Sea Level: Not So Fast

  • Date: 14/12/12

Rising sea level has become an icon of global warming with claims that by 2100 many cities on the coast will face severe problems. In 2009 the Met Office, the Natural Environmental Research Council (NERC) and the Royal Society released a joint pre-Copenhagen Conference statement that included as one of its five main scientific points: “There is increasing evidence of continued and accelerating sea-level rises around the world.”

At the same time the Royal Society said in a press statement touching on sea level changes that, “…estimates generally larger than those previously projected including evidence of continued and accelerating sea-level change around the world.”

What a difference a few years makes.

Sea levels have been rising since the end of the last Ice Age as the Northern Hemisphere’s ice burden was lifted. From Roman times however there is no evidence of significant changes in sea level until about 1750 – 1800 when sea levels started to rise linearly until about 1910 when the rate of change increased. Since 1910 the rate of sea level rise has been, within the errors, constant, despite the statements made by the institutions listed above.

Recently however there have been signs things are changing in the opposite direction from the statements made in 2009. It is early days, but there does appear to be a lowering in the rate of rise, contrary to the IPCC’s assessment and predictions.

In 2009 – the year the Royal Society, the Met Office and NERC made their statements about the acceleration of sea level rise – researchers analysed TOPEX/Poseidon and Jason-1 altimeter satellite data between January 1993 and June 2008. These calculations showed a reduction in the rate of sea level rise since 2005, by ~2 mm/yr. This is a 60% reduction compared to the 3.3 mm/yr sea level rise (glacial isostatic adjustment correction applied) measured between 1993 and 2005.

In 1997 Bruce Douglas of the University of Maryland looked at this data set and concludes that sea level rose at 1.8 mm per year between 1880 – 1980. The data was based on 24 measurement sites with the average data set length being 83 years. Douglas detected no acceleration at all between 1880 – 1980.

The Forth Assessment Report for the IPCC states that sea level rise was 1.8 mm per year between 1961 – 2003 and that the rate was faster, 3.1 mm/yr, between 1993 – 2003 (the satellite era).

Now a new report by the US National Oceanic and Atmospheric Administration shows that global sea levels rose by only 1.1 – 1.3 mm/year from 2005 – 2012, which is less than half of the rate claimed by the IPCC. This is less than 5 inches per century.

A new paper in the Journal of Climate examines global sea level and shows that it is possible to reconstruct changes in global sea level from known contributors apart from a constant residual term that is small enough to be explained as a long-term contribution from the Antarctic ice-sheet.

The interesting thing about this work is that it shows that sea level has remained constant this century confirming what others have pointed out. There has been little, or very small acceleration in sea levels, despite the increasing anthropogenic forcing.

This observation that the rate of sea level rise has decreased has important implications for forecasting the future. Many models for projecting global sea levels rise depend on a relationship between global climate change and the rate of sea level rise, but the implication of the paper in the Journal of Climate Change is that such a relationship is weak.