Introduction to Global Warming

Measurements of temperature taken by instruments all over the world, on land and at sea have revealed that during the 20^th century the Earth’s surface and lowest part of the atmosphere warmed up on average by about 0.6°C. During this period, man-made emissions of greenhouse gases, including carbon dioxide, methane and nitrous oxide have increased, largely as a result of the burning of fossil fuels for energy and transportation, and land use changes including deforestation for agriculture. In the last 20 years, concern has grown that these two phenomena are, at least in part, associated with each other. That is to say, global warming is now considered most probably to be due to the increases in greenhouse gas emissions and concurrent increases in atmospheric greenhouse gas concentrations, which have enhanced the Earth's natural greenhouse effect. Whilst other natural causes of climate change can cause global climate to change over similar periods of time, computer models demonstrate that in all probability there is a real discernible human influence on the global climate.

If the climate changes as current computer models have projected, global average surface temperature could be anywhere from 1.4 to 5.8°C higher by the end of the 21^st century than in 1990. To put this temperature change into context, the increase in global average surface temperature which brought the Earth out of the last major ice age 14,000 years ago was of the order of 4 to 5°C. Such a rapid change in climate will probably be too great to allow many ecosystems to suitably adapt, and the rate of species extinction will most likely increase. In addition to impacts on wildlife and species biodiversity, human agriculture, forestry, water resources and health will all be affected. Such impacts will be related to changes in precipitation (rainfall and snowfall), sea level, and the frequency and intensity of extreme weather events, resulting from global warming. It is expected that the societies currently experiencing existing social, economic and climatic stresses will be both worst affected and least able to adapt. These will include many in the developing world, low-lying islands and coastal regions, and the urban poor.

The Framework Convention on Climate Change (1992) and the Kyoto Protocol (1997) represent the first steps taken by the international community to protect the Earth's climate from dangerous man-made interference. Currently, nations have agreed to reduce greenhouse gas emissions by an average of about 5% from 1990 levels by the period 2008 to 2012. The UK, through its Climate Change Programme, has committed itself to a 12.5% cut in greenhouse gas emissions. Additional commitments for further greenhouse gas emission reduction will need to be negotiated during the early part of the 21^st century, if levels of greenhouse gas concentrations in the atmosphere are to be stabilised at reasonable levels. Existing and future targets can be achieved by embracing the concept of sustainable development - development today that does not compromise the development needs of future generations. In practical terms, this means using resources, particularly fossil-fuel-derived energy, more efficiently, re-using and recycling products where possible, and developing renewable forms of energy which are inexhaustible and do not pollute the atmosphere.

British Isles

Climate change has potential risks for the British Isles. Most critical of these risks is an increase in frequency and intensity of extreme weather such as hot spells, drought and storms. Accompanying a projected rise in average surface temperature of between 0.9 and 2.4°C by 2050 will be the increased occurrence of hot, dry summers, particularly in the southeast. Mild wet winters are expected to occur more often by the middle of the 21^st century, especially in the northwest, but the chance of extreme winter freezing should diminish.

Higher temperatures may reduce the water-holding capacity of soils and increase the likelihood of soil moisture deficits, particularly if precipitation does not increase as well. These changes would have a major effect on the types of crops, trees or other vegetation that the soils can support. The stability of building foundations and other structures, especially in central, eastern and southern England, where clay soils with a large shrink-swell potential are abundant, would be affected if summers became drier and winters wetter.

Any sustained rise in mean surface temperature exceeding 1°C, with the associated extreme weather events and soil water deficits, would have marked effects on the UK flora and fauna. There may be significant movements of species northwards and to higher elevations. Predicted rates of climate change may be too great for many species, particularly trees, to adapt genetically. Many native species and communities would be adversely affected and may be lost to the UK, especially endangered species which occur in isolated damp, coastal or cool habitats. It is likely that there would be an increased invasion and spread of alien weeds, pests, diseases and viruses, some of which may be potentially harmful. Increased numbers of foreign species of invertebrates, birds and mammals may out-compete native species.

Climate changes are likely to have a substantial effect on agriculture in the UK. In general, higher temperatures would decrease the yields of cereal crops (such as wheat) although the yield of crops such as potatoes and sugar beet would tend to increase. However, pests such as the Colorado beetle on potatoes and rhizomania on sugar beet, currently thought to be limited by temperature, could become more prevalent in the future. The length of the growing season for grasses and trees would increase by about 15 days per degree Celsius rise in average surface temperature, an increase that could improve the viability of crops such as maize and sunflower, which are currently grown more in warmer climates.

Increases in sea level, and the frequency and magnitude of storms, storm surges and waves would lead to an enhanced frequency of coastal flooding. A number of low-lying areas are particularly vulnerable to sea level rise, including the coasts of East Anglia, Lancashire, Lincolnshire and Essex, the Thames estuary, parts of the North Wales coast, the Clyde/Forth estuaries and the Belfast Lough. Flooding would result in short-term disruption to transport, manufacturing and housing, and long-term damage to engineering structures such as coastal power stations, rail and road systems. In addition, long-term damage to agricultural land and groundwater supplies, which provide about 30% of the water supply in the UK, would occur in some areas due to salt water infiltration.

