Earth's temperature could be reaching its highest level in a million years, American scientists said yesterday.

Researchers at Nasa's Goddard Institute for Space Studies said a further one degree celsius rise in the global temperature could be critical to the planet, and there was already a threat of extreme weather resulting from El Niño.

The scientists said that in the 30 years to the end of 2005, temperatures increased at the rate of 0.2 degrees per decade, a rate they described as "remarkably rapid".

Comparison of the current global temperature with estimates of historical temperatures - based on a study of ocean sediment - showed the current temperature was now within 1C of the maximum temperature of the past million years.

Dr James Hansen, who led the study, said further global warming of just 1C could lead to big changes to the planet.

"If warming is kept less than that, effects of global warming may be relatively manageable," he said.

"But if further global warming reaches two or three degrees celsius, we will likely see changes that make Earth a different planet [to] the one we know.

"The last time it was that warm was in the middle Pliocene, about 3m years ago, when sea level was estimated to have been about 25 meters (80 feet) higher than today."

The study showed that global warming was greatest at high latitudes in the northern hemisphere. The study attributed this to the effect of snow and ice melting, leaving dark areas that absorb more sunlight.

Warming was lower over sea than on land, but the research team said the temperature of the western Pacific had been increasing, and was becoming much warmer than the eastern Pacific. This increased difference could boost the likelihood of strong El Niño weather conditions such as those seen in 1983 and 1998, when many countries around the world experienced devastating floods and tornadoes.

Dr Hansen said that the increasing temperature could lead to the extinction of some species, which would find it increasingly difficult to find viable habitats.

Plants and animals have been observed migrating towards the poles to find areas where the temperature better suits them. However, Dr Hansen said that they were not keeping up with the pace at which temperature zones are moving - this movement had reached 25 miles per decade between 1975 and 2005.

He said: "Rapid movement of climatic zones is going to be another stress on wildlife.

"It adds to the stress of habitat loss due to human developments. If we do not slow down the rate of global warming, many species are likely to become extinct.

"In effect we are pushing them off the planet."

The environment group Earthwatch today warned that a change in climate could make the difference between survival and extinction for endangered lemurs in Madagascar.

A 20-year study of the Milne-Edward's sifakas lemur found that during dry spells, older females were unable to chew enough leaves to provide their infants with milk.

Lemurs are able to give birth up to five times during the final decade of their lives, but as they get older their teeth become worn, making it harder for them to eat.

If dry periods become more common, they will struggle to feed their young, and the numbers of animals who make it to adulthood will fall.