Global temperature rise confirms warming trend

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THE ERUPTION of Mount Pinatubo in the Philippines cooled the world last year, according to researchers in Britain and the United States. None the less, the 1992 global average temperature remained well above the average for the century and may signal that a man-made climate change is under way.

The 1980s was the hottest decade since world-wide temperature records began in the mid-19th century. The trend continued into this decade, with 1990 the hottest year on record and 1991 the third warmest.

Each year of high temperatures gives a little more support to the theory that pollution is altering the climate by trapping heat in the atmosphere. According to the Meteorological Office and the University of East Anglia's Climatic Research Unit, 1992 was two-tenths of a degree centigrade cooler than 1991, making it the 10th warmest year in the 140-year record. That was still warmer than 1982, 1984, 1985 and 1986 and means that the overall warming trend continues.

The chief cause of the drop is almost certainly the huge volcanic eruption in June 1991, which spread fine airborne dust and chemicals over much of the Earth's upper atmosphere. This caused the formation of microscopic, shiny liquid particles, which reflect the sun's warmth back into space.

The El Nino phenomenon may also have contributed to the cooling. Every three to seven years there is a major change in the ocean currents that flow around the tropics in the Pacific. This shift alters the weather around almost the entire globe, causing droughts, torrential rainfall and an overall rise in temperatures, which lasts up to 18 months. The latest cycle of El Nino was ending about a year ago, causing temperatures to fall.

Dr Phil Jones, of the Climatic Research Unit, said that the underlying trend seemed to point to warm years continuing. 'It's still too early to say for sure that this is due to the build up of greenhouse gases in the atmosphere, but it becomes more likely with each passing year.'

Both the climatic unit and the Meteorological Office rely on temperature records from more than 1,400 recording stations on every continent, including Antarctica, and readings made at sea. Researchers compile a monthly global average based on the average monthly temperature at thousands of points on land and sea.

In the US, Nasa's Goddard Institute for Space Studies in New York, does similar work on world- wide temperature data but relies almost entirely on land-based measurements. Helene Rind, one of the reseachers there, yesterday confirmed that they had also found a cooling in 1992. However, the global average temperature last year was still well above the average for 1951 to 1980, and in line with the values for the 1980s.

The most abundant and important of the man-made greenhouse gases is carbon dioxide, which is produced when coal, oil, gas and also forests are burnt. There is now unequivocal evidence that the concentrations of carbon dioxide and other greenhouse gases associated with industry and agriculture are rising throughout the atmosphere. This is forecast to lead to a gradual rise in temperature and changes in climate because the gases absorb infra-red radiation which would otherwise beam out into space, thereby cooling the planet.