences, while ecosystems could gradually adapt? Not so, say scientists studying the world's weakest links.
John Schellnhuber, research director at the Tyndall Center for Climate Change Research in Norwich, U.K., played a key role in identifying the dozen systems where global warming could produce sudden and dramatic environmental damage. He calls them the "tipping points," the Achilles' heels of the planet. At a conference earlier this year, Schellnhuber and other scientists called for a concerted, global effort to investigate the Earth's known tipping points and to search for new ones. Only then, he says, will we be able to identify where the consequences will be felt first.
"It'll take a global effort to understand these, and we have to make sure that none are activated through human actions," he says. Here, we present a list of the tipping points and what might happen if they are triggered.
Sahara desert
Occupying some 3.5 million square miles of northern Africa, the Sahara Desert is expected to shrink with global warming as more plentiful rain brings a flourish of vegetation to its southernmost reaches. For those on the edge of the desert, the fertile land will undoubtedly be a boon, but the Sahara plays a broader role in the health of the planet. The dry dust that is whipped up from the desert by strong prevailing winds contains crucial nutrients that seed the Atlantic and may even help fertilize the Amazon.
As the Sahara turns from brown to green, the flux of nutrients into the ocean is expected to drop, restricting food available for plankton, the smallest of links in the marine food chain. As the number of plankton falls, so does food for aquatic creatures further up the food chain.
That's not the only knock-on effect. Plankton lock up the greenhouse gas carbon dioxide from the atmosphere, and so help counter global warming. With fewer plankton, the oceans will take less of the gas from the Earth's atmosphere.
Dust from the Sahara has other, more subtle influences. When blown out over the Atlantic, clouds of Saharan dust act to stabilize the atmosphere, suppressing the formation of hurricanes. A greener Sahara could mean more frequent, or more severe, hurricanes slamming into the Caribbean, parts of Central and South America and the Southeastern U.S. Meanwhile, the now wetter Saharan regions of Sudan, Morocco and Algeria could become more prone to infestations of locusts, such as the swarms that have devastated crops in the region this year.
Amazon forest
The size of western Europe, the Amazon forest is one of the most biodiverse regions on Earth. Models suggest that with global warming will come a drop in Amazonian rainfall, leading to the gradual death of the forest and subsequent collapse of the myriad ecosystems it supports.
The extinction of species is only one consequence of a warmer planet. As the trees die off, they will fall and rot, releasing carbon dioxide. In the worst-case scenario, the quantities of carbon dioxide emitted could be of the same order of magnitude as from the 20th century's total fossil fuel output.
"It's the biggest biodiversity pool on Earth, and if it were lost, it would be an incredible loss for our nature capital. This is not just a fantasy. There's clearly a vulnerability here," says Schellnhuber.
Greenland ice sheet
The Greenland ice sheet holds about 2.6 cubic kilometers of fresh water, which is some 6 percent of the planet's supply. It is imperative that this water remain frozen. If global warming sees temperatures rise by more than about 3 degrees Celsius, Greenland is likely to begin a slow thaw, steadily releasing all that water -- currently resting on land -- into the north Atlantic Ocean.
Climate models suggest that a more drastic temperature increase of some 8 degrees Celsius could see the Greenland ice sheet disappear almost entirely, a thaw that would see the seven seas rise by seven meters. Such a dramatic rise in sea level would cause flooding that is bound to have a devastating impact on people living on unprotected shorelines around the globe.
And the drowned land wouldn't reappear for some time. "If the Greenland ice sheet goes, it probably will not come back for the next 60,000 years," says Schellnhuber.
North Atlantic current
The North Atlantic current is one of the strongest ocean currents in the world, of which the Gulf Stream is the precursor. It works like a conveyer belt. Surface water in the North Atlantic is first cooled by westerly winds from North America, making the water more dense and salty so it sinks to the ocean floor before moving toward the equator. Driven by winds and replacing the cold water moving south, warm water from the Gulf of Mexico moves upward into the Atlantic. The effect of the current on climate is dramatic. It brings to Europe the equivalent of 100,000 large power stations' worth of free heating, propping up temperatures by in excess of 10 degrees Celsius in some parts.
Global warming could change all that, though not very quickly. Computer models predict that as global warming increases, so will rainfall in the North Atlantic. Gradually, the heavier rains will dilute the sea water and make it less likely to sink, a process that could bring the whole conveyer to a gradual halt. "It won't happen in a matter of weeks, like in the movie 'The Day After Tomorrow,' but it could happen over a few decades," says Schellnhuber.
In the past 50,000 years, the current has stopped at least seven times. Collapse of the North Atlantic current would hit Iceland, Scotland and Norway most, where temperatures could drop 10 degrees Celsius or more. If it happened soon, we would notice a difference. But in 100 years, when global temperatures may be a few degrees higher anyway, temperatures may simply revert back to today's levels. Total shutdown of the current would lead to a rapid regional sea level rise of around one meter along the U.K. coast as the ocean adjusts to the change.
Methane clathrates
Deep within the Siberian permafrost and ocean floor sediments lie vast deposits of gas-filled ice called methane clathrates. At Siberian temperatures, or under the weight of icy oceans, the clathrates are stable. But as global warming takes effect, the icy crystals that clutch the gas could rupture, releasing it into the oceans and atmosphere. According to the U.S. Geological Survey, some 10 to 11 trillion tons of carbon are locked up in clathrates in ocean floor deposits, the equivalent of 20 times the known reserves of natural gas. Some scientists believe sudden releases of methane from clathrates caused a severe environmental impact in the past.
