Global warming has been a popular topic among scientists

The Earth's average temperature over the past quarter century has been the hottest in four centuries -- and part of the world has been warmer during the past 25 years than any period in the past 1,000 years, according to the National Academy of Sciences in a recently published report.  The data published in the report contains "additional supporting evidence: "that human activities are responsible for much of the recent warming."  Congressman Sherwood Boehlert, chairman of the House Science Committee, requested the 141-page report by the National Academy of Sciences.

However, since proper thermometer records were not kept until the mid-19th century, scientists have had to use a number of interesting methods to infer temperature averages from different eras.  Scientists preferred to measure the width of rings inside trees that were growing during the middle ages and also checked the ratios of oxygen isotopes in polar ice cores.

Climate scientist Michael Mann's "hockey stick" chart of past temperatures, which shows the constant temperature rise since A.D. 900, has been a popular target of global warming critics.  The recently published report supports the work that Mann and two of his colleagues did while at the University of Virginia.

The report was published a few days after astrophysicist Stephen Hawking said that humans need to colonize a planet or moon because the Earth potentially faces destruction due to a man made disaster.

"The Space Elevator will be built about 50 years after everyone stops laughing" -- Sir Arthur C. Clarke
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