Another study, this one from NASA, calls for curbs on global warming. 

The globe is at its warmest in the past 12,000 years, triggering environmental changes that could become "dangerous" if it continues, said the new study published Tuesday.

Researchers led by NASA scientist James Hansen found that the earth has heated up by 0.2 degrees Celsius each decade in the past 30 years. This is consistent with predictions made in 1980s global warming simulations based on greenhouse gas increases.

Further increases could dramatically change sea level and exterminate species, the study concludes.

"This evidence implies that we are getting close to dangerous levels of human-made pollution," said Hansen, of NASA's Goddard Institute for Space Studies in New York.

The warming is greatest over land in the northern hemisphere, the study found. The problem is compounded by melting snow and ice at higher latitudes, which uncovers darker surfaces that absorb more sunlight. This increases warming, a process known as positive feedback.

The study also found that the planet has warmed to within one degree Celsius of the hottest temperature of the last million years.

Dramatic change

"If further global warming reaches two or three degrees Celsius, we will likely see changes that make Earth a different planet than the one we know," Hansen said. "The last time it was that warm was ... about three million years ago, when sea level was estimated to have been about 25 metres higher than today."

The researchers noted that the impact of global warming is already becoming evident, citing a 2003 study published in the magazine Nature. It found 1,700 species of plants, animals and insects, that survive within a climatic range, began to move closer to the poles at an average rate of six kilometres a decade through the latter half of the 20th century.

That rate of migration is too slow to keep pace with the climatic zone, which is moving at a rate of about 40 km a decade.

"If we do not slow down the rate of global warming, many species are likely to become extinct," Hansen said. "In effect we are pushing them off the planet."

The study's results were published in the scientific journal Proceedings of the National Academy of Sciences.