A new contributor to sea level rise

This is certainly, in theory at least, believable.

But a new study shows that global warming is not the only cause of swelling seas. Much comes from \”water mining\” – the pumping of vast amounts of groundwater from beneath the earth, mainly to irrigate crops. This inevitably ends up in the oceans after it evaporates from farmland and comes down as rain.

The study – to be published in a forthcoming issue of the journal Geophysics Research Letters – reckons that this accounts for about quarter of global sea-level rise, as much as the melting ice from all the glaciers outside Greenland and Antarctica.

OK, so, underground water, once it\’s pumped up and used ends up in the ocean somehow or other. Yes, this will have an effect on sea level.

The question is how much?

Hmm, finding out how much is withdrawn doesn\’t seem to be easy as one single global number. But this old (1990) number seems to say about 50 thousand million gallons a day for the US. With 1.1x 10 12 gallons to a cubic mile this is one cubic mile every 22 days? 16 cubic miles a year? That doesn\’t sound right but I\’ve checked my zeroes a couple of times now and that does seem to be correct.

Take the US as being 10% of fossil water mining (and \”ground water\” as being a synonym for \”fossil water\”) and say, ooh, just for the sake of it, 200 cubic miles a year globally.

Surface area of the oceans is 139 million square miles, number of inches in a mile is 63,360. So we\’ve got 200x 63,360 inches to be spread over 139 million miles of ocean. 139 million divided by 12 million….eh? A foot a year?

Hmm, no, obviously something wrong with my figures (hey, how back of the envelope do I have to be here?) as observed sea level rises are much lower than this. If the above is correct then global warming is shrinking the oceans which we know isn\’t true.

But, to stick with these (wrong, but perhaps not wrong by more than one or two orders of magnitude) figures, it does seem that such fossil water mining could be having an effect upon sea levels. Estimates are tens of centimetres a century, aren\’t they?

4 thoughts on “A new contributor to sea level rise”

  1. Seems to me that you’re messing your units up.

    What you’re trying to ask is

    “If I poured 200 cubic miles of water over a sheet that is 139 million square miles, how deep would the water be”

    200 / 139 x 10^6 = 1.44 x 10 -6 miles

    which is about 0.09 inches

    “So we’ve got 200x 63,360 inches to be spread”
    You’re multiplying a cubc length by a length here, essentially giving a Length ^4, which is clearly wrong.

    Tim adds: and 0.09 inches is about 2 mm…..which does get us into the correct sort of range for annual sea level rise.

    Hurrah!

  2. This bothered me for a bit as it didn’t feed into the Climate Change agenda, then I realised fossil fuels were probably being used to pump the water up, phew, so it’s still that bastard carbon’s fault.

  3. It’s only that part of it which evaporates and is rained into the oceans that can cause a direct change to sea levels. I might have misunderstood the sums, but they seem to assume all the wet stuff pumped onto the land ends up in the oceans, which doesn’t seem correct.

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