There are some beneficial effects of global warming such as the extension toward the poles of the area of agricultural land in Canada and Northern Europe, and possibly New Zealand. Most of the effects however range from inconvenience to disaster. They include extreme weather conditions, melting of the polar ice caps and glaciers, and disruption or extinction of flora and fauna.
The Report by Sir Nicholas Stern in 2006 examined the current scientific consensus on the possible consequences of global warming and the economic costs. He detailed some of the likely consequences of different temperature rises:
1°C: Smaller mountain glaciers disappear in the Andes, threatening the water supply of 50 million people. More than 300,000 extra people die from increase in climate-related diseases in tropical regions.
2°C: Water scarcity increases in southern Africa and the Mediterranean. Significant decline in food production in Africa, where malaria affects up to 60 million people. Up to 10 million extra people affected by coastal flooding each year.
3°C: Serious droughts in southern Europe occur once every ten years. Between 1 and 4 billion people suffer water shortages and a similar number suffer from floods. Many millions of people are at risk of malnutrition, as agricultural yields at higher latitudes reach peak output. More than 100 million people are affected by the risk of coastal flooding.
4°C: Sub-Saharan Africa and the southern Mediterranean suffer between 30 and 50% decrease in availability of water. Agricultural yields decline by 15-35% in Africa. Crops fail in entire regions. Up to 80 million extra people are exposed to malaria.
5°C: There is a possible disappearance of the large glaciers in the Himalayas, affecting the supply of 25% of the population of China and hundreds of millions more in India. Ocean acidity increases with threat of total collapse in the global fisheries industry. Sea levels rise inexorably, inundating vast regions of Asia and about half of the world's major cities, including London, New York and Tokyo.'
This implies that the USA, Brazil, the Mediterranean countries, East Russia and the Middle East, Southern Africa and Australia are likely to lose up to 30% of their rainfall by 2050, and increase by a similar amount in Central Asia, Pakistan, India, Siberia and most of China. This will have a large effect of the people of India because the monsoon is very sensitive to rainfall: fluctuations of only 10% from the average value can lead to drought or flooding. The melting of the Himalayan glaciers could ultimately lead to extensive desertification. Many of the effects of global warming are very long-term, but are nevertheless devastating.
The conclusion of the Report is that if no action is taken the world could lose at least 5% and possibly up to 20% of global GDP each year indefinitely equivalent to £3.68 trillion. If, however, action is taken to reduce greenhouse gas emissions this could be limited to 1% of GDP each year, equivalent to £148 billion. This 'is an international problem requiring international co-operation and leadership' (Nuclear Issues 28, November 2006).
There are several other possible effects of climate change. There are many cases of communities that have flourished for centuries before being destroyed. One example is the Anasazi who lived in Colorado and were finally forced by major droughts in 1130-1180 and 1275-1299 to abandon their cities and to move away to areas with a climate that allowed them to continue their lifestyle. Greenland, as its name implies, was at one time a fertile land, and supported a colony of Vikings from about 1250 to 1650 when colder weather forced them to move elsewhere. The same cold spell around the fourteenth century caused parts of the Baltic Sea to freeze, and also the river Thames. In the nineteen thirties the reduction in rainfall on the Great Plains in the USA, followed by winds that removed the topsoil and created a dustbowl, forced farmers to pack up and move away. In other parts of the world weather patterns are subject to violent changes. The normally regular monsoons in India, for example, can sometimes fail, causing catastrophic famines.
One of these catastrophic famines occurred in 1899 and stimulated Sir Gilbert Walker, the Director General of Observatories in India, to study monsoons in more detail. He found that monsoons are not just a local phenomenon but are part of a vast global climate fluctuation called the Southern Oscillation. This consists of a cyclic process whereby high pressures in the Pacific are correlated with low pressures in the Indian ocean, and vice-versa. The period of this oscillation is about three or four years (Philander 1998).
Walker's work was recognised in the 1950s, when Bjerknes realised that the well-known El Nino current is part of the Southern Oscillation. He suggested that the Southern Oscillation is a periodic oscillation between El Nino and a complementary state called El Nina. El Nino has long been known as a warm ocean current that can have disastrous effects on the eastern Pacific shores. Satellite observations and computer modelling now enable some predictions of its effects to be made (Fagan 2004). Thus for example severe floods and storms were predicted to occur in California in 1997-1998. During autumn and over Christmas the weather was fine, but in January and February hurricane force winds battered San Francisco, floods rose and mudslides swept houses away. Floodwaters submerged the freeway to Los Angeles and swept away the Southern Pacific railroad bridge. Fourteen years earlier another El Nino caused floods and landslides that caused a billion dollars worth of damage. In other tropical regions, the 1997-1998 El Nino caused over $10 billion in damage. There were severe droughts in Australia and Southeast Asia, vast forest fires in Indonesia and Mexico and famine in Brazil.
