Impact Of Groundwater Extraction On Sea Levels
By Paul Homewood
While we’re on the topic of sea level, it is worth taking a look at this paper from 2010 by Wada et al.
In regions with frequent water stress and large aquifer systems groundwater is often used as an additional water source. If groundwater abstraction exceeds the natural groundwater recharge for extensive areas and long times, overexploitation or persistent groundwater depletion occurs. Here we provide a global overview of groundwater depletion (here defined as abstraction in excess of recharge) by assessing groundwater recharge with a global hydrological model and subtracting estimates of groundwater abstraction. Restricting our analysis to sub-humid to arid areas we estimate the total global groundwater depletion to have increased from 126 (±32) km3 a−1 in 1960 to 283 (±40) km3 a−1 in 2000. The latter equals 39 (±10)% of the global yearly groundwater abstraction, 2 (±0.6)% of the global yearly groundwater recharge, 0.8 (±0.1)% of the global yearly continental runoff and 0.4 (±0.06)% of the global yearly evaporation, contributing a considerable amount of 0.8 (±0.1) mm a−1 to current sea-level rise. [My bold]
The authors conclude:
Most of the groundwater released from storage due to groundwater depletion will end up in the ocean, partly by runoff and, as most of the groundwater use is for irrigation purposes, predominantly through evaporation and then precipitation. Based on the ratio of groundwater recharge (15 · 103 km3 · a−1) to total precipitation on earth (574 · 103 km3 · a−1) [German Advisory Council on Climate Change, 1999] and assuming all other stores (atmospheric moisture and surface waters) to remain constant, we can thus estimate which fraction of the depleted groundwater returns to the groundwater store by additional recharge (15/574 = 0.03) and which part ends up in the ocean (559/574 = 0.97) and contributes to sea level rise. We estimate the contribution of groundwater depletion to sea level rise to be 0.8 (±0.1) mm a−1, which is 25 (±3) % of the current rate of sea level rise of 3.1 mm a−1 reported in the last IPPC report [Bindoff et al., 2007] and of the same order of magnitude as the contribution from glaciers and ice caps (without Greenland and Antarctica). Our estimate (0.6–1.0 mm a−1 in terms of range) sits in the upper region of the range of 0.2–1.0 mm a−1 reported by Gornitz et al.  and is larger than the 0.55 mm a−1 given by Postel  (see Huntington  for a recent overview). The possible contribution of groundwater over-exploitation to sea level rise is mentioned in the IPCC Third Assessment Report [Church et al., 2001, p. 657]. However, it is also mentioned that uncertainty is large and that the positive contribution of groundwater depletion may be offset by impoundment in reservoirs and associated recharge of surrounding aquifers. For this reason, anthropogenic contributions to sea level rise are not quantified in Fourth Assessment Report, although they are mentioned as the possible cause for the discrepancy between observed sea-level rise and the sum of the known sources [Church et al., 2001]. However, global groundwater depletion has been increasing since the 1960 and is likely to increase further in the near future, while the increase of impoundment by dams has been tapering off since the 1990s [Chao et al., 2008]. Consequently, the contribution of groundwater depletion to sea-level rise may become increasingly important in the coming decades.
The interaction of groundwater and dam building is an interesting one, because, as they point out, the latter has been tapering off since the 1990’s, while the former has been increasing. Consistently, around the world, we find a pattern of faster sea level rise in the early 20thC, then a slowdown, and then a recovery back to earlier levels.
One of the reasons for the apparent slowdown in the post war years was dam building, and as this tapers off, we naturally see sea levels rising that bit faster. If we then add in 0.8mm/year from groundwater extraction, we can explain much, if not all, of the recent acceleration in sea level rise.
Given that the 3.1mm/year sea level rise, claimed in the paper, includes a glacial isostatic adjustment of 0.3mm (to account for ocean basins getting larger, as the ocean bottom sinks), the actual sea level rise, as measured by tide gauges, is only 2.8mm/year.
Take away the 0.8mm from groundwater, and we are back to the 2.0mm/year, generally accepted to be the rate of rise during the 20thC.