If released into the atmosphere, methane from clathrates could exacerbate global warming. Some estimates suggest that since methane is such a strong greenhouse gas, a significant release could increase global warming by up to 25 percent. More likely, some scientists say, is that released methane would poison the oceans or oxidize and dissolve as carbon dioxide as it rises, which would still be toxic to some species.
The monsoon
During March and April, the Indian subcontinent begins to heat up, reaching some of the highest surface temperatures of the year by May. The hot land produces a sharp temperature gradient between the land and sea, which causes an abrupt reversal of the winds from seaward to landward.
As the winds strike the Himalayas and are deflected upward, they create a low pressure system, forcing rain clouds to release their stores of water. While the monsoon season can cause incredible flood damage, local populations are largely adapted and to some extent reliant on the weather.
If global warming has the expected effect of heating India even more, the monsoon season could become far more severe. What happens will be influenced by the level of pollution in the region. Sulfur dioxide and even dust make rain droplets smaller and so diminish overall rainfall. These substances also increase the reflectivity of clouds, which prevents the ground from heating up so much. Both of these factors would weaken the monsoon, causing havoc for Indian agriculture, with serious consequences for food production.
Ozone hole
Decades after many countries introduced a ban on CFCs, the danger of the ozone hole causing a nasty surprise remains very real. "In a sense, this is the mother of all tipping points, and it's one that has been activated already," says Schellnhuber.
Scientists are now generally agreed that global warming may drastically amplify the power of ozone-destroying chemicals, which linger in the stratosphere for decades. At high altitude, ozone acts as a shield against the sun's damaging radiation. Global warming, while heating the lower atmosphere, can lead to cooling in the stratosphere where the ozone layer forms. Cooling this band of air has a complex knock-on effect, disrupting a chemical process that prevents ozone from breaking down. The result is a loss of ozone as the world warms up.
Though the ozone hole is often associated with Antarctica and Australia, ozone loss due to global warming could see a hole appear over parts of Europe. "If it were to stretch beyond Antarctica, it would increase, in an intolerable way, the risk of skin cancer and blindness," says Schellnhuber.
Tibetan plateau
Spanning one-quarter of China's entire landmass lies the Tibetan plateau. Because the region is permanently under snow and ice, it behaves like a giant mirror, reflecting the sun's rays back into space. The effect is to keep a lid on global warming, at least locally, as the darker soils are unable to bask in the sun's radiation and increase in temperature.
In a warmer world, the white of the Tibetan plateau will slowly turn to brown and gray as the snow retreats to reveal the ground beneath. As the ground warms, melting will accelerate. Tibet will become a much warmer place.
Salinity valves
In some parts of the world, local geography conspires to pinch the waters between adjacent seas into separate bodies of water. If one is saltier than the other, a flux of salt, nutrients and oxygen can be set up across the gap, producing what scientists refer to as a salinity valve. Arguably the most significant salinity valve is the Strait of Gibraltar, acting as a pinch between the Mediterranean and the north Atlantic Ocean. The gradients across the valves give rise to unique ecosystems that are highly adapted to local conditions.
As global warming is expected to disrupt ocean currents, by warming the seas and diluting the surface waters that drive other water circulations, marine life around salinity valves could in turn face major disruption. "Everything is in a balance now; all the ecosystems have adapted to a certain salinity," says Schellnhuber. If conditions around salinity valves change rapidly, those ecosystems might not be able to adapt quickly enough to survive. "The Mediterranean is very fragile already. It could have an extremely negative effect for several decades," adds Schellnhuber.
El Niño The disruption caused by El Niño is well known, from droughts in Asia and Australia to flooding in regions such as Ecuador and northern Peru. The Peruvians gave El Niño its name, derived from the Spanish for "the boy child." El Niqo was originally used to describe a warm ocean current that arrived around Christmas time.
Nowadays, the term refers to a general warming of the central and Asian Pacific, which causes a major shift in weather pattern and in particular responds sensitively to changes at the western boundary of the Pacific. El Niños are already somewhat erratic, occurring every two to seven years, but some models suggest global warming could make El Niño not just more severe but more frequent.
The impact on agriculture and so food production could be serious. Indonesia, the Philippines, Southeast Asia and eastern Australia could face damaging droughts while the heavy rains and flooding cause problems for the northwestern regions of South America.
West Antarctic ice sheet
The giant West Antarctic ice sheet is not about to melt anytime soon -- the ice is up to a kilometer thick -- but two years ago a vast chunk, the Larsen B ice shelf, broke off the eastern side of the Antarctic peninsula and fragmented into icebergs. In just 35 days, about 3,250 square kilometers of ice was lost; the size of the entire shelf is now roughly 40 percent of the size at which it had previously stabilized.
Some predictions suggest that the rest of the sheet could feel the force of global warming quickly. Should the entire sheet melt, it is estimated the sea level rise around the world would top six meters. Once more, coastal regions would be under threat.
The Atlantic circumpolar current
Some scientists believe the Atlantic circumpolar current to be the most significant on the planet. It swirls some 140 million cubic meters of water around Antarctica every second, mixing water from the Pacific, Atlantic and Indian oceans as it goes. The current taps into another circulation that sees cold surface water sink while warmer water rises, bringing with it vital nutrients from dead plankton and other marine life on the ocean floor.
Global warming is expected to produce more rainfall over the poles, which could slow the rise of nutrients for dispersal by the Atlantic circumpolar current. "For marine life, any change in the currents is extremely important," says Schellnhuber.
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