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The predicted effects of global warming are many and various, both for the environment and for human life. The primary effect is an increasing global average temperature. From this flow a variety of secondary effects, including rising sea levels, altered patterns of agriculture, increased extreme weather events, and the expansion of the range of tropical diseases. In some cases, the effects may already be being experienced, although it is generally difficult to attribute specific natural phenomena to long-term global warming.

The extent and likelihood of these consequences is a matter of considerable political controversy. A summary of possible effects and our current understanding can be found in the report of the IPCC Working Group II .

Effects

Oceans

Sea level rise

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 and the polar ice caps. An increase of 1.5 to 4.5 °C is estimated to lead to an increase of 15 to 95 cm (IPCC 2001).

The sea level has risen more than 120 metres since the peak of the last ice age about 18,000 years ago. The bulk of that occurred before 6000 years ago. From 3000 years ago to the start of the 19th century sea level was almost constant, rising at 0.1 to 0.2 mm/yr; since 1900 the level has risen at 1–2 mm/yr ; since 1992 satellite altimetry from TOPEX/Poseidon indicates a rate of about 3 mm/yr .

Temperature rise

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 . As well as effects on ecosystems (eg by melting sea ice, affecting algae that grow on its underside), warming could reduce the ocean's ability to absorb CO₂.

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 metric 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 (120,000,000,000 tonnes or 120 petagrams of carbon) .

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 on the 14-point scale, to 8.2. Predicted emissions could lower it by a further 0.5 by 2100, to a level not seen for millions of years.

There are concerns that increasing acidification could have a particularly detrimental effect on corals (16% of the world's coral reefs have died from bleaching since 1998 ) and other marine organisms with calcium carbonate shells. Increased acidity may also directly affect the growth and reproduction of fish as well as the plankton on which they rely on for food .

Shutdown of Thermohaline circulation

There is some speculation that global warming could trigger localised cooling in the North Atlantic and lead to cooling, or lesser warming, in that region, affecting in particular areas like Scandinavia and Britain that are warmed by the North Atlantic drift. The chances of this occurring are unclear, but probably not large, based on climate model results.

Heat is transported from the equator polewards mostly by the atmosphere but also by ocean currents, with warm water near the surface and cold water at deeper levels. The best known segment of this Thermohaline circulation (THC) is the Gulf Stream. Warm water from the Caribbean is transported to the North Atlantic, where its effect in warming the atmosphere contributes to warming Europe. The evaporation of ocean water in the North Atlantic increases the salinity (relative saltiness) of the water as well as cooling it, both actions increasing the density of water at the surface. The formation of sea ice further increases the salinity. This dense water then sinks and the circulation stream continues in a southerly direction. Global warming could lead to an increase in freshwater in the northern oceans, by melting glaciers in Greenland and by increasing precipitation. It is by no means clear that sufficient freshwater could be provided to interrupt the THC - climate models indicate not, but research continues.

Some even fear that global warming may be able to trigger the type of abrupt massive temperature shifts which occurred during the last glacial: a series of Dansgaard-Oeschger events—rapid climate fluctuations—may be attributed to freshwater forcing at high latitude interrupting the THC. The Younger Dryas event may have been of this sort too (See the discussion of chaos theory for related ideas.). However, these events are believed to have been triggered by massive freshwater discharges from the Laurentide ice sheet, rather than from the precipitation changes associated with global warming. Also, in coupled AOGCMs the warming effects outweigh the cooling, even locally: the IPCC Third Annual Report notes that even in models where the THC weakens, there is still a warming over Europe ; see also .

Recently the hypothesis that the Gulf Stream is switching off received a boost when a retrospective analysis of U.S. satellite data seemed to show a slowing of the North Atlantic Gyre, the northern swirl of the Gulf Stream.NASA: Slowing of North Atlantic Gyre

In May 2005, Peter Wadhams reported to The Times about the results of investigations in a submarine under the Arctic ice sheet measuring the giant chimneys of cold dense water, in which the cold dense water normally sinks down to the sea bed and is replaced by warm water, forming one of the engines of the North Atlantic Drift. He and his team found the chimneys to have virtually disappeared. Normally there are seven to twelve giant columns, but Wadhams found only two giant columns, both extremely weak .

