An "unprecedented" March heat wave in much of the continental United States has set or tied more than 7,000 high temperature records, and signals a warming climate, health and weather experts said on Friday.

While natural climate variability plays a major role, it is the addition of human-spurred climate change that makes this particular hot spell extraordinary, the scientists said in a telephone and web briefing.

"This heat wave is essentially unprecedented," said Heidi Cullen of the nonprofit science and communication organization Climate Central. "It's hard to grasp how massive and significant this is."

Since March 12, more than 7,000 high temperature records have been equaled or exceeded, Cullen said, citing figures from the U.S. National Climatic Data Center.

These records include daytime high temperatures and record-high low temperatures overnight, which in some cases are higher than previous record highs for the day, Cullen said.

"When low temperatures are breaking previous record highs, that's when you see this is incredibly special," she said.

Cullen noted that this warmth is part of a trend that is pushing the spring season earlier by an average of three days in the contiguous 48 U.S. states.

The date of first leafing—the day when buds burst open—has moved forward from March 20, where it was during the 30-year period from 1951 to 1980, to March 17, where it has been for the period from 1981 to 2010.

An online report accompanying the briefing shows that some states have an even earlier spring, with Montana, Washington state, Oregon, Nevada, Utah and Connecticut seeing spring arrive five days or more ahead of the previous average.

"Most likely the weird weather arises from natural variation on top of a warming climate," said Michael Oppenheimer, a geoscientist at Princeton and a veteran participant in the Intergovernmental Panel on Climate Change. "What we're seeing now is not surprising in the greenhouse world ... It's just the beginning of our experience with the new atmosphere."

Oppenheimer was a lead author of the panel's path-breaking 2007 report that analyzed research by hundreds of scientists and found there was a 90 percent probability that climate change is occurring and human activities contribute to it.

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