The last few decades of the 20th century were warmer than any comparable time in at least the past four centuries, a new report says.

The report from the U.S. National Academy of Sciences was requested by Congress after a controversy last year over climate reconstructions published in the late 1990s.

That work, by University of Virginia climatologist Michael Mann and others, found the warming of the Northern Hemisphere in the last decades of the 20th century was unprecedented in the past thousand years. They concluded that the 1990s were the warmest decade, and 1998 the warmest year.

Scientists believe they can glean evidence of past temperatures from tree rings, boreholes, retreating glaciers, corals, cave deposits and other sources, although the evidence becomes weaker further back in time.

Such evidence agrees with the globally averaged warming of about 1 degree Fahrenheit (0.6 degrees Celsius) that instruments have recorded during the last century, according to members of a committee that drafted the new report.

The committee of the National Research Council, part of the National Academy of Sciences, called “plausible” the Mann team’s conclusion that warming in the last few decades of the 20th century was unprecedented over the last millennium.

But the panel said it had less confidence that the warming was unprecedented before 1600, and in the Mann team’s conclusions about the 1990s and 1998 in particular.

Scientists’ reconstructions of Northern Hemisphere surface temperatures for the past thousand years are generally consistent, the dozen-member committee reported.

These reconstructions, members added, show relatively warm conditions centered around the year 1000, and a relatively cold period, or “Little Ice Age,” from roughly 1500 to 1850. None of the reconstructions show temperatures were warmer during medieval times than during the past few decades, the committee added.

Most scientists blame global warming on greenhouse gases, substances such as carbon dioxide emitted as a result of human activities and that collect in the atmosphere.

Scientist are predicting a host of evils for human health and the environment as a result of global warming. Among many other problems, they have reported that polar bears are being driven to cannibalism as their icy habitat melts away, and baby walruses are possibly being orphaned for similar reasons.

The committee wrote that surface temperature reconstructions for periods before the Industrial Revolution—when levels of atmospheric greenhouse gases were much lower—are one of many lines of evidence suggesting human activities are responsible for the warming.

The National Research Council is a private, nonprofit institution that provides science and technology advice under a congressional charter.

ABOUT THE AUTHOR

Science writer Jack Lucentini is founder and editor of the World Science science news webzine. He has worked as a staff writer at three daily newspapers, and as a freelance science writer for a range of publications including The Washington Post, Discover magazine and The Scientist magazine. He earned his bachelor's degree at Oberlin College in Oberlin, Ohio in 1993.