The weather cycles in Peru and Bolivia are quite regular, unless they are interrupted by El Nino, which is unpredictably variable. Mostly the result is torrential rainstorms, warmer seas and changes in fish populations. Occasionally, however, an El Nino event causes significant changes to the climate and brings ruin to fishermen and farmers. Thus in 1925 the sea temperature off northern Peru rose over six degrees in ten days. Millions of seabirds perished as the anchovies on which they fed moved to cooler nutrient-rich waters. Cloudbursts turned dry ravines into raging torrents and the city of Trujillo received 396 mm of rain instead of the normal 1.7 mm. Farmlands and irrigation systems were destroyed by a sea of mud, and hundreds of people starved. Many other examples could be given of the devastating effects of El Nino.
It is conjectured that El Nino events played an important role in the decline and fall of ancient civilisations when they were already seriously stressed by other economic and political factors. Particular examples are the Maya civilisation in Yucatan and the Moche civilisation in Peru. In these cases the devastation caused by El Nino was the final event that caused the once-flourishing societies to collapse.
The rapid changes in climate associated with El Nino events took place long before the temperature increase associated with global warming and still continue. They are not yet fully understood and make it more difficult to ascribe climate changes to anthropogenic emissions.
Recent studies (Ker 2005; Webster et al. 2005; Witze 2006; Smith 2006) have shown greatly increased frequencies during the last 35 years for the more devastating hurricanes like the one called Katrina that struck New Orleans and the surrounding states in 2005. This increase has been attributed by some scientists to the rising temperatures of the oceans due to global warming (Witze 2006). If this is the case, some regions of the earth will be liable to more devastating hurricanes in the future. The cost of the damage due to the hurricane Katrina has been estimated to be around $100 billion.
On a much longer timescale, the mathematician Milutan Milankovitch identified cold and warm periods alternating every 100,000 years, with smaller cycles every 41,000 and 10,000 years. These cycles are attributed to perturbations of the earth's orbit by the moon that cause the precession of the equinoxes and by other small effects due to the planets and are confirmed by worldwide measurements of glaciers, coral reefs, peat bogs and polar ice caps.
There are thus many ways the climate can be changed in addition to the effects of the greenhouse gases. Careful scientific analysis is therefore needed before their contribution can be established. Many scientists worldwide are making detailed calculations using increasingly sophisticated models of the atmosphere. This is obviously a very complicated task. What, for example, do we mean by the temperature of the atmosphere? We can measure the temperature at a particular place and height, but this needs to be done over the whole surface of the earth and for heights up to several miles. The best we can do is to establish a grid of points and measure the temperatures at these points as a function of the time. Even a coarse grid contains millions of points and the calculations are very time-consuming even on a fast modern computer. The more accurate we want our calculations to be the longer they will take. In addition, the results may be very sensitive to the initial conditions; this is called the butterfly effect. The main uncertainty at present seems to be the effects of water vapour, which are greater than those of all the other gases combined. These are sensitively affected by changes in the cloud cover which in turn changes the amount of solar energy absorbed or reflected.
The results of such calculations are published periodically by the Intergovernmental Panel on Climate Change, consisting of about two thousands of the world's leading climate scientists, under the Chairmanship of Sir John Houghton. With many qualifications, the conclusion of the latest assessment (2007) is that there is unequivocal evidence that world temperature is increasing, and it is predicted that the average temperature will rise by about 0.2°C per decade. The sea level is predicted to rise by about 17 cm in the next century and this will eliminate many islands such as the Maldives in the Indian Ocean, and will inundate much of Bangladesh and some of Holland. Already the sea level has risen by 0.1 to 0.2 cm per year during the twentieth century.
The connection between the rise in temperature and the rise in sea level has been attributed to the melting of the polar ice caps. However, the ice immediately around the North Pole and in the ice shelves around Antarctica is floating, and so when it melts it has very little effect on the sea level, as Archimedes knew very well. There may, however, be some small effects due to differences in salinity between the ice and the sea.
