In its most recent Form 10-K, Alpha Natural Resources warned investors that the increasing difficulty of locking in long-term contracts could reduce the company’s protection from short-term coal price volatility.

Alpha said that in 2013, about 45% of its steam coal was delivered through long-term coal contracts.

“In large part as a result of increasing and frequently changing regulation … and natural gas pricing, electric power generators are increasingly less willing to enter into long-term coal supply contracts, instead purchasing higher percentages of coal under short-term supply contracts,” Alpha wrote in its filing. “This industry shift away from long-term supply contracts could adversely affect us and the level of our revenues.”

Arch Coal expresses a similar concern in a section of its Form 10-K.

“Changes in the coal industry may cause some of our customers not to renew, extend or enter into new long-term coal supply agreements with us or to enter into agreements to purchase fewer tons of coal than in the past or on different terms or prices,” the filing states. “In addition, uncertainty caused by federal and state regulations, including the Clean Air Act, could deter our customers from entering into long-term coal supply agreements.”

According to an SNL Energy analysis of fuel contract data from the US Energy Information Administration supplied on Form EIA-923, the long-term coal contract appears to have become more elusive in the past four years.

In March, 16% of coal delivered came from contracts with remaining terms beyond five years, down from 20% in the same period in 2010.

In increasingly competitive markets, a long-term coal contract can offer price, volume and earnings stability that allows companies to make long-term planning decisions regarding their mines, according to the Kentucky Coal Association.