Weather

More extreme weather

As the climate grows warmer, evaporation will increase. This will cause heavier rainfall and more erosion, and in more vulnerable tropical areas (especially in Africa), desertification. Many scientists think that it could result in more extreme weather as global warming progresses. The IPCC Third Annual Report says: "...global average water vapour 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" .

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" .

The duration and wind speed of tropical storms has increased by 50% in the last 30 years. The measures are "very well correlated with the surface temperature of the tropical oceans" . 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. Precipitation hitting the US from hurricanes increased by 7% over the twentieth century . See also Time Magazine's "Global Warming: The Culprit?".

A substantially higher risk of extreme weather does not necessarily mean a noticeably greater risk of slightly-above-average weather (eg loading a dice to double the odds of a six quadruples a chance of a double six) .

Destabilisation of local climates

The total surface area of glaciers worldwide has decreased by 50% since the end of the 19th century . As of March 2005, the snow cap that has covered the top of Mount Kilimanjaro for the past 11,000 years since the last ice age has almost disappeared . The loss of glaciers not only directly causes landslides, flash floods and glacial lake overflow, but also increases annual variation in water flows in rivers. 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 .

In the northern hemisphere, the southern part of the Arctic region (home to 4,000,000 people) has experienced a temperature rise 1° to 3° Celsius 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 . 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 . 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. Western Siberia is the world's largest peat bog, and the melting of its permafrost is likely to lead to the release, over decades, of large quantities of methane—creating an additional source of greenhouse gas emissions .

Hurricanes were thought to be an entirely north Atlantic phenomenon. In April 2004, the first Atlantic hurricane to form south of the Equator hit Brazil with 40 m/s (144 km/h) winds; monitoring systems may have to be extended 1,600 km (1000 miles) further south .

Reduced ozone layer

One of the lesser-known effects of global warming is a reduction in the thickness of the ozone layer, which protects life on Earth from harmful radiation. Global warming appears to be partially countering the improvements in the ozone layer caused by the reduction in the use of ozone-destroying chemicals achieved through the Montreal Protocol. "What appears to have caused the further loss of ozone is the increasing number of stratospheric clouds in the winter, 15 miles above the earth. These clouds, in the middle of the ozone layer, provide a platform which makes it easier for rapid chemical reactions which destroy ozone to take place" . The ecological impact may have further knock-on effects, as it reduces photosynthesis in plants (with potential impacts on agriculture) and damages the DNA of plankton, which play a significant role in the world's carbon cycle .

Ecosystems

Rising temperatures are beginning to impact on ecosystems. 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 highways. In Britain, spring butterflies are appearing an average of 6 days earlier than two decades ago . 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 do not hunt on land .

Two 2002 studies in Nature (vol 421) 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 . Grass has become established in Antarctica for the first time.

Forests potentially face an increased risk of forest fires. 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. .

Productivity

Increasing average temperature and carbon dioxide may have the effect, up to a point, of improving ecosystems' productivity. Atmospheric carbon dioxide is rare in comparison to oxygen (less than 1% of air compared to 21% of air). This carbon dioxide starvation becomes apparent in photorespiration, where there is so little carbon dioxide, that oxygen can enter a plant's chloroplasts and takes the place where carbon dioxide normally would be in the Calvin Cycle. This causes the sugars being made to be destroyed, badly suppressing growth. 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 increase in growth.

Further global warming (positive feedback)

Some effects of global warming themselves contribute directly to further global warming.

Methane release from melting permafrost peat bogs

Climate scientists reported in August 2005 that a one million square kilometer region of permafrost peat bogs in western Siberia is starting to melt for the first time since it was formed 11,000 years ago at the end of the last ice age. This will release methane, an extremely effective greenhouse gas, possibly as much as 70,000 million tonnes, over the next few decades. An earlier report in May 2005 reported similar melting in eastern Siberia .

