Temperature in the northern hemiphere warmed by about one degree Fahrenheit (0.6 degrees Celsius) during the last century, the report by the National Academy of Sciences found.

Scientists have argued that the change was sufficient to alter hurricane patterns and see Polar ice caps melt down.

"There is sufficient evidence ... to say with a high level of confidence that the last few decades of the 20th century were warmer than any comparable period in the last 400 years," the academy's National Research Council report said.

The Republican-led House Science Committee commissioned the study in November after controversial findings by climatologist Michael Mann showed that the 1990s were the warmest decade and 1998 the warmest year of the 20th century.

The administration of US President George W. Bush has argued that evidence on global warming was not strong enough to support expensive new measure to curb pollution that the White House argues could also cost American jobs.

But the latest report -- while stressing that data from before 1600 is for the large part sketchy -- said Mann's conclusion that a graphic charting temperatures over the past century looked like a "hockey stick" was correct.

The academy's National Research Council said Mann's findings were "plausible," but that there was insufficient evidence to pinpoint 1998 as the hottest year.

Existing data did illustrate that temperatures were lower before the industrial revolution and that human activity was responsible for the warming, the study said.

"The committee pointed out that surface temperature reconstruction for periods before the Industrial Revolution ... are only one of multiple lines of evidence supporting the conclusion that current warming is occurring in response to human activities," the report said.

For the northern hemisphere, reconstructions showed that warm conditions centered around the year 1000 and that a relatively cold period -- dubbed the "Little Ice Age" -- followed from 1500 to 1850.

Things started heating up again with the industrial revolution before spiking up in the past decades.

Although temperatures 1,000 years ago were warm, they do not compare with those now, the report said.

"None of the reconstruction indicates that temperatures were warmer during medieval times than during the past few decades," it said.

The report called for more research into proxy data for the period prior to 1600 so that comparisons for the past two millennia could be made, and for improved global access to existing to information on which temperature studies could be conducted.

It noted that far more research existed for the northern hemisphere than for the southern hemisphere, and that this vacuum limited the reliability of data overall.

Source: Space Daily