WikiPortallus: Effects of global warming.html

Graphical description of risks and impacts from global warming from the Third Assessment Report of the Intergovernmental Panel on Climate Change.

Overview

Climate changes characterized as global warming are leading to large-scale irreversible effects at continental and global scales. The likelihood, magnitude, and timing is observed to be increasing and accelerating.

Many consequences of global warming once thought controversial are now being observed. Large reductions in the Greenland and West Antarctic Ice Sheets, accelerated global warming due to carbon cycle feedbacks in the terrestrial biosphere, and releases of terrestrial carbon from permafrost regions and methane from hydrates in coastal sediments are accelerating.¹²

The Woods Hole proposal that melting ice might bring fresh water to the Gulf Stream bringing significant slowing of the ocean circulation that transports warm water to the North Atlantic isn't occurring.^{citation needed}

The probability of warming having unforeseen consequences increases with the rate, magnitude, and duration of climate change. Additionally, the United States National Academy of Sciences has stated, "greenhouse warming and other human alterations of the earth system may increase the possibility of large, abrupt, and unwelcome regional or global climatic events…. Future abrupt changes cannot be predicted with confidence, and climate surprises are to be expected."³

The IPCC reports that the effects of global warming will be mixed across regions. For smaller values of warming (of up to 3°C) or about 5°F), changes are expected to produce net benefits in some regions and for some activities, and net costs for others. Greater warming may produce net costs (or to reduce the benefits from smaller warming) in all regions. Developing countries are vulnerable to reduced economic growth as a result of warming.⁴

Most of the consequences of global warming would result from physical changes: sea level rise, higher local temperatures, and changes in rainfall patterns. Sea level is expected to rise 18 to 59 cm (7.1 to 23.2 inches) by the end of the 21st century, not including the unknown contribution from non-linear changes to large ice sheets.⁵

Physical impacts

Effects on weather

Global warming may be responsible in part for some trends in natural disasters such as extreme weather.

Increasing temperature is likely to lead to increasing precipitation ⁶⁷ but the effects on storms are less clear. Extratropical storms partly depend on the temperature gradient, which is predicted to weaken in the northern hemisphere as the polar region warms more than the rest of the hemisphere.⁸

Extreme weather

Storm strength leading to extreme weather is increasing, such as the power dissipation index of hurricane intensity.⁹ Kerry Emanuel writes that hurricane power dissipation is highly correlated with temperature, reflecting global warming.¹⁰. However, a further study by Emanuel using current model output concluded that the increase in power dissipation in recent decades cannot be completely attributed to global warming¹¹. Hurricane modeling has produced similar results, finding that hurricanes, simulated under warmer, high-CO₂ conditions, are more intense, however, hurricane frequency will be reduced.¹² Worldwide, the proportion of hurricanes reaching categories 4 or 5 – with wind speeds above 56 metres per second – has risen from 20% in the 1970s to 35% in the 1990s.¹³ Precipitation hitting the US from hurricanes has increased by 7% over the twentieth century.¹⁴¹⁵¹⁶ The extent to which this is due to global warming as opposed to the Atlantic Multidecadal Oscillation is unclear. Some studies have found that the increase in sea surface temperature may be offset by an increase in wind shear, leading to little or no change in hurricane activity.¹⁷

Increases in catastrophes resulting from extreme weather are mainly caused by increasing population densities, and anticipated future increases are similarly dominated by societal change rather than climate change.¹⁸ The World Meteorological Organization explains that “though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point.”¹⁹ They also clarified that “no individual tropical cyclone can be directly attributed to climate change.”¹⁹ However, Hoyos et al. (2006) have linked the increasing trend in number of category 4 and 5 hurricanes for the period 1970-2004 directly to the trend in sea surface temperatures.²⁰

This image shows the conclusions of Knutson and Tuleya (2004) that maximum intensity reached by tropical storms is likely to undergo an increase, with a significant increase in the number of highly destructive category 5 storms.

Thomas Knutson and Robert E. Tuleya of NOAA stated in 2004 that warming induced by greenhouse gas may lead to increasing occurrence of highly destructive category-5 storms.²¹ Vecchi and Soden find that wind shear, the increase of which acts to inhibit tropical cyclones, also changes in model-projections of global warming. There are projected increases of wind shear in the tropical Atlantic and East Pacific associated with the deceleration of the Walker circulation, as well as decreases of wind shear in the western and central Pacific.²² The study does not make claims about the net effect on Atlantic and East Pacific hurricanes of the warming and moistening atmospheres, and the model-projected increases in Atlantic wind shear. ²³

A substantially higher risk of extreme weather does not necessarily mean a noticeably greater risk of slightly-above-average weather.²⁴ However, the evidence is clear that severe weather and moderate rainfall are also increasing. Increases in temperature are expected to produce more intense convection over land and a higher frequency of the most severe storms.²⁵

Stephen Mwakifwamba, national co-ordinator of the Centre for Energy, Environment, Science and Technology — which prepared the Tanzanian government's climate change report to the UN — says that change is happening in Tanzania right now. "In the past, we had a drought about every 10 years", he says. "Now we just don't know when they will come. They are more frequent, but then so are floods. The climate is far less predictable. We might have floods in May or droughts every three years. Upland areas, which were never affected by mosquitoes, now are. Water levels are decreasing every day. The rains come at the wrong time for farmers and it is leading to many problems"²⁶.

