June 22, 2006 The Earth is the hottest it has been in the at least the last 400 years—and potentially the last 1,000—according to a new National Academy of Sciences (NAS) report, which largely attributes the spike in temperature in the last few decades to human activity.

Brenda Ekwurzel of the Union of Concerned Scientists applauded the study, which compiled data from large-scale temperature reconstructions to chronicle conditions for the last 2,000 years.

Ekwurzel emphasized the magnitude of such an undertaking, adding that the abundance and variety of so many “archives of temperature,” proxy evidence such as tree rings, corals, lake sediments, and cave deposits, has been cited by global warming naysayers in the past.

“People who were skeptical about climate change and global warming would point to the uncertainty that we were already well aware of,” said Ekwurzel, a climate specialist. “This report means that skeptics can no longer point to this.”

Requested by Congress, the independent review was summoned after recent controversy arose over a study published in 1998 by climatologist Michael Mann and his colleagues, who concluded the northern hemisphere was warmer during the late 20th century than at any other time in the past millennium.

The NAS report didn’t quite go that far, but, according to Ellen Druffel, an oceanographer and co-writer of the report, “It’s likely.”

Evidence suggesting so includes both additional large-scale surface temperature studies and pronounced changes in a variety of local proxy indicators, the report said.

Based on these indicators—some of which appear to be unprecedented during at least the last 2,000 years, such as the melting of icecaps and glaciers—the committee called the statements by Mann et al. to be “plausible.”

There is less evidence to suggest that the current warming period was unprecedented prior to 1600 due to the scarcity of proxy evidence. Records of temperatures measured with instruments date back only about 150 years.

The new report says there are multiple lines of evidence supporting the conclusion that climate change is largely human-induced, caused in part by carbon dioxide emissions that get trapped in the atmosphere, consequently increasing global temperatures.

Temperature data for periods prior to the Industrial Revolution—when levels of greenhouse gases in the atmosphere were much lower—is only one example, and not the primary evidence, supporting the conclusion that warming is occurring in response to human activities, the report states.

Primary evidence is rooted in the concentrations of carbon dioxide measured in places such as Antarctica and Greenland, which can go as far back as 650,000 years, Ekwurzel said.

The “tight” correlation between levels of carbon dioxide and levels of temperature was clearly demonstrated in Al Gore’s new movie, An Inconvenient Truth, which debuted widely earlier this month.

Since carbon dioxide levels today are higher than any other period during that time frame and continue to be on the rise, Ekwurzel says, continuing in that direction will see rising temperatures.

“Which is what we call global warming,” said Ekwurzel.

Still, there are some critics.

William Gray, who has led a team of hurricane forecasters at Colorado State University for more than 20 years, wrote in a BBC article that “human kind has little or nothing to do with the recent temperature changes.”

For others, like Druffel, “It’s staring us right in the face.”

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