Tuesday, May 22, 2012 Riddle of rising sea levels said solved

PARIS — Massive extraction of groundwater can resolve the puzzle over rising sea levels seen in past decades, Japanese scientists said Sunday.

Global sea levels rose an average of 1.8 mm per year from 1961 to 2003, according to data from tide gauges. But the big question is how much of this can be pinned on global warming?

In its landmark 2007 report, the U.N.'s Nobel Prize-winning Intergovernmental Panel on Climate Change ascribed 1.1 mm per year to thermal expansion of the oceans — water expands when it is heated — and to meltwater from glaciers, icecaps and the Greenland and Antarctica icecaps.

That left 0.7 mm per year unaccounted for, a mystery that left many scientists wondering if the data were correct or if there were sources that eluded them.

In a study published in the journal Nature Geoscience, a team led by Yadu Pokhrel of the University of Tokyo says the answer lies in water that is extracted from underground aquifers, rivers and lakes for human development but is never replenished.

The water eventually makes its way to the sea via rivers and evaporation in the soil, they noted.

Groundwater extraction is the main component of additions that account for the mystery gap, according to their paper, which was based on computer modeling.

"Together, unsustainable groundwater use, artificial reservoir water impoundment, climate-driven change in terrestrial water storage and the loss of water from closed basins have contributed a sea-level rise of 0.77 mm per year between 1961 and 2003, about 42 percent of the observed sea-level rise," the paper said.

The probe sought to fill one of the knowledge gaps in the complex science of climate change.

Researchers admit to many unknowns about how the oceans respond to warming, and one of them is sea-level rise, an important question for hundreds of millions of coastal dwellers.

Just a tiny rise, if repeated year on year, can eventually have a dramatic impact in locations that are vulnerable to storm surges or the influx of saltwater into aquifers or coastal fields.

In its 2007 Fourth Assessment Report, the IPCC said the oceans will rise by between 18 and 59 cm by the end of the century.

But this estimate did not factor in meltwater from the mighty Greenland and Antarctic ice sheets.