25% of sea-level rise is due to groundwater depletionOctober 24, 2010
We are told that sea level rise is one of the more dire consequences of global warming. It is posited as a bellwether to the fate of the planet. So it may come as a surprise to learn that about 25% of the yearly sea level rise comes from pumping water from the ground and adding it to the oceans, not from melting ice. And this percentage has been rapidly increasing since 1950. Mistaking this groundwater as ice-melt-water in calculations designed to ferret out the effects of global temperature on sea level greatly prejudices the calculations toward higher sea levels in the future.
Remember the “correction” to the sea level from Chao that made the sea level rise rate for the second half of the 20th century appear to be higher (surprise, surprise) than the commonly referred to Church and White sea level data (Church, J. A., and N. J. White , “A 20th century acceleration in global sea-level rise”, Geophys. Res. Lett., 33, 2006)? I wrote about it back in May. Martin Vermeer and Stefan Rahmstorf were all too happy to include this “correction” in their model relating sea level to temperature, of which I have written about extensively.
Chao’s idea was that water stored in the increasing number of artificial reservoirs built around the world in the last half of the 20th century is water that would otherwise be in the oceans. Therefore, he said, the effect of global warming on sea levels was underestimated, and needed to be corrected by adding the stored water to the sea level. I pointed out that his analysis was both useful and flawed. Useful because he did a nice job of researching the construction of reservoirs around the world and their total capacity. Flawed because he neglected to consider the balancing effect of groundwater depletion.
I argued that water pumped out of the planet’s aquifers ultimately makes its way to the oceans, and raises the sea level. This results in an overestimation of the effect of global warming on sea levels. My quick calculation showed that the depleted groundwater could “entirely counteract the effect of artificial reservoirs.”
This effect of groundwater depletion was briefly mentioned and quickly dismissed by Vermeer and Rahmstorf as they drew a very different conclusion than mine. They said…
We have corrected the sea-level data for the reservoir storage component, but a further non-climatic effect of relevant magnitude is the mining of groundwater for human uses in arid regions. No time series of this is available, so it cannot be included in the above analysis…but in recent decades groundwater mining could have contributed 0.2–0.3 mm/year to sea level.
Lo and behold! A new Geophysical Research Letters paper (Wada, Y., L. P.H. van Beek, C. M. van Kempen, J. W.T.M. Reckman, S. Vasak, and M.F.P. Bierkens (2010), Global depletion of groundwater resources, Geophysical Research Letters, in press) confirms my estimate (and more) and shows that Vermeer and Rahmstorf were low-balling the effect of groundwater depletion. In fact, Wada’s data shows the effect of ground water depletion at the present time to be GREATER than the effect of artificial reservoir storage. They say…
We estimate that since the 1960s groundwater abstraction has more than doubled (from 312 ± 37 to 734 ± 84 km3 a-1) resulting in an increase in groundwater depletion of from 126 ± 32 to 283 ± 40 km3 a-1. 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…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… and the same order of magnitude as the contribution from glaciers and ice caps.
It seems preposterous not to include a correction for groundwater depletion when its effect is “the same order of magnitude as the contribution from glaciers and ice caps.”
The following plot (figure 2) from Wada shows the total number of km3 of water removed from the ground each year (top plot) and the depleted fraction (bottom plot) each year from 1960 to 2000. The depleted ground water is the fraction of the removed water that is not naturally recharged by rain, snow, etc.
The depletion rate fits an exponential very nicely. In figure 3 I have digitized the groundwater depletion data from figure 2, converted it to sea level rise rate in mm ( one km3 of water yields 2.78 x 10-3 mm of sea level rise), and fit it to an exponential (R2= 0.98).
The Wada groundwater depletion data only covers 1960 to 2000. However, it is reasonable to assume that prior to 1960 (and after 2000) the groundwater depletion approximates an exponential. As Wada says…
“Increasing population numbers, expanding areas of irrigated agriculture and economic development are drivers for an ever-increasing demand for water worldwide.”
and these drivers have all been moving along more or less exponential trajectories for the last century. So, in figure 4 I have extrapolated the Wada groundwater depletion back to 1880 along its exponential fit, and overlaid it with the Chao reservoir correction.
Figure 5 shows the uncorrected Church and White sea rise rate, as well as the Chao reservoir corrected version, and the Chao reservoir plus Wada groundwater depletion versions. Vermeer and Rahmstorf used only the Chao reservoir corrected version. The version that has both the reservoir and groundwater depletion corrections is further divided into two parts: prior to 1960 and after 1960. The pre-1960 data is based on the exponential extrapolation of the 1960 to 2000 data. Those who feel incredulous about this extrapolation can simply ignore it – it has no effect on the groundwater delpletion correction after 1960.
Does it make any difference?
Does the Wada groundwater correction make any difference? Look at figure 4 and notice that around 1985 the groundwater depletion correction overtakes the reservoir correction. Before 1985 the combination of the two corrections yield a sea level rise rate that is greater than the plain Church and White data, but after that the sea level rise rate is lower. The groundwater depletion data only goes to the year 2000, but if the exponential extrapolation holds, then by 2010 the reduced sea level rise rate will be even more pronounced.
This difference is huge in the scheme of things. It takes brutal mathematical contortions to turn this…
The following animation gives some idea of the effect of going from figure 6 to figure 7.
What do you lying eyes tell you about the sea level from about 1930 to the 2000 in figure 6? Here is what I see: a sea-level rise rate that does not change much, with a pretty good fit to a line, despite changes to the global temperature. Any significant increase in the future sea level rise rate that can be divined from it must arise from obscure effects that only the most powerful mathematical minds (like Vermeer’s and Rahmstorf’s) can discern. From figure 5 it can be seen that the inclusion of the Wada groundwater depletion correction decreases the sea level rise rate by a not so obscure 20% at the end of the 20th century, compared to Vermeer’s and Rahmstorf’s calculations.
Let’s face it, Vermeer’s and Rahmstorf’s sea-level rise predictions come from the forced confession of innocent data. Every effort is made, no stone unturned, in a quest to wring out as much sea level rise as the most gullible audience will believe. The review and publication of their model by the National Academy of Science stands as a monument to the supreme reign of the global warming dogma.