Greg Holland, director of the Mesoscale and Microscale Meteorology Division at the National Center for Atmospheric Research in Boulder, Colorado, said on April 24, 2006, "The hurricanes we are seeing are indeed a direct result of climate change," and that the wind and warmer water conditions that fuel storms when they form in the Caribbean are, "increasingly due to greenhouse gases. There seems to be no other conclusion you can logically draw." Holland said, "The large bulk of the scientific community say what we are seeing now is linked directly to greenhouse gases." ²⁷ (See also "Global warming?" in tropical cyclone)

Increased evaporation

Increasing water vapor at Boulder, Colorado.

Over the course of the 20th century, evaporation rates have reduced worldwide ²⁸; this is thought by many to be explained by global dimming. As the climate grows warmer and the causes of global dimming are reduced, evaporation will increase due to warmer oceans. Because the world is a closed system this will cause heavier rainfall, with more erosion. This erosion, in turn, can in vulnerable tropical areas (especially in Africa) lead to desertification. On the other hand, in other areas, increased rainfall lead to growth of forests in dry desert areas.

Scientists have found evidence that increased evaporation could result in more extreme weather as global warming progresses. The IPCC Third Annual Report says: "...global average water vapor concentration and precipitation are projected to increase during the 21st century. By the second half of the 21st century, it is likely that precipitation will have increased over northern mid- to high latitudes and Antarctica in winter. At low latitudes there are both regional increases and decreases over land areas. Larger year to year variations in precipitation are very likely over most areas where an increase in mean precipitation is projected."⁶²⁹

Cost of more extreme weather

As the World Meteorological Organization explains, “recent increase in societal impact from tropical cyclones has largely been caused by rising concentrations of population and infrastructure in coastal regions.”¹⁹ Pielke et al. (2008) normalized mainland U.S. hurricane damage from 1900–2005 to 2005 values and found no remaining trend of increasing absolute damage. The 1970s and 1980s were notable because of the extremely low amounts of damage compared to other decades. The decade 1996–2005 has the second most damage among the past 11 decades, with only the decade 1926–1935 surpassing its costs. The most damaging single storm is the 1926 Miami hurricane, with $157 billion of normalized damage.¹⁸

The American Insurance Journal predicted that “catastrophe losses should be expected to double roughly every 10 years because of increases in construction costs, increases in the number of structures and changes in their characteristics.”³⁰ The Association of British Insurers has stated that limiting carbon emissions would avoid 80% of the projected additional annual cost of tropical cyclones by the 2080s. The cost is also increasing partly because of building in exposed areas such as coasts and floodplains. The ABI claims that reduction of the vulnerability to some inevitable effects of climate change, for example through more resilient buildings and improved flood defences, could also result in considerable cost-savings in the longterm.³¹

Destabilization of local climates

The first recorded South Atlantic hurricane, "Catarina", which hit Brazil in March 2004

In the northern hemisphere, the southern part of the Arctic region (home to 4,000,000 people) has experienced a temperature rise of 1 °C to 3 °C (1.8 °F to 5.4 °F) over the last 50 years. Canada, Alaska and Russia are experiencing initial melting of permafrost. This may disrupt ecosystems and by increasing bacterial activity in the soil lead to these areas becoming carbon sources instead of carbon sinks ³². A study (published in Science) of changes to eastern Siberia's permafrost suggests that it is gradually disappearing in the southern regions, leading to the loss of nearly 11% of Siberia's nearly 11,000 lakes since 1971 ³³. At the same time, western Siberia is at the initial stage where melting permafrost is creating new lakes, which will eventually start disappearing as in the east. Furthermore, permafrost melting will eventually cause methane release from melting permafrost peat bogs.

Hurricanes were thought to be an entirely North Atlantic phenomenon. In late March 2004, the first Atlantic cyclone to form south of the equator hit Brazil with 40 m/s (144 km/h) winds, although some Brazilian meteorologists deny that it was a hurricane.³⁴ Monitoring systems may have to be extended 1,600 km (1,000 miles) further south. There is no agreement as to whether this hurricane is linked to climate change,³⁵³⁶ but one climate model exhibits increased tropical cyclone genesis in the South Atlantic under global warming by the end of the 21st century.³⁷

Glacier retreat and disappearance

A map of the change in thickness of mountain glaciers since 1970. Thinning in orange and red, thickening in blue.

Lewis Glacier, North Cascades, WA USA is one of five glaciers in the area that melted away

In historic times, glaciers grew during a cool period from about 1550 to 1850 known as the Little Ice Age. Subsequently, until about 1940, glaciers around the world retreated as the climate warmed. Glacier retreat declined and reversed in many cases from 1950 to 1980 as a slight global cooling occurred. Since 1980, glacier retreat has become increasingly rapid and ubiquitous, and has threatened the existence of many of the glaciers of the world. This process has increased markedly since 1995.³⁸

The loss of glaciers not only directly causes landslides, flash floods and glacial lake overflow,⁴⁰ but also increases annual variation in water flows in rivers. Glacier runoff declines in the summer as glaciers decrease in size, this decline is already observable in several regions.⁴¹ Glaciers retain water on mountains in high precipitation years, since the snow cover accumulating on glaciers protects the ice from melting. In warmer and drier years, glaciers offset the lower precipitation amounts with a higher meltwater input.³⁹