Antarctica occupies 13.2 million sq.km, about 1.3 times the area of Europe and the ice cap is up to 4 km thick. No less than 90% of the world's ice is in Antarctica, and if this were all to melt the world sea level would rise by 70 to 90 m. However the ice in central Antarctica is at a temperature from -40 to -60°C and so is unaffected by a rise in temperature of a few degrees. The same applies to central Greenland which occupies an area about one-sixth of Antarctica. The ice shelves surrounding Antarctica are somewhat warmer, but are floating like the Arctic sea ice and so melting them has little effect on the sea level. However the sea level is affected by warmer coastal glaciers flowing into the sea from the ice caps of Antarctica, Greenland and Northern Canada. The glaciers are very thick, and the pressure on the ice where it rides over the ground is enough to liquefy it, and this makes it easier for it to slide down into the sea. An additional effect has been suggested: when the surface of the ice melts lakes are formed and this water flows down through cracks in the ice until it reaches the bedrock. There it spreads and reduces the friction between the ice and the bedrock, further increasing the rate of travel of the ice towards the sea. When this happens it causes the sea level to rise, but by an amount that is difficult to estimate. There is increasing evidence that the ice shelves are breaking up and this reduces the pressure on the glaciers and so their rate of flow increases (Shepherd 2006; Murray et al. 2008). The sea level rise is offset by increased evaporation from the sea, and so the net effect is difficult to determine.
The melting of the Arctic ice due to the rising temperature is already having a devastating effect on people living in the far north. In several places their traditional method of hunting seals is no longer possible because the ice has melted. The dry weather has caused widespread forest fires in Alaska, and the temperature of the permafrost has risen by two or three degrees. This leads to increased emission of methane, a greenhouse gas much more damaging than carbon dioxide. As the ice melts the albedo, a measure of the fraction of sunlight that is reflected falls rapidly from 0.8 to 0.9 to less than 0.1. The result is that more sunlight is absorbed, melting more ice in a continuing positive feedback effect. This is one of the main reasons why the Arctic ice is melting so rapidly, thus providing a sensitive indicator of global warming. Some computer models indicate that by 2080 the Arctic ocean will be ice-free in summer, making it impossible for the polar bears to survive (Comiso and Parkinson 2004). The Arctic ice has shrunk by more than a third since the late twentieth century, and the rate has accelerated faster than predicted by climate models.
Satellite observations have shown that the perennial Arctic sea ice covers about seventeen billion acres. This area varies from year to year, but in recent years the overall trend has been strongly downward, particularly in the Beaufort and Chukchi seas, and also to a lesser extent in the Siberian and Laptev seas. The shrinkage now amounts to about 250 million acres, and recent satellite and other observations show that the retreat is more rapid than estimated by any of the eighteen computer models used by the IPCC.
Many glaciers in Iceland and other mountainous countries are also retreating rapidly. Some of those in Iceland have retreated by as much as 1000 ft in a decade. Climate models predict that by the end of the century there will be ice only on the highest mountains.
The effects of global warming are not necessarily a smooth function of time. Thus according to one theory of Antarctic deglaciation at the start of the Holocene about 10,000 years ago a small change in temperature increased the temperature and hence raised the sea level. This destabilized the floating ice shelves which then broke up, exposing the margin of the ice sheet to attack by waves. This allowed the glaciers behind the ice shelves to flow faster into the sea, thus further raising the sea level. This process continued until feedback mechanisms eventually slowed it down. In this way a relatively small event can initiate a runaway series of events with significant consequences.
Another possible effect is due to the expansion of the oceans when they are heated. It is seldom mentioned that the land also expands when heated, and so whether this affects the sea level depends on the relative expansion coefficients of the land and the sea. This is further complicated by times that it takes for the warming to take place, which depends on the thermal conductivities of land and sea, and by the presence of currents in the sea.
Global warming is such a complicated problem, with many contributing causes and feedback loops, that there is still much debate about whether it is real or not. The climatologist Stephen H. Schneider has considered this question, and has pointed out that it depends on what we mean by proof. He considers that a strict deductive proof of the course of future events is impossible, but that 'my reading of the many lines of evidence puts global warming well over the minimum threshold of belief — far enough to assert that it is already proved to the point where we need to consider it seriously' (Brockman 2005, p. 76).
A recent report from the Environmental Protection Agency in the United States has highlighted the health effects of global warming, in particular concerning the food, energy and water on which society depends. They suggest that 'extreme weather events and diseases carried by ticks could kill more people as temperatures rise and allergies could worsen because climate change could produce more pollen. Smog, a leading cause of respiratory illness and lung diseases could become more severe'. These effects are already affecting human health and welfare.
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