Tree planting is no climate solution at northern high latitudes

Planting trees has become a popular solution for climate change mitigation, owing to the ability of trees to accumulate carbon in biomass and thereby reduce anthropogenic atmospheric CO2 enrichment. As conditions for tree growth expand with global warming, tree-planting projects have been introduced in regions of the highest northern latitudes. However, several lines of evidence suggest that high-latitude tree planting is counterproductive to climate change mitigation. In northern boreal and Arctic regions, tree planting results in net warming due to increased surface darkness (decreased albedo), which counteracts potential mitigation effects from carbon storage in areas where biomass is limited and of low resilience. Furthermore, tree planting disturbs pools of soil carbon, which store most of the carbon in cold ecosystems, and has negative effects on native Arctic biota and livelihoods. Despite the immediate economic prospects that northern tree planting may represent, this approach does not constitute a valid climate-warming-mitigation strategy in either the Arctic or most of the boreal forest region. This has been known for decades, but as policies that incentivize tree planting are increasingly adopted across the high-latitude region, we warn against a narrow focus on biomass carbon storage. Instead, we call for a systems-oriented consideration of climate solutions that are rooted in an understanding of the whole suite of relevant Earth system processes that affect the radiative balance. This is crucial to avoid the implementation of ineffective or even counterproductive climate-warming mitigation strategies in the Arctic and boreal regions.