Of particular importance are the Hindu Kush and Himalayan glacial melts that comprise the principal dry-season water source of many of the major rivers of the Central, South, East and Southeast Asian mainland. Increased melting would cause greater flow for several decades, after which "some areas of the most populated regions on Earth are likely to 'run out of water'" as source glaciers are depleted.⁴²

According to a UN climate report, the Himalayan glaciers that are the sources of Asia's biggest rivers—Ganges, Indus, Brahmaputra, Yangtze, Mekong, Salween and Yellow—could disappear by 2035 as temperatures rise.⁴³ Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers.⁴⁴ India, China, Pakistan, Bangladesh, Nepal and Myanmar could experience floods followed by droughts in coming decades. In India alone, the Ganges provides water for drinking and farming for more than 500 million people.⁴⁵⁴⁶⁴⁷ It has to be acknowledged, however, that increased seasonal runoff of Himalayan glaciers led to increased agricultural production in northern India throughout the 20th century.⁴⁸

The recession of mountain glaciers, notably in Western North America, Franz-Josef Land, Asia, the Alps, the Pyrenees, Indonesia and Africa, and tropical and sub-tropical regions of South America, has been used to provide qualitative support to the rise in global temperatures since the late 19th century. Many glaciers are being lost to melting further raising concerns about future local water resources in these glacierized areas. The Lewis Glacier, North Cascades pictured at right after melting away in 1990 is one of the 47 North Cascade glaciers observed and all are retreating.⁴⁹

Despite their proximity and importance to human populations, the mountain and valley glaciers of temperate latitudes amount to a small fraction of glacial ice on the earth. About 99% is in the great ice sheets of polar and subpolar Antarctica and Greenland. These continuous continental-scale ice sheets, 3 kilometres (1.9 mi) or more in thickness, cap the polar and subpolar land masses. Like rivers flowing from an enormous lake, numerous outlet glaciers transport ice from the margins of the ice sheet to the ocean.

Retreat of the Helheim Glacier, Greenland

Glacier retreat has been observed in these outlet glaciers, resulting in an increase of the ice flow rate. In Greenland the period since the year 2000 has brought retreat to several very large glaciers that had long been stable. Three glaciers that have been researched, Helheim, Jakobshavns and Kangerdlugssuaq Glaciers, jointly drain more than 16% of the Greenland Ice Sheet. Satellite images and aerial photographs from the 1950s and 1970s show that the front of the glacier had remained in the same place for decades. But in 2001 it began retreating rapidly, retreating 7.2 km (4.5 mi) between 2001 and 2005. It has also accelerated from 20 m (66 ft)/day to 32 m (100 ft)/day.⁵⁰ Jakobshavn Isbræ in west Greenland is generally considered the fastest moving glacier in the world. It had been moving continuously at speeds of over 24 m (79 ft)/day with a stable terminus since at least 1950. The glacier's ice tongue began to break apart in 2000, leading to almost complete disintegration in 2003, while the retreat rate doubled to over 30 m (98 ft)/day.⁵¹

Glacier retreat and acceleration is also apparent on two important outlet glaciers of the West Antarctic Ice Sheet. Pine Island Glacier, which flows into the Amundsen Sea thinned 3.5 ± 0.9 m (11 ± 3.0 ft) per year and retreated 5 kilometres (3.1 mi) in 3.8 years. The terminus of the glacier is a floating ice shelf and the point at which it is afloat is retreating 1.2 km (0.75 mi)/year. This glacier drains a substantial portion of the West Antarctic Ice Sheet and has been referred to as the weak underbelly of this ice sheet.⁵² This same pattern of thinning is evident on the neighboring Thwaites Glacier cliff.

Oceans

The role of the oceans in global warming is a complex one. The oceans serve as a sink for carbon dioxide, taking up much that would otherwise remain in the atmosphere, but increased levels of CO₂ have led to ocean acidification. Furthermore, as the temperature of the oceans increases, they become less able to absorb excess CO₂. Global warming is projected to have a number of effects on the oceans. Ongoing effects include rising sea levels due to thermal expansion and melting of glaciers and ice sheets, and warming of the ocean surface, leading to increased temperature stratification. Other possible effects include large-scale changes in ocean circulation.

Sea level rise

Sea leve rise during the Holocene.

Sea level has been rising 0.2 cm/year, based on measurements of sea level rise from 23 long tide gauge records in geologically stable environments.

With increasing average global temperature, the water in the oceans expands in volume, and additional water enters them which had previously been locked up on land in glaciers, for example, the Greenland and the Antarctic ice sheets. For most glaciers worldwide, an average volume loss of 60% until 2050 is predicted.⁵³ Meanwhile, the estimated total ice melting rate over Greenland is –239 ± 23 cubic kilometers per year, mostly from East Greenland.⁵⁴ The Antarctic ice sheet, however, is expected to grow during the 21st century because of increased precipitation.⁵⁵ Under the IPCC Special Report on Emission Scenarios (SRES) A1B scenario by the mid-2090s, for instance, global sea level reaches 0.22 to 0.44 m above 1990 levels, and is rising at about 4 mm per year.⁵⁵ Since 1900, the sea level has risen at an average of 1.7 mm/yr.;⁵⁵ since 1993, satellite altimetry from TOPEX/Poseidon indicates a rate of about 3 mm/yr.⁵⁵

