Greenhouse Gases (GHGs)

The greenhouse gases that matter most today are carbon dioxide, chlorofluorocarbons, methane, nitrous oxide and ozone. A few others, like nitric oxide and carbon monoxide, do not act as greenhouse gases themselves but react easily with the actual GHGs and alter their concentration in the atmosphere.

How effective a given greenhouse gas is depends on three things: how much its concentration has increased, how long it stays in the atmosphere, and the wavelength of radiation it absorbs. Chlorofluorocarbons are highly effective on this count. Ozone is interesting because its role changes with altitude — in the stratosphere it absorbs ultraviolet radiation, but when it appears in the lower troposphere it becomes very effective at absorbing terrestrial radiation. There is also a recovery problem built into the system: the longer a GHG molecule lingers in the atmosphere, the longer the earth's atmospheric system will take to recover from any change that gas has caused.

Carbon dioxide has the largest concentration among all greenhouse gases. Its emission comes mainly from burning fossil fuels — oil, gas and coal. Forests and oceans act as its sinks. Forests absorb carbon dioxide in their growth, so deforestation caused by changes in land use removes a sink and thereby pushes the concentration up. The atmosphere takes roughly 20 to 50 years to adjust carbon dioxide levels to changes in sources and sinks, and the gas is currently rising at about 0.5 per cent annually. In climate models, doubling the concentration of carbon dioxide over its pre-industrial level is used as the standard index for estimating climate change.

Chlorofluorocarbons are entirely products of human activity. Ozone, by contrast, forms naturally in the stratosphere, where ultraviolet rays convert oxygen into ozone — and in doing so, those rays are absorbed and never reach the earth's surface. When CFCs drift up into the stratosphere, they destroy this ozone. The depletion is especially large over Antarctica, and this thinning of stratospheric ozone is called the ozone hole. With the ozone layer damaged, ultraviolet rays pass through the troposphere and reach the ground.

International efforts to cut GHG emissions have been underway for decades. The most important of these is the Kyoto Protocol, proclaimed in 1997 and brought into effect in 2005, ratified by 141 nations. It bound 35 industrialised countries to reduce their emissions by the year 2012 to 5 per cent less than their 1990 levels.

The long-run danger is that rising GHG concentrations will warm the earth, and once global warming sets in, it will be very difficult to reverse. The warming will not be uniform everywhere, but its adverse effects will hit the life-supporting systems of the planet. Melting glaciers and ice-caps, together with the thermal expansion of the sea, will raise sea levels and inundate large parts of coastal areas and islands, creating serious social problems. This is why the world community treats global warming as a major concern and has already begun efforts to control emissions. The hope is that the world responds to this challenge and adopts a lifestyle that leaves behind a livable world for future generations. …