Greenhouse gases trapped in the upper atmosphere are acting like natural climatic forces that made some ancient droughts last for 1,000 years, UCLA researchers say.

Global warming created by these gases could be making a more arid climate, like what California has seen in its current five-year drought, “the new normal,” said UCLA geography professor Glen MacDonald, the study’s lead author.

Released today, the study in Scientific Reports — part of the Nature Publishing Group — found that carbon dioxide and methane emitted from the burning of fossil fuels may be mimicing the effects of some catastrophic environmental phenomena the planet has previously experienced.

The consequences of the buildup of greenhouse gases in the Earth’s atmosphere look a lot like the effects of storms on the surface of the sun, decreases in volcanic activity or wobbles in the Earth’s orbit.

Events such as those precipitated 60-year periods of drought in the 12th century, MacDonald wrote. Some prehistoric droughts created by those phenomena lasted as long as 1,000 years.

Man-made activity causing the Earth’s temperature to rise may take the place of these natural phenomena — known as “radiative forcing” — in creating what scientists identify as extended arid periods.

“Radiative forcing in the past appears to have had catastrophic effects in extending droughts,” said MacDonald, an international expert on drought and climate change, in a statement.

“When you have arid periods that persist for 60 years, as we did in the 12th century, or for millennia, as we did from 6,000 to 1,000 B.C., that’s not really a ‘drought,’ ” MacDonald said. “That aridity is the new normal.”

While many scientists suspected that California had experienced extended dry periods in the ancient past, MacDonald’s study found new, decisive evidence of mega-droughts occurring before recorded history.

Much like how researchers can use tree rings to understand what happened to an ancient tree over the course of its life, MacDonald used sediment samples from Kirman Lake in central-eastern California to document previous droughts. He concluded that these dry periods related to sea surface temperatures in the Pacific Ocean.

Ocean temperatures help climatologists predict weather patterns like El Niño and La Niña.

Californians awaiting this year’s El Niño knew they could anticipate months of wetter weather. That didn’t happen, with scientists saying a dome of high pressure kept the storm track of the most recent El Niño confined to the northern part of the state.

If the current La Niña period, which usually means drier weather, combines with radiative forcing of global warming, this double-whammy could produce droughtlike conditions that may extend indefinitely, the study says.

If levels of greenhouse gases continue to rise, the resulting effects could wither the state’s forest preserves and decrease snowfall and rainfall, the study’s authors concluded.

The U.S. Forest Service in June said a combination of drought, a bark beetle infestation and hotter temperatures killed an estimated 66 million trees in California’s forests of the Sierra Nevada over the last six years. The mass of dead trees raised concerns that there would be more fuel for wildfires in the state this summer.

The costs of fighting wildfires during the state’s 2015-16 fiscal year, which ended June 30, jumped to $366 million, according to the California Department of Forestry and Fire Protection. That’s compared to $209 million for the previous fiscal year.

Dry brush as a result of the drought also quickened fires in Southern California, like the Blue Cut fire that raged in the San Bernardino Mountains in August.

“When we go out to hike in the Sierra Nevada mountains or the Santa Monica mountains, I worry that we will see very different wildlands by the end of this century,” MacDonald said in his statement.