By Deborah Zabarenko, Environment Correspondent

WASHINGTON, April 23 (Reuters) - In a vicious cycle made worse by humans, scientists now believe fires spur climate change, which in turn makes blazes bigger, more frequent and more damaging to the environment.

Climate experts have known that a warmer world would spawn more fires, but in research published on Thursday in the journal Science, scientists reported that fires -- especially those set by humans to clear forests -- influence climate change.

Smoke particles sent into the atmosphere by fires inhibit rainfall, which makes the land drier and encourages more fires to start, said study co-author Jennifer Balch of the University of Santa Barbara in California.

On a global scale, burning releases vast amounts of heat-trapping greenhouse gases like carbon dioxide, making fires more likely in a warming world, Balch said in a video news briefing.

The report's authors estimate that greenhouse emissions from the world's fires equal about 50 percent of emissions that come from the burning of fossil fuels.

Deforestation fires, like those set to clear forest for pasture in tropical areas like the Amazon, are part of an unintentional "extreme experiment," Balch said: "We're testing how burning forests will influence the climate system."

'THE SCARY BIT'

These deliberately set forest fires contributed up to one-fifth of all human-generated warming in industrial times, she said.