Boreal forests are heading for an open state

The boreal forest biome is warming four times faster than the global average. Changes so far are moderate, but time lags in responses may transiently maintain forest states which are no longer supported by current environmental conditions. Here, we explore whether tree cover dynamics hint at the state to which the biome may be shifting. Our results suggest that global boreal tree cover is heading for a unimodal state of open forest (30 to 50%), contrasting starkly to the bimodal distribution of dense forests (>60%) and sparse woodlands (5 to 15%) found today. A future open forest in a warmer world would likely increase the risk of forest fires, leading to potentially substantial releases of stored carbon.