Sea level is a very sensitive index of climate change and variability. As the oceans warm in response to global warming, sea waters expand raising sea levels; mountain glaciers melt in response to increasing air temperature, and fresh water mass input to the oceans increase, again raising the sea level; and, ice mass loss from the ice sheets, also results in sea level rise. In addition, redistribution of heat inside the oceans by the ocean circulation causes regional variability in the rates of sea level change.
After the ~130 m sea level rise associated with the deglaciation that followed the Last Glacial Maximum ~20,000 years ago, geological and archeological observations indicate that the mean sea level remained almost stable during the last 2-3 millennia. However, since the late 19th century, tide gauge records have shown significant sea level rise during the 20th century, especially after 1950, with a mean rate of 1.7-1.8 mm per year over the past 50 years. From the early 1990s, satellite altimetry has become the main tool for precisely and continuously measuring sea level with quasi global coverage and a few days revisit time. Compared to tide gauges which provide sea level relative to the ground, satellite altimetry measures ‘absolute’ sea level variations. The temporal evolution of the global mean sea level from satellite altimetry since early 1993 is characterised by an almost linear increase of ~ 3.3 +/- 0.4 mm per year. This rate is significantly higher than the mean rate recorded by tide gauges over the past decades. Satellite altimetry has also revealed that sea level is not rising uniformly
(Fig. 1). In some regions, e.g., western Pacific, the rates of sea level rise have been faster by a factor up to 3 of the global mean rate over the past two decades. In the Indian Ocean, the northern part has been rising at almost the same rate (~ 1.5-3 mm per year) similar to the global mean, but south of 10°S, the rate was about twice as large. In some other regions, e.g., eastern Pacific, rates have been slower than the global mean.
The main factors causing current global mean sea level rise are thermal expansion of sea waters and land ice loss. These contributions vary in response to natural climate variability and to global climate change induced by anthropogenic greenhouse gas emissions.
Analyses of in situ ocean temperature data collected over the past 50 years by ships and recently by Argo profiling floats indicate that ocean heat content, and hence ocean thermal expansion, has significantly increased since 1950. On an average, over the satellite altimetry era 1993-2010, the contribution of ocean warming to sea level rise accounts for ~30-40 per cent.
Sensitive to global warming, mountain glaciers and smaller ice caps have retreated worldwide during the recent decades, with significant acceleration during the 1990s. From mass balance studies of a large number of glaciers, estimates have been made of the contribution of glacier’s ice melt to sea level. For the period 1993-2010, glaciers and ice caps have accounted for ~ 30 per cent of sea level rise.
Total melting of the Greenland and West Antarctic ice sheets would raise the sea level by about 7 m and 3-5 m respectively. In fact even a small amount of ice mass loss from the ice sheets is able to produce substantial sea level rise, with adverse societal and economical impacts on vulnerable low lying coastal regions. Since the early 1990s, different remote sensing observations – airborne and satellite radar and laser altimetry, Interferometric Synthetic Aperture Radar (InSAR), and since 2002, space gravimetry from the Gravity Recovery and Climate Experiment (GRACE) mission, have provided important observations of the mass balance of the ice sheets. These data indicate that Greenland and West Antarctica are loosing mass at an accelerated rate. For the period 1993-2003, <15 per cent of the rate of global sea level rise was due to the ice sheets, but since 2003-2004 their contribution has increased up to ~75 per cent, equally split between Greenland and Antarctica. Although not constant through time, on an average, over the period 2003-2010, ice sheets mass loss explains ~30 per cent of the sea level rise.
The regional variability in sea level trends is mainly due to large scale changes in the density structure of the oceans in response to forcing factors (e.g., heat and fresh water exchange at the sea-air interface) and its interaction with the ocean circulation. The largest regional changes in sea level trends result from ocean temperature change – non uniform thermal expansion, but in some regions, change in water salinity is also important. Observations of ocean temperature over the past few decades show that trend patterns in thermal expansion are not stationary but fluctuate both in space and time in response to internal perturbations of the climate system such as El Nino-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), North Atlantic Oscillation (NAO) and Pacific Decadal Oscillation (PDO). As a result, sea level trends patterns observed by satellite altimetry over the last 18 years are transient features. On longer time scales, these patterns may be different from what is presently observed.
Interannual sea level variability in the Indian Ocean has dominant quasi-periodic oscillation in the 3–7 year waveband similar as the ENSO frequency, suggesting that interannual sea level variations in the Indian Ocean are linked to ENSO. However, Indian Ocean sea level variations are also driven by other natural climate modes such as the IOD. Figure 2 shows tide gauge-based sea level fluctuations at Mumbai, India since 1880. The Southern Oscillation Index (SOI), a proxy of ENSO is superimposed (with positive/negative SOI corresponding to La Nina/El Nino, ENSO cold/warm phases). The figure shows that sea level at Mumbai is higher/lower than average during La Nina/El Nino. However, we can also note that sea level at Mumbai displays lower frequency fluctuations on a multi-decadal time scale.
Long term tide gauge records along Indian coastlines indicate an average 20th century sea level rise of ~1.5 mm per year, of the same order as the global mean rate over the same time span, except at Diamond Harbour (West Bengal) on the eastern coast that shows more than 5 mm per year rise and appears to be related to local factors.
Future sea level rise
There is little doubt that sea level will continue to rise in the future with global warming. The 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4) published in 2007 indicated that sea level would be higher by ~40 cm than today’s value by the year 2100 (±15 cm due to model results dispersion and uncertainty on future greenhouse gases emissions).
Since a decade, Greenland and West Antarctic have displayed dynamical instabilities in their coastal regions that has led to accelerated ice mass loss. Such processes were not taken into account in the IPCC AR4 sea level projections. Recent studies suggest that ice sheet mass loss may be the largest contributor of future sea level rise and a rise in the range 50 cm to 1 m by the year 2100 (above the present level) is now considered plausible.
Present-day sea level rise is not uniform; and is expected to be the same in the future. Projections of future regional changes suggest higher than average sea level rise in the Arctic Ocean. Also, according to the climate models, sea level rise larger than the global mean is expected in the Indian Ocean.
Sea level rise is a major concern for the populations of low-lying coastal regions – with inundation, wetland loss, shoreline erosion, and saltwater intrusion in surface water bodies and aquifers increasing manifold. Moreover, in many coastal regions of the world, the effects of rising sea act in combination with other natural and/or anthropogenic factors, such as decreased rate of fluvial sediment deposition in deltaic areas, ground subsidence due to sediment compactions, ground water pumping and hydrocarbon extraction. These phenomena often produce ground subsidence, of the same order of magnitude as climate-related sea level rise, amplifiying the vulnerability of low-lying, highly populated coastal areas.
Considering the highly negative impact of future sea level rise for society, multidisciplinary aspects of sea level rise (observations, modelling, coastal impact studies) should remain a major area for future climate research.