Global warming and past management practices are making forests in the Western United States more susceptible to fire, according to a report released Thursday.

Large wildfires, like two that burned thousands of acres in Utah last year, are blamed for making climate change worse and putting unnatural stress on ecosystems, the National Wildlife Federation report said.

The report, "Increased Risk of Catastrophic Wildfires: Global Warming's Wake-Up Call for the Western United States," claims global warming is increasing the risk of fires because of rising temperatures, drier conditions, more lightning from stronger storms, added dry fuel for fires and a longer fire season.

Those factors have combined with decades of fire-suppression tactics that allowed unsafe fuel loads to accumulate, as well as severe bark-beetle infestations that are rapidly decimating trees and ever-expanding human settlements in and near forests, the report said. "The result is increasing vulnerability to major fires."

During a teleconference Thursday, National Wildlife Federation climate scientist Amanda Staudt said the number of wildfires has increased fourfold each year since the mid-1980s. Their impact on global warming is considered significant throughout the country and in Utah.

Last year's 363,000-acre Milford Flat fire in Utah burned for about two weeks in July and is estimated to have released into the atmosphere more than 186,400 tons of carbon dioxide, considered the most abundant of greenhouse-gas emissions impacting climate change.

"I think CO2 emissions from fires are significant," said Brock LeBaron, technical analysis manager for the Utah Division of Air Quality. "It's something that needs to be considered in greenhouse-gas emissions inventory as we go forward."

The department reported that in 2005, vehicles on Utah roads emitted about 15.1 million tons of carbon dioxide, including methane gases. To help curb emissions, members of the Western Climate Initiative are calling for more regulation of carbon emitters, including industrial polluters. The Western Climate Initiative is a collaboration of several Western states and two provinces of western Canada to find ways to work together and reduce greenhouse gases in the region.

Watchdogs say hotter and longer summers in the West aren't helping fight the global-warming problem.

The University of Montana College of Forestry and Conservation's Steven Running said during the teleconference that forests are starting their summer "dry down" early in the spring, melting away snow that he called "the best fire retardant ever invented."

Summer rains throughout the West are doing little to hydrate forest ecosystems, while areas that have burned don't get enough water and come back as grasslands or landscapes defined by shrubs, he said.