The sea level has risen more than 120 metres since the Last Glacial Maximum about 20,000 years ago. The bulk of that occurred before 7000 years ago.⁵⁶ Global temperature declined after the Holocene Climatic Optimum, causing a sea level lowering of 0.7 ± 0.1 m between 4000 and 2500 years before present.⁵⁷ From 3000 years ago to the start of the 19th century, sea level was almost constant, with only minor fluctuations. However, the Medieval Warm Period may have caused some sea level rise; evidence has been found in the Pacific Ocean for a rise to perhaps 0.9 m above present level in 700 BP.⁵⁸

In a paper published in 2007, the climatologist James Hansen et al. claimed that ice at the poles does not melt in a gradual and linear fashion, but flips suddenly from one state to another according to the geological record. In this paper Hansen et al. state:

Temperature rise

From 1961 to 2003, the global ocean temperature has risen by 0.10°C from the surface to a depth of 700 m. There is variability both year-to-year and over longer time scales, with global ocean heat content observations showing high rates of warming for 1991 to 2003, but some cooling from 2003 to 2007.⁵⁵ The temperature of the Antarctic Southern Ocean rose by 0.17 °C (0.31 °F) between the 1950s and the 1980s, nearly twice the rate for the world's oceans as a whole ⁶⁰. As well as having effects on ecosystems (e.g. by melting sea ice, affecting algae that grow on its underside), warming reduces the ocean's ability to absorb CO₂.^{citation needed}

Acidification

The world’s oceans soak up much of the carbon dioxide produced by living organisms, either as dissolved gas, or in the skeletons of tiny marine creatures that fall to the bottom to become chalk or limestone. Oceans currently absorb about one tonne of CO₂ per person per year. It is estimated that the oceans have absorbed around half of all CO₂ generated by human activities since 1800 (118 ± 19 petagrams of carbon from 1800 to 1994).⁶¹

But in water, carbon dioxide becomes a weak carbonic acid, and the increase in the greenhouse gas since the industrial revolution has already lowered the average pH (the laboratory measure of acidity) of seawater by 0.1 units, to 8.2. Predicted emissions could lower it by a further 0.5 by 2100, to a level probably not seen for hundreds of millennia and, critically, at a rate of change probably 100 times greater than at any time over this period.⁶²⁶³

There are concerns that increasing acidification could have a particularly detrimental effect on corals ⁶⁴ (16% of the world's coral reefs have died from bleaching caused by warm water in 1998,⁶⁵ which coincidentally was the warmest year ever recorded) and other marine organisms with calcium carbonate shells.⁶⁶

Shutdown of thermohaline circulation

There is some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localized cooling in the North Atlantic and lead to cooling, or lesser warming, in that region. This would affect in particular areas like Scandinavia and Britain that are warmed by the North Atlantic drift. More significantly, it could lead to an oceanic anoxic event.

The chances of this near-term collapse of the circulation are unclear; there is some evidence for the short-term stability of the Gulf Stream and possible weakening of the North Atlantic drift. However, the degree of weakening, and whether it will be sufficient to shut down the circulation, is under debate. As yet, no cooling has been found in northern Europe or nearby seas.

Abrupt and irreversible effects

The scientific consensus in the IPCC Fourth Assessment Report is that "Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change."

Abrupt effects

Partial loss of ice sheets on polar land could imply metres of sea level rise, major changes in coastlines and inundation of low-lying areas, with greatest effects in river deltas and low-lying islands. Such changes are projected to occur over millennial time scales, but more rapid sea level rise on century time scales cannot be excluded.⁶⁷

Positive feedback effects

Some of the observed and potential effects of global warming are positive feedbacks, contributing directly to further global warming.

Methane release from melting permafrost peat bogs

Western Siberia is the world's largest peat bog, a one million square kilometer region of permafrost peat bog that was formed 11,000 years ago at the end of the last ice age. The melting of its permafrost is likely to lead to the release, over decades, of large quantities of methane. As much as 70,000 million tonnes of methane, an extremely effective greenhouse gas, might be released over the next few decades, creating an additional source of greenhouse gas emissions ⁶⁸. Similar melting has been observed in eastern Siberia ⁶⁹.

Methane release from hydrates

Methane clathrate, also called methane hydrate, is a form of water ice that contains a large amount of methane within its crystal structure. Extremely large deposits of methane clathrate have been found under sediments on the ocean floors of Earth. The sudden release of large amounts of natural gas from methane clathrate deposits, in a runaway greenhouse effect, has been hypothesized as a cause of past and possibly future climate changes. The release of this trapped methane is a potential major outcome of a rise in temperature; it is thought that this might increase the global temperature by an additional 5° in itself, as methane is much more powerful as a greenhouse gas than carbon dioxide. The theory also predicts this will greatly affect available oxygen content of the atmophere. This theory has been proposed to explain the most severe mass extinction event on earth known as the Permian-Triassic extinction event. In 2008, a research expedition for the American Geophysical Union detected levels of methane up to 100 times above normal in the Siberian Arctic, likely being released by methane clathrates being released by holes in a frozen 'lid' of seabed permafrost, around the outfall of the Lena River and the area between the Laptev Sea and East Siberian Sea.⁷⁰⁷¹⁷²

