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

Background

This study investigates how a 2°C increase in global mean temperature above pre-industrial levels could alter above-ground carbon storage in forests across the western United States, a region already experiencing climate-driven tree mortality, reduced regeneration, and more frequent fire and insect outbreaks. Forest carbon density is treated as a “carbon carrying capacity” controlled by climate and disturbance regimes, and there is concern that warming and drying will shift many areas towards lower carbon forest or non-forest states, with implications for climate mitigation and carbon offset projects.

Goals and Methods

The authors estimated how much above-ground forest carbon storage would change region-wide under a 2°C warming scenario, distinguishing losses driven by reduced carbon density from those driven by conversion to non-forest vegetation. They use a spatial climate analogs approach at 4 km resolution, combining data from TerraClimate datasets for four biophysically important variables (i.e., mean maximum temperature of the warmest month, mean minimum temperature of the coldest month, mean annual actual evapotranspiration and mean annual climate water deficit) with the TreeMap 2016 interpolated Forest Inventory and Analysis dataset to associate present-day carbon density with climatic conditions, and then mapping those relationships onto a 2°C climate.

Conclusions and Takeaways

This study projects an 18% decline, about 796 teragrams of carbon, in above-ground forest carbon storage across western US forests under 2°C warming, driven by both reduced carbon density in currently high-biomass areas and climate-driven transitions from forest to non-forest in warmer, drier ecoregions. Many regions retain similar median carbon densities but lose high-density tails and forest extent, suggesting that productive forests commonly targeted for offset projects will face declining carbon carrying capacity and heightened risk from compounded drought, insects, and fire. The authors argue that offset accounting and forest management must integrate dynamic baselines and climate-informed species and site selection, since disturbances and regeneration lags mean that realized carbon losses could exceed climate-only projections in a 2°C world.