The peaks of Peru's Cordillera Blanca are home to the densest array of tropical glaciers in the world.

Runoff from that ice feeds the Rio Santa, providing most of the river's flow during the annual summer dry season -- water that is used for drinking, irrigating fields and generating power along a broad swath of Peru's Pacific coast.

It's a bounty that scientists and government officials have long expected to decrease as climate change shrinks the region's mountain glaciers. But a new study suggests that is already happening -- that decades earlier than anyone expected, the glacial runoff that feeds into the Rio Santa has passed "peak water."

"Two years ago, I had a discussion with the head of the glaciology office at Huaraz," said the study's lead author, McGill University doctoral student Michel Baraer. "We talked about the consequences of melting glaciers and water resource management. ... He said, 'We are before the peak, and we have only 20-30 years to adapt to these changes before we really see a negative trend.' What we have discovered now is that these 20-30 years do not exist. The peak is already past."

Baraer is part of an international team of researchers who used historical stream gauge records and satellite data to assess the state of the Cordillera Blancato's glaciers and their runoff into the system of tributaries that feeds the Rio Santa.

Their work, published in the January issue of the Journal of Glaciology, reveals that runoff from those glaciers has peaked and is now declining. Although the rate at which the glaciers are melting continues to increase, Baraer said, there is so little ice left that its melt will produce less and less runoff as time goes on.

And that poses a thorny problem for communities that depend on the Rio Santa.

'No way back' for these glaciers
During the region's wet season, which stretches from October to April, glacial runoff from the Cordillera Blanca accounts for a small portion of the water that reaches the river. But during the summer dry season, when there is little or no rainfall, that glacial runoff is what keeps the river running.

If the new study is right, and the glacial runoff is past "peak water," communities that draw from the Rio Santa will have to find new ways to supplement or stretch their water supply during the dry months as glacial runoff diminishes.

The scientist said the problem could be compounded by the current practice of diverting up to 80 percent of glacial runoff for irrigation before it actually reaches the river.

"The difficulty that population will face is that during the dry season, the demand for water is high," Baraer added.

He and his colleagues believe that if the glaciers disappear completely, the river's water level during the annual summer dry season would drop up to 30 percent below today's level.

They are interested now in using the same technique they applied to the runoff from the Cordillera Blanco glaciers to examine whether mountain ice in other areas -- including glaciers in Bolivia, Ecuador and Argentina -- has also passed "peak water."

For the glaciers of the Cordillera Blanco, "most probably there is no way back, even if we stop our emissions of carbon dioxide," Baraer said. "There is probably nothing we can do to reverse the phenomenon, because it's too far along. But for all the regions we still probably have where we are far from peak water, there is still some hope."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500