Decreasing landscape carbon storage in western US forests with 2 °C of warming

Changing climate is altering the amount of carbon that can be sustained in forest ecosystems. Increasing heat and drought is already causing increased mortality and decreased regeneration in some locations. These changes have implications for landscape carbon storage with ongoing climate change. We used a climate analogs approach to project aboveground forest carbon density under +2 °C warming above pre-industrial climate for western US forests. We calculated analogs for current climate and under +2 °C warming and associated carbon density for each time period. We found that in most ecoregions, maximum carbon density values are projected to decline and the interquartile range of carbon density values is projected to narrow. Using mean carbon density values, we project a 796 Tg decline in landscape carbon storage across the western US. As tree mortality increases, the transition from live to dead carbon will increase fuel buildup and fire hazard in many ecosystems. Greater fire hazard and increased susceptibility to insects from drought could cause carbon density changes to occur more rapidly than our climate-only projections. This may have substantial implications for forest-based carbon offset projects.