Carbon cycle feedbacks

There have been predictions, and some evidence, that global warming might cause loss of carbon from terrestrial ecosystems, leading to an increase of atmospheric CO₂ levels. Several climate models indicate that global warming through the 21st century could be accelerated by the response of the terrestrial carbon cycle to such warming ⁷³. All 11 models in the C4MIP study found that a larger fraction of anthropogenic CO₂ will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO₂ varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100 ppm. The higher CO₂ levels led to an additional climate warming ranging between 0.1° and 1.5 °C. However, there was still a large uncertainty on the magnitude of these sensitivities. Eight models attributed most of the changes to the land, while three attributed it to the ocean ⁷⁴. The strongest feedbacks in these cases are due to increased respiration of carbon from soils throughout the high latitude boreal forests of the Northern Hemisphere. One model in particular (HadCM3) indicates a secondary carbon cycle feedback due to the loss of much of the Amazon rainforest in response to significantly reduced precipitation over tropical South America⁷⁵. While models disagree on the strength of any terrestrial carbon cycle feedback, they each suggest any such feedback would accelerate global warming.

Observations show that soils in England have been losing carbon at the rate of four million tonnes a year for the past 25 years⁷⁶ according to a paper in Nature by Bellamy et al. in September 2005, who note that these results are unlikely to be explained by land use changes. Results such as this rely on a dense sampling network and thus are not available on a global scale. Extrapolating to all of the United Kingdom, they estimate annual losses of 13 million tons per year. This is as much as the annual reductions in carbon dioxide emissions achieved by the UK under the Kyoto Treaty (12.7 million tons of carbon per year).⁷⁷

It has also been suggested (by Chris Freeman) that the release of dissolved organic carbon (DOC) from peat bogs into water courses (from which it would in turn enter the atmosphere) constitutes a positive feedback for global warming. The carbon currently stored in peatlands (390-455 gigatonnes, one-third of the total land-based carbon store) is over half the amount of carbon already in the atmosphere.⁷⁸ DOC levels in water courses are observably rising; Freeman's hypothesis is that, not elevated temperatures, but elevated levels of atmospheric CO₂ are responsible, through stimulation of primary productivity.⁷⁹⁸⁰

Forest fires

The IPCC Fourth Assessment Report predicts that many mid-latitude regions, such as Mediterranean Europe, will experience decreased rainfall and an increased risk of drought, which in turn would allow forest fires to occur on larger scale, and more regularly. This releases more stored carbon into the atmosphere than the carbon cycle can naturally re-absorb, as well as reducing the overall forest area on the planet, creating a positive feedback loop. Part of that feedback loop is more rapid growth of replacement forests and a northward migration of forests as northern latitudes become more suitable climates for sustaining forests. There is a question of whether the burning of renewable fuels such as forests should be counted as contributing to global warming.⁸¹⁸²⁸³

Retreat of sea ice

Northern Hemisphere ice trends

Southern Hemisphere ice trends

The sea absorbs heat from the sun, while the ice largely reflects the sun rays back to space. Thus, retreating sea ice will allow the sun to warm the now exposed sea water, contributing to further warming. The mechanism is the same as when a black car heats up faster in sunlight than a white car. This albedo change is also the main reason why IPCC predict polar temperatures in the northern hemisphere to rise up to twice as much as those of the rest of the world. In September 2007, the Arctic sea ice area reached about half the size of the average summer minimum area between 1979 to 2000. ⁸⁴⁸⁵ Based on the accelerated loss, predictions are that by 2030 the Arctic could be ice-free part of the year. ⁸⁶ Also in September 2007, Arctic sea ice retreated far enough for the Northwest Passage to become navigable to shipping for the first time in recorded history.⁸⁷ The polar amplification of global warming is not predicted to occur in the southern hemisphere.⁸⁸ The Antarctic sea ice reached its greatest extent on record since the beginning of observation in 1979,⁸⁹ but the gain in ice in the south is exceeded by the loss in the north. The trend for global sea ice, northern hemisphere and southern hemisphere combined is clearly a decline.⁹⁰

Negative feedback effects

Following Le Chatelier's principle, the chemical equilibrium of the Earth's carbon cycle will shift in response to anthropogenic CO₂ emissions. The primary driver of this is the ocean, which absorbs anthropogenic CO₂ via the so-called solubility pump. At present this accounts for only about one third of the current emissions, but ultimately most (~75%) of the CO₂ emitted by human activities will dissolve in the ocean over a period of centuries: "A better approximation of the lifetime of fossil fuel CO₂ for public discussion might be 300 years, plus 25% that lasts forever"⁹¹. However, the rate at which the ocean will take it up in the future is less certain, and will be affected by stratification induced by warming and, potentially, changes in the ocean's thermohaline circulation.

Also, the thermal radiation of the Earth rises in proportion to the fourth power of temperature, increasing the amount of outgoing radiation as the Earth warms. The impact of this negative feedback effect is included in global climate models summarized by the IPCC.

