Mean Sea Level issues: questions in discussion
Sea level budget over the altimetry era (1993-2010) and Regional variabilty
Prepared with Anny Cazenave and Benoît Meyssignac, Legos
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. Terrestrial waters little contribute to sea level rise.
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 (Levitus et al., 2009, Ishii and Kimoto, 2009, Lymann et al., 2010). On average, over the satellite altimetry era (1993-2010), the contribution of thermal expansion to sea level rise is on the order of 1 mm/yr (Church et al., 2011, Cazenave and Llovel, 2011).
Being very sensitive to global warming, mountain glaciers have retreated worldwide during the recent decades, with significant acceleration since the beginning of 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, the glacier contribution to sea level rise is estimated to 1 mm/yr (Steffen et al., 2010, Church et al., 2011, Pfeffer, 2011).
Evolution of glaciers mass balances (in gigatons per year). A zero balance means that the glacier has neither gained nor lost mass during the studied period. A negative balance indicates a loss of the glacier, which water is added to the ocean. To make easier the comparison, the same area was shaded on the three panels (see mass balance for Greenland and Antarctica bottom). The glaciers balances (Cogley J.G. compilation) are averaged over a 5-year period; the vertical lines represent the dispersion of measurements (standard deviation) which is lower than the uncertainty, this latest being about 70.106 / year. Credits LEGOS/Berthier et al.
While little was known before the 1990s on the mass balance of the ice sheets because of inadequate and incomplete observations, different remote sensing techniques available since then (e.g., airborne and satellite radar and laser altimetry, Synthetic Aperture Radar Interferometry -InSAR-, and since 2002, space gravimetry from the GRACE mission) have provided important results on the changing mass of Greenland and (west) Antarctica.
These data indicate that both ice sheets are currently loosing mass at an accelerated rate (e.g., Steffen et al., 2010). In effect, most recent mass balance estimates from space-based observations (e.g., GRACE and InSAR) show ice mass loss acceleration in the recent years (e.g., Chen et al., 2009, Velicogna, 2009, Rignot et al., 2011).
For the period 1993-2003, <15% of the rate of global sea level rise was due to the ice sheets (IPCC, 2007). But their contribution has increased up to ~75% since 2003-2004. Although not constant through time, on average over 1993-2010, ice sheets mass loss explains ~20% (0.6 mm/yr) of the rate of sea level rise (Steffen et al., 2010, Cazenave and Remy, 2011, Church et al., 2011).
Evolution of ice sheets mass balances, in Greenland (left) and Antarctica (right) measured by various remote sensing methods since 1992: radar and laser altimetry (yellow), InSAR radar interferometry (blue) and space gravimetry Grace (red). These measurements were recorded over various periods : horizontal bars indicate this period whereas the vertical lines indicate the uncertainties on the measurements. A zero balance means that ice sheet has neither gained nor lost mass during the studied period. A negative balance indicates a loss of the ice sheet, which water is added to the ocean. The horizontal dashed lines correspond to the equivalent sea level rise (mm / year). Credits LEGOS/Berthier et al.
Sea level budget of the ~last two decades
Although none of the climate factors discussed above evolve linearly with time, on average over the 1993-2010 time span, ocean warming and glaciers melting have roughly contributed by ~30% each. The ice sheet contribution is slightly less (~20%). Over the altimetry era, the sea level budget is almost closed given the uncertainties of each contribution.
The regional variability in sea level trends is mainly due large-scale changes in the density structure of the oceans in response to forcing factors (e.g., wind stress, heat and fresh water exchange at the sea-air interface) and associated changes in ocean circulation.
The largest regional changes in sea level trends result from ocean temperature changes (i.e., from non uniform thermal expansion) but in some regions, changes in water salinity is also important (Bindoff et al., 2007).
Self-gravitation, elastic and visco-elastic deformations of the solid Earth in response to water mass redistribution associated with present day and past land ice melt (the latter being called Glacial Isostatic Adjustment -GIA-) also cause regional variability in sea level (Milne et al., 2009, Tamisiea and Mitrovica, 2011). These effects are currently very small compared to steric effects but may become important in the future if the ice sheet contribution increases.
Observations over the past few decades show that trend patterns in thermal expansion are not stationnary but fluctuate both in space and time in response to internal/natural modes of variability of the climate system such as ENSO (El Nino-Southern Oscillation), IOD (Indian Ocean Dipole), NAO (North Atlantic Oscillation) and PDO (Pacific Decadal Oscillation) (Bindoff et al., 2007, Meyssignac et al. 2012a,b).
As a result, sea level trends patterns observed by satellite altimetry are transient features. On longer time scales, these patterns are expected to be different from what is observed over the altimetry era. Past sea level reconstructions covering the second half of the 20th century confirm this (e.g., Ray and Douglas, 2011, Hamlington et al., 2011, Meyssignac et al., 2012a).
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