Environment

Global sea level rise temporarily dampened by 2010-11 Australia floods

Three atmospheric patterns came together above the Indian and Pacific Oceans in 2010 and 2011. When they did, they drove so much precipitation over Australia that the world's ocean levels dropped measurably.

Unlike other continents, the soils and topography of Australia prevent almost all its precipitation from flowing into the ocean.

Australia's vast interior, called the Outback, is ringed by coastal mountains and is often quite dry.

Because of the low-lying nature of the continent's eastern interior, and the lack of river runoff in its western dry environment, most of the heavy rainfall of 2010-11 remained inland rather than flowing to the oceans.

While some of it evaporated in the desert sun, much of it sank into the dry, granular soil of the Western Plateau or filled the Lake Eyre basin in the east.

The 2010-11 event temporarily halted a long-term trend of rising sea levels caused by higher temperatures and melting ice sheets, according to a team of researchers at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., and other institutions.

Now that the atmosphere's circulation has returned to its previous patterns, the seas are again rising.

The scientists conclude that the Outback region in Australia played a crucial role in trapping a large amount of rainfall when widespread floods occurred over the continent.

The researchers concluded that the smallest continent in the world can affect sea level worldwide. Its influence is so strong that it can temporarily overcome the background trend of rising sea levels we see with climate change.

As the climate warms, the world's oceans have been rising in recent decades by just over three millimetres annually.

This is partly because heat causes water to expand, and partly because runoff from retreating glaciers and ice sheets is making its way into the oceans.

But for an 18-month period beginning in 2010, the oceans mysteriously dropped by about seven millimetres, more than offsetting the annual rise.

Fasullo and co-authors published research results last year demonstrating that the reason was related to the increased rainfall over tropical continents.

They also showed that the drop coincided with the atmospheric oscillation known as La Niña, which cooled tropical surface waters in the eastern Pacific and suppressed rainfall there--while enhancing it over portions of the tropical Pacific, Africa, South America and Australia.

However, an analysis of the historical record showed that past La Niña events only rarely accompanied such a pronounced drop in sea level.