by Alok Jha
The volume of water in the sea can go up for two reasons: water expands as it gets warmer; or extra water enters the sea because ice sheets melt. Both these things have been happening for the past century, partly due to natural cycles, but also driven by man-made global warming, as we burn fossil fuels and throw greenhouse gases into the atmosphere.
“Measurements from tide gauge stations around the world show that the global sea level has risen by almost 20 cm since 1880,” says Stefan Rahmstorf, a climate scientist and oceanographer at the Potsdam Institute for Climate Impact Research. “Since 1993, global sea level has been measured accurately from satellites; since 1993 figures have shown levels rising at a rate of 3.2 cm per decade.”
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GLOBAL SEA RISE
20 cm across the globe since 1880
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One of the biggest ice sheets in the world sits near the Arctic Circle, on the land mass of Greenland. The land here was not always covered—around 60 million years ago, the Earth was far warmer than today, and Greenland was a grassy tundra populated by ancient mammals. Today’s ice sheet, which is three kilometers (almost two miles) thick in some parts, was formed in the last ice age, around 20,000 years ago, and the reason it is still there is because average temperatures between then and now have not risen high enough to melt it. Any ice that has melted into the surrounding sea has been replaced by regular snowfall in that time.
But Greenland’s equilibrium might now be on the rocks. Under the worst climate scenarios predicted for this century, the average temperature around Greenland might increase by 8°C by 2100. If all the ice there melted into the sea, global sea levels could rise by around seven meters.
Even if the average temperature in Greenland increased by 3°C, its ice sheet would largely disappear, though it would take a long time—around 1,000 years—for it to melt completely. In 2004, Jonathan Gregory of the University of Reading showed that by 2100, concentrations of greenhouse gases would probably have reached levels sufficient to raise the temperature past this warming threshold.
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A sea-level rise of this order would decimate the world’s big cities and much of their populations along with it.
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At the other end of the world is an even bigger continent of ice—Antarctica. Overall it is more stable than Greenland, but if just a small part of the west Antarctic ice sheet were to melt (which is not an unreasonable possibility, given its various major collapses during the past decade), the world’s sea levels would jump by up to five meters.
Of course, if temperatures keep going up, ever more ice will fall into the sea and the water will continue to rise. If all the ice in the world melted, from mountain ranges, Greenland and Antarctica, the result would be catastrophic. “The land-based ice sheets of Greenland and Antarctica hold enough water to raise global sea level by more than 200 feet,” says Robin Bell, an expert on Antarctica at Columbia University’s Lamont-Doherty Earth Observatory. A sea-level rise of this order would decimate the world’s big cities and much of their populations along with it.
Which areas are at risk from higher water?
Any low-lying countries and anyone living by the coast is at risk from a global rise in sea levels. Major cities including London and New Orleans already need protection against storm surges, and would need even better defenses if sea levels rose. A onemetre rise would swamp cities on the eastern seaboard of the US. A six-meter rise would drown most of Florida.
Places such as the Maldives, Tuvalu (where the highest point is currently four meters above sea level) and scores of Pacific islands would be subsumed entirely with sea-level rises of not much more than a few meters.
In his economic analysis of the impacts of climate change, Nicholas Stern of the London School of Economics calculated that 200 million people live within one meter above the present sea level. This includes eight out of ten of the world’s largest cities, and all the megacities of the developing world.
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200 million people live within one meter above the present sea level. This includes eight out of ten of the world’s largest cities.
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“Miami tops the list of most endangered cities in the world, as measured by the value of property that would be threatened by a three-foot rise,” say Rob Young, a geoscientist at Western Carolina University, and Orrin Pilkey of Duke University. “This would flood all of Miami Beach and leave downtown Miami sitting as an island of water, disconnected from the rest of Florida.” Other threatened US cities include New York, Newark, New Orleans, Boston, Washington, Philadelphia and San Francisco. Outside North America, major risks of flood and water rise are faced by Osaka, Kobe, Tokyo, Rotterdam, Amsterdam and Nagoya.
Vivien Gornitz of Columbia University’s Center for Climate Systems Research worked out that rising oceans would eat away almost 2,400 km (1,500 miles) of shoreline in and around New York, which is home to around 20 million people. “Sea level has already climbed around 27 cm in New York City and 38.5 cm along the New Jersey coast during the 20th century,” she says. “These local rates exceed the global average of 10–25 cm per century because the East Coast is slowly sinking, as the earth’s crust continues to readjust to the removal of the ice from the last glaciation, around 15,000 years ago. But present rates of sea level rise could accelerate several fold, as mountain and polar glaciers melt and upper ocean layers heat up and expand, due to global warming.”
Rising seas also increase the chances of storm surges and dangerous floods. The severity of a flood is often defined in terms of the probability of its occurring once every 100 years. At the beginning of the 21st century, the height of a 100-year flood around New York was just under three meters (ten feet), an event that would devastate the city and its surrounding area. According to Gornitz, if the seas rose, smaller surges could produce 100-year floods. By the 2080s, given present rates of sea-level rise, the likelihood of such an event would be once in 50 years. It would rise to once every four years in the very worst-case scenario.
In eastern Asia, 18 million people live in the Vietnamese portion of the Mekong Delta, which accounts for around a fifth of Vietnam’s population and more than 40 percent of its cultivated land surface. They grow half the country’s rice, produce 60 percent of the fish and seafood and harvest 80 percent of the fruit crop. A onemetre sea-level rise would displace 7 million people, and a two-meter rise would displace twice that number. Not to mention the devastating impact of increased floods, beyond anything the locals are used to. The government, mindful of the coming changes, is already moving people away from some parts of the Mekong River’s main branch in An Giang province.
Is it likely?
Back in 2007, the Intergovernmental Panel on Climate Change (IPCC) issued its fourth report on the impacts of global warming, in which it suggested that by 2100, seas might rise by between 180 and 590 mm. Subsequent research, which took into account more advanced models of how ice sheets move and melt, suggested that the end-of-century rise might well be twice the IPCC’s estimate, with a likely limit of around two meters.
In the 20th century, sea-level rise was primarily due to the expansion of ocean water as it warmed up. Contributions from melting mountain glaciers and the large ice sheets were minor components. “But most climate scientists now believe that the main drivers of sea level rise in the 21st century will be the melting of the West Antarctic Ice Sheet (a potential of a 16-foot rise if the entire sheet melts) and the Greenland Ice Sheet (a potential rise of 20 feet if the entire ice cap melts),” say Young and Pilkey. “The nature of the melting is non-linear and is difficult to predict.”
There is an added complication, revealed by scientists in recent years. Underneath ice sheets there are complex systems of rivers, lakes and meltwater, all of which can enable the flow of vast chunks of ice toward the ocean. “For millennia, the outgoing discharge of ice has been balanced by incoming snowfall,” says Robin Bell. “But when warming air or surface meltwater further gre
ases the flow or removes its natural impediments, huge quantities of ice lurch seaward. Models of potential sea-level rise from climate change have ignored the effects of subglacial water and the vast streams of ice on the [overall] flow of ice entering the sea.”
It is fair to say that the global sea level is rising much faster than expected. “A commission of 20 international experts, called on by the Dutch government to help plan its coastal defences, has recently given a high-end estimate of 55 cm to 110 cm by 2100,” says Stefan Rahmstorf. “Equally important, this commission has highlighted the fact that sea level rise will not stop in the year 2100. By 2200, they estimate a rise of 1.5 to 3.5 m unless we stop the warming. This would spell the end of many of our coastal cities.” And that’s just the scenario the scientists can predict.
The Gulf Stream Shuts Down
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Our planet’s climate might seem slow-moving. But the records show that it has a disturbing ability to make abrupt shifts from one climate to another in record time. What happens to life if the world around it changes overnight?
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Climate change is usually understood to mean an overall gradual rise in temperature around the world. Often the term is used interchangeably with global warming. In any case, its implications are clear: environmental change, pressure on resources and a gradual transition to a warmer world.
But this description masks some of the more horrifying potential effects. Evidence gathered from ancient episodes of climate change on Earth reveals that our planet has gone from warm to cold in a matter of just a few years, rather than the centuries scientists usually expect for climactic changes.
The disaster scenario that keeps scientists awake at night concerns a huge current of water that flows around our oceans, carrying energy from one part of the Earth to another. The thermohaline circulation is indescribably crucial in keeping temperature and energy balanced across the globe and making the northern latitudes, including Europe, more habitable. Unfortunately, it seems as though it would not take much to knock this conveyor belt of water out of kilter and ruin the lives of billions of people.
If the thermohaline circulation were to shut down, the heat-bearing Gulf Stream would stop, and winters in the North Atlantic, Europe and North America would become twice as severe as anything on record, with average temperatures dropping by up to 5°C, moisture in soil falling and winds becoming more intense. Since these are areas that provide a significant fraction of the world’s food, our ability to support the global human population would plummet.
“Fossil evidence clearly demonstrates that Earth’s climate can shift gears within a decade, establishing new and different patterns that can persist for decades to centuries,” says Robert B. Gagosian, president of the Consortium for Ocean Leadership and former director of the Woods Hole Oceanographic Institution in Massachusetts. “In addition, these climate shifts do not necessarily have universal, global effects. They can generate a counterintuitive scenario: Even as the Earth as a whole continues to warm gradually, large regions may experience a precipitous and disruptive shift into colder climates.”
Geological records confirm that an abrupt thermohaline-induced climate change would generate severe winters in the North Atlantic. A few bad winters might be inconvenient, but we are able to deal with them. A persistent series of them over decades or even a century, however, could cover countries in ice, make rivers freeze, and cause sea ice to grow and spread. The Western world as we know it, the center of so much commerce, agriculture and political power, would be uninhabitable.
What is the thermohaline circulation?
The Sun shines on to the Earth at different intensities at different latitudes. The equator gets more sunshine than the poles, and global ocean currents move the energy around the Earth. One result of the equatorial sunshine is that the warmer water here evaporates more, leaving behind a saltier sea. The thermohaline circulation then moves huge volumes of warm, salty water from the tropics up the east coast of the US and on to Europe.
The heat-bearing Gulf Stream would stop, and winters in the North Atlantic, Europe and North America would become twice as severe as anything on record.
“This oceanic heat pump is an important mechanism for reducing equator-to-pole temperature differences,” says Gagosian. “It moderates Earth’s climate, particularly in the North Atlantic region. Conveyor circulation increases the northward transport of warmer waters in the Gulf Stream by about 50 percent. At colder northern latitudes, the ocean releases this heat to the atmosphere—especially in winter when the atmosphere is colder than the ocean and ocean-atmosphere temperature gradients increase. The conveyor warms North Atlantic regions by as much as 5°C and significantly tempers average winter temperatures.”
Around the waters of the northern Atlantic—the Labrador, Irminger and Greenland Seas—the thermohaline circulation helps to release large amounts of heat into the atmosphere. After it passes through these seas, the cold winds around Iceland cool the water, which sinks and moves south, eventually flowing around the Antarctic. As the water sinks in the North Atlantic, salty tropical surface waters are drawn northwards to replace it.
It is a truly huge movement of water and energy. But records contained in ice cores and sediments show that this conveyor belt has not run steadily all the time. “Variations in the conditions governing the density of high-latitude surface waters can lead to abrupt reorganizations of the ocean’s circulation. The surprise revealed to us by the climatic record is the extent, rapidity, and magnitude of these atmospheric changes,” wrote Wallace Broecker, a climatologist at the Lamont-Doherty Earth Observatory of Columbia University, in a 1997 paper in Science.
How could it be shut down?
The thermohaline circulation operates because of temperature and salt differences in the world’s oceans, which act like natural pumps to move the water from one place to another. Disrupt any part of that, and the overall conveyor belt will be adversely affected.
“Salty water is denser than fresh water. Cold water is denser than warm water. When the warm, salty waters of the North Atlantic release heat to the atmosphere, they become colder and begin to sink,” says Gagosian. “If cold, salty North Atlantic waters did not sink, a primary force driving global ocean circulation could slacken and cease. Existing currents could weaken or be redirected. The resulting reorganization of the ocean’s circulation would reconfigure Earth’s climate patterns.”
The way to mess up the system is to add large amounts of fresh water into the North Atlantic part. This would dilute the salinity of the water, and at a certain threshold, it would not be dense enough to sink. This part of the conveyor belt would therefore stop.
A report by Peter Schwartz and Doug Randall for the Climate Institute in Washington DC plays out a scenario of what would happen next. “The North Atlantic Ocean continues to be affected by fresh water coming from melting glaciers, Greenland’s ice sheet, and perhaps most importantly increased rainfall and runoff. Decades of high-latitude warming cause increased precipitation and bring additional fresh water to the salty, dense water in the North, which is normally affected mainly by warmer and saltier water from the Gulf Stream. That massive current of warm water no longer reaches far into the North Atlantic. The immediate climatic effect is cooler temperatures in Europe and throughout much of the Northern Hemisphere and a dramatic drop in rainfall in many key agricultural and populated areas.
The thermohaline circulation is a global system of water currents that moves energy, in the form of warm water, around the world. Around different parts of the world it takes on different, more local, names: the Gulf Stream is the specific movement of warm air and water from the tropics to the North Atlantic.
However, the effects of the collapse will be felt in fits and starts, as the traditional weather patterns reemerge only to be disrupted again—for a full decade.”
It has happened before. When the Earth came out of its most recent ice age, around 13,000 years ago, the thermohaline c
irculation was disrupted. The resulting cold period, called the Younger Dryas, lasted more than 1,000 years, with icebergs as far south as Portugal.
Another rapid change in ocean circulation occured 8,200 years ago, leading to widespread drought in the American West, Africa and Asia. “Regional cooling events also have been linked with changes in the Southwest Asian monsoon, whose rains are probably the most critical factor supporting civilizations from Africa to India to China,” says Gagosian.
The effects of a shutdown today
Schwartz and Randall’s scenario shows that there would be global effects if the thermohaline circulation were to shut down. Annual average temperatures would drop across Asia, North America and northern Europe, and droughts would persist for at least a decade in important agricultural and population centers. Temperatures would rise, however, across Australia, South America and southern Africa.
Winter storms and winds would intensify across western Europe and the northern Pacific. Access to energy supplies would be disrupted because of extensive sea ice and storms. The combination of wind and drought would cause widespread dust storms and loss of soil. By the end of a decade, the majority of Europe would feel more like modern Siberia.
“As global and local carrying capacities are reduced, tensions could mount around the world, leading to two fundamental strategies: defensive and offensive,” wrote Schwartz and Randall. “Nations with the resources to do so may build virtual fortresses around their countries, preserving resources for themselves. Less fortunate nations, especially those with ancient enmities with their neighbors, may initiate struggles for access to food, clean water, or energy. Unlikely alliances could be formed as defense priorities shift and the goal is resources for survival rather than religion, ideology, or national honor.”