What are the possible future impacts of global warming

As discussed in previous chapters, there is strong evidence to suggest that humanity's greenhouse gas emissions have already started to influence our climate. The most sophisticated and powerful computer models suggest global warming will cause major climatic changes by the end of the 21st century. These changes will potentially have wide-ranging effects on the natural environment as well as on human societies and our economies. Estimates have been made concerning the potential direct impacts on various socio-economic sectors, but in reality the full consequences are complicated to predict because impacts on one sector have an indirect effect on others. To assess these potential impacts, it is necessary to estimate the extent and magnitude of climate change, especially at national and local levels. For example, the latest IPCC 2001 reports look at the impacts on a continental level. There are also a number of excellent national reports, such as the National Assessment Synthesis Team 2001, which assesses climate change in the United States, dealing with the impacts on a region-by-region basis. Although much progress has been made in understanding the climate system and climate change, it must be remembered that projections of climate change and its impacts still contain huge uncertainties, particularly at the regional and local levels. The single biggest problem with global warming is our inability to predict the future. Although it is clear that humanity can live, survive, and even flourish in extreme climates from the Arctic to the Sahara, what causes problems is when the predictable extremes of the local climate are exceeded. Many of the future climate change problems are associated with water, either too much or too little compared with the usually expected amount. Unfortunately, changes in precipitation are even harder to predict than temperature. However, the most important influence on the relative impact of global warming-induced climate change is how regional economies develop and adapt in the future. So all the impacts discussed below can be mitigated to a significant degree by changes in the global economy.

The IPCC 2001 report estimates that global mean surface temperature could rise by between 1.4 and 5.8° C by 2100, which would mean that, in addition, global mean sea level would rise between 20 and 88 cm by 2100. Future climate change will have impacts on all factors affecting human society, including coastal og regions, storms and floods, health and water resources, agriculture, | and biodiversity. Below are reviewed each of these key areas of j| concern and the possible impact of climate change as assessed by 3 the IPCC. What cannot be assessed are the impacts if climate change occurs abruptly. This is discussed in Chapter 7.

Coastline

As we have seen, the IPCC reports that under a business-as-usual scenario (i.e. continued increase of burning fossil fuels) sea level could rise between 20 and 88 cm in the next 100 years, primarily through the thermal expansion of the oceans. This is a major concern to all coastal areas as it will decrease the effectiveness of coastal defences against storms and floods and increase the instability of cliffs and beaches. In Britain, the USA, and the rest of the developed world the response to this danger has been to add another few feet to the height of sea walls around property on the coast, the abandoning of some poorer-quality agricultural land to the sea (as it is no longer worth the expense of protecting it), and to add additional legal protection to coastal wetlands, being nature's best defence against the sea. However, globally, there are some nations based on small islands and river deltas, which face a much more dire situation.

For small island nations, such as the Maldives in the Indian Ocean and the Marshall Islands in the Pacific, a 1 m rise in sea level would flood up to 75% of the dry land, making the islands uninhabitable. Interestingly, it is also these countries, which rely on tourism, which have some of the highest fossil-fuel emissions per head of population than any other country in the world. However, there is a different twist to the story if we consider nations where a significant portion of the population lives by river deltas; these include, for example, Bangladesh, Egypt, Nigeria, and Thailand. A World Bank report in 1994 concluded that human activities on the deltas, such as freshwater £ extraction, were causing these areas to sink much faster than »

any predicted rise in sea level, increasing their vulnerability to S

storms and floods. f ua o a

In the case of Bangladesh, over three-quarters of the country is w within the deltaic region formed by the confluence of the Ganges, | Brahmaputra, and Meghna rivers. Over half the country lies less J' than 5 m above sea level; thus flooding is a common occurrence. During the summer monsoon a quarter of the country is flooded. Yet these floods, like those of the Nile, bring with them life as well as destruction. The water irrigates and the silt fertilizes the land. The fertile Bengal Delta supports one of the world's most dense populations, over 110 million people in 140 thousand square kilometres. But the monsoon floods have been getting worse throughout the 1990s. Every year the Bengal Delta should receive over 1 billion tonnes of sediment and a thousand cubic kilometres of freshwater. This sediment load balances the erosion of the delta both by natural processes and human activity. However, the Ganges River has been diverted in India into the Hooghly Channel for irrigation. The reduced sediment input is causing the delta to subside. Exacerbating this is the rapid extraction of fresh water

