Sea level could rise faster than we think

Svetlana Jevrejeva of the Proudman Oceanographic Lab in Liverpool and colleagues from the University of Lapland found that sea level acceleration – the change in rate of sea level rise – appears to have started at the end of the 18th century and has been about 0.01 mm/yr² from then until now. Sea level rose by 6 cm in the 19th century and by 19 cm in the 20th century. If the conditions responsible for such acceleration continue, sea level will rise by 34 cm over the 21st century. The researchers say that the acceleration will depend on the actual rate of temperature increase in the 21st century.

Most evidence for global sea level acceleration comes from climate models, with IPCC sea level acceleration estimates ranging from 0.22 mm/yr² to 0.014 mm/yr². A recent IPCC report suggests a 20th century acceleration of about 0.013 mm/yr². Other evidence comes from actual observational records but there are two main problems with these: the difficulty in overcoming year-to-year and decadal variations in sea level, and the lack of global long-term tide gauge records.

Now, Jevrejeva and co-workers have analysed how global sea acceleration has evolved over the past 300 years using tide gauge records from Amsterdam (available since 1700), Liverpool (since 1768) and Stockholm (since 1774). Although these records had been analysed individually before, the researchers combined them to construct a global picture. They used a method called Monte-Carlo Singular Spectrum Analysis to separate time variable trends. The team also removed the linear part of each record, which contains a component that takes land movement into account.

In addition, the scientists found quasi-periodic fluctuations with a period of around 60 years that were superimposed onto the long-term sea level acceleration. These may be related to an underlying variability in the thermohaline circulation, they say.

The fastest sea level rise during the 20th century was between 1920 and 1950 and might have happened because of a combination of the 60-year cycle peak and a period of low volcanic activity. Major volcanic eruptions can inject scattering aerosols into the stratosphere and thus lead to significant radiative cooling of the atmosphere and ocean. In turn, this means lower sea levels. When there are no volcanic eruptions sea level could rise faster, explained Jevrejeva.

An overall analysis shows a sea level rise of 28 cm from 1700 to 2000. Simply extrapolating this figure leads to a 34 cm increase between 1990 and 2090. In contrast, IPCC sea level projections range from between 18 and 38 cm.

"Ours is a scientific study," Jevrejeva told environmentalresearchweb. "We are mainly trying to understand the reasons for sea level rise and its variability and I would not go too far with implications just yet. One of the main results in our work is that sea level is rising faster than we expected even if there are a lot of temporal variability and regional differences."

Another interesting point: why did sea level acceleration start earlier than the second half of the 19th century, when the industrial revolution occurred? "We will try to find possible answers to this question," declared Jevrejeva.

In general, sea level rise is a combination of changes in ocean volume – changes in ocean temperature and heat content – and changes in the mass of the ocean because of continental ice melting and filling of continental reservoirs.

The results were reported in Geophysical Research Letters.