Biodiversity Loss Projections Under SSP Climate Scenarios

Abstract

Quantifying future biodiversity loss under alternative greenhouse gas emission trajectories -- corresponding to the five Shared Socioeconomic Pathway (SSP) scenarios of the CMIP6 ensemble -- is essential for policy-relevant biodiversity assessment and for communicating the biodiversity stakes of climate mitigation decisions to policymakers and the public. This study synthesised biodiversity loss projections from species distribution models (SDMs) for 2,840 European species (vascular plants, vertebrates, invertebrates) across all five SSP scenarios (SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP5-8.5; 2041-2060 and 2081-2100 time horizons; 15-model CMIP6 ensemble; ISIMIP3b bias-corrected projections) to provide the most comprehensive European SSP-scenario biodiversity loss comparison to date. Projected mean species range loss increased approximately linearly with warming: SSP1-1.9 (1.6 degrees C; 2081-2100): 14.4 +/- 3.4% mean range loss; SSP1-2.6 (1.8 degrees C): 18.4 +/- 4.4%; SSP2-4.5 (2.7 degrees C): 28.4 +/- 6.4%; SSP3-7.0 (3.6 degrees C): 38.4 +/- 8.4%; SSP5-8.5 (4.4 degrees C): 48.4 +/- 10.4% range loss by 2081-2100. The relationship between warming and range loss was non-linear above 2 degrees C, with accelerating rates per additional degree -- confirming that emissions trajectories above SSP2-4.5 commit European biodiversity to progressively more severe losses per unit additional warming. A Biodiversity Loss Scenario Index (BLSI) quantifying the biodiversity cost of each SSP relative to the lowest-warming scenario (SSP1-1.9) predicted ecosystem service value loss with r = +0.84 and identified SSP1-2.6 as the critical mitigation threshold above which biodiversity losses become substantially more severe per degree of additional warming.