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Impact Of Groundwater Extraction On Sea Levels

January 4, 2016
tags:

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.

 

ABSTRACT

 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]

http://onlinelibrary.wiley.com/doi/10.1029/2010GL044571/full

 

 

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. [1997] and is larger than the 0.55 mm a−1 given by Postel [1999] (see Huntington [2008] 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.

For instance:

 

index

ny

fd

fr

http://tidesandcurrents.noaa.gov/sltrends/sltrends_global.htm

 

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.

8 Comments leave one →
  1. January 4, 2016 10:34 pm

    Interesting facts…. I think that a similar analysis should be conducted in the case of offshore wind-parks. I have read, here (http://climate-ocean.com/2015/K.html), that they seem to play a role in climate change. After all, any use of the oceans by mankind has an influence on thermo-saline structures within the water column from a few cm to 10 m and more.

  2. January 4, 2016 10:43 pm

    I think there are two close but separate issues:
    -Whether changes in terrestrial storage have contributed to the ‘acceleration’ in sea level rise.
    -Whether changes in terrestrial storage are currently contributing to sea level rise.

    The first point is undeniable: lots of dam-building in the 50s and 60s and 70s, very little dam-building now. That, by itself, will cause apparent sea level rise to accelerate: water impoundments went from being a substantial negative SLR (perhaps 1mm/year at peak) to being a small factor now.
    http://www.sciencemag.org/content/320/5873/212.abstract


    (Impossible to miss that SLR, after accounting for impoundments, shows pretty much the same trend since 1930 – whether temps were going up or down)

    The second point is different. To show that terrestrial storage contributes to sea level rise, you have to show that extraction has overtaken impoundments. In principle Wada et al with 0,8mm show this but…

    Unfortunately the Wada paper is very speculative. I’d have to read it again to know exactly what they did, but it’s pretty clear that they DIDN’T use actual groundwater extraction data – because such data isn’t public. Rather they indexed groundwater to total water use or something like that. It’s clear that groundwater extraction has grown a lot, but it’s not clear at all how much is taking place in absolute terms.

    Still, they do mention that irrigation consumes 2050 km3 per year, which is 5.5mm worth of sea level rise. So there is definitely potential for water consumption to contribute meaningfully to sea level rise, it’s just that there isn’t enough data to assert so. But yes, the current 3mm or so SLR, minus some groundwater extraction, would fit very well with the 2.5mm/year we’ve had since 1930.

    Furthermore, lakes and inland seas have probably contributed to SLR as well.
    https://www.ipcc.ch/ipccreports/tar/wg1/421.htm

    By the way, I don’t think it makes sense to ‘remove’ the 0.3mm from GIA when discussing the SLR budget. After all, presumably the ocean’s bottom was also sinking back in the 60s. Removing it would make sense if talking about SLR ‘impacts’, i.e. how much is sea actually rising vs coastal cities.

    • January 4, 2016 10:48 pm

      By the way, I don’t think it makes sense to ‘remove’ the 0.3mm from GIA when discussing the SLR budget. After all, presumably the ocean’s bottom was also sinking back in the 60s. Removing it would make sense if talking about SLR ‘impacts’, i.e. how much is sea actually rising vs coastal cities.

      Yes, the problem comes when comparing satellite measurements, which include the 0.3mm, with prior data from tide gauges, which obviously don’t.

  3. tom0mason permalink
    January 5, 2016 1:36 am

    Did they factor in that the human population increase over the 20th century was around 5 billion people and each person about 60% water? Is that an insignificant amount of water being stored over the surface of the planet in those natural little organic bladders of humanity?🙂

  4. January 5, 2016 1:36 am

    Paul, while this might be true, I do not think so. The multiple reasons are given in essay Pseudo Precision. With footnotes.

  5. John F. Hultquist permalink
    January 5, 2016 2:13 am

    While there doesn’t seem to be much impact on the world-ocean, the local extraction does have an impact. This is an old issue (Mexico City is famous) and many places have been studied.
    Here are the first 2 paragraphs of a story [link below] from the San Jose Mercury News:

    DOS PALOS, Calif. (AP) — A canal that delivers vital water supplies from Northern California to Southern California is sinking in places. So are stretches of a riverbed undergoing historic restoration. On farms, well casings pop up like mushrooms as the ground around them drops.

    Four years of drought and heavy reliance on pumping of groundwater have made the land sink faster than ever up and down the Central Valley, requiring repairs to infrastructure that experts say are costing billions of dollars.

    Sinking in California

  6. January 5, 2016 11:21 am

    O/T –

    Wettest Dec (record from 1910 only)

    http://www.bbc.co.uk/news/uk-35230696

    Harrabin:

    [the Met Office] say climate change has raised UK temperatures by around 1C (34F) so far so it will be many decades before this level of extreme weather becomes the new winter norm.

    http://www.bbc.co.uk/news/uk-35228779

    So a 1°C rise = 34°F rise in temps?😀

    Presumably also no natural warming was involved in the one degree rise it was merely a mirage – let’s leave aside the inability of numerical models to reproduce the climate system (*coughs oceans*) just add fudge and all is hunky dory.

    To his credit Scafe as quoted does say that CC played a ‘minor’ part in Dec attributing El Nino and the jetstream.

    But the 34°F rise – phew positively roasting out there😉

  7. January 5, 2016 11:23 am

    Reblogged this on Climate Collections.

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