COPENHAGEN - The impacts of climate change have worsened almost every year since 1980, according to a study on Wednesday inspired by the Dow Jones stock index that distils global warming into a single number.

The new climate change index is based on world temperatures, Arctic sea ice extent in summer, and concentrations of carbon dioxide in the atmosphere and sea levels, the International Geosphere-Biosphere Programme (IGBP) said.

The climate system is changing in the direction of a warming planet, Sybil Seitzinger, executive director of the IGBP, told a news conference on the sidelines of December 7-18 talks in Copenhagen due to agree a U.N. deal to fight climate change.

She said that the idea was to give the public a snapshot of global warming to help understand the issues.

She said that scientists had taken four easily understood factors, reported by governments, and denied favoring elements that might bias the findings. The index went back to 1980, when satellite records begin.

The idea was inspired by stock market indices such as the Dow Jones or the FTSE 100, she said.

According to the index, climate change got worse every year since 1980 except 1982, 1992 and 1996, perhaps because large volcanic eruptions those years threw sun-dimming dust high into the atmosphere and curbed temperatures.

The dip in the curve in 1992 may have been caused by the massive Mount Pinatubo volcanic eruption in the Philippines in 1991, a statement said. Other eruptions occurred in Mexico in 1982 and Monserrat in 1996.

Scientists might in future expand the index to other factors such as deforestation, ocean acidity or the frequency of extreme weather events, she said.