Climate change is making the planet hotter and drier, and has about doubled the area burned by forest fires in the western United States in the past three decades, a study said Monday.

Researchers found that since 1984, drier conditions and higher temperatures have caused fires to spread across an additional 16,000 square miles (41,500 square kilometers) -- an area about 30 times the size of Los Angeles.

This "approximately doubled the western US forest fire area beyond that expected from natural climate variability alone during 1984-2015," said the study in the October 10 issue of the Proceedings of the National Academy of Sciences, a peer-reviewed US journal.

Even more scorching wildfires can be expected in the years to come, warned the study authors.

"No matter how hard we try, the fires are going to keep getting bigger, and the reason is really clear," said co-author Park Williams, a bioclimatologist at Columbia University's Lamont-Doherty Earth Observatory.

Wildfires have been on the rise since the 1980s.

So far this year, some three million acres of forest (1.2 hectares) have burned in the western United States. While not a record-breaking year, the most dangerous conditions could come in the next two months.

Last year, a record-setting 10.1 million acres burned across the United States, the largest number since the National Interagency Fire Center began documenting wildland fire area in 1983, said the study.

Federal firefighting costs for 2015's fire season also reached a record-high $2.1 billion.

"A lot of people are throwing around the words climate change and fire -- specifically, last year fire chiefs and the governor of California started calling this the 'new normal,'" said lead author John Abatzoglou, a professor of geography at the University of Idaho.

"We wanted to put some numbers on it."

Researchers arrived at their figures by examining eight different systems for rating forest aridity, including the Palmer Drought Severity Index, the MacArthur Forest Fire Danger Index and the Canadian Forest Fire Danger Rating System.

They did not look at other contributing factors, including the impact of millions of trees killed by beetle infestations, changes in soil moisture due to an earlier snowmelt, or the potential for more frequent lightning -- as is expected in a warming world -- to ignite fires.

Therefore, the estimates may be lower than reality, they said.