Tim Wallace
Greenland’s melting ice sheet is making a much bigger contribution to rising global sea levels than previously thought, according to a new analysis of satellite and other data that calculates the rate of sea-level rise has jumped 50% in two decades.
This acceleration highlights the “importance and urgency of mitigating climate change and formulating coastal adaption plans to mitigate the impacts of ongoing sea-level rise”, says the analysis, authored by scientists associated with the Centre for Southern Hemisphere Oceans – an Australian-Chinese collaborative research venture based in Hobart, Tasmania – and published in Nature Climate Change.
The results, based on the first two decades of relatively precise satellite altimetry records, indicate some notable differences in the constituent contributors to sea-level rise over the past few decades compared with the assumed contributions that underpin future projections made in the Intergovernmental Panel on Climate Change’s Fifth Assessment Report (5AR). {%recommended 4952%}
Sea level rise can result from thermal expansion of the ocean; loss of mass from glaciers, the Greenland ice sheet and the Antarctic ice sheet; and changes in land water storage due to both climate variability and anthropogenic effects.
The 5AR projects ocean thermal expansion as making the greatest contribution (between 30% and 55%) to future sea level rise. This new analysis suggests thermal expansion actually diminished in significance between 1993 and 2014, contributing about half of the annual increase in global sea level at the beginning of the time period but less than a third at the end.
This, the paper suggests, might be due to the volcanic eruption of Mount Pinatubo in 1991. Climate model simulations show ocean temperature falls following such eruptions and is suppressed for more than 15 years. “Thus, the underlying acceleration of thermal expansion in response to the anthropogenic forcing may emerge over the next decade or so,” the researchers write, “resulting in a further acceleration in the rate from that reported here and recent estimates.”
That said, the rate of ocean thermal expansion did not change much. What really affected its significance to overall sea-level rise was the increasing contribution of ice melt from glaciers and the Antarctic and Greenland ice sheets. The Greenland ice sheet’s increased contribution to total average global sea level was particularly stark, rising from 5% in 1993 to 25% in 2014.
“Our thermal expansion estimate is consistent with recent Argo float observation available since 2006,” says study co-author Xuebin Zhang, a senior oceans and atmosphere research scientist with Australia’s CSIRO. “As far as I understand, climate models have done a reasonable job to represent thermal expansion impact, as reported in the IPCC’s AR5. The largest uncertainty in further sea-level rise is not from thermal expansion, but from melting of ice sheets.”
Another small but significant increase came from land water storage sources, which includes changes associated with human activities such as groundwater extraction, irrigation, dams, wetland drainage and deforestation.
To evaluate satellite and other data sets, the researchers used what is called ensemble empirical mode decomposition (EEMD), a method useful for analysing ‘non-linear’ and ‘non-stationary’ natural signals but which has only recently been applied to sea-level trend estimation. This method, the authors write, “can separate non-stationary oscillations (such as natural variations on different timescales) from the long-term trend, and the trend is found empirically without any assumptions about its shape.”
The study affirms a recent reassessment of satellite altimetry data to which co-authors John Church, Christopher Watson, Matt King, and Benoit Legresy also contributed. That study explained an apparent decrease in the rate of sea level rise – from 3.2 mm a year in the first decade to 2.8 mm a year in the second – as the result of a systematic drift error within the altimeter record, particularly affecting the first six years, inflating the trend by about 0.9 to 1.5 mm a year between 1993 and 1996.
This new study’s analysis reports findings in “approximate agreement” with those adjusted satellite observations, with the sum increase of all observed contributions to global average sea levels being 2.2 ± 0.3 mm a year in 1993, rising to 3.3 ± 0.3 mm a year in 2014.
The Centre for Southern Hemisphere Oceans Research in Hobart, Tasmania is a collaborative research venture between Australia’s CSIRO, University of New South Wales and University of Tasmania and China’s Qingdao National Laboratory for Marine Science and Technology.
