Around the world agriculture has boomed due to groundwater irrigation. It's a major part of how we've been keeping our mortally bloated population fed. How much water have we been bringing to the surface from deep underground? Trillions of tonnes of it. And we've been pumping it up far faster than nature has been refilling, "recharging", those underground reservoirs or aquifers. And where does it wind up? In our oceans.
Water is a constant. It's always somewhere, in some form, doing some thing or another. It's been estimated that every refreshing glass of water you draw from the tap passed through some dinosaur at least four times eons ago. Today water is held in the soil, in the air, in vegetation and other organisms or concentrated in oceans, lakes, rivers and, yes, aquifers.
"'The water being taken from deep wells is geologically old – there is no replenishment and so it is a one way transfer into the ocean,' said sea level expert Prof Robert Nicholls, at the University of Southampton. 'In the long run, I would still be more concerned about the impact of climate change, but this work shows that even if we stabilise the climate, we might still get sea level rise due to how we use water.' He said the sea level would rise 10 metres or more if all the world's groundwater was pumped out, though he said removing every drop was unlikely because some aquifers contain salt water. The sea level is predicted to rise by 30-100cm by 2100, putting many coasts at risk, by increasing the number of storm surges that swamp cities.
"The new research was led by Yadu Pokhrel, at the University of Tokyo, and published in Nature Geoscience. 'Our study is based on a state-of-the-art model which we have extensively validated in our previous works,' he said. 'It suggests groundwater is a major contributor to the observed sea level rise.' The team's results also neatly fill a gap scientists had identified between the rise in sea level observed by tide gauges and the contribution calculated to come from melting ice.
"The drawing of water from deep wells has caused the sea to rise by an average of a millimetre every year since 1961, the researchers concluded. The storing of freshwater in reservoirs has offset about 40% of that, but the scientists warn that this effect is diminishing.
"...The researchers compared the contribution of groundwater withdrawal and reservoir storage to the more familiar causes of rising sea level: ice melted by global warming and the expansion of the ocean as it warms. The pumping out of groundwater is five times bigger in scale than the melting of the planet's two great ice caps, in Greenland and Antarctica, and twice as great as both the melting of all other glaciers and ice or the thermal expansion of seawater.
"The scale of groundwater use is as vast as it is unsustainable: over the past half century 18 trillion tonnes of water has been removed from underground aquifers without being replaced. In some parts of the world, the stores of water have now been exhausted. Saudi Arabia, for example, was self-sufficient in wheat, grown in the desert using water from deep, fossil aquifers. Now, many of the aquifers have run dry and most wheat is imported, with all growing expected to end in 2016. In northern India, the level of the water table is dropping by 4cm every year."