Global warming, also known as the 'greenhouse effect', is the term used to describe the gradual rise in the world's average temperature, largely during the 20th Century. The effect is also known, somewhat less informatively, as 'climate change'.

The greenhouse effect is caused by certain gases in the atmosphere trapping infra-red radiation from the sun. Incoming sunlight passes through the atmosphere relatively easily, whereupon it warms the surface of the earth, thus losing energy. Heat energy of lower energy, and therefore longer wavelength, is reflected back and is absorbed by molecules of certain gases in the atmosphere (called greenhouse gases, or GHGs). These gas molecules then have extra energy, causing extra vibrations of the chemical bonds. This excess energy is then dissipated in all directions causing the atmosphere, and therefore the planet, to warm up. As a result, the atmosphere is a one-way filter for heat energy and therefore the average temperature of the Earth is higher than it would otherwise be. Indeed, without these so-called 'greenhouse gases' our world would be some 30°C cooler, and life as we know it would not exist. For example, the average surface temperature of the moon, which has no atmosphere, and is essentially the same distance as the Earth from the Sun, is -18°C. By contrast, the average surface temperature of the Earth is 15°C.

Composition of the Atmosphere

The atmosphere consists of 78% nitrogen, 21% oxygen and 0.9% argon. The atmosphere also consists of about 0.03% carbon dioxide and variable amounts of water vapour. In addition to these there are trace amounts of hydrogen, ozone, methane, carbon monoxide, and the noble gases helium, neon, krypton and xenon. Other gases created as a result of human activities may also be present, and chief among these have been increased levels of carbon dioxide as a result of burning fossil fuels, and the chlorinated fluorocarbons (CFCs).

Arrhenius' Insight

By the middle of the 18th Century, the Industrial Revolution was well underway, especially in Britain, and the Swedish chemist Svante Arrhenius recognised that increasing amounts of fossil fuels were being burned as an energy source, and that this was releasing carbon dioxide into the atmosphere. He further appreciated that the atmospheric carbon dioxide concentration would continue to increase as the world's consumption of fossil fuels, particularly coal, accelerated. From this he was able to make the prediction that if the carbon dioxide concentration doubled, the Earth would become several degrees warmer.

Following this, the average temperature of the Earth rose by about 0.25°C between 1880 and 1940, and rose overall by about 0.6°C in the last century. Seven of the ten hottest years worldwide, in a record stretching back to 1860, occurred during the 1990s; and temperature increases are now being measured at depth in the oceans.

Role of Carbon Dioxide

Part of the evidence for the link between atmospheric carbon dioxide concentration and global warming is given by the fact that a graph of atmospheric carbon dioxide concentration since 1850 is paralleled by a graph showing a steady rise in the Earth's average surface temperature over the same period. Since the beginning of the Industrial Revolution 200 years ago, huge swathes of forest have been cleared. This is harmful as, during the process of photosynthesis, trees consume carbon dioxide and 'fix' it as carbohydrate, in the process producing oxygen as a fortunate waste product. At the same time, the combustion of fossil fuels releases approximately 24 billion tonnes of carbon dioxide into the atmosphere each year, and the net result of these two opposing processes has been a 26% increase of this gas in the atmosphere since the start of the Industrial Revolution.

Currently, the atmospheric carbon dioxide concentration is rising at about 0.4% per annum, and this rise is caused almost entirely by the combustion of fossil fuels (coal, oil and natural gas). Since we can choose how much fossil fuel we burn, the rise in atmospheric carbon dioxide concentration is, in principle, controllable.

So, do any other gases behave as greenhouse gases?

Although carbon dioxide is the most significant of the greenhouse gases, it is not the only one; methane and ozone are also implicated.

Methane levels have risen by 11% since 1978; this is produced from diverse sources, principally agricultural. About 80% of atmospheric methane is produced by decomposition in rice paddies, swamps and the intestines of grazing animals such as cattle. Tropical termites are also a significant producer of methane. Furthermore, the world's coalmines are estimated to emit about 25 million tonnes of methane per year, which is roughly the same as is emitted from the world's oil and gas fields. Thus the level of atmospheric methane is inherently less controllable than carbon dioxide.

Molecule for molecule, methane is twenty times more powerful a greenhouse gas than is carbon dioxide, so, although considerably less methane is released into the atmosphere, it is the second-largest cause of global warming after carbon dioxide. The IPCC consider that methane increases in our atmosphere account for about one sixth of the total effect of well-mixed greenhouse gases on warming. However, a report published in 2005 suggests that the contribution of methane may be double this and, in fact, account for one-third of the global warming between the 1750s and today.

The rising methane level is also the main cause of rising ozone levels.

Ozone

The presence of ozone in the stratosphere is essential to life, filtering out harmful ultraviolet light from the incoming solar radiation. However, lower down in the troposphere it is problematic; being involved in the formation of photochemical smog. Ozone is also an irritant toxic gas, and high concentrations near ground level are detrimental to human health. Of greater relevance to this Entry is that it is a greenhouse gas and so contributes to global warming.

Formation of ozone

Tropospheric ozone is not directly emitted, but is formed as a result of the action of sunlight on a mixture of two primary pollutants, nitrogen oxides (collectively known as NO_x) and hydrocarbons (including methane), together with oxygen and water vapour.

Chlorinated Fluorocarbons (CFCs)

Since their discovery in the 1930s by Thomas Midgeley, who demonstrated the use of dichlrodifluoromethane as a refrigerant, chlorinated fluorocarbons (CFCs have been used also as propellants, as 'blowing agents' in the manufacture of expanded plastics, and as fire-retardants. However, a problem with the CFCs is that they are very unreactive and therefore persist in the atmosphere. The estimated lifetime for CFCs in the troposphere is 100 years, and this allows plenty of time for them to be transported to the stratosphere where, under the influence of ultraviolet light, they react with ozone.

Another problem with CFCs is that, as greenhouse gases, they are thousands of times more active than carbon dioxide; so very small concentrations can have very large effects. Fortunately, due to their effect in depleting the ozone layer, since the Montreal Protocol in 1987, there have been a series of international agreements to restrict the production and release of CFCs into the atmosphere. The aim is to eventually eliminate emissions of ozone-depleting substances as a result of human activity.

Role of Water Vapour

As mentioned above, the atmosphere contains variable amounts of water vapour. Water vapour behaves differently from the greenhouse gases in that water is a liquid, with some vapour associated with it. Depending on the conditions, H₂O_(l) and H₂O_(g) are readily interconverted. If human activities such as burning fossil fuels puts H₂O_(g) into the atmosphere, most of it will condense to H₂O_(l) and eventually return to Earth as precipitation. So, from this point of view, water is not as much of a greenhouse problem as CO_2(g).

However, one needs to consider what will happen as the Earth continues to warm. Firstly, H₂O_(g) will evaporate from the oceans, thus accelerating the global warming. On the other hand, droplets of H₂O_(l) in clouds will tend to block out the Sun, thus causing global cooling. This sort of contradictory effect is one reason why it is so difficult to create an accurate mathematical climate model.

Consequences of Global Warming

The average temperature in Europe has increased by 0.8°C over the last century, and it is predicted that the average global temperature could rise by between 1.5 to 4.5°C by the year 2100. At first glance, this might not seem to be very much, but it is enormous if one considers that only 5°C separates us from the last Ice Age!

Based on this information, scientists have produced various models to study the effect of this on, for example, rainfall and other precipitation patterns and humidity; and have tried to predict the effects on our everyday lives.

Mean Sea Level and the Arctic Ice Sheet

The global mean sea level has risen by 10 - 20cm in the past century, which is ten times faster than the average over the past 2,000 years. At this rate, it is predicted that the mean sea level will rise by another 47cm in the next 100 years. Furthermore, the warmer temperatures will cause thermal expansion of oceans.

Scientists fear that in the worst-case scenario, the temperatures of the Arctic and Antarctic Oceans could exceed 0°C. If this were to happen then the polar ice caps would start to melt. Evidence for this has been found in both Alaska and in the Weddell Sea in Antarctica.

The Intergovernmental Panel on Climate Change (IPCC), in a report published in 2001, predicted that the total melting of Greenland's ice sheet alone could add over 6.1m to mean global sea level, but thought that this degree of catastrophic melting was centuries away.

Considering more immediate effects, findings from Britain's Hadley Centre for Climate Change indicate that by 2050 there could be a 21cm rise in global mean sea level. This would result in the loss of large areas of fertile land. Furthermore, some of the capital cities of the world would be flooded, thus depriving 100 million people of their homes. According to Earthpulse, a publication of National Geographic magazine, over one third of the world’s population lives within 100km of a shoreline, and 13 of the world's 20 largest cities are located on a coast. One such city at risk is the capital of the USA, Washington DC. For low-lying countries like Bangladesh, rising sea levels would undoubtedly mean worse floods than those which already regularly afflict the Ganges basin.

In parallel with this data, new research published in 2009 shows that melting of the Arctic ice sheet is, in fact, accelerating and that sea ice coverage has declined almost visibly by a staggering 45,000 square kilometers per year over the last 20 - 30 years.

It is thought that industry, transportation, and the burning of biomass in North America, Europe, and Asia are all contributing trace gases and microparticulates into the atmosphere, and that these are causing an 'Arctic Haze' to form every winter and spring. It is this that is thought to be accelerating the polar melt. Some models are now suggesting that Arctic sea ice will completely disappear during the summer months by 2080.

As the thickness and expanse of the ice sheet declines, so the habitat available to such species as the polar bear, Pacific walrus, ringed and hooded seals, narwhal and ivory gull is reduced. Indeed, habitat loss means that the polar bear is now facing possible extinction within the next 70 years, whereas in 2003 it was predicted that they still had another 100 years to go. The problem is being exacerbated by the migration of other species northwards as the Earth heats up, thus displacing the indigenous species.

Impact on Tundra

The word 'tundra', meaning 'ice desert' comes from the Finnish word tunturia, which means a barren land. It occurs in the far north of Eurasia (Finland, Russia and Siberia), North America (Alaska and Canada) and the high plateau of Tibet.

The ground is permanently frozen 25 to 100cm down and means that trees are unable to grow. The barren, often rocky, ground is able to support only low-growing plants such as mosses, heaths, and lichen.

Tundra has a yearly average temperature in the range of -5°C to -15°C. Thus, in the winter it is cold and dark and in the summer the top layer of permafrost melts, thus becoming very soggy. Then the tundra becomes covered with marshes, lakes, bogs and streams that enables insects to breed, thus attracting many migrating birds.

The tundra is of vast importance to global ecology since it is one of Earth's three major carbon dioxide 'sinks' ie, a biomass which takes in more carbon dioxide than it releases. During the short summer, tundra's plants photosynthesise, thus taking in carbon dioxide and water. Normally plants eventually die and decompose, thus releasing carbon dioxide back into the atmosphere. However because of the short, cool summer and freezing winter temperatures, plants do not decompose in the tundra.

Today global warming is causing the permafrost to melt. As the tundra melts, the plant mass is decomposing, thus returning carbon dioxide to the atmosphere. The rotting vegetation also releases methane gas. It is estimated by the IPCC that by 2050, 16% of permafrosted areas will have melted.

Desertification

To date, most studies on global warming have focused on the effects on the frozen regions of the world, and comparatively few have focused on the hotter drier regions. However, desert sand dunes currently cover 5% of the global land surface, and up to 30% percent of Africa. So any effect that climate change has on such regions could be very significant.

Changed rainfall patterns could mean new deserts in Africa and dustbowls in Mid-Western states of America, the Commonwealth of Independent States, and perhaps the densely-populated Nile delta.

As an example, a paper by scientists from Oxford University and published in the prestigious journal, Nature on 30 June, 2005 warns that the Kalahari sand dunes, which are currently stable and covered by vegetation, will undergo widespread reactivation during the 21st Century as a result of declining rainfall, increasing droughts, and rising wind strengths. The Kalahari region covers an area of a 2.5 million km², stretching from northern South Africa to Zambia and beyond.

Impact on World Forests

Forests are essential for consuming atmospheric carbon dioxide and converting it into breathable oxygen by the process of photosynthesis.

Under the direct influence of global warming, tropical forests are predicted to decline, with a major loss of species; and this does not include the pressures put on them by logging and agriculture. This has very serious implications! Edward O Wilson, an eminent naturalist from Harvard University, estimates that two-thirds of all species on Earth live in the African rainforests - and rainforests make up just 6% of our planet's surface.

On the other hand, temperate and boreal (northern) forests are predicted to increase their cover if left to their own devices. Boreal forests, which occur in North America and Eurasia, cover 27% of the Earth's surface; and consist of such species as spruce, birch and fir. These species grow in places where the yearly average temperature ranges between -5 to +3°C. They do not absorb as much carbon dioxide as the tropical rainforests.

Different tree species favour different temperature ranges, and will therefore be affected by global warming. Many tree species have narrow temperature niches and a warming of as little as 1°C would have an impact. It is thought that trees whose seeds are spread by birds have the fastest mobility and therefore have the best chance for survival; whereas trees whose seeds are spread either by the wind or via nut have decreased viability, as they can take a long time to mature and establish themselves. For example, Scots pines in Britain move only about 4km per century.

Projections by the Hadley Centre show also that tropical grasslands, which support grazing and agriculture in areas of high population, will decline.

Effects on Human Health

According to The World Health Organisation (WHO) global warming could lead to a major increase in vector-borne diseases¹ in Britain and Europe.

Many animal vectors of disease prefer warmer and more humid conditions for breeding, and global warming could enable such animals to become established further north. Vectors of concern include mosquitoes, which transmit (among other diseases) malaria, yellow fever, dengue fever and West Nile fever; ticks, which spread tick-borne encephalitis; and sandflies which spread visceral leishmaniasis.

According to WHO, there are three European regions within its remit which are already as risk from mosquito-borne malaria, these being Azerbaijan, Tajikistan and Turkey. WHO believes that the disease is likely to spread to further areas within eastern Europe, and from there, possibly, to western areas.

Summary

There now seems to be no doubt that global warming is a very real phenomenon, and that the causes are anthropogenic - mainly the result of carbon dioxide emissions due to the burning of fossil fuels.

In 1990, scientists estimated that a 60% reduction in carbon dioxide emissions was needed just to stabilise the Earth's climate. The last global agreement, known as the Kyoto Protocol, which came into force in 2005, was to bring about a 5% reduction over 15 years.

The British government is very proactive and has set the ambitious target of a 20% cut in carbon dioxide emissions by the year 2010, and by between 60% and 80% by 2050

Everybody, as individuals needs to be aware of the causes and consequences of global warming so that they can play their own part in helping to reduce carbon dioxide emissions.