Natural and Anthropogenic Climate Variations in the Arctic

Reductions in sea ice extent and ice thickness have been recently detected in the Arctic Ocean, based on data primarily from the past 22 years (for satellite data) and 42 years (for submarine data). Since the observed trends are statistically significant, they are often considered to be evidence of the Arctic response to global-scale warming, though they may also be the result of Arctic climatic variability on multi-decadal time scales. A global climate model is used here to compare simulated Arctic climate variability and long-term ice trends.

A global atmosphere-ocean-sea ice general circulation model called the Parallel Climate Model (PCM) is used in simulations of climate with greenhouse gas concentrations and sulfate aerosols prescribed from observational data (1870 through 1995), and future projections (1995 through 2100). Simulations that include the variability in solar flux over 1870-1995 are also performed. The PCM uses elastic-viscous-plastic sea ice dynamics and two-layer ice and snow thermodynamics. Additional PCM simulations are being initiated that include a sea-ice thickness distribution model and improved enthalpy-conserving thermodynamics.

Arctic surface temperatures in the PCM over 1870-1995 are compared to temperature anomalies derived from Arctic paleoclimatic proxy indicators, such as tree rings, lake varves, ice cores, and the limited instrumental data. The PCM and the proxy data exhibit similar variations on decadal and longer time periods, in response to solar flux variations and greenhouse gas changes. The PCM exhibits larger decadal variability in sea ice thickness and area than the trend over the last 100 years. The PCM’s ice trends become as large as the variability at around year 2040, in response to greenhouse gas-induced warming. An ensemble of independent simulations produces a wide range of Arctic ice trends over the next 50 years. The results illustrate the difficulty in detecting and attributing Arctic climate change to particular climate forcing, as significant periods of record are required, and decadal variability can mask or oppose the long-term trends.