Exploring the Influence of Internal Climate Variability and Forced Change on Arctic Greening

Abstract

Warming Arctic temperatures are increasing terrestrial ecosystem productivity and allowing plant species to advance poleward. This Arctic greening has extensively altered regional ecosystems and climate, and it is projected to be a central element of future Arctic change. Existing fieldwork has identified increases in growing degree days (the annual sum of daily temperatures above 5 ˚C) as a key driver of Arctic greening. We use the Community Earth System Model 2 Large Ensemble to describe how climate variability and change affect Arctic growing degree days over historical and future periods. 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 allow us to connect these events to coupled climate variability. For the Brooks Range--a region where recent observations illustrate the treeline is rapidly advancing--we show that crossover in this region is associated with Pacific and Arctic sea surface temperature variability. Crossover into no-analog climate states relative to a preindustrial simulation begins in the mid-21st century around the circumpolar region. Mechanistic insights into the influence of coupled climate variability are robust to biases in the simulated high-latitude climate that stem from uncertainties in aerosol emissions. The methods we demonstrate provide a framework to incorporate contributions from internal variability and climate change together in future projections of ecosystem impacts.