The Earth is running a slight fever from greenhouse gases, after enjoying relatively stable temperatures for 2,000 years. The US National Academy of Sciences, after reconstructing global average surface temperatures for the past two millennia, said Thursday the data are "additional supporting evidence ... that human activities are responsible for much of the recent warming."

Other new research showed that global warming produced about half of the extra hurricane-fueled warmth in the North Atlantic in 2005, and natural cycles were a minor factor, according to Kevin Trenberth and Dennis Shea of the National Center for Atmospheric Research, a research lab sponsored by the US National Science Foundation and universities.

This satellite image released by NASA shows the minimum concentration of Arctic sea ice in 2005. On September 21, 2005, the sea ice extent dropped to 2.05 million square miles. The National Academy of Sciences said the Earth is running a slight fever from greenhouse gases. [AP/file]

The academy had been asked to report to US Congress on how researchers drew conclusions about the Earth's climate going back thousands of years, before data was available from modern scientific instruments. The academy convened a panel of 12 climate experts, chaired by Gerald North, a geosciences professor at Texas A&M University, to look at the "proxy" evidence before then, such as tree rings, corals, marine and lake sediments, ice cores, boreholes and glaciers.

Combining that information gave the panel "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 panel wrote. It said the "recent warmth is unprecedented for at least the last 400 years and potentially the last several millennia," though it was relatively warm around the year 1000 followed by a "Little Ice Age" from about 1500 to 1850.

Their conclusions were meant to address, and they lent credibility to, a well-known graphic among climate researchers ¡ª a "hockey-stick" chart that climate scientists Michael Mann, Raymond Bradley and Malcolm Hughes created in the late 1990s to show the Northern Hemisphere was the warmest it has been in 2,000 years.

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