Effects of continental waters
An estimate of continental water contribution (soil waters, underground water, surface water and snow) to the rise in sea level over the past 50 years has been calculated using the global land surface model ORCHIDEE (developed at IPSL for climate simulations), in collaborative work carried out between the Laboratoire de Météorologie Dynamique (LMD/IPSL) and LEGOS. The results (Ngo-Duc et al., 2005b) indicate that:
On average, changes in land water storage do not present any significant trends over the past 50 years, and accordingly do not contribute to the long-term sea level rise.
However, continental waters make a very significant contribution to decadal sea level fluctuations. A very strong negative correlation (of -0.9) has also been observed between the decadal oscillations of the continental water contribution to sea level and those of thermal expansion (figure 1). The following interpretation can be given to this result: when the ocean warms up, evaporation over the ocean increases as does rainfall on the continents. More water is stored in the continental reservoirs, which causes the sea level to drop - and vice-versa. This finding suggests that decadal fluctuations in ocean heat content (as evidenced by in situ ocean temperature observations) are probably real, even if they are not reproduced by climate models (and often attributed to artifacts linked to poor coverage of ocean temperature data).
Another conclusion of this work is that at the decadal time scale, the effects of thermal expansion and continental waters on sea level compensate each other, at least in part. This result shows that the ocean's thermal state has an influence on the continental water cycle, with negative feedback on sea level.
Figure 1 : Contribution of continental waters on sea level for 1950-2000, by the ORCHIDEE model (black curve) and detrended thermal expansion calculated using historical ocean temperature data for the same period (green curve) (Ngo-Duc et al., 2005b).
In a recent analysis, the effect of continental waters on sea level has been estimated using GRACE data. A contribution ~0.2 mm/yr for 2003-2006 has been estimated (Ramillien et al., 2007). Figure 2 below, presents the contribution (expressed in ‘equivalent water level') of the major river basins. Note that some tropical river basins (Amazon, Congo, etc.) contribute significantly to sea level.
Figure 2 : Contribution of land waters to sea level for 2002-2006 from GRACE.