Anthropogenic warming exacerbates natural precipitation deficits through heat-induced increases in evaporative demand, which further dries soils and stresses vegetation. Roughly 15-20% of soil moisture loss during the strong 2012-2015 U.S. West Coast drought, for example, was directly attributable to anthropogenic warming. While this drought clearly reduced primary production and increased mortality of vegetation, the proximate climatic drivers (e.g., extreme heat vs. soil moisture loss) and relative contributions of natural variability vs. anthropogenic forcing remain unknown. Using solar-induced fluorescence (SIF) with observed and modeled climate data, we quantified the proportion of drought-induced primary production loss that was directly attributable to anthropogenic climate change, as well as the proximate climatic drivers of primary production loss. We hypothesized that anthropogenic contributions to lost primary production during the 2012-2015 West Coast drought were even greater than anthropogenic contributions to loss of soil moisture due to the direct negative effects of higher temperatures and vapor pressure deficits (VPD) on stomatal conductance and photosynthesis.
Results/Conclusions
Approximately 30-50% (depending on year) of reduced SIF was attributable to anthropogenic warming. Averaged across the four drought years, the single greatest driver of drought-induced loss of primary production was reduced soil moisture from natural variability (contributing ~60% of lost SIF). However, the second largest driver of reduced SIF during the drought was anthropogenic increases in VPD (contributing approximately 20-30% of reduced SIF), exacerbated by anthropogenic losses of soil moisture and increases in temperature (roughly 10% each). Increases in temperature and VPD from natural variability contributed to ~10-15% of the reduced SIF, while natural increases in solar radiation (which on its own should increase primary production and SIF) partially offset reduced SIF from other factors. Taken together, these results demonstrate the substantial contribution of anthropogenic warming to drought-induced loss of primary production. While droughts are generally initiated by natural variability of precipitation, anthropogenic warming exacerbates these conditions, especially for vegetation due to the direct responses of stomata to higher VPD. These anthropogenic effects on vegetation activity could both increase the likelihood of plant mortality during drought and exacerbate drought conditions via biogeophysical feedbacks.