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27.06.2017 02:46 Age: 28 days
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Contributions to sea-level rise have increased by half since 1993, largely because of Greenland’s ice

The IMOS satellite altimeter time series was used in a paper published in the journal Nature Climate Change today.

Water mass enters the ocean from glaciers such as this along the Greenland coast. Image credit: NASA/JPL-Caltech

The study shows that the sum of contributions increased from 2.2mm per year to 3.3mm per year. This is consistent with, although a little larger than, the observed increase in the rate of rise estimated from satellite observations.

Globally, the rate of sea-level rise has been increasing since the 19th century. As a result, the rate during the 20th century was significantly greater than during previous millennia. The rate of rise over the past two decades has been larger still.

The rate is projected to increase still further during the 21st century unless human greenhouse emissions can be significantly curbed.

However, since 1993, when high-quality satellite data collection started, most previous studies have not reported an increase in the rate of rise, despite many results pointing towards growing contributions to sea level from the ice sheets of Greenland and Antarctica. This study was partly aimed at explaining how these apparently contradictory results fit together.

In 2015, the authors completed a careful comparison of satellite and coastal measurements of sea level. This revealed a small but significant bias in the first decade of the satellite record which, after its removal, resulted in a slightly lower estimate of sea-level rise at the start of the satellite record. Correcting for this bias partially resolved the apparent contradiction.

In their new research, the authors compared the satellite data from 1993 to 2014 with what they know has been contributing to sea level over the same period. These contributions come from ocean expansion due to ocean warming, the net loss of land-based ice from glaciers and ice sheets, and changes in the amount of water stored on land.

Previously, after around 2003, the agreement between the sum of the observed contributions and measured sea level was very good. Before that, however, the budget didn’t quite balance.

Using the satellite data corrected for the small biases identified in their earlier study, the authors found agreement with the sum of contributions over the entire time from 1993 to 2014. Both show an increase in the rate of sea-level rise over this period.

In summary:

In the paper, the altimeter records from various laboratories, including the authors, were analysed in an attempt to improve the instantaneous closure of the sea-level budget (i.e. the agreement between the total observed signal and the sum of the observed constituent components), over the satellite era.

The main findings were:

  • The mass contributions to global mean sea level (GMSL) (i.e. the contributions from the Antarctic and Greenland ice sheets, mountain glaciers, and terrestrial water storage) increase from 45% in 1993 to 70% of the total in 2014.
  • The largest and statistically significant increase in contributions to GMSL comes from the contribution from the Greenland ice sheet, which is less than 5% of the GMSL rate during 1993 but more than 25% during 2014.
  • This approximate but improved closure of the sea-level budget over the satellite era is progress with respect to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. The result increases confidence in our altimetry observations (including the associated calibration and validation supported by IMOS) and improves our understanding of recent changes and emerging increases to the rate of sea-level rise.

The paper can be viewed here: ‘The increasing rate of global mean sea-level rise during 1993–2014’ 

An accompanying piece in The Conversation can be viewed here: Contributions to sea-level rise have increased by half since 1993, largely because of Greenland’s ice