The slowdown in sea-level rise reported over the past decade is due to natural variability in the climate and is not indicative of a slowdown in the effects of global warming, a new study published online in Nature Climate Change claims. The research is also consistent with suggestions that heat is being accumulated in the deep oceans below 700m.
To separate natural variability from anthropogenic influences, Anny Cazenave, Laboratoire d’Etudes de Océanographie Spatiales, Toulouse, France, and colleagues analysed time-series data of global mean sea level from five prominent research groups that processed satellite radar altimeter measurements for the periods 1994–2002 and 2003–2011. Satellite radar altimeters measure the distance between the surface and the satellite, and can be used to make sensitive measurements of changes in sea level over time.
The researchers note that the largest cause of interannual sea-level variability is the El Niño–Southern Oscillation (ENSO) Pacific Ocean warming and cooling cycle, which impacts the global water cycle through changes in land water and atmospheric water vapour content. Removal of the natural variability shows recent sea-level rise has not slowed compared with the earlier satellite period.
The data for 2003–2011 shows an average rise in ocean levels of 2.4 mm per year compared with a rise of 3.5 mm per year for the preceding decade, representing a decrease of about 30%. Correcting for the interannual variability gives an underlying sea-level rise rate of 3.3 ± 0.4 mm per year for the period 2003-2011, the authors of the paper report, which is comparable to the rate observed in the 1990s of about 3.1 mm per year. This highlights the need to quantify and correct records for short-term natural variability to extract the climate change signal, they say.
Although it has been suggested that several decades of satellite altimetry-based global mean sea level data would be needed to isolate a long term global warming signal, the authors state that their “result also shows that this may be already achievable by removing the (mainly ENSO-driven) interannual variability”.
Equally, significantly, the authors believe that their result has implications for the so called “missing energy”. The larger than previously believed rate of global mean sea level increase calculated during the past decade “would be compatible with a significant warming contribution from the deep ocean,” they state. They allude to recent studies of ocean heat content that suggest around 30% of the ocean warming has occurred below 700m.
“This heat may be sequestered into the deep ocean during decades of large ocean-atmosphere natural variability, highlighting once more, as shown here, the role of short-term natural variability on longer-term change, probably associated with global warming,” they write.
Present day sea-level rise is a major indicator of climate change. Since the early 1990s, sea level rose at a mean rate of 3.1mm/yr. However, over the last decade a slowdown of this rate, of about 30 per cent, has been recorded. It coincides with a plateau in Earth's mean surface temperature evolution, known as the recent pause in warming. Here we present an analysis based on sea-level data from the altimetry record of the past 20 years that separates interannual natural variability in sea level from the longer-term change probably related to anthropogenic global warming. The most prominent signature in the global mean sea level interannual variability is caused by El Nino-Southern Oscillation, through its impact on the global water cycle. We find that when correcting for interannual variability, the past decade's slowdown of global sea mean sea level dissapears, leading to a similar rate of sea-level rise (of 3.3+/- 0.4 mm/yr) during the first and second decade of the altimetry era. Our results confirm the need for quantifying and further removing from the climate records the short-term natural climate variability if one wants to extract the global warming signal.
The rate of sea-level rise by Anny Cazenave, Habib-Boubacar Dieng, Benoit Meyssignac, Karina von Schuckmann, Bertrand Decharme and Etienne Berthier published in Nature Climate Change online on 23 March 2014. DOI: 10.1038/NCLIMATE2159.
Read the abstract and get the paper here.
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