We know how much water is on our blue planet, but what about in it? A new study published in Nature Geoscience estimates that earth’s total supply of groundwater is about 23 million cubic kilometers. If that sounds like a lot, it’s because it is—enough to cover the world’s land surface in 590 feet of water.
But here’s the catch: Only 6 percent of that volume is close-to-the-surface “modern” groundwater. That’s the stuff that can be renewed within a human lifetime by natural water cycles and also the stuff that’s more vulnerable to contamination and effects of climate change like drought and flooding.
Previous studies have shown that we’re overdrawing our aquifers, but hopefully quantifying this finite resource will help keep us from spending water like...well, water. The authors plan to calculate how much time we have left before we tap out.
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