26. Flooding of Bangladesh in 1998. These scenes could be more common with sea-level rise and heavier monsoons from the delta for agriculture and drinking water. In the 1980s, 100,000 tube wells and 20,000 deep wells were sunk, increasing the freshwater extraction sixfold. Both these projects are essential to improving the quality of life for people in this region but have produced a subsidence rate of up to 2.5 centimetres per year, one of the highest rates in the world. Using estimates of subsidence rate and global warming sea-level rise, the World Bank has estimated that by the end of the 21st century the relative sea level in Bangladesh could rise by as much as 1.8 metres. In a worst-case scenario they estimated that this would result in a loss of up to 16% of land, supporting 13% of the population, and producing 12% of the current gross domestic product (GDP). Unfortunately, this scenario does not take any account of the devastation of the mangrove forest and the associated fisheries. Moreover, increased landward intrusions of salt water would further damage water ¡r quality and agriculture. This is a worst-case scenario and the »

greater part of the relative sea-level rise is not caused by global S warming. f ua o a

Another example of a threatened coastline is the Nile Delta, w which is one of the oldest intensely cultivated areas on Earth. It is | very heavily populated, with population densities up to 1,600 J'

inhabitants per square kilometre. Deserts surround the low-lying, fertile floodplains. Only 2.5% of Egypt's land area, the Nile Delta and the Nile valley, are suitable for intensive agriculture. Most of a 50 km wide land strip along the coast is less than 2 m above sea level and is only protected from flooding by a 1-10 km wide coastal sand belt, shaped by discharge of the Rosetta and Damietta branches of the Nile. Erosion of the protective sand belt is a serious problem and has accelerated since the construction of the Aswan dam in the south of Egypt. A rising sea level would destroy weak parts of the sand belt, which are essential for the protection of lagoons and the low-lying reclaimed lands. These impacts could be very serious. About one-third of Egypt's fish catches are made in the lagoons, and sea-level rise would change the water quality and affect most freshwater fish; valuable agricultural land would be inundated; vital, low-lying installations in Alexandria and Port Said would be threatened; recreational tourism beach facilities would be endangered; and essential groundwater would be salinated. All these effects are preventable, as dykes and protective measures would stop the worst flooding up to a 50 cm sea-level rise. However, there may still be serious groundwater salination and the impact of increasing wave action would be serious.

The most important influence on the impact of sea-level rise on coastal regions is the rate of change. At the moment the predicted rise of about 50 cm in the next hundred years can be dealt with if there is the economic foresight to plan for the protection and adaptation of coastal regions. This then comes back to the development of regional economies and the availability of resources to implement appropriate changes. If sea level rises by over 1 m in og the next hundred years, which is thought to be unlikely according to

| IPCC, then humanity would have major problems adapting to it.

Storms and floods

Storms and floods are major natural hazards, which between 1951 and 1999 were responsible for 76% of the global insured losses, 58% of the economic losses, and 52% of fatalities from natural catastrophes. It is, therefore, essential we know what is likely to happen in the future. We know from historic records that during periods of rapid climate change, weather patterns can become erratic and the number of storms can increase. One example of this is the Little Ice Age, which lasted from the end of the 16th to the beginning of the 18th century, and is mainly remembered for the ice fairs that were held on the frozen River Thames. However, what is not remembered is that going into and coming out of the Little Ice Age there were some apocalyptic tempests in Europe. For example, at the end of Little Ice Age, as climate was finally warming in 1703, there was the worst recorded storm in British history, which killed over 8,000 people. There is some evidence that the temperate regions, particularly in the northern hemisphere, have become more stormy over the last fifty years. The model simulations for the future of mid-latitude storms differ widely for the next hundred years. The computer models do, however, suggest that the proportion of rainfall occurring as heavy rainfall has and will continue to increase, as will the year-to-year variability. This will increase the frequency of flooding events.