This positive feedback was not known about in 2001 when the IPCC issued its last major report on climate change. The discovery of permafrost peat bogs melting in 2005 implies that warming is likely to happen faster than was predicted in 2001.

Carbon cycle feedbacks

There have been predictions, and some evidence, that global warming might cause loss of carbon from ecosystems and soils, leading to an increase of atmospheric CO₂ levels. England's soils have been losing carbon at the rate of four million tonnes a year for the past 25 years 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 emmisions achieved by the UK under the Kyoto Treaty (12.7 million tons of carbon per year).

Some model predictions (from HadCM3) indicate that changes in precipitation patterns could lead to deforestation in the Amazon, accompanied by extra CO₂ release.

References

"How Climate Evolved on the Terrestrial Planets," by James F. Kasting, Owen B. Toon, and James R. Pollack, Scientific American February 1988, p. 90.

Forest Fires

Rising Global temperature may cause 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.

(Climate Change and Fire)

(Climate Roulette: Loss of Carbon Sinks & Positive Feedbacks)

(EPA: Global Warming: Impacts: Forests)

(Feedback Cycles linking forests, climate and landuse activities)

Consequences

See also Mitigation of global warming

Economic

Decline of agriculture

For some time it was hoped that a positive effect of global warming would be increased agricultural yields, because of the role of CO₂ in photosynthesis. 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 have also affected fish stocks .

Whilst 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" (The Independent, April 27, 2005, "Climate change poses threat to food supply, scientists say" - report on this event).

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 .

Insurance

An industry very directly affected by the risks is the insurance industry; the number of major natural disasters has trebled since the 1960s, and insured losses increased fifteen-fold in real terms (adjusted for inflation) . According to one study, 35–40% of the worst catastrophes have been climate change related (ERM, 2002). 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 (ERM, 2002).

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". 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 risk insurance will become unaffordable for some.

In the United States, insurance losses have also greatly increased, but according to one study those increases are attributed to increased population and property values in vulnerable coastal areas.(Science, Vol 284, Issue 5422, 1943-1947).

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, and, in areas with permafrost, subject to subsidence .

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 will be worst hit by any sea level rise, in terms of floods or the cost of preventing them.

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 .

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 .

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 percent of 2050’s predicted 10 billion world population.)

Northwest Passage

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 relevance for supertankers which are too big to fit through the Panama Canal and currently have to go around the tip of South America. According the Canadian Ice Service, the amount of ice in Canada's eastern Arctic Archipelago decreased by 15 percent between 1969 and 2004 .

Development

The combined effects of global warming may impact particularly harshly on people and countries without the resources to mitigate those effects. This may slow economic development and poverty reduction, and make it harder to achieve the Millennium Development Goals , .

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.

Environmental

Secondary evidence of global warming — lessened snow cover, rising sea levels, weather changes — provides examples of consequences of global warming that may influence not only human activities but also ecosystems. Increasing global temperature means that ecosystems may change; some species may be forced out of their habitats (possibly to extinction) because of changing conditions, while others may flourish. Few of the terrestrial ecoregions on Earth could expect to be unaffected.

Increasing carbon dioxide may (up to a point) increase ecosystems' productivity; but the interaction with other aspects of climate change, means the environmental impact of this is unclear. An increase in the total amount of biomass produced is not necessarily all good, since biodiversity can still decrease even though a smaller number of species are flourishing.

Water scarcity

Sea level rises threaten to contaminate groundwater, affecting drinking water and agriculture in coastal zones. Increased evaporation will reduce the effectiveness of reservoirs. Increased extreme weather means more water falls on hardened ground unable to absorb it - leading to flash floods instead of a replenishment of soil moisture or groundwater levels. In some areas, shrinking glaciers threaten the water supply .

Higher temperatures will also increase the demand for water for cooling purposes.

In the Sahel, there has been on average a 25 per cent decrease in annual rainfall over the past 30 years .

Health

Direct effects of temperature rise

Rising temperatures have two opposing direct effects on mortality: higher temperatures in winter reduce deaths from cold; higher temperatures in summer increase heat-related deaths. The distribution of these changes obviously differs. Palutikof et al calculate that in England and Wales for a 1 °C temperature rise the reduced deaths from cold outweigh the increased deaths from heat, resulting in a reduction in annual average mortality of 7000.

