The scientists told lawmakers that surface temperatures in the Northern Hemisphere rose about 1 degree during the 20th century, and that "human activities are responsible for much of the recent warming," reported the Associated Press.
The National Research Council, operating for the National Academy of Sciences, issued the 155-page report at the request of Congress.
House Science Committee Chairman Sherwood Boehlert, R-N.Y., requested the report after another committee launched an investigation into three climate scientists who said the Northern Hemisphere was the warmest its been in 2,000 years -- a finding that yielded a "hockey-stick" graphic showing temperatures rising.
The National Academy scientists concluded that the work of the three scientists, Michael Mann, Raymond Bradley and Malcolm Hughes, done in the late 1990s was "likely" true and supported by more recent data.
Temperatures have been recorded for the past 150 years, and prior to that, the scientists used tree rings, corals, glaciers and ice cores, cave deposits, ocean and lake sediments, boreholes and other sources to track surface temperatures.
Combining these sources gave the panel a "high level of confidence" that the last few decades of the 20th century were the warmest in 400 years and that the warming during those decades was unprecedented over the last 1,000 years. Relatively warm conditions persisted around the year 1000, followed by a "Little Ice Age" from about 1500 to 1850.
Prior to 1600, the scientists said they were less certain about the temperature but were able to detect spikes in carbon dioxide and methane, two major "greenhouse" gases blamed for trapping heat in the Earth's atmosphere.
Congress charters the National Academy of Sciences to advise the government on scientific issues.