Other consequences

As recent estimates of the rate of global warming have increased, so have the financial estimates of the damage costs.⁹²

Economic and social

Many estimates of aggregate net economic benefits and costs from climate change across the globe have been published. One metric, the social cost of carbon (SCC), expressed in terms of future net benefits and costs that are discounted to the present, has been estimated. Peer-reviewed estimates of the SCC for 2005 have an average value of US$43 per tonne of carbon (tC) (i.e., US$12 per tonne of carbon dioxide) but the range around this mean is large. For example, in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide.)⁴

Nicholas Stern, the former Chief Economist and Senior Vice-President of the World Bank, states in an October 29, 2006, review that climate change could affect growth, which could be cut by one-fifth unless drastic action is taken. ⁹³ Stern has warned that one percent of global GDP is required to be invested in order to mitigate the effects of climate change, and that failure to do so could risk a recession worth up to twenty percent of global GDP.⁹⁴ Stern’s report⁹⁵ suggests that climate change threatens to be the greatest and widest-ranging market failure ever seen. The report has had significant political effects: Australia reported two days after the report was released that they would allott AU$60 million to projects to help cut greenhouse gas emissions.⁹⁶

The Stern Review has been criticized by some economists, saying that Stern did not consider costs past 2200, that he used an incorrect discount rate in his calculations, and that stopping or significantly slowing climate change will require deep emission cuts everywhere.⁹⁷ Other economists have supported Stern's approach ⁹⁸ ⁹⁹, or argued that Stern's estimates are reasonable, even if the method by which he reached them is open to criticism. ¹⁰⁰.

In a 2004 comment on the economic effect of global warming in Copenhagen Consensus, Professor Robert O. Mendelsohn of Yale School of Forestry and Environmental Studies, stated that

Insurance

An industry very directly affected by the risks is the insurance industry; the number of major natural disasters has tripled since the 1960s, and insured losses increased fifteenfold in real terms (adjusted for inflation).¹⁰² According to one study, 35–40% of the worst catastrophes have been climate change related. Over the past three decades, the proportion of the global population affected by weather-related disasters has doubled in linear trend, rising from roughly 2% in 1975 to 4% in 2001. ¹⁰³

According to a 2005 report from the Association of British Insurers, limiting carbon emissions could avoid 80% of the projected additional annual cost of tropical cyclones by the 2080s.¹⁰⁴ A June 2004 report by the Association of British Insurers declared "Climate change is not a remote issue for future generations to deal with. It is, in various forms, here already, impacting on insurers' businesses now."¹⁰⁵ It noted that weather risks for households and property were already increasing by 2-4 % per year due to changing weather, and that claims for storm and flood damages in the UK had doubled to over £6 billion over the period 1998–2003, compared to the previous five years. The results are rising insurance premiums, and the risk that in some areas flood insurance will become unaffordable for some.

Financial institutions, including the world's two largest insurance companies, Munich Re and Swiss Re, warned in a 2002 study that "the increasing frequency of severe climatic events, coupled with social trends" could cost almost US$ 150 billion each year in the next decade.¹⁰⁶ These costs would, through increased costs related to insurance and disaster relief, burden customers, taxpayers, and industry alike.

In the United States, insurance losses have also greatly increased. According to Choi and Fisher (2003) each 1% increase in annual precipitation could enlarge catastrophe loss by as much as 2.8%.¹⁰⁷ Gross increases are mostly attributed to increased population and property values in vulnerable coastal areas, though there was also an increase in frequency of weather-related events like heavy rainfalls since the 1950s ¹⁰⁸.

Transport

Roads, airport runways, railway lines and pipelines, (including oil pipelines, sewers, water mains etc) may require increased maintenance and renewal as they become subject to greater temperature variation. Regions already adversely affected include areas of permafrost, which are subject to high levels of subsidence, resulting in buckling roads, sunken foundations, and severely cracked runways. ¹⁰⁹

Effects on agriculture

For some time it was hoped that a positive effect of global warming would be increased agricultural yields, because of the role of carbon dioxide in photosynthesis, especially in preventing photorespiration, which is responsible for significant destruction of several crops. In Iceland, rising temperatures have made possible the widespread sowing of barley, which was untenable twenty years ago. Some of the warming is due to a local (possibly temporary) effect via ocean currents from the Caribbean, which has also affected fish stocks.¹¹⁰

While local benefits may be felt in some regions (such as Siberia), recent evidence is that global yields will be negatively affected. "Rising atmospheric temperatures, longer droughts and side-effects of both, such as higher levels of ground-level ozone gas, are likely to bring about a substantial reduction in crop yields in the coming decades, large-scale experiments have shown" ¹¹¹.

Moreover, the region likely to be worst affected is Africa, both because its geography makes it particularly vulnerable, and because seventy per cent of the population rely on rain-fed agriculture for their livelihoods. Tanzania's official report on climate change suggests that the areas that usually get two rainfalls in the year will probably get more, and those that get only one rainy season will get far less. The net result is expected to be that 33% less maize—the country's staple crop—will be grown.¹¹²

In 2007, higher incentives for farmers to grow non-food biofuel crops¹¹⁵ combined with other factors (such as rising transportation costs, climate change, growing consumer demand in China and India, and population growth)¹¹⁶ to cause food shortages in Asia, the Middle East, Africa, and Mexico, as well as rising food prices around the globe.¹¹⁷¹¹⁸ As of December 2007, 37 countries faced food crises, and 20 had imposed some sort of food-price controls. Some of these shortages resulted in food riots and even deadly stampedes.¹¹⁹¹²⁰¹²¹

