My first guess on the title was : “this should be caused by a positive feedback with forest fires in the loop”.
…so, I went and read this Wikipedia article : en.m.wikipedia.org/wiki/Climate_change_feedbacks
… and it says :
(…) Carbon cycle uncertainty is driven by the large rates at which CO2 is both absorbed into plants and released when biomass burns or decays. For instance, permafrost thaw produces both CO2 and methane emissions, in ways that are difficult to model. (…)
But it also says that this one is not the most important feedback loop.