SAN FRANCISCO (AP) — California's beloved giant redwood trees are in the midst of an unprecedented growth spurt, according to a climate study released Wednesday.

The ancient trees have produced more wood over the past century than they have during any other time in their lives, according to findings by the Save the Redwoods League and a team of renowned scientists.

The $3 million four-year study, called the Redwoods and Climate Change Initiative, was the most intensive research project ever conducted on the giant trees. The researchers climbed, poked and prodded 137 coast redwoods and giant sequoias on 16 research plots throughout the trees' geographic range.

The study said changing environmental conditions are triggering spurts in coast redwoods and the giant sequoias. Researchers added that ancient redwood forests can store up to three times more carbon above ground than any other forest in the world.

Emily Burns, science director at the Save the Redwoods League said the findings are a happy surprise.

"We have found ancient forests where climate conditions are accelerating growth and we predict these places will stay vibrant habitat refuges for other plants and animals in the foreseeable future," Burns said.

One researcher, Humboldt State forestry professor Stephen Sillett, said it's unclear what's prompted the growth spurts.

It could be that rising temperatures have lengthened the growing season or that the redwoods are getting more sun. Or it could be something more mundane, such as a reduction in air pollution.

"Our hypothesis is that it's because it is warmer. That lengthens the growth season," Burns told the San Francisco Chronicle.

Sillet told the Los Angeles Times that, "We really do not know. What we can say is that . it's not like a doom and gloom scenario by any means."

Whatever the reason, Sillett says that when it comes to climate change, he thinks redwoods will hold their own.

Though global warming is generally expected to make wet areas wetter and arid regions drier, its effects on regional precipitation are less clear. Tree records from the study show that precipitation has been highly variable but has not declined overall in the study area in recent decades.

Sillett said old giant sequoias could be growing faster because rising temperatures have lengthened the growing season in the Sierra Nevada.

The redwoods could be getting more sun as a result of reduced fog related to climate change, Burns said, while still getting the precipitation they need.