Records show temperatures topped or equaled historic highs at 7,755 locations in the United States in March, climate scientists say.

The warm March kept up a winter trend, with the National Climatic Data Center declaring December, January and February collectively the fourth-warmest winter on record in the lower 48 states.

Accumulating greenhouse gases likely played a role in the "year without winter" experienced by a significant portion of North America, climate scientists said.

"Clearly, this is outstanding and well outside any expectation under an unchanging climate. The magnitude and duration of the events in March certainly indicate that some unusual factors are afoot," Kevin Trenberth at the National Center for Atmospheric Research in Boulder, Colo., told LiveScience.com.

However, even in a warming world there will be regions with below-normal temperatures, Trenberth said, noting 287 locations out of 174,643 reporting had record lows or tied historic ones.

The cold sites were clustered largely in the western United States, he said.

Still, it was the record highs that had scientists' attention.

"Extremes are always expected to happen as the climate record gets longer, but certain extremes related to heating are becoming more evident," Trenberth said.