As global warming thaws the permafrost, the frozen land that covers nearly six million square miles of the earth, a big question for scientists is: How much will be lost?

The answer, according to a new analysis: more than many of them thought.

A study published Tuesday in the journal Nature Climate Change suggests that as the planet warms toward two degrees Celsius (3.6 degrees Fahrenheit) above preindustrial levels, each degree Celsius of warming will lead to the thawing of about 1.5 million square miles of permafrost.

That figure is at least 20 percent higher than most previous studies, said Sarah E. Chadburn, a researcher at the University of Leeds in England and the lead author of the study.