Water resources would generally benefit from wetter winters, but warmer summers with longer growing seasons and increased evaporation would lead to greater pressures on water resources, especially in the southeast of the UK. Increased rainfall variability, even in a wetter climate, could lead to more droughts in any region in the UK. Higher temperatures would lead to increased demand for water and higher peak demands, requiring increased investment in water resources and infrastructure. An increase in temperature would increase demand for irrigation, and abstraction from agriculture would compete with abstractions for piped water supply by other users.

Higher temperatures would have a pronounced effect on energy demand. Space heating needs would decrease substantially but increased demand for air conditioning may entail greater electricity use. Repeated annual droughts could adversely affect certain manufacturing industries requiring large amounts of process water, such as paper-making, brewing and food industries, as well as power generation and the chemical industry.

Sensitivity to weather and climate change is high for all forms of transport. Snow and ice present a very difficult weather related problem for the transport sector. A reduction in the frequency, severity and duration of winter freeze in the British Isles would be likely under conditions associated with global warming and could be beneficial. However, any increase in the frequency of severe gale episodes could increase disruption to all transport sectors.

The insurance industry would be immediately affected by a shift in the risk of damaging weather events arising from climate change in the British Isles. If the risk of flooding increases due to sea level rise, this would expose the financial sector to the greatest potential losses.

UK tourism has an international dimension which is sensitive to any change in climate which alters the competitive balance of holiday destinations worldwide. If any changes to warmer, drier summer conditions occur, this could stimulate an overall increase in tourism in the UK. However, any significant increase in rainfall, wind speed or cloud cover could offset some of the general advantages expected from higher temperatures.

Methane

Methane (CH₄) is a colourless, odourless non-toxic gas consisting of molecules of four hydrogen atoms and one carbon atom. Methane is combustible, and mixtures of about 5 to 15% in air are explosive. It is the main constituent of natural gas, a fossil fuel. It is released into the atmosphere when organic matter decomposes in environments lacking sufficient oxygen. Natural sources include wetlands, swamps and marshes, termites, and oceans. Man-made sources include the mining and burning of fossil fuels, digestive processes in ruminant animals such as cattle, rice paddies and the burying of waste in landfills. Most methane is broken down in the atmosphere by reacting with hydroxyl (OH) radicals.

Like carbon dioxide, methane is a greenhouse gas whose molecules absorb heat trying to escape to space. Methane contributes to the Earth's natural greenhouse effect. Man-made emissions of methane are helping to enhance the greenhouse effect. Since the beginning of the Industrial Revolution, atmospheric methane concentration has more than doubled, and has contributed 20% to the enhancement of the greenhouse effect, second only to carbon dioxide.

Chlorofluorocarbons

Chlorofluorocarbons, commonly known as CFCs, are a group of man-made compounds containing chlorine, fluorine and carbon. They are not found anywhere in nature. The production of CFCs began in the 1930s for the purpose of refrigeration. Since then they have been extensively utilised as propellants in aerosols, as blowing agents in foam manufacture and in air conditioning. There are no removal processes or sinks for CFCs in the lowest part of the atmosphere called the troposphere. As a result they are transported up into the stratosphere, between 10 to 50 km above the Earth's surface, where they are broken down by ultraviolet (UV) radiation from the Sun, releasing free chlorine atoms which cause significant ozone depletion.

Although the amounts of CFCs in the atmosphere are very small, measured in parts per trillion (million million), they do contribute significantly to the enhancement of the natural greenhouse effect, because they are very good at trapping heat. Molecule for molecule some CFCs are thousands of times stronger than carbon dioxide as greenhouse gases.

Since the dangers caused by CFCs to the ozone layer were first identified, their use has gradually been phased out, according to international agreements made in Montreal, Canada, in 1987. However, CFCs have long lifetimes in the atmosphere before they are broken down by sunlight, and consequently they will continue to enhance the greenhouse effect well into the 21^st century.

Water

Global warming will lead to an intensification of the global water or hydrological cycle through increases in surface temperature and rates of evaporation, and in some regions, increases in precipitation. Changes in the total amount of precipitation and its frequency and intensity directly affect the magnitude and timing of run-off and the intensity of floods and droughts. Such changes will have significant impacts on regional water resources.

It is not certain how individual water catchment areas will respond to changing evaporation rates and precipitation. It is likely however, that currently dry regions will be more sensitive to changes in climate. Relatively small changes in temperature and precipitation could cause relatively large changes in run-off. Arid and semi-arid regions will therefore be particularly sensitive to reduced rainfall and to increased evaporation.

An increase in the duration of dry spells will not necessarily lead to an increased likelihood of low river flows and groundwater levels, since increases in precipitation may be experienced during other seasons. More probably, increased rainfall will lead to an increased likelihood of river flooding. Changes in seasonal patterns of rainfall may affect the regional distribution of both ground and surface water supplies.