Flood defense

For historical reasons to do with trade, many of the world's largest and most prosperous cities are on the coast, and the cost of building better coastal defenses (due to the rising sea level) is likely to be considerable. Some countries will be more affected than others—low-lying countries such as Bangladesh and the Netherlands would be worst hit by any sea level rise, in terms of floods or the cost of preventing them. Still, in 180 of 192 littoral countries worldwide, coastal protection will cost less than 0.1% of the country's gross domestic product.¹²²

In developing countries, the poorest often live on flood plains, because it is the only available space, or fertile agricultural land. These settlements often lack infrastructure such as dykes and early warning systems. Poorer communities also tend to lack the insurance, savings or access to credit needed to recover from disasters.¹²³

Migration

Some Pacific Ocean island nations, such as Tuvalu, are concerned about the possibility of an eventual evacuation, as flood defense may become economically unviable for them. Tuvalu already has an ad hoc agreement with New Zealand to allow phased relocation.¹²⁴

In the 1990s a variety of estimates placed the number of environmental refugees at around 25 million. (Environmental refugees are not included in the official definition of refugees, which only includes migrants fleeing persecution.) The Intergovernmental Panel on Climate Change (IPCC), which advises the world’s governments under the auspices of the UN, estimated that 150 million environmental refugees will exist in the year 2050, due mainly to the effects of coastal flooding, shoreline erosion and agricultural disruption (150 million means 1.5% of 2050’s predicted 10 billion world population).¹²⁵¹²⁶

Northwest Passage

Arctic ice thicknesses changes from 1950s to 2050s simulated in one of GFDL's R30 atmosphere-ocean general circulation model experiments

Melting Arctic ice may open the Northwest Passage in summer, which would cut 5,000 nautical miles (9,000 km) from shipping routes between Europe and Asia. This would be of particular benefit for supertankers which are too big to fit through the Panama Canal and currently have to go around the tip of South America. According to the Canadian Ice Service, the amount of ice in Canada's eastern Arctic Archipelago decreased by 15% between 1969 and 2004.¹²⁷

In September 2007, the Arctic Ice Cap retreated far enough for the Northwest Passage to become navigable to shipping for the first time in recorded history.¹²⁸

In August, 2008, melting sea ice simultaneously opened up the Northwest Passage and the Northern Sea Route, making it possible to sail around the Arctic ice cap. Scientists estimate that this hasn't happened in 125,000 years.¹²⁹ The Northwest Passage opened August 25, 2008, and the remaining tongue of ice blocking the Northern Sea Route dissolved a few days later. Because of arctic shrinkage, the Beluga group of Bremen, Germany, announced plans to send the first ship through the Northern Sea Route in 2009.¹³⁰

Development

In October 2004 the Working Group on Climate Change and Development, a coalition of development and environment NGOs, issued a report Up in Smoke on the effects of climate change on development. This report, and the July 2005 report Africa - Up in Smoke? predicted increased hunger and disease due to decreased rainfall and severe weather events, particularly in Africa. These are likely to have severe impacts on development for those affected.

Ecosystems

Unchecked global warming could affect most terrestrial ecoregions. Increasing global temperature means that ecosystems will change; some species are being forced out of their habitats (possibly to extinction) because of changing conditions, while others are flourishing. Secondary effects of global warming, such as lessened snow cover, rising sea levels, and weather changes, may influence not only human activities but also the ecosystem. Studying the association between Earth climate and extinctions over the past 520 million years, scientists from University of York write, "The global temperatures predicted for the coming centuries may trigger a new ‘mass extinction event’, where over 50 per cent of animal and plant species would be wiped out."¹³²

Many of the species at risk are Arctic and Antarctic fauna such as polar bears¹³³ and emperor penguins¹³⁴. In the Arctic, the waters of Hudson Bay are ice-free for three weeks longer than they were thirty years ago, affecting polar bears, which prefer to hunt on sea ice.¹³⁵. Species that rely on cold weather conditions such as gyrfalcons, and snowy owls that prey on lemmings that use the cold winter to their advantage may be hit hard.¹³⁶ ¹³⁷Marine invertebrates enjoy peak growth at the temperatures they have adapted to, regardless of how cold these may be, and cold-blooded animals found at greater latitudes and altitudes generally grow faster to compensate for the short growing season.¹³⁸ Warmer-than-ideal conditions result in higher metabolism and consequent reductions in body size despite increased foraging, which in turn elevates the risk of predation. Indeed, even a slight increase in temperature during development impairs growth efficiency and survival rate in rainbow trout.¹³⁹

Rising temperatures are beginning to have a noticeable impact on birds¹⁴⁰, and butterflies have shifted their ranges northward by 200 km in Europe and North America. Plants lag behind, and larger animals' migration is slowed down by cities and roads. In Britain, spring butterflies are appearing an average of 6 days earlier than two decades ago ¹⁴¹.

