Ecological limits to terrestrial biological carbon dioxide removal

Terrestrial biological atmospheric carbon dioxide removal (BCDR) through bioenergy with carbon capture and storage (BECS), afforestation/reforestation, and forest and soil management is a family of proposed climate change mitigation strategies. Very high sequestration potentials for these strategies have been reported, but there has been no systematic analysis of the potential ecological limits to and environmental impacts of implementation at the scale relevant to climate change mitigation. In this analysis, we identified site-specific aspects of land, water, nutrients, and habitat that will affect local project-scale carbon sequestration and ecological impacts. Using this framework, we estimated global-scale land and resource requirements for BCDR, implemented at a rate of 1 Pg C y(-1). We estimate that removing 1 Pg C y(-1) via tropical afforestation would require at least 7 x 10(6) ha y(-1) of land, 0.09 Tg y(-1) of nitrogen, and 0.2 Tg y(-1) of phosphorous, and would increase evapotranspiration from those lands by almost 50 %. Switchgrass BECS would require at least 2 x 10(8) ha of land (20 times U.S. area currently under bioethanol production) and 20 Tg y(-1) of nitrogen (20 % of global fertilizer nitrogen production), consuming 4 x 10(12) m(3) y(-1) of water. While BCDR promises some direct (climate) and ancillary (restoration, habitat protection) benefits, Pg C-scale implementation may be constrained by ecological factors, and may compromise the ultimate goals of climate change mitigation.