Two-fifths of the world's population lives under the monsoon belt which brings life-giving rains. Monsoons are driven by the temperature contrast between continents and oceans. For example, moisture-laden surface air blows from the Indian Ocean to the Asian continent and from the Atlantic Ocean into West Africa during northern hemisphere summers, when the land masses F

become much warmer than the adjacent ocean. In winter the £

continents become colder than the adjacent oceans and high »

pressure develops at the surface, causing surface winds to blow S towards the ocean. Climate models indicate an increase in the f strength of the summer monsoons as a result of global warming a over the next hundred years. There are three reasons to support why w this should occur. (1) Global warming will cause continents to warm | more than the ocean in summer and this is the primary driving J' force of the monsoon system. (2) Decreased snow cover on Tibet, expected in a warmer world, will increase this temperature difference between land and sea, increasing the strength of the Asian summer. (3) Warmer climate means the air can hold more water vapour, so the monsoon winds will be able to carry more moisture. For the Asian summer monsoon this could mean an increase of 10-20% in average rainfall, with an interannual variability of 25-100% and a dramatic increase in the number of days with heavy rain. The most worrying model finding is the predicted increase in rain variability between years, which could double, making it very difficult to predict how much rainfall will occur each year - essential knowledge for farmers. An exception to this increase is given by the Met Office Hadley Centre GCM which predicts reduced rainfall over Amazonia, but increased rainfall in the other monsoon systems. This case study is discussed in more detail in the next chapter.

The good news is that currently there is no evidence from the last hundred years to show any increase in the number of hurricanes or cyclones. Most model predictions about future frequency and intensity of hurricanes are ambivalent, some suggesting increases while others suggest decreases. Most suggest that decadal and multi-decade variations will be larger than any trend caused by global warming.

Even if the numbers and the intensity of hurricanes and extratropical cyclones do not increase in the next century, global warming may influence our ability to predict these events, because our predictive capability is based on both the fundamental physics of the climate system and repetitive patterns of past weather events.

For example, storms are given a return time based on their | frequency in the past. This provides a means of managing coastal j| defences, river flood control, and water reserves. If these return 3 times become unpredictable, then new methods will have to be adopted to deal with storm and flood events. This view is supported by many of the climate models, which show that in a warmer world the year-to-year variability of storm occurrence and other extreme climate events becomes larger. A possible example of this was in the winter of 2000 when Britain experienced two floods in one month, both of which were classified as one-in-30-year events. Again, the low cost option in most developed countries for dealing with this increased variability is better weather prediction, tighter building regulations, stricter controls on the use of coastal regions and flood plains, and greater protection for coastal wetlands.

In terms of loss of human life, the frequency and intensity of storms are not the only controlling factors. The single major control on the number of deaths and cost of damage of a storm is the level of development of the region or country that is affected. This is shown by comparing two of the worst hurricanes that hit in the 1990s. In

August 1992 Hurricane Andrew hit the United States and caused record damage, estimated at $20 billion, but killed only 53 people. In 1998 Hurricane Mitch hit Central America and killed at least 20,000 people, made 2 million people homeless, and set back the economic growth of the region by decades. Therefore, even if global warming does increase the number of storms globally, economic development of the poorer countries could very quickly reduce the death rate but of course correspondingly increase the cost of the associated damage.

