Carbon released from frozen ground (permafrost) is at risk of release into the atmosphere as Arctic temperatures warm. This could lead to still further warming, initiating a catastrophic positive feedback. Our best estimates of how significant this feedback is likely to be are still very uncertain; here, we take a new approach to try to understand how this process operated during a past period of widespread global warming during the end of the last ice age. Using geologic evidence of permafrost degradation and paleoclimate data, we compare time periods of intense thawing to atmospheric greenhouse gas records preserved in ice cores. The expanse of carbon-rich peatlands and boreal forests occurred at the same time as elevated permafrost thaw, which helped offset emissions of carbon into the atmosphere and prevented a runaway feedback. A similar mechanism may help forestall runaway warming in the coming centuries--however, there are important distinctions between past and present. Understanding the importance and extent of these differences can help further constrain future risk.