The atmospheric levels of carbon dioxide and other greenhouse gases are increasing and a rise in global temperatures seems imminent
The present targets set by various countries to limit carbon dioxide emissions are not enough to avert an increase in its atmospheric levels, concludes the latest report by the Intergovernmental Panel for Climate Change (IPCC). The report says that an accumulation of carbon dioxide and other greenhouse gases in the atmosphere continues to threaten the globe with drastic climatic changes.
The IPCC is a worldwide network of about 2,500 scientists that advises governments on the effects of greenhouse gases on the world's climate. It was set up in 1988 by the World Meteorological Organisation and the United Nations Environment Programme to collate data on global warming and its likely implications.
The latest IPCC report, released in Maastricht in the Netherlands in mid-September, has been prepared by the Working Group I, which has been instituted to adjudge the possible impacts on global climate of changes in the carbon cycle. On the IPCC's own admission, "These new findings add to the detail of our knowledge, but do not substantially change the essential results which appeared in the 1990 and 1992 IPCC scientific assessments."
The earlier assessments estimated that a doubling of carbon dioxide concentrations in the atmosphere, from anthropogenic and other sources, would raise global temperatures by 1.5 to 4.5 0 C. The new report warns that even to stabilise atmospheric carbon dioxide levels at 750 parts per million by volume (ppmv), which is more than twice the present levels of 360 ppmv, and the highest level studied in the climate change scenarios, carbon dioxide emissions will have to be drastically cut.
Even if more ambitious emission targets are set, the IPCC's experts are doubtful that they would achieve much success in averting global warming, because the carbon dioxide molecule stays in the atmosphere for a long time. The report points out that though anthropogenic carbon dioxide is small compared to natural sources of the gas such as respiration by plants and animals, the net uptake of the gas, particularly by the deep ocean, occurs slowly over millennia; so the addition of anthropogenic carbon dioxide has a long lasting effect on atmospheric concentrations. If carbon dioxide concentrations are held constant at the present levels, the atmospheric levels would continue to rise for at least 2 centuries.
However, on the brighter side, the report reveals that the growth rate of atmospheric carbon dioxide concentration slowed down during 1991 to mid-1993 to as low as 0.5 ppmv per year as compared to the average rates of increase of more than 1.5 ppmv per year over the previous decade. But in the latter half of 1993, this rate of growth again started increasing.
The IPCC experts claim that their understanding of the carbon cycle has now improved. In the past, experts were unable to account for some 1.4 billion tonnes of carbon each year. This they reckoned found its way into a so-called "missing sink". The new report indicates that scientists have located this sink in the regenerating forests of the Northern hemisphere.
The IPCC report further reveals that the global warming potential of greenhouse gases other than carbon dioxide are 10 to 35 per cent greater than previously believed (see table). It has been found that the global warming potential of methane is twice as much as previously thought, because it stays in the atmosphere for longer than previously supposed. Methane, which accounts for 20 per cent of the greenhouse gases, is released mainly from garbage dumps and paddy fields and traps 25 times more heat in the atmosphere than a carbon dioxide molecule, over a period of 100 years.
But the good news about methane is that it stays in the atmosphere only for a short time compared to other greenhouse gases, and cutting emissions by only 10 per cent would stabilise atmospheric methane at present concentrations. Additionally, scientists reveal that the rate of methane emissions has declined over the past decade.
The new report provides a better estimate of the cooling role of sulphate aerosols. These particles are produced by burning vegetation and fossil fuels and they reflect the sun's radiation, thus cooling Earth. Aerosols released into the atmosphere with the eruption of Mt Pinatubo in the Philippines in June 1991 have been quantified, and experts contend that this has resulted in a large but short-lived global cooling of 0.4 to 0.6 0 C. These particles, according to the IPCC, could be responsible for offsetting about half the global warming caused by anthropogenic greenhouse gases. However, because aerosols are not evenly distributed over the globe, experts caution that their effect should not be considered a direct antidote to greenhouse gases.
|Hotter than ever|
|Global warming potential (GWP) of various greenhouse gases over a 100-year period, with reference to CO2 whose GWP value is taken as 1|
|GWP (' 92 report)||GWP (' 94 report)|
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