El Nino-Southern Oscillation

One of the most important and mysterious elements in global climate is the periodic switching of the direction and intensity F of ocean currents and winds in the Pacific. Originally known as £ El Nino ('Christ child' in Spanish), as it usually appears at Christmas, » and now more normally known as ENSO (El Nirio-Southern S

Oscillation), this phenomenon typically occurs every three to seven f years. It may last from several months to more than a year. The a

1997-8 El Nino conditions were the strongest on record and caused w droughts in southern USA, East Africa, northern India, north-east | Brazil, and Australia. In Indonesia, forest fires burned out of control J' in the very dry conditions. In California, parts of South America, Sri Lanka, and east-central Africa there were torrential rains and terrible floods.

ENSO is an oscillation between three climates, the 'normal' conditions, La Nina, and 'El Nirfo' (see Figure 27). El Nirfo conditions have been linked to changes in the monsoon, storm patterns, and occurrence of droughts all over the world. The state of the ENSO has also been linked to the position and occurrence of hurricanes in the Atlantic. For example, it is thought that the poor prediction of where Hurricane Mitch made landfall was because the ENSO conditions were not considered and the strong trade winds helped drag the storm south across Central America instead of west as predicted.

27. El Niño - Southern Oscillation (ENSO) a) normal conditions and b) El Niño conditions

Predicting El Niño events is very difficult but getting steadily better. For example, there is now a large network of both ocean and satellite monitoring systems over the Pacific Ocean, primarily aimed at recording sea-surface temperature, which is the major indicator of the state of the ENSO. By using this climatic data in both computer circulation models and statistical models, predictions are made of the likelihood of an El Ni~o or La Ni~a event. We are really still in the infancy stage of developing our understanding and predictive capabilities of the ENSO phenomenon.

There is also considerable debate over whether ENSO has been affected by global warming. The El Ni~o conditions generally occur every three to seven years; however, they have returned for three years out of four: 1991-2, 1993-4, and 1994-5. El Niño then ü

returned again to wreak havoc on global weather in 1997-8. m

Reconstruction of past climate using coral reefs in the western S Pacific shows sea-surface temperature variations back 150 years, f well beyond our historic records. The sea-surface temperature shows the shifts in ocean current, which accompany shifts in the W ENSO and reveal that there have been two major changes in the | frequency and intensity of El Ni~o events. First, was a shift at the ¿ beginning of the 20th century from a 10-15-year cycle to a 3-5-year cycle. The second shift was a sharp threshold in 1976 when a marked shift to more intense and even more frequent El Ninño events occurred. These are sobering results considering the huge weather disruption and disasters caused by recent El Niñno events. Modelling results also suggest that the current 'heightened' state of El Ninño can permanently shift weather patterns. For example, it seems that the drought region in the USA could be shifting eastward. However, as we have seen, to predict an El Ni~o event six months from now is hard enough without trying to assess whether or not ENSO is going to get more extreme over the next 100 years. Most computer models of ENSO in the future are inconclusive; some have found an increase and others have found none. This is, therefore, one part of the climate system which we do not know how global warming will affect. Not only does ENSO have a direct impact on global climate but it also affects the numbers, intensity, and pathways of hurricanes and cyclones, and the strength and timing of the Asian monsoon. Hence, when discussing the potential impacts of global warming one of the largest unknowns is the variation of ENSO and its knock-on effects on the rest of the global climate system.

Another possibility that we must consider is that in the early Holocene no evidence has been found for ENSO. In fact, it is thought that ENSO began sometime between 4,000 and 5,000 years ago. So Bj0rn Lomborg radically suggests in his book The Skeptical Environmentalist that a 2-3°C warming could be a good thing for the future as it may switch off ENSO. None of the computer models used to look at future climate has found this effect, and it must be remembered that the position of the Earth's og orbit compared to the sun was very different in the early Holocene,

| but it is something else to consider.