In August 2003 a heatwave in Europe killed 22,000–35,000 people, based on normal mortality rates (Schär and Jendritzky, 2004). It can be said with 90% confidence that past human influence on climate was responsible for at least half the risk of the 2003 European summer heat-wave (Stott et al 2004).

If average temperatures increase by 1 degree Celsius, there will be an estimated 24,000 additional murders in the U.S. each year (as the additional heat stress leads to more frequent rage). (New Scientist 11/5/02, review of Body Heat by Mark Blumberg.)

Spread of disease

Global warming is expected to extend the favourable zones for vectors conveying infectious disease such as malaria . In poorer countries, this may simply lead to higher incidence of such diseases. In richer countries, where such diseases have been eliminated or kept in check by vaccination, draining swamps and using pesticides, the consequences may be felt more in economic than health terms, if greater spending on preventative measures is required .

Skin cancer / Vitamin D

A reduced ozone layer has negative impacts on human health—notably skin cancer and eye problems such as cataracts. However, the net effect of the thinning of the ozone layer on human health may be positive. Research by Dr. Edward Giovannucci, a Harvard University professor of medicine and nutrition who gave a keynote lecture at a recent American Association for Cancer Research, suggests that vitamin D might help prevent 30 deaths for each one caused by skin cancer. Vitamin D is nicknamed the "sunshine vitamin" because the skin makes it from ultraviolet rays , .

See also

• Global warming
• National Assessment on Climate Change for the United States
• Loop Current of the Gulf of Mexico

References

• ERM (2002). Predicted impact of global climate change on poverty and the sustainable achievement of the Millennium Development Goals. Report prepared for DFID by Environmental Resources Management. Reference 8409. London.
• McMichael et al. (2003). Climate Change and Human Health – Risk and Responses. WHO, UNEP, WMO, Geneva. ISBN 9241590815.
• Müller, B. (2002) Equity in Climate Change. The Great Divide. Oxford Institute for Energy Studies. Oxford, UK (Executive Summary)
• J.P. Palutikof, S. Subak and M.D. Agnew. "Impacts of the exceptionally hot weather in the UK", Climate Monitor 25(3)
• Schär and Jendritzky (2004). Hot news from summer 2003. Nature 432: 559-560.
• Stott, et al. (2004). Human contribution to the European heatwave of 2003. Nature 432, 2 December 2004
• Thomas, C. D., A. Cameron, R. E. Green, M. Bakkenes, L. J. Beaumont, Y. C. Collingham, B. F. N. Erasmus, M. Ferreira de Siqueira, A. Grainger, L. Hannah, L. Hughes, B. Huntley, A. S. Van Jaarsveld, G. E. Midgely, L. Miles, M. A. Ortega-Huerta, A. T. Peterson, O. L. Phillips, and S. E. Williams. 2004. Extinction risk from climate change. Nature 427: 145-148 (and a reply from the authors to some comments).

External links

• Munich Re: World Map of Natural Hazards
• Oliver James, The Guardian, June 30, 2005 Face the facts: For many people climate change is too depressing to think about, and some prefer to simply pretend it doesn't exist
• Mark Lynas, The Guardian, March 31, 2004, "Vanishing worlds"
• Peter Schwartz and Doug Randall, Global Business Network, "An Abrupt Climate Change Scenario and Its Implications for United States National Security", A report commissioned by the U.S. Defense Department, October 2003 (executive summary)
• Effects of global warming on skiing
• Stabilisation 2005 conference: survey of scientific papers
• Elevated levels of atmospheric CO₂ decrease the nutritional value of plants
• Royal Society, 30 June 2005, "Ocean acidification due to increasing atmospheric carbon dioxide"
• Time Magazine's "Global Warming: The Culprit?" (Time Magazine, October 3, 2005, pages 42-46)
• Hurricanes and Global Warming Review of most recent Nature and Science data.

• Climate change