For the past couple of decades, the oceans have been steadily rising. Each year, sea-level increases by about 3 millimeters, a constant and ominous creep responding to climate warming.
Scientists have been measuring this rise from satellites since 1993, using instruments called altimeters. But for an 18-month period that began in the middle of 2010, something surprising happened. Instead of rising, sea levels fell.
"Every few months we check in on sea level and try to get some idea as to what's happening and why. For most of the altimeter record, it's been a fairly bland story. But some years have really thrown some curveballs," said John Fasullo, a climate scientist at the National Center for Atmospheric Research.
The one-and-a-half-year, 7-mm fall in sea levels was certainly a curveball. At the time, global warming skeptics used it to support arguments against climate change.
Fasullo, who was trying to balance out the Earth's "water budget," sought an explanation for where that water, normally