It has been suggested that global warming will have an adverse effect on human health. Initial suggestions have been that increased global temperatures will increase the death rate. A recent study shows that the population in Europe has successfully adapted their lifestyle to take into consideration the high summer temperatures. This is a classic case of individual risk assessment and adaptation, because most heat-related mortality occurs when the temperature goes above a usual temperature. For example, in London heat-related mortality starts at 22.3°C while in Athens it starts at 25.7°C. So it seems that providing the correct information and continued increased accessibility to air conditioning will mean that the world will be able to adapt to warmer conditions. In fact, it has also been suggested that the death rate may even drop, since more people die from cold weather than warm weather, thus warmer winters would reduce this cause of death.

By far the most important threat to human health, however, is access to fresh drinking water. At present, rising human populations, particularly growing concentrations in urban areas, are putting great stress on water resources. The impacts of climate change - including changes in temperature, precipitation, and sea levels - are expected to have varying consequences for the availability of fresh water around the world. For example, changes in river run-off will affect the yields of rivers and reservoirs and thus the recharging of groundwater supplies. An increase in the rate of evaporation will also affect water supplies and contribute to the salinization of irrigated agricultural lands. Rising sea levels may result in saline intrusion in coastal aquifers. Currently, approximately 1.7 billion people, a third of the world's population, live in countries that are water-stressed. IPCC reports suggest that with the projected global population increase, and the expected £ climate change, assuming present consumption patterns, 5 billion » people will experience water stress by 2025. Climate change is likely S to have the greatest impact in countries with a high ratio of relative f use to available supply. Regions with abundant water supplies will a get more than they want with increased flooding. As suggested w above, computer models predict much heavier rains and thus major | flood problems for Europe, whilst, paradoxically, countries that J' currently have little water (e.g. those relying on desalinization) may be relatively unaffected. It will be countries in between, which have no history or infrastructure for dealing with water shortages, which will be the most affected. For in central Asia, North Africa, and southern Africa there will be even less rainfall and water quality will become increasingly degraded through higher temperatures and pollutant run-off. Add to this the predicted increased year-to-year variability in rainfall, and droughts will become more common. Hence, it is those countries that have been identified as most at risk which need to start planning now to conserve their water supplies and/or deal with the increased risks of flooding, because it is the lack of infrastructure to deal with drought and floods rather than the lack or abundance of water which causes the threat to human health. Therefore, economic development of areas most at risk is essential in the next century to provide resources to mitigate the effects of global warming.

Another possible future threat to human health is the increased transmission of many infectious diseases, as these are directly affected by climatic factors. Infective agents and their vector organisms (e.g. mosquitoes) are sensitive to factors such as temperature, surface water, humidity, wind, soil moisture, and changes in forest distribution. For example, there is a strong correlation between increased sea-surface temperature and sea level and the annual severity of the cholera epidemics in Bangladesh. With predicted future climate change and the rise in Bangladesh's relative sea level, cholera epidemics could increase. Climate change will particularly influence vector-borne diseases (VBD), i.e. diseases which are carried by another organism, such as malaria carried by mosquitoes. It is, therefore, projected that climate change and og altered weather patterns would affect the range (both altitude and | latitude), intensity, and seasonality of many vector-borne and other j| infectious diseases. In general, increased warmth and moisture 3 caused by global warming will enhance transmission of diseases. While the potential transmission of many of these diseases increases in response to climate change, we should remember that our capacity to control the diseases will also change. New or improved vaccination can be expected; some vector species can be constrained by use of pesticides. Nevertheless, there are uncertainties and risks here, too: for example, long-term pesticides use breed-resistant strains and kill many predators of pests.

