Past land use decisions have increased mitigation potential of reforestation

[1] Anthropogenic land cover change (ALCC) influences global mean temperatures via counteracting effects: CO₂ emissions contribute to global warming, while biogeophysical effects, in particular the increase in surface albedo, often impose a cooling influence. Previous studies of idealized, large-scale deforestation found that albedo cooling dominates over CO₂ warming in boreal regions, indicating that boreal reforestation is not an effective mitigation tool. Here we show the importance of past land use decisions in influencing the mitigation potential of reforestation on these lands. In our simulations, CO₂ warming dominates over albedo cooling because past land use decisions resulted in the use of the most productive land with larger carbon stocks and less snow than on average. As a result past land use decisions extended CO₂ dominance to most agriculturally important regions in the world, suggesting that in most places reversion of past land cover change could contribute to climate change mitigation. While the relative magnitude of CO₂ and albedo effects remains uncertain, the historical land use pattern is found to be biased towards stronger CO₂ and weaker albedo effects as compared to idealized large-scale deforestation.