Sea Level Rises Under Estimated
New scientific research has shown that the recent estimates of sea level increases have been under estimates of the actual rate of sea level change. This recent paper entitled “Comparing climate projections to observations up to 2011” (Rahmstorf et al 2012) has shown that the third and fourth assessment reports of the Intergovernmental Panel on Climate Change (IPCC) have tended to underestimate the rate of sea level rise over the last few decades.
Climate projections, as supplied by the Intergovernmental Panel on Climate Change (IPCC) are used for policy making by many countries. The fact that the actual increases in sea level have been predicted to be lower than observational data suggests underestimation of sea-level projections of the last 2001 and 2007 IPCC reports. The increased rate of global sea level rises may be due to the loosing of ice mass from the Greenland and Antarctic ice sheets. Information on the sea level has been obtained from satellite measurements. The linear trend over the period 1993–2011 is a rate of increase of 3.2mm per annum (± 0.5 mm yr−1). This trend is 60% faster than the best IPCC estimate of 2.0 mm yr−1 over the same period. Other studies (such as Vermeer and Rahmstorf cited in 2009) have shown an observed sea level change rate over many decades correlates with global temperatures to a highly significant level. This was based upon 130 years of observational data. The trend identified by satellite observations therefore mirrors this longer term trend.
Another study in the journal Nature entitled “Probabilistic Reanalysis of Twentieth Century Sea-level Rise” (Hay et al, 2015) also highlights a recent increase in the rate sea level rise. This work considers a re-assessment of the global mean sea level over the twentieth century based upon 622 tidal records. It is a probabilistic study mainly due to the lack of quality global data and considers the rate of seal level increase from 1901-1990 to be in the region of 1.2mm per year (± 2mm). This figure has a 90% confidence level and also shows the rate of increase from 1993-2010 to be 3mm per annum (± 0.7mm). This figure is in line with the Rahmstorf et al study outlined above. The 1901-1990 figures of sea level rise are below previous estimates: if this is the case then recent global sea level increases of around 3mm are more likely to be larger. The methods employed in the study used a technique called Kalman Smoothing (or KS). The technique allows for sparse data (which is the case in the early twentieth century) from around the globe over a time period. It provides a rigorous probabilistic framework, an analysis of time-series residuals and comparisons with observed results.
These findings suggest that recent accelerations in the increase of sea level are around 25% higher than thought. This area of science is really challenging, especially when trying to estimate for global mean sea levels. These two studies have helped to provide better understandings of the rate of changes and the rate of increase forced by anthropogenic warming of the planet.
The last IPCC report projected global mean sea-level rises between 26cm and 82cm this century for 2081-2100. The figures are based upon greenhouse gas emission pathway estimates that vary. There are likely to be implications for these estimates if the studies outlined here are found to be correct. Recent satellite data from Jason spacecraft have provided reliable and robust measurements from 1992 onwards that have helped with our understanding of this complex area of scientific measurement. New satellites such as Jason-3 and Sentinel-3 will provide better ocean measurements.
The measurements should be used by policy makers to re-assess the previously published IPPC reports. The close correlation between temperature increases and the sea level rises should be noted as this may lead to wider impacts upon communities living in low-lying coastal zones.