Hydrological regimes in high latitude or mountain areas are often determined by winter snowfall and spring snowmelt. Most climate models predict that global warming will reduce the amount of precipitation falling as snow in these regions, increasing the rate of water run-off and enhancing the likelihood of flooding. Climatic effects on tropical hydrological regimes are harder to predict. In the mid-latitudes, including the UK, wintertime soil moisture is expected to increase whilst summertime soil moisture may decrease. There will however, be regional variations.

Freshwater ecosystems, including lakes, streams and non-coastal wetlands will be influenced by changes to the hydrological cycle as a result of global warming. These influences will interact with other man-made changes in land use, waste disposal and water extraction. In general, freshwater organisms will tend to move towards higher latitudes as temperatures increase, whilst extinctions may be experiences at the lower latitudes.

Changes in surface water availability and run-off will influence the recharging of groundwater supplies and, in the longer term, aquifers. Water quality may also respond to changes in the amount and timing of precipitation. Rising seas could invade coastal freshwater supplies. Coastal aquifers may be damaged by saline intrusion as salty groundwater rises. Reduced water supplies would place additional stress on people, agriculture, and the environment. Regional water supplies, particularly in developing countries, will come under many stresses in the 21^st century. Global warming will exacerbate the stresses caused by pollution and by growing populations and economies. The most vulnerable regions are arid and semi-arid areas, some low-lying coasts, deltas, and small islands.

Water availability is an essential component of human welfare and productivity. Much of the world’s agriculture, hydroelectric power production, water needs and water pollution control is dependent upon the hydrological cycle, and the natural recharching of surface and groundwater resources. Changes in the natural water availability as a result of global warming would result in impacts which are generally most detrimental in regions already under existing climatic stresses. Even in more benign climates, the effective management of water resources will receive increasing attention as climate change increases the level of competition between potential users for water.

Trees

A change in global climate would be accompanied by shifts in climatic zones, thereby altering the suitability of a region for the growth of distinctive species. Trees in particular have long reproductive cycles, and many species may not be able to respond to the climatic changes quickly enough. A shift in climatic zones not only affects the vegetation but also affects the incidence of tree pests such as insects and diseases. These pests have less difficulty in migrating with their climatic zones than vegetation and may damage tree species with lower immunity.

As well as the effects of temperature and precipitation variations, and changes to weather patterns, forest growth may also respond to increased atmospheric concentrations of carbon dioxide. Studies with immature forest plantations suggest that an increase in atmospheric carbon dioxide would be beneficial to tree growth. The elevated carbon dioxide concentrations enhance photosynthesis rates with increased utilisation of carbon dioxide. This is called the carbon fertilisation effect. As a consequence of carbon fertilisation, water use efficiency may also increase. Increase in growth rates however, would vary enormously within ecosystems and between species. In general, it is expected that the negative impacts of climate change on forests will have a greater impact than any positive effect due to an increase in growth rates as a result of elevated atmospheric carbon dioxide concentrations. With unmitigated emissions of greenhouse gases, substantial dieback of tropical forests and tropical grasslands is predicted to occur by the 2080s, especially in northern South America and central southern Africa. If emissions are reduced enabling atmospheric carbon dioxide concentrations to stabilise at 550 ppm (double the pre-industrial level), this loss would be substantially reduced, even by the 2230s. Considerable growth of forests is predicted to occur in North America, northern Asia and China.

As well as the effects on forests themselves, climate change is expected to influence societies and economies dependent upon forestry. Forest products make up the third most valuable international commodity after oil and gas. Trade is expected to increase in the 21^st century along with demand, particularly in the developing countries. Global warming may well affect the development of such developing economies, particularly if current rates of deforestation remain unchecked and the unsustainable management of forests continues. As a consequence, societies dependent upon the income, food and shelter their forests provide them may well face increasing stresses due to crop failures, soil nutrient depletion and the effects of extreme weather events in the years to come.

The Greenhouse Effect

The Sun, which is the Earth's only external form of heat, emits solar radiation mainly in the form of shortwave visible and ultraviolet (UV) energy. As this radiation travels toward the Earth, 25% of it is absorbed by the atmosphere and 25% is reflected by the clouds back into space. The remaining radiation travels unimpeded to the Earth and heats its surface. The Earth releases a lot of energy it has received from the Sun back to space. However, the Earth is much cooler than the Sun, so the energy re-emitted from the Earth's surface is much weaker, in the form of invisible longwave infrared (IR) radiation, sometimes called heat.

Greenhouse gases like water vapour, carbon dioxide, methane and nitrous oxide trap the infrared radiation released by the Earth's surface. The atmosphere acts like the glass in a greenhouse, allowing much of the shortwave solar radiation to travel through unimpeded, but trapping a lot of the longwave heat energy trying to escape back to space. This process makes the temperature rise in the atmosphere just as it does in the greenhouse. This is the Earth's natural greenhouse effect and keeps the Earth 33°C warmer than it would be without an atmosphere, at an average 15°C. In contrast, the moon, which has no atmosphere, has an average surface temperature of -18°C.

During the last 200 years mankind has been releasing extra quantities of greenhouse gases which are trapping more heat in the atmosphere. Over the same time period the climate of the Earth has warmed, and many scientists now accept that there is a direct link between the man-made enhancement of the greenhouse effect and global warming.