Conditions leading to extrinsic and intrinsic ecosystem change across large ensembles of climate futures

Abstract

Natural climate variability and forced change influence ecosystems through the direct impacts of changing environmental conditions ("extrinsic change"), and by altering internal ecosystem dynamics ("intrinsic change"). While simulating complex ecosystems and species-level change remains challenging, Earth system models are often capable of capturing patterns of the regional-scale climate conditions which lead to ecological change. Investigating these climate conditions allows models to be leveraged in studying ecosystem change without requiring direct simulation of ecological processes. In this dissertation, we explore conditions driving extrinsic and intrinsic ecosystem change in large ensembles of climate futures with external forcings from anthropogenic warming and stratospheric aerosol injection, a hypothetical method of climate intervention. In the first project, we use the Community Earth System Model 2 Large Ensemble to describe how climate variability and change affect Arctic growing season warmth. Using a novel statistical metric, we find that many simulated Arctic ecoregions have already entered a state in which the warming trend dominates over internal variability. Storylines of cases where this "crossover" occurs earlier or later connect these events to coupled climate variability. The second study uses climate speeds--a metric of the rate of movement of thermal niches--to explore possible ecosystem impacts from design choices in stratospheric aerosol injection scenarios. We find highly distinct profiles of ecological risk in two simulations with similar global temperature targets but a 10-year delay in deployment. In the final study, we explore intrinsic change by using an ecological niche model to project future changes to habitat suitability for the Gyrfalcon (Falco rusticolus), a large predatory bird which is a top consumer in the tundra. Climate warming leads to a poleward contraction in suitability over the 21st century; a climate intervention scenario with global temperature reduction rapidly reverses overall trends but yields distinct regional patterns. Storyline methods reveal a substantial role for internal variability even under very strong external forcings. This dissertation provides new methods to use climate models to probe extrinsic and intrinsic ecosystem change, and reveals insights into potential ecological impacts from climate intervention methods.