A 2002 article in Nature¹⁴² surveyed the scientific literature to find recent changes in range or seasonal behaviour by plant and animal species. Of species showing recent change, 4 out of 5 shifted their ranges towards the poles or higher altitudes, creating "refugee species". Frogs were breeding, flowers blossoming and birds migrating an average 2.3 days earlier each decade; butterflies, birds and plants moving towards the poles by 6.1 km per decade. A 2005 study concludes human activity is the cause of the temperature rise and resultant changing species behaviour, and links these effects with the predictions of climate models to provide validation for them ¹⁴³. Scientists have observed that Antarctic hair grass is colonizing areas of Antarctica where previously their survival range was limited. ¹⁴⁴

Mechanistic studies have documented extinctions due to recent climate change: McLaughlin et al. documented two populations of Bay checkerspot butterfly being threatened by precipitation change.¹⁴⁵ Parmesan states, "Few studies have been conducted at a scale that encompasses an entire species"¹⁴⁶ and McLaughlin et al. agreed "few mechanistic studies have linked extinctions to recent climate change."¹⁴⁵ Daniel Botkin and other authors in one study believe that projected rates of extinction are overestimated.¹⁴⁷

Many species of freshwater and saltwater plants and animals are dependent on glacier-fed waters to ensure a cold water habitat that they have adapted to. Some species of freshwater fish need cold water to survive and to reproduce, and this is especially true with Salmon and Cutthroat trout. Reduced glacier runoff can lead to insufficient stream flow to allow these species to thrive. Ocean krill, a cornerstone species, prefer cold water and are the primary food source for aquatic mammals such as the Blue whale¹⁴⁸. Alterations to the ocean currents, due to increased freshwater inputs from glacier melt, and the potential alterations to thermohaline circulation of the worlds oceans, may affect existing fisheries upon which humans depend as well.

The white lemuroid possum, only found in the mountain forests of northern Queensland, has been named as the first mammal species to be driven extinct by man-made global warming. The White Possum has not been seen in over three years. These possums cannot survive extended temperatures over 30 degrees C, which occurred in 2005. A final expedition to uncover any surviving White Possums is scheduled for 2009.¹⁴⁹

Forests

Pine forests in British Columbia have been devastated by a pine beetle infestation, which has expanded unhindered since 1998 at least in part due to the lack of severe winters since that time; a few days of extreme cold kill most mountain pine beetles and have kept outbreaks in the past naturally contained. The infestation, which (by November 2008) has killed about half of the province's lodgepole pines (33 million acres or 135,000 km³)¹⁵⁰¹⁵¹ is an order of magnitude larger than any previously recorded outbreak¹⁵² and passed via unusually strong winds in 2007 over the continental divide to Alberta. An epidemic also started, be it at a lower rate, in 1999 in Colorado, Wyoming, and Montana. The United States forest service predicts that between 2011 en 2013 "virtually all 5 million acres of Colorado’s lodgepole pine trees over five inches in diameter will be lost"¹⁵¹.

As the northern forests are a carbon sink, while dead forests are a major carbon source, the loss of such large areas of forest has a positive feedback on global warming. In the worst years, the carbon emission due to beetle infestation of forests in British Columbia alone approaches that of an average year of forest fires in all of Canada or five years worth of emissions from that country's transportation sources ¹⁵²¹⁵³.

Besides the immediate ecological and economic impact, the huge dead forests provide a fire risk. Even many healthy forests appear to face an increased risk of forest fires because of warming climates. The 10-year average of boreal forest burned in North America, after several decades of around 10,000 km² (2.5 million acres), has increased steadily since 1970 to more than 28,000 km² (7 million acres) annually.¹⁵⁴. Though this change may be due in part to changes in forest management practices, in the western U. S., since 1986, longer, warmer summers have resulted in a fourfold increase of major wildfires and a sixfold increase in the area of forest burned, compared to the period from 1970 to 1986. A similar increase in wildfire activity has been reported in Canada from 1920 to 1999.¹⁵⁵

Forest fires in Indonesia have dramatically increased since 1997 as well. These fires are often actively started to clear forest for agriculture. They can set fire to the large peat bogs in the region and the CO₂ released by these peat bog fires has been estimated, in an average year, to be 15% of the quantity of CO₂ produced by fossil fuel combustion. ¹⁵⁶

Mountains

Mountains cover approximately 25 percent of earth's surface and provide a home to more than one-tenth of global human population. Changes in global climate pose a number of potential risks to mountain habitats¹⁵⁷. Researchers expect that over time, climate change will affect mountain and lowland ecosystems, the frequency and intensity of forest fires, the diversity of wildlife, and the distribution of water.

Studies suggest that a warmer climate in the United States would cause lower-elevation habitats to expand into the higher alpine zone.¹⁵⁸ Such a shift would encroach on the rare alpine meadows and other high-altitude habitats. High-elevation plants and animals have limited space available for new habitat as they move higher on the mountains in order to adapt to long-term changes in regional climate.

Changes in climate will also affect the depth of the mountains snowpacks and glaciers. Any changes in their seasonal melting can have powerful impacts on areas that rely on freshwater runoff from mountains. Rising temperature may cause snow to melt earlier and faster in the spring and shift the timing and distribution of runoff. These changes could affect the availability of freshwater for natural systems and human uses.¹⁵⁹

Ecological productivity

Increasing average temperature and carbon dioxide may have the effect of improving ecosystems' productivity. In photorespiration, carbon dioxide that oxygen can enter a plant's chloroplasts and take the place of carbon dioxide in the Calvin cycle. This causes the sugars being made to be destroyed, suppressing growth. Higher carbon dioxide concentrations tend to reduce photorespiration. Satellite data shows that the productivity of the northern hemisphere has increased since 1982 (although attribution of this increase to a specific cause is difficult).

IPCC models predict that higher CO₂ concentrations would only spur growth of flora up to a point, because in many regions the limiting factors are water or nutrients, not temperature or CO₂; after that, greenhouse effects and warming would continue but there would be no compensatory

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