The thinning of the West Antarctic Ice Sheet (WAIS) is currently contributing to sea level rise at a rate of about 4.5 cm per century, and the situation will probably only get worse (see Physics Today, July 2014, page 10). Though increasing greenhouse gas concentrations from human activity may seem to be the obvious cause, the details have been clouded by periodic fluctuations in the rate of ice loss. That natural variability stems from year-to-year change associated with the tropical Pacific Ocean’s El Niño–Southern Oscillation (see the article by David Neelin and Mojib Latif, Physics Today, December 1998, page 32) and the Interdecadal Pacific Oscillation. Now Paul Holland of the British Antarctic Survey and his colleagues have found a direct link between WAIS loss and human-induced climate change. Model simulations indicate that a reversal in the direction of local winds is destabilizing the ice sheet.
When eastward-flowing winds drive warmer ocean water onto the continental shelf, the exposed underside of the ice sheet melts. The researchers analyzed winds from 1979 to 2017, and they used sea-surface temperature data to extend the record further back in time. Variations in tropical Pacific temperatures cause wind perturbations that move from the tropics to West Antarctica. Holland and his colleagues used that connection in climate simulations to study wind speeds as far back as 1920.
The graph above shows the strong decadal variability in the simulated wind speed (black solid line) and the observations (blue solid line). The trend line (black dashed line) indicates that the average wind speed increased by about 0.7 m/s per century. That corresponds to a decrease in westward winds (negative values) and an increase in eastward winds (positive values) over the 20th century. The reversal in wind direction brings more warm water and consequently more WAIS ice loss.
To separate the long-term trend from the noisy time series, Holland and his colleagues examined simulations from the same climate model but without the effect that the decadal ocean temperature variability has on the winds. The analysis shows that the once predominant westward winds have weakened by about 0.5 m/s as human activity has warmed air and ocean temperatures, which has allowed warmer eastward winds to carry heat to the WAIS and exacerbate ice loss. (P. R. Holland et al., Nat. Geosci. 12, 718, 2019.)
Editor’s note, 9 September: The first sentence of the article was updated to clarify the contribution of WAIS thinning to sea level rise.
