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 on Thursday the data is "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-fuelled 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 National Science Foundation and universities.

The academy had been asked to report to 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 "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.

It compared the sharp curve of the hockey blade to the recent up-tick in temperatures a 1-degree rise in global average surface temperatures in the Northern Hemisphere during the 20th century and the stick's long shaft to centuries of previous climate stability.

That research is "likely" true and is supported by more recent data, said John "Mike" Wallace, an atmospheric sciences professor at the University of Washington and a panel member.

The academy panel said it had less confidence in the evidence of temperatures before 1600.

But it considered the evidence reliable enough to conclude there were sharp spikes in carbon dioxide and methane, the two major "greenhouse" gases blamed for trapping heat in the atmosphere, beginning in the 20th century, after remaining fairly level for 12,000 years.

Between 1 AD and 1850, volcanic eruptions and solar fluctuations had the biggest effects on climate. But those temperature changes "were much less pronounced than the warming due to greenhouse gas" levels by pollution since the mid-19th century, the panel said.

Source: China Daily/agencies