California’s cap-and-trade system, designed to reduce carbon emissions that contribute to climate change, is being undermined by forest fires caused in part by climate change, according to scientists.

Launched in 2013, California’s cap-and-trade allows businesses to offset carbon emissions by purchasing a limited number of credits, most of which are used to protect trees that store carbon dioxide from being cut down. California and a group of East Coast states were the first to develop such a system in the U.S. Similar programs exist in the European Union, South Korea, and China.

But over the past two years, wildfires along the West Coast have consumed as many as 6.8 million metric tons of stored carbon dioxide in forests enrolled in the program, according to preliminary estimates by the Climate Trust, a Portland, Ore., nonprofit that manages carbon offset acquisition projects.

That is more than one fifth of the 29 million tons California regulators have set aside so far to potential losses due to wildfires or insects over a full century. The state’s Air Resources Board designated that amount as a buffer for the program, which currently aims to preserve a total of 156 million tons of forest carbon credits.

Researchers now warn the state’s stockpile of carbon dioxide credits is rapidly depleting because of the growing size and intensity of wildfires. The two biggest wildfires in California history have burned in the past two years and 14 of the top 20 took place over the past decade.

“These risks are pretty big, and they are probably much larger than what the buffer pool currently has,” said William Anderegg, associate professor of biology at the University of Utah. He co-headed a study released in November that predicted the area of U.S. forests affected by fire would grow by a factor of as much as 14 by the end of the century.

A spokesman for the California Air Resources Board said the program is protected in part by the fact that California businesses can buy offsets in forests in 29 states, from Alaska to Maine. He also said the size of the forest buffer will increase as more projects are added.