The phrase "thawing tundra" refers to the process of frozen land in the Arctic regions, known as tundra, starting to melt and become softer due to rising temperatures. Full definition
Thawed tundra just isn't the same as Iowa top soil. (realclimate.org)
Many amplifying feedbacks are closely monitored by scientists, including thawing tundra and dying forest releasing massive amounts of heat - trapping CO2, and the way in which the growing amount of darkened land absorbs more of the sun's heat, thus further warming the ground and air. (abcnews.go.com)
First, most climate simulations, including ours above and those of IPCC [1], do not include slow feedbacks such as reduction of ice sheet size with global warming or release of greenhouse gases from thawing tundra. (journals.plos.org)