The most important vector-borne disease is malaria, with currently 500 million infected people worldwide, which is about twice the population of the USA. Plasmodium vivax, which is carried by the Anopheles mosquito, is an organism which causes malaria. The main climate factors that have a bearing on the malarial transmission potential of the mosquito population are temperature and precipitation. Assessments of the potential impact of global climate change on the incidence of malaria suggest a widespread increase of risk because of the expansion of the areas suitable for malaria transmission. Mathematical models mapping out the suitable temperature zones for mosquitoes suggest that by the 2080s the potential exposure of people could increase by 2-4% (260-320 million people). The predicted increase is most pronounced at the borders of endemic malarial areas and at higher altitudes within malarial areas. The changes in malaria risk must be interpreted on the basis of local environmental conditions, the effects of socio-economic development, and malaria control programmes or capabilities. The incidence of infection is most sensitive to climate changes in areas of South-East Asia, South America, and parts of Africa. Global warming will also provide excellent conditions for Anopheles mosquitoes to breed in southern England, Europe, and the northern USA.

It should, however, be noted that the occurrence of most tropical » diseases is related to development. An example was major epidemic t disease in much of Europe during the Little Ice Age. As recently as f the 1940s malaria was endemic in Finland, Poland, Russia, and 36

a states in the USA including Washington, Oregon, Idaho, Montana, w North Dakota, New York, Pennsylvania, and New Jersey. So though | global warming has the potential to increase the range of many J' of these tropical diseases, the experience of Europe and the USA suggests that combating malaria is strongly linked to development and resources: development to ensure efficient monitoring of the disease and resources to secure a strong effort to eradicate the mosquitoes and their breeding grounds.

Biodiversity

The IPCC report lists the following species as those most at threat from climate change as a result of global warming: the mountain gorilla in Africa, amphibians that only live in the cloud forests of the neotropics, the spectacled bear of the Andes, forest birds of Tanzania, the Resplendent Quetzal in Central America, the Bengal tiger, and other species only found in the Sundarban wetlands, rainfall-sensitive plants found only in the Cape Floral Kingdom of South Africa, polar bears, and penguins. Natural habitats that are threatened include coral reefs, mangroves, other coastal wetlands, mountain ecosystems found in the upper 200-300 m of mountainous areas, prairie wetlands, permafrost ecosystems, and ice edge ecosystems which provide a habitat for polar bears and penguins. The primary reason for the threat to these species or ecosystems is that they are unable to migrate in response to climate change because of their particular geographical location or the encroachment of human activity, particularly farming and urbanization. An example of the former is the cloud forests of the neotropics: as climate changes, this particular climatic zone will migrate up the mountainside until the point where there is no more mountain.

One example of an ecosystem under threat is the coral reefs. Coral og

•E reefs are a valuable economic resource for fisheries, recreation, E

| tourism, and coastal protection. In addition, reefs are one of the j| largest global stores of marine biodiversity, with untapped genetic 3 resources. Some estimate that the global cost of losing the coral reefs runs into hundreds of billions of dollars each year. The last few years have seen unprecedented declines in the health of coral reefs. In 1998 El Nino was associated with record sea-surface temperatures and associated coral bleaching, which is when the coral expels the algae that live within them and that are necessary to their survival. In some regions, as much as 70% of the coral may have died in a single season. There has also been an upsurge in the variety, incidence, and virulence of coral disease in recent years, with major die-offs in Florida and much of the Caribbean region. In addition, increasing atmospheric carbon dioxide concentrations could decrease the calcification rates of the reef-building corals, resulting in weaker skeletons, reduced growth rates, and increased vulnerability to erosion. Model results suggest these effects would be most severe at the current margins of coral reef distribution.

On a more theoretical note, a recent study by Thomas et al. (Nature,

427, 145-8, 2004) investigated the possible increase in the likely extinction rate over the next 50 years in key regions such as Mexico, Amazonia, and Australia. The theoretical models suggest that by 2050 the climatic changes predicted by the IPCC would commit 18% (warming of 0.8-1.7°C), 24% (1.8-2.0°C), and 35% (above 2.0°C) of the species studied to extinction in these regions. That means a quarter of all species in these regions may become extinct by the middle of this century. There are many assumptions in their models, which may or may not be true; for example, they assume we know the full climatic range in which each species can persist and the precise relationship between shrinking habitat and extinction rates. So these results can only be seen as the likely direction of extinction rates, not necessarily the exact magnitude. However, these predictions do represent a huge future threat to regional and global biodiversity and illustrate the sensitivity of the biological system to the amount and rate of warming that will occur in the future.

