Climate scientists have been puzzling over the answer to a large question: At what concentration of carbon dioxide in the atmosphere does global warming cause the loss of the Antarctic ice sheet and the large rise in sea level around the world?

U.S. geologists say they may have found the answer — or at least an important clue — in a bone-dry desert in Egypt, of all places.

In the Western Desert of Egypt, geologists from the University of Wisconsin and University of Michigan have been studying the exposed sediment along an ancient valley that was Mediterranean coastal land at a time — known to geologists as the Eocene — about 34 million years ago when Earth’s climate was swinging dramatically from warm to cool.

Like a flickering switch, temperatures and carbon dioxide concentrations were gyrating up and down during this climate transition. The Antarctic Ice Sheet waxed and waned, and global sea level rose and fell.

Sea level falls, and the volume of the world’s oceans decline, when a cooling climate allows vast amounts of evaporated ocean water to fall as snow and remain as permanent ice on the Antarctic continent. The Antarctic Ice Sheet, shown above in this photo by Ted Scambos of the National Snow and Ice Data Center in Boulder, Colo., is the largest ice mass on Earth and the key to the question of global sea level.

In fact, writing in the journal Geology, Shanan Peters and colleagues report that the level of the world’s oceans during this period rose and fell some 40 meters (130 feet) between the buildup and collapse of the ice on Antarctica.

The sharp rises and falls in carbon dioxide concentrations in the atmosphere during this volatile transition period allowed the researchers to zero in on the level of atmospheric CO2 that constituted a critical “threshold” for the Antarctic Ice Sheet.

“Growth and decay of the ice sheet appear to have been paced by changes in atmospheric CO2 concentration that oscillated around ~750 ppm,” Peters wrote. “Furthermore, sea level fluctuated tens of meters during this interval, suggesting a highly volatile East Antarctic Ice Sheet when CO2 concentrations were near this threshold value.”

Current concentrations of this dominant greenhouse gas have reached 390 parts per million — up from 280 ppm in recent pre-industrial times — and scientists tracking this buildup see no end in sight.

“Because many of the carbon emission scenarios for the coming century predict that atmospheric CO2 will cross this same 750 ppm threshold,” Peters writes, “our results raise the possibility that global climate could rapidly transition to a state not unlike the Late Eocene, when a large, permanent East Antarctic Ice Sheet was not sustainable.”

Photo credit: Ted Scambos/National Snow and Ice Data Center