Excessive Withholding

One of the most talked-about outcomes of climate change is global sea-level rise--perhaps because the effects are straightforward and tangible: If sea level rises by this much, wipe this much of Florida (Bangladesh, Venice, Vancouver, Togo, the U.K., etc.) off the map. That's a lot more immediate than envisioning the effect of a 3 degree rise in temperature on, say, the location of the world's intertropical convergence zones.​

Records show that on average, sea level has risen by about 1.7 millimeters (the thickness of a quarter) per year over the last century, for a total of more than 6 inches so far. But like many natural records, a graph of sea-level rise over time gives you a jittery line. Sea-level rise accelerated around 1930, slowed in 1960, and sped up again around 1990.

That is, until a correction arrived last week, when two Taiwanese scientists, writing in the journal Science, calculated that worldwide dam construction in the 20th century had kept nearly 11,000 cubic kilometers of water from reaching the ocean. The effect of all that withholding has been to slow the ocean's rise by about 0.55 millimeters, or nearly a third of the total, per year.

The researchers went on to trace the timeline of dam construction, using a database of more than 29,000 of the world's largest dams. They allowed for contradictory effects such as reservoirs not filling completely, water seeping into the ground below and small dams not being reported in the database. When they were finished, they added up the dam volumes year by year and superimposed the amounts on the historical, jagged graph of observed sea level rise.

The result: a much straighter line. It seems that the great dam-building bonanzas of the 1950s through the 1980s changed the Earth's runoff patterns enough to be felt (admittedly, somewhat minutely) at sea level. Without dams, sea level would have risen at an average 2.46 millimeters per year. You can take this news as good or bad.

Good: it means sea-level rise may not have accelerated as sharply in recent years as it seems to have done. Bad: scientists can't account for where all the rising seawater is coming from, and these new numbers mean there's even more water to be accounted for. Which reminds me: new models suggest melting ice could raise sea levels by 4 feet this century, New Scientist reports. That's nearly three times more than in the worst-case estimates from the Intergovernmental Panel on Climate Change.