Agriculture

One of the major worries concerning future climate change is the effect it will have on agriculture, both globally and regionally. The main question is whether the world can feed itself under the predicted future global warming conditions. Predictions of cereal production for 2060 suggest that there are still huge uncertainties about whether climate change will cause global agricultural production to increase or decrease. If the predicted temperature increases are considered, then we expect there to be a drop in food production in both the developed and less-developed countries. But if other effects are taken into consideration, then this effect of temperature is greatly reduced, or in the case of the developed world becomes an increase. One of the most important additional factors is that increased atmospheric carbon dioxide acts as a fertilizer; thus scientific studies have shown that plants in an atmosphere which contains more carbon dioxide grow faster and better, because the CO2 is essential for photosynthesis and the prime source of carbon for plants. So plants like more atmospheric CO2 and thus farm yields may increase in the future in many regions. In addition, if it is assumed that farmers can take action to adapt to changing climate this also boosts or at least maintains agricultural production in many regions. For example, farmers could vary the planting time and/or switch to a different variety of the same plant to respond to changing conditions. Therefore, models suggest that with reasonable assumptions on a worldwide scale, the change is expected to be small or moderate. But this does not mean the amount of cereal produced worldwide will be the same or lower in 2060 compared with today. Since 1960 world grain production has doubled and is predicted to continue to rise at a similar rate. So even a pessimistic 1999 study using the Met Office Hadley Centre climate model estimated that cereal production in 2080 would only increase by 90% compared with today, not by 94% which would have occurred in the absence of

•E global warming.

j| This, however, masks the huge changes that will occur in different

3 regions, with both winners and losers, the poorest countries, of course, which are least able to adapt, being the losers. Also the results of all these studies are heavily dependent on the assumed trade models and market forces used, as, unfortunately, agricultural production in the world has very little to do with feeding the world's population and much more to do with trade and economics. Hence, this is why the EU has stockpiles of food, while many underdeveloped countries export cash crops (e.g. sugar, cocoa, coffee, tea, rubber, etc.) but cannot adequately feed their own populations. A classic example is the West African state of Benin, where cotton farmers can obtain cotton yields of four to eight times per hectare greater than their US competitors in Texas. The USA subsidizes their farmers, however, which means that US cotton is cheaper than that coming from Benin. Currently, US cotton farmers receive $3.9 billion in subsidies, almost twice the total GDP of Benin. So even if global warming makes Texan cotton yields even lower, it still does not change the biased market forces.

So in the computer models, markets can reinforce the difference between agricultural impacts in developed and developing countries and, depending on the trade model used, agricultural exporters may gain in money even though the supplies fall, because when a product becomes scarce the price rises. The other completely unknown factor is the extent to which a country's agriculture can be adaptable. For example, the models assume that production levels in developing countries will fall more compared with those in the developed countries because their estimated capability to adapt is less than that of developed countries. But this is just another assumption that has no analogue in the past, and as these effects on agriculture will occur over the next century, many developing countries may catch up with the developed world in terms of adaptability.

One example of the real regional problems that global warming » could cause is the case of coffee-growing in Uganda. Here, the total t area suitable for growing Robusta coffee would be dramatically f reduced, to less than 10%, by a temperature increase of 2°C. Only a higher areas would remain; the rest would become too hot to grow w coffee. This demonstrates the vulnerability to the effects of global | warming of many developing countries, whose economies often rely J' heavily on one or two agricultural products. Hence, one major adaptation to global warming should be the broadening of the economic and agricultural base of the most threatened countries. This, of course, is much harder to do in practice than on paper and it is clear that the EU and US agricultural subsidies and the current one-sided World Trade Agreements have a greater effect on global agricultural production and the ability of countries to feed themselves than global warming will ever have.

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