Main causes of global warming

The build-up of greenhouse gases in the atmosphere during the 20th century has resulted from the growing use of energy and expansion of the global economy. Over the century, industrial activity grew 40-fold, and the emissions of gases such as carbon dioxide (CO2) and sulphur dioxide (SO2) grew 10-fold.

The amount of CO2 in the air increased from some 280 parts per million by volume (ppmv) at the beginning of the century to 383 ppmv at the end of 2007. The amount of CO2 varies within each year as the result of the annual cycles of photosynthesis and oxidation (see graph). Of the other greenhouse gases, methane (CH4), which is formed by anaerobic decomposition of organic matter, rose from a preindustrial atmospheric concentration of around 700 parts per billion by volume (ppbv) to about 1 789 ppbv by 2007. Other important greenhouse gases include the oxides of nitrogen, notably nitrous oxide (NO2) and halo carbons, including the chlorofluorocarbons (CFCs) and other chlorine and bromine containing compounds.

The build-up of greenhouse gases in the atmosphere alters the radiative balance of the atmosphere. The net effect is to warm the Earth's surface and the lower atmosphere because greenhouse gases absorb some of the Earth’s outgoing heat radiation and reradiate it back towards the surface. The overall warming from 1850 to the end of the 20th century was equivalent to about 2.5 W/m2; CO2 contributed some 60 per cent of this figure and CH4 about 25 per cent, with N2O and halo carbons providing the remainder. The warming effect that would result from a doubling of CO2 from pre-industrial levels is estimated to be 4 W/m2

OZONE DEPLETION

In 1985 Joe Farman, of the British Antarctic Survey, published a paper showing the decline of ozone levels over Antarctica during the early 1980s. The response was dramatic: large-scale international scientific programmes were mounted to prove that CFCs (used as aerosol propellants, in industrial cleaning fluids and in refrigeration equipment) were the cause of the problem. Even more important was immediate international action to curb the emissions of CFCs.

Plummeting ozone levels in the stratosphere over Antarctica during September and October are the result of complex chemical processes. The return of the Sun at the end of winter triggers photo chemical reactions that lead to the destruction of ozone in the stratosphere. The October values of ozone have declined by up to 70 per cent compared to the pre-ozone hole years, and the size of the ozone hole had grown to more than 25 million km2 (twice the size of Antarctica) by 2000.

Over the Arctic the gradual development of an annual decline during the 1990s is a significant trend. More generally, over northern middle latitudes the concentration of stratospheric ozone has decreased since 1979 by 5.4 per cent in winter and spring, and by about 2.8 per cent in summer and autumn. There has been no discernible trend in the tropics and subtropics.

The scale and suddenness of the ozone decline shocked the scientific world, and led to the 1985 Vienna Convention for the Protection of the Ozone Layer and the 1987 Montreal Protocol and subsequent amendments to eliminate certain CFCs from industrial production. As a result of this rapid action the global consumption of the most active gases fell by 40 per cent within five years and the levels of certain chlorine-containing chemicals in the atmosphere have started to decline. It will be decades before the CFCs already in the atmosphere fully decay. In the meantime, the substantial destruction of ozone in the stratosphere over Antarctica during September and October will continue.

AEROSOLS IN THE ATMOSPHERE

Atmospheric aerosols are able to alter climate in two important ways. First, they scatter and absorb solar and infrared radiation and, second, they may change the micro physical and chemical properties of clouds and possibly their lifetime and extent. The scattering of solar radiation acts to cool the planet, while absorption of solar radiation by aerosols warms the air directly instead of allowing sunlight to be absorbed by the surface of the Earth.

The human contribution to the amount of aerosols in the atmosphere takes many forms. Dust is a biproduct of agriculture. Biomass burning produces a combination of organic droplets and soot particles. Industrial processes produce a wide variety of aerosols depending on what is being burned or produced in the manufacturing process. In addition, exhaust emissions from transport generate a rich cocktail of pollutants that are either aerosols from the outset, or are converted by chemical reactions in the atmosphere to form aerosols.

The concentrations of condensation nuclei are about three times higher in the Northern Hemisphere than in the Southern Hemisphere. This higher concentration is estimated to result in radiation forcing that is only about 50 per cent higher for the Northern Hemisphere.