In the late 1990s, climatologist Michael Mann and colleagues Raymond Bradley and Malcolm Hughes published historical surface temperature reconstructions, concluding that the Northern Hemisphere was the warmest it has been in 2,000 years. Their research became known as the "hockey-stick" graph, because it compared the sharp curve of the hockey blade to the recent uptick in temperatures and the stick's long shaft to centuries of previous climate stability.

Their findings weren’t universally well received. Last year, House Energy and Commerce Committee chairman Rep. Joe Barton (R-Texas), a skeptic on global warming, launched an investigation of the three climate scientists. Prompted by a February 2005 Wall Street Journal article, Barton sent letters in June 2005 to Drs. Mann, Bradley and Hughes, as well as the Intergovernmental Panel on Climate Change and the National Science Foundation "questioning many aspects of a global warming study." This move was an obvious and blatant attack on the scientists rather than a serious attempt to understand the science.

Legislators of both parties criticized Barton's approach as "misguided and illegitimate" and "a transparent effort to bully and harass climate change experts who have reached conclusions with which you disagree." House Science Committee chairman Rep. Sherwood Boehlert (R-N.Y.) said Barton should try to learn from scientists, not intimidate them. In November, Boehlert requested an independent report from the National Research Council to address the question whether global warming was a major threat or not.

Well, guess what? It turns out that the Research Council committee found Mann's conclusion that warming in the last few decades of the 20th century was unprecedented over the last thousand years to be plausible, although it had less confidence that the warming was unprecedented prior to 1600 - fewer proxies, in fewer locations, provide temperatures for periods before then.

The Research Council committee found that there is sufficient evidence from tree rings, boreholes, retreating glaciers, and other "proxies" of past surface temperatures 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. However, less confidence can be placed in proxy-based reconstructions of surface temperatures for A.D. 900 to 1600, although the available proxy evidence does indicate that many locations were warmer during the past 25 years than during any other 25-year period since 900. Very little confidence can be placed in statements about average global surface temperatures prior to A.D. 900 because the proxy data for that time frame are sparse, the committee added.

Scientists rely on proxies to reconstruct paleoclimatic surface temperatures because geographically widespread records of temperatures measured with instruments date back only about 150 years. Other proxies include corals, ocean and lake sediments, ice cores, cave deposits, and documentary sources, such as historic drawings of glaciers. The globally averaged warming of about 1 degree Fahrenheit (0.6 degrees Celsius) that instruments have recorded during the last century is also reflected in proxy data for that time period, the committee noted.

The committee noted that scientists' reconstructions of Northern Hemisphere surface temperatures for the past thousand years are generally consistent. The reconstructions show relatively warm conditions centered around the year 1000, and a relatively cold period, or "Little Ice Age," from roughly 1500 to 1850. The exact timing of warm episodes in the medieval period may have varied by region, and the magnitude and geographical extent of the warmth is uncertain, the committee said. None of the reconstructions indicates that temperatures were warmer during medieval times than during the past few decades, the committee added.

The scientists said the evidence was 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 A.D. and 1850, volcanic eruptions and solar fluctuations were the main causes of changes in greenhouse gas levels. But those temperature changes "were much less pronounced than the warming due to greenhouse gas" levels by pollution since the mid-19th century, it said.