UNFPA - United Nations Population Fund

State of World Population 2009

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1  Elements of climate change

The temperature of the earth's surface has risen 0.74 degrees Celsius in the past 100 years. This increase may not seem much, but this warming has been sufficient to disrupt many of the planet's ecosystems to pose significant risks to human well-being. More importantly, if recent trends continue or accelerate as many climate scientists predict, the earth's temperature may rise another four to six degrees by 2100, with likely catastrophic effect on the environment, habitats, economies and people.(1)

Lightning streaks across the night sky in Hefei, China. Storms will become more frequent and intense as the earth's atmosphere continues to warm.
© Xinhua/Xinhua Press/Corbis

With growing confidence, climate scientists around the world attribute the bulk of recent warming to the greenhouse gases injected into the atmosphere as a result of the activities of an increasingly wealthy human population, particularly in the industrialized countries. Natural climate variation may explain some of the increase in temperatures since 1900. But scientists have so far found no natural causes—not changing solar conditions or an ongoing recovery from past ice ages—that can fully explain such a dramatic rise in temperatures. Nor can any natural forces explain why the 10 warmest years globally since 1880 have been in the last 13 years. (See Box 7)

Greenhouse gases, such as carbon dioxide and methane, occur naturally and create a "greenhouse effect"—so called because of an imperfect comparison to the glass walls of a greenhouse—that keeps the earth's surface warm. Without greenhouse gases, much more of the heat radiated from the sun would bounce back into space, and the earth's surface would be too cold to sustain life.

The additional greenhouse gases that come from intense burning of fossil fuels, modern farming methods that rely on fertilizers, and the industrial use of chloro-fluorocarbons, particularly in the past 40 years, have thrown the earth's natural greenhouse effect into a state of disequilibrium. In addition, deforestation, clearing of other vegetation and the accumulation of carbon dioxide in the oceans have reduced the capacity of the world's "carbon sinks," which have for millennia absorbed excess carbon from the atmosphere. Less capacity to absorb carbon means there is more carbon dioxide in the atmosphere, exacerbating what now appears to be a runaway greenhouse effect.


As the earth's surface warms, weather patterns shift. Unreliable rains hamper food production. Melting ice in the Arctic is contributing to rising sea levels, endangering the lives of millions of people living in low-lying coastal areas around the world. Human-induced climate change threatens to cause shortages of fresh water for human consumption and agriculture. More frequent and severe storms are likely, leading to devastating floods. And the warming atmosphere may be allowing diseases and pests once confined to tropical areas to spread north and south towards the poles.

Ecosystems as diverse as the Amazon rainforest and the Arctic tundra may be approaching thresholds of dramatic change through warming and drying. Mountain glaciers are in retreat, and the downstream effects of reduced water supply in the driest months will have repercussions that transcend generations.

There is alarming evidence that important "tipping points," leading to irreversible changes in climate or other earth systems, may already have been reached—or passed. Ecosystems as diverse as the Amazon rainforest and the Arctic tundra may be approaching thresholds of dramatic change through warming and drying. Mountain glaciers are in retreat, and the downstream effects of reduced water supply in the driest months will have repercussions that transcend generations.

[9] Climate tipping elements

Scientists believe that several tipping elements could destabilize the planet's climate by setting off chain reactions—positive feedbacks—that accelerate other climate changes. Once a tipping element is triggered by crossing a threshold or tipping point, there is no turning back, even if all greenhouse-gas emissions were to end. Some tipping elements, such as the loss of Arctic summer sea ice, may be triggered within the next decade if climate change continues at the same rate. Others, such as the collapse of the Atlantic Ocean current, are thought to be many decades away, while the loss of Antarctic ice is unlikely to be complete for several centuries.(4)

Tipping element Expected consequences

Loss of Arctic summer sea ice

Higher average global temperatures and changes to ecosystems

Melting of Greenland ice sheet

Global sea level rise up to six metres

Collapse of West Antarctic ice sheet

Global sea level rise up to five metres

Increase in El Niño events

Changes to weather patterns, including increased droughts, especially in Southeast Asia

Destruction of Amazon forests

Massive extinctions and decreased rainfall

Changes to India's summer monsoon

Widespread drought and changes in weather patterns

Changes to the Sahara/Sahel and the West African monsoon

Changes to weather patterns, including potential greening of the Sahara/Sahel—one of the few positive tipping elements

In 2008, researchers using data from four different climate models found that changes in Arctic and Antarctic temperatures are not consistent with natural variability and are directly attributable to human influence.(2) Evidence grew in 2008 that the Arctic sea ice is disappearing more rapidly than previously expected because of higher air and ocean temperatures.

For the second year in a row, there has been an ice-free channel in the Northwest Passage through the islands of northern Canada. This year also saw the opening of the Northern Sea Route along the Arctic Siberian coast. The two passages have probably not been open simultaneously in about 100,000 years, before the last ice age.

The overall declining trend of sea-ice in the Arctic has lasted at least three decades. The loss is greatest in summer, but is also evident in the reduced thickness of the winter ice packs. With less ice surviving the summer, the amount of thick ice that has built up over several years is decreasing. This leaves the whole sea-ice system more vulnerable to future warming and brings closer the prospect of an ice-free Arctic.(3)

The lake is all that remains of a glacier near the rural town of Botijlaca, Bolivia. Bolivia's glaciers are melting rapidly, jeopardizing water supplies to rural and urban communities.
© Andi Gitow/UNTV

In the Arctic the atmosphere is warming twice as fast as in most other parts of the world. In the far north, warming is amplified by a decrease in the reflectivity of the Earth's surface as ice and snow melt. Ice and snow reflect solar energy back into space, while darker surfaces like bare tundra and open ocean absorb more solar energy and then radiate it to heat the air above. So as the reflective surfaces disappear, the darker surfaces release heat into the immediate environment that results in more melt.

However, there may be other factors contributing to accelerated warming in the Arctic Ocean. In 2007, there was an especially large loss of ice in the Beaufort Sea, north of Canada and Alaska. This was due to incursions of warm water from the south that melted the ice from beneath.(5) Also, local atmospheric conditions amplified ice loss. Unnaturally clear, sunny skies in 2007, for example, increased melting in the 24-hour sun part of the year, and strong winds during the early part of the summer drove ice into seasonal packs, creating enlarged patches of open ocean.(6)

The largest mass of ice in the Arctic covers the island of Greenland. In places, the ice sheet is three kilometres thick. If all of it melts, it will raise sea levels by an estimated six metres. Until recently, glaciologists presumed that the ice would thaw slowly over millennia, as warming at the surface of the ice sheet permeates downward and gradually melts the ice. But the ice sheet is losing mass much faster than would be expected if normal melting alone was to blame. Current losses are more than 100 cubic kilometres a year. New findings in 2008 revealed that the flow into the ocean of the Jakobshavn Isbrae glacier in western Greenland, one of the most important routes for ice loss, has doubled since 1997.(7) A recent analysis of historical data on the extent of the Greenland ice sheet shows that total meltdown is quite possible as a result of warming on the scale that is being forecast for the next few decades.(8)

Antarctica is losing ice, too, particularly from the West Antarctic ice sheet. This sheet contains enough ice to raise sea levels by about five metres. Researchers estimated in 2008 that loss of ice from the West Antarctic ice sheet increased by 60 per cent from 1996 to 2006.(9) Ice loss from the Antarctic Peninsula, which extends from West Antarctica toward South America, increased by 140 per cent.

The most recent assessment by the Intergovernmental Panel on Climate Change forecast that global sea levels would rise by between 18 and 59 centimetres in the coming century—just from the thermal expansion of warmer oceans and the melting of mountain glaciers. But since the report was completed, many researchers involved in that assessment have predicted that a much larger rise is possible or probable. The new prediction—of a one-metre rise by 2100—originates in part from reassessments of the potential for physical breakup of the ice sheets of Greenland and Antarctica.(10)

A 2008 study on the dynamics of ice-sheet loss argued that sea levels could rise by as much as two metres in the coming century as a result of outflows of ice from Greenland, Antarctica and other glaciers and ice caps.(11) Such a rise would be far beyond anything seen in the recent past. Sea levels rose 2 centimetres in the 18th century, 6 centimetres in the 19th century, and 19 centimetres in the 20th century; a rise equivalent to 30 centimetres is projected for the 21st century based on rates observed in the century's first few years.(12) The magnitude of scale for sea-level rise now being forecast would be in line with what happened at the end of the last ice age. Then, as ice sheets disintegrated, sea levels rose by between 70 and 130 centimetres per century.(13) Given current population densities of the areas affected, a one-metre rise in sea levels worldwide would displace around 100 million people in Asia, mostly eastern China, Bangladesh, and Vietnam; 14 million in Europe; and 8 million each in Africa and South America.(14)

Research in 2008 indicates that sea-level rise—from thermal expansion, mountain glacier retreat, and ice sheet melt—is likely to be much greater and to arrive much sooner than believed even two years ago. No matter how quickly climate change is mitigated, sea levels will rise. So, efforts to adapt to rising seas are more urgent than ever.

The Arctic contains very large stores of greenhouse gases in the form of methane locked in ice lattices in permafrost or beneath the bed of the Arctic Ocean, methane that may be released as the planet warms. Large-scale methane releases would exacerbate global warming and could turn natural ecosystems from carbon sinks to carbon sources, triggering a rapid and uncontrollable temperature increase.

Climate scientists are concerned that methane hydrates could escape into the atmosphere either as permafrost melts or as warmer waters destabilize frozen offshore deposits. In 2008, a study of the Siberian Shelf reported elevated methane concentrations offshore from the Lena River Delta.(15) Meanwhile, researchers showed that, once under way, thawing of east Siberian permafrost—thought to contain 500 billion tons of carbon—would be irreversible; 250 billion tons could be released in a century.(16) Northern peatland soils that are not frozen also contain large amounts of carbon and are vulnerable to warming. The peat's ability to store carbon is highly dependent on its moisture content. Warming will dry out the peat, lowering water tables. A new modelling study showed that this would lead to massive loss of organic carbon in the soil. In northern Manitoba, Canada, a 4-degree Celsius warming would release 86 per cent of the carbon that is sequestered, or stored away, in deep peat.(17)

One reason for fears about the ability of forests to soak up carbon dioxide is that forest cover itself is declining and is now contributing to emissions—1.5 billion tons of carbon a year enter the atmosphere from changes in land use, almost entirely from deforestation in the tropics.(18) Another reason is that even intact forests may be in trouble; the ability of forests to store carbon may have peaked, and rising temperatures may already be decreasing carbon uptake by vegetation in the northern hemisphere. Higher temperatures impose significant stress on trees during the summer season, and photosynthesis halts sooner. Once photosynthesis halts, carbon is no longer sequestered, and stressed forests are vulnerable to damage from pollution, fires, pests and disease that can turn them into carbon sources.(19)

The other carbon sink—the oceans—is also in jeopardy. Oceans absorb carbon, helping maintain equilibrium in the earth's atmosphere. Over the past 150 years, the oceans have absorbed between one-third and one-half of the atmosphere's added carbon dioxide. As the absorbed gas combines with carbonate ions in seawater and forms carbonic acid, the oceans have become 30 per cent more acidic. The acidification inhibits marine life's ability to calcify, threatening shellfish and coral, which are an important source of food and incomes for many of the world's people.

Evidence is mounting for significant consequences to climate variability from soot, or black carbon that originates from fires, coal plants, diesel engines and burning by households. Dark particles that remain suspended in the atmosphere absorb radiant energy and warm the air they occupy. Global emissions of black carbon are rising fast.

There are other important human-induced influences on climate besides greenhouse gases. Evidence is mounting for significant consequences to climate variability from soot, or black carbon, that originates from fires, coal plants, diesel engines and burning by households. Dark particles that remain suspended in the atmosphere absorb radiant energy and warm the air they occupy. Global emissions of black carbon are rising fast, and Chinese emissions may have doubled since 2000. The warming influence of black carbon could be three times greater than estimates from the Intergovernmental Panel on Climate Change's latest report, making it the second-most important climatic agent after carbon dioxide.(20) These findings remain controversial because black soot can cool as well as warm. But when black carbon falls onto ice it darkens the surface, absorbing more of the sun's energy which leads to local warming and melting. Soot may be a contributor to the disappearance of glaciers in some regions and could even explain the accelerated rates of melt in the Himalaya-Hindu Kush.(21)

New research demonstrates that winds in the strong-est cyclones have become more intense in all oceans.(22) The increase has been greatest in relatively cool ocean basins that have seen the largest increases in sea temperatures, notably the North Atlantic, but also the eastern North Pacific and southern Indian oceans. Tropical cyclones form only when ocean temperatures exceed about 26 degrees Celsius. Therefore it is possible that warmer oceans may generate more frequent and more intense tropical cyclones.

Growing concern about world water shortages highlight new findings on the possible impacts that climate change will have on the earth's hydrologic cycle, including rainfall, soil evaporation, and loss of glacial meltwater flows in rivers. New findings predict empty reservoirs in the Mediterranean and American Midwest, dry rivers in China and the Middle East, and less predictable river flows characterized by flash floods in a glacier-free South Asia.(23)

The Intergovernmental Panel on Climate Change laid out a range of possible increases in global temperatures and possible impacts on society and the environment, ranging from modest and manageable to ones that can only be described as catastrophic. Unless action is taken soon to stabilize and then decrease concentrations of greenhouse gases in the atmosphere, there is a great risk that temperature increases could cause widespread damage to ecosystems, natural resources, human populations and disrupt economic activities. Such damage could certainly end prosperity in developed countries and threaten basic human livelihoods in developing countries.

Uncertainties remain in climate-change science. Still, the evidence available so far suggests that we may be within a few years of crossing tipping points, with potential to permanently disrupt seasonal weather patterns that have supported agricultural activities of half the human population, sustained carbon sinks, and prevented major ice sheets from melting.

[10] Climate change risks

The average global temperature could rise by as much as 6.4 degrees Celsius by the end of this century.(24)

As much as 30 per cent of plant and animal species could become extinct if the global temperature increase exceeds 2.5 degrees Celsius.

One-third of the reef-building corals around the world could become extinct because of warming and acidifying waters.

Global average sea levels could rise by as much as 43 centimetres by the end of this century.

Arctic ice could disappear altogether during the summer by the second half of this century.

One in six countries could face food shortages each year because of severe droughts.

By 2075, between 3 billion and 7 billion people could face chronic water shortages.


Anita Cano, a 20-year-old woman with a quick smile, works the front desk at the Ambergris Diving Company in San Pedro, Belize. But she says she may not stay there for long. "It's not stable, because of the economy," she says.

Under normal circumstances, San Pedro is one of Belize's more vibrant tourism spots, bustling with foreigners in search of diving and snorkeling adventures on the nearby reef.
Tourism has taken a nosedive this year—most people say because of the global economic crisis. But there is also concern that dying coral reefs will make tourist destinations like Belize less attractive for the masses of visitors that each year help inject cash into the Belizean economy.

The tourism industry in Belize employs 15,000 people—about one in four jobs. Tourism is the country's largest employer of women, many of them single heads of households.
Scientists say that the corals off the coast of Belize and neighbouring countries are gradually dying due to effects of the climate change, such as higher water temperatures, stronger and more frequent hurricanes, as well as acidification of the ocean as it absorbs more and more carbon dioxide from the air.

Anita Cano.
© Trygve Olfarnes/UNFPA

If there is a dramatic reduction of live corals along the Caribbean coast, tourism will not be the only sector to suffer. The country's 2,200 fishermen could find their livelihoods in peril. The fishing industry generates between 6 per cent and 8 per cent of Belize's gross domestic product. In addition, the likelihood of catastrophic consequences of stronger and more frequent hurricanes would increase as the protection the coral reefs provide would literally erode.

Anita says she doesn't know much about the dying coral reef and other possible effects of climate change, but "90 per cent of people here depend on the ocean for their living, so of course it's important," she acknowledges.

"Business is down this year by 60 per cent compared to three to five years ago," says Andre Paz, a tour guide at Amigos del Mar Dive Shop in San Pedro, Belize. He, too, attributes the current decline in business to the global financial crisis, but also to the dying reef. "We see less fish, less coral and fewer colors out there," he says. Andre and his colleague Robert Zelaya believe climate change is the culprit.

Scientists say that corals are dying due to higher water temperatures, acidification of the ocean—a result of increased carbon-dioxide being absorbed into the water—and stronger and more frequent hurricanes.

As a result of the decline in business, Amigos del Mar has laid off four people and sold one of its 10 boats. "On an average day, we used to get 30 people going out diving, fishing or snorkeling. Now we get about 15," Paz says.

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