First evidence of human-caused climate change melting the West Antarctic Ice Sheet

One century of wind forcing and ice-sheet response. Grey and black show PACE ensemble-mean and ERA-Interim winds, respectively. Green and blue show anomalies in discharge of Thwaites Glacier and PIG3, relative to 102 and 108 Gt yr−1 median values, respectively. Red shows anomalies in combined melt rates from PIG5 (triangles) and the Dotson Ice Shelf6 (squares), relative to a median of 48 Gt yr−1. Orange events include initial (A) and final (B) ungrounding of PIG from a submarine ridge8,12, and the onset of recent thinning of Pine Island (C) and Thwaites (D) glaciers4 (mean ± 1 s.d. of dates for tributaries). Graphic: Holland, et al., 2019 / Nature Geoscience — One century of wind forcing and ice-sheet response. Grey and black show PACE ensemble-mean and ERA-Interim winds, respectively. Green and blue show anomalies in discharge of Thwaites Glacier and PIG3, relative to 102 and 108 Gt yr⁻¹ median values, respectively. Red shows anomalies in combined melt rates from PIG5 (triangles) and the Dotson Ice Shelf (squares), relative to a median of 48 Gt yr⁻¹. Orange events include initial (A) and final (B) ungrounding of PIG from a submarine ridge, and the onset of recent thinning of Pine Island (C) and Thwaites (D) glaciers (mean ± 1 s.d. of dates for tributaries). Graphic: Holland, *et al*., 2019 / *Nature Geoscience*

12 August 2019 (British Antarctic Survey) – A new study published this week reveals the first evidence of a direct link between human-induced global warming and melting of the West Antarctic Ice Sheet. UK-US researchers say that curbing greenhouse gas emissions now could reduce the future sea-level contribution from this region.

Ice loss in West Antarctica has increased substantially in the last few decades and is ongoing. Scientists have known for some time that ice loss in this region is caused by ocean-driven melt, and that varying winds in the region cause transitions between relatively warm and cool ocean conditions around key glaciers. However, until now it was not clear how these naturally-occurring variations in the winds could cause ongoing ice loss.

Published this week in the journal Nature Geoscience the UK-US science team report that, in addition to the natural variations, which last about a decade, there has been a longer-term change in the winds that can be linked with human activities. This is an important result because continued ice loss could cause tens of centimetres of sea-level rise by the year 2100.

The research team combined satellite observations and climate model simulations to understand how winds over the ocean near West Antarctica have changed since the 1920s in response to rising greenhouse gas concentrations. Their investigation reveals that human-induced climate change has caused a long-term change in the winds, and that warm ocean conditions have gradually become more prevalent as a result.

Trends in Amundsen Sea winds within climate model ensembles. The 1920–2005 trends in the PACE ensemble mean. Colours (contours) show trend in zonal winds (SLP). Contours have a spacing of 0.5 hPa century−1, with black positive, grey negative and the zero contour omitted. Graphic: Holland, et al., 2019 / Nature Geoscience — Trends in Amundsen Sea winds within climate model ensembles. The 1920–2005 trends in the PACE ensemble mean. Colours (contours) show trend in zonal winds (SLP). Contours have a spacing of 0.5 hPa century⁻¹, with black positive, grey negative and the zero contour omitted. Graphic: Holland, *et al*., 2019 / *Nature Geoscience*

Lead author, Professor Paul Holland from the British Antarctic Survey (BAS), says:

“The impact of human-induced climate change on the West Antarctic Ice Sheet is not simple. This is the first evidence for a direct link between human activities and the loss of ice from West Antarctica. Our results imply that a combination of human activity and natural climate variations have caused ice loss in this region, accounting for around 4.5 cm of sea-level rise per century.”

The team also looked at model simulations of future winds. Prof Holland continues:

“An important finding is that if high greenhouse-gas emissions continue in future, the winds keep changing and there could be a further increase in ice melting. However, if emissions of greenhouse gases are curtailed, there is little change in the winds from present-day conditions. This shows that curbing greenhouse gas emissions now could reduce the future sea-level contribution from this region.”

Linkages between Amundsen Sea winds and global SST and SLP. Time series of zonal wind and zonal total stress over the PITT box, the SOI and the IPO. The legend shows the unit for each time series, and scaling for the axis values where appropriate. Graphic: Holland, et al., 2019 / Nature Geoscience — Linkages between Amundsen Sea winds and global SST and SLP. Time series of zonal wind and zonal total stress over the PITT box, the SOI and the IPO. The legend shows the unit for each time series, and scaling for the axis values where appropriate. Graphic: Holland, *et al*., 2019 / *Nature Geoscience*

Co-author, Professor Pierre Dutrieux from Lamont-Doherty Earth Observatory, Columbia University Earth Institute in New York, says:

“We knew this region was affected by natural climate cycles lasting about a decade, but these didn’t necessarily explain the ice loss. Now we have evidence that a century-long change underlies these cycles, and that it is caused by human activities.”

Co-author, Professor Eric Steig from the University of Washington, says:

“These results solve a long-standing puzzle. We have known for some time that varying winds near the West Antarctic Ice Sheet have contributed to the ice loss, but it has not been clear why the ice sheet is changing now. Our work with ice cores drilled in the Antarctic Ice Sheet have shown, for example, that wind conditions have been similar in the past. But the ice core data also suggest a subtle long-term trend in the winds. This new work corroborates that evidence and, furthermore, explains why that trend has occurred.”

West Antarctic ice loss influenced by anthropogenic forcing and internal climate variability is published by Paul R. Holland, Thomas J. Bracegirdle, Pierre Dutrieux, Adrian Jenkins and Eric J. Steig in the journal Nature Geoscience.

First evidence of human-caused climate change melting the West Antarctic Ice Sheet

West Antarctic ice loss influenced by internal climate variability and anthropogenic forcing

ABSTRACT: Recent ice loss from the West Antarctic Ice Sheet has been caused by ocean melting of ice shelves in the Amundsen Sea. Eastward wind anomalies at the shelf break enhance the import of warm Circumpolar Deep Water onto the Amundsen Sea continental shelf, which creates transient melting anomalies with an approximately decadal period. No anthropogenic influence on this process has been established. Here, we combine observations and climate model simulations to suggest that increased greenhouse gas forcing caused shelf-break winds to transition from mean easterlies in the 1920s to the near-zero mean zonal winds of the present day. Strong internal climate variability, primarily linked to the tropical Pacific, is superimposed on this forced trend. We infer that the Amundsen Sea experienced decadal ocean variability throughout the twentieth century, with warm anomalies gradually becoming more prevalent, offering a credible explanation for the ongoing ice loss. Existing climate model projections show that strong future greenhouse gas forcing creates persistent mean westerly shelf-break winds by 2100, suggesting a further enhancement of warm ocean anomalies. These wind changes are weaker under a scenario in which greenhouse gas concentrations are stabilized.

West Antarctic ice loss influenced by internal climate variability and anthropogenic forcing