Is Sea Level Rise Accelerating?

Figure 1: IPCC sea level rise projections and more recent satellite observations [1] Figure 1 portrays the root of the debate; recent satellite observations have place observed sea level rise in the uppermost echelons of the IPCC sea level rise predictions, at a gradient that indicates acceleration in sea levels during the late 20th century and throughout the 21st century. However, the debate is ongoing, with conclusions ranging according to the type and length of study undertaken and some claims of the impossibility of a global prediction of present and future sea level change [2]. The reasons for the continuing debate and the varying conclusions of sea level rise are outlined below. Varying Lengths of Study  An example of the problems with varying lengths of study can be seen with the use of tide gauge data to predict sea level rise. Short term tide gauge data can be easily affected by short term changes, due to weather events, such as El Nino and isostatic sea level change, caused by tectonic movement or glacial rebound [2]. This issue has led researchers to study tide gauge data of 60 years or more in length [3], in an effort to avoid any short term changes obscuring the general trend. However, this increased length of observation created the need to average each data set, in an effort to remove the noise of short term, tidal changes. Such an averaging has been found to obscure any acceleration that may be evident [4] and has led to the general assumption that there was no acceleration in sea levels, during the 20th century [4].  As well as the accusations of the loss of any meaningful data, there have also been claims that the study of tide gauge data alone is not long enough to identify any acceleration that may have occurred [4 & 5], this has forced other researchers to combine tide gauge data and satellite observations, an effort that has produced more intriguing results (figure 2).  Figure 2: Sea levels between 1880 and 2001, showing an increase in the rate of rise after 1930 [4]   Figure 2 shows the merits of studying longer periods of time and combining methods of measurement; the sharpening of the incline after 1930 may have gone unnoticed if the length of study was shortened or the data of both tide gauges and satellites were not used.  Ice Sheet and Glacier Knowledge Many of the predictions of accelerated sea level rise are based around future ice sheet and glacier contributions. Claims have been made that half the present sea level rise can be attributed to the melting of terrestrial ice [6] and that glacier and ice cap melting makes up 60% of the ‘new water’ component of sea level rise (1.8 mm/year) [7]. A second point regularly made in the literature concerns the rise in sea levels, as a result of temperature rise and increased ice melt [7 & 8]; sea levels have been predicted to increase by 3.4 mm/ year per degree Celsius increase in temperature [8]. Such claims may make more of an impact, if the uncertainties of ice sheet dynamics and the response to warming were not so wide ranging and well publicised.  Such uncertainties include a lack of understanding of ice sheet dynamics, polar ice thickness, depth of bed rock, speed of movement towards the sea, the potential of mountain glacier disappearance and Antarctic contribution to sea level rise [6 & 8]. Such a spectrum of unknowns has prevented the creation of solid conclusions of the future increase in sea levels and the potential of acceleration (figure 3). Papers are forced to conclude with very open statements, covering the possibilities of no change, continued present day speed and rapid sea level rise [5, 6, 7 & 8].  Figure 3: Ranging predictions of sea level rise by 2100, upper most and lowest predictions include error bars, further increasing the range of possibilities [5]   The compounding factors tested in each scenario of figure three and the uncertainties of ice sheet reactions to heating have caused sea level predictions for 2100 to range between 0.1 and 0.7 meters. Despite the uncertainties and on going debates of future acceleration and sea levels, the current evidence does point towards 21st century acceleration in increase and generally higher sea levels across the globe (figure 1). With the advent of satellite observation, the global coverage, quality and temporal resolution of observations are only going to increase, making any future predictions and observation much more reliable.  After consideration of the findings and in full knowledge of the problems with any predictions, we must now decide how to react to the chance of sea levels rise, the steps we are willing to take to protect ourselves and cities and what, if any, change we should make now, in an  effort to prevent the potential sea level rise.  The remaining posts will focus on such matters. References  [1] Allison, I. , N.L. Bindoff, R.A. Bindschadler, P.M. Cox, N. de Noblet, M.H. England, J.E. Francis, N. Gruber, A.M. Haywood, D.J. Karoly, G. Kaser, C. Le Quéré, T.M. Lenton, M.E. Mann, B.I. McNeil, A.J. Pitman, S. Rahmstorf, E. Rignot, H.J. Schellnhuber, S.H. Schneider, S.C. Sherwood, R.C.J. Somerville, K. Steffen, E.J. Steig, M. Visbeck, A.J. Weaver, (2009) The Copenhagen Diagnosis, Updating the World on the Latest Climate Science. The University of New South Wales Climate Change Research Centre (CCRC), Sydney, Australia. [2] Douglas, D.C., M.S. Kearney, S.P. Leatherman eds. (2001) Sea Level Rise: History and Consequences, Volume 1 London: Academic [3] Douglas, B.C., (1997) Global sea rise: a redetermination. Surveys in Geophysics, 18, 279-292.  [4] Church, J.A., N.J. White (2006) A 20th Century Acceleration in Global Sea-level Rise. Geophysical Research Letters, 33, L01602 [5] Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. Van Der Linden, X. Dai, K. Maskell, C.A. Johnson eds. (2001) Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change.  [6] Thomas, R., E. Rignot, G. Casassa, P. Kanagaratnam, C. Acun, T. Akins,H. Brecher, E. Frederick, P. Gogineni, W. Krabill, S. Manizade, H. Ramamoorthy, A. Rivera, R. Russell, J. Sonntag, R. Swift, J. Yungel, J. Zwally (2004) Accelerated Sea-Level Rise from West Antarctica. Science 306, 255  [7] Meier, M.F. M.B. Dyurgerov, U.K. Rick, S. O’Neel, W.T. Pfeffer, R.S. Anderson, S.P. Anderson,A.F. Glazovsky (2007) Glaciers Dominate Eustatic Sea-Level Rise in the 21st Century. Science 317, 1064    [8] Rahmstorf, S. (2007) A Semi-Empirical Approach to Projecting Future Sea-Level Rise. Science 315, 368