As Congress headed home for the Christmas break after passing a budget deal, Agriculture Department Secretary Tom Vilsack presented lawmakers with an angry ultimatum: Put up more cash if you want the U.S. Forest Service to keep putting out huge wildfires.

Vilsack is fuming because Congress set aside $1.6 billion to pay for wildfire suppression in 2016 despite the service, which he controls, spending $100 million more than that to fight blazes this year. Year after year, Congress has underfunded the firefighting effort, forcing the Forest Service to borrow hundreds of millions of dollars from other departments in the agency to pay for equipment and firefighters.

Congress allocated $1 billion for fire suppression in 2015 — a year fast approaching another record for most acres burned. As fires continued, Vilsack pleaded for more money to avoid the frantic inter-agency borrowing of fire seasons, such as $999 million in 2002; $695 million in 2003; $200 million in 2006, along with four other years when the budget came up short.

In a smoldering letter to lawmakers Thursday, Vilsack put his foot down. The 2016 budget “fails to provide a long-term solution to address the critical and growing problem of paying for catastrophic wildfire and instead leaves the Forest Service hobbled by the current untenable budget situation,” he wrote. He issued what amounts to a threat, saying he will no longer rob from other departments to pay for firefighting efforts that Congress doesn’t fund.

“If the amount Congress appropriated in 2016 is not sufficient to cover fire suppression costs, Congress will need to appropriate additional funding on an emergency basis,” Vilsack wrote.

Ironically, money was diverted this year from the department devoted to prevent wildfires by removing debris that helps them burn. The Forest Service pays loggers to remove trees left dead and dying from insect infestation, as well as foliage that grew in previous years but dried and turned to kindling during long periods without snow or rain.

“We will continue to protect lives, property and our natural resources, but it is the responsibility of Congress to ensure those resources are sufficient each year so that the Forest Service is able to accomplish the full complement of its work,” said Matt Herrick, a USDA spokesman. In the future, the agency “will notify Congress well in advance” if emergency funding is needed when the budget is short.

The nation’s worst fire season in recorded history was 2006, when 9.87 million acres were scorched. This year’s season is only a few thousand acres below that mark with nearly two weeks remaining and two large fires still going in Kentucky. Herrick said the Forest Service has estimated that the record will be surpassed. Another record was broken in August when the agency paid $243 million to suppress fires in only one week.

In an era of climate change, fire seasons that once started in May now start in March. They end in December, not October. Alaska, dry from a lack of snow, saw its second-worst fire season when a freakish series of lightning strikes ignited blazes that burned more than 5 million acres.

“The future trend will be hotter, longer, more severe, and ultimately more costly fire seasons,” Vilsack wrote, with little money to pay firefighters to suppress them and for support such as fire engines, airplanes and logistics personnel. By 2025, the Forest Service estimates, fighting fires will eat 67 percent of its budget, “a dramatic increase from 16 percent in 1995,” according to his letter.

“This directly impacts the Forest Service’s ability to fund other critical work such as restoration that can reduce wildfire threat, drinking water area protection, and recreation investments, not just in the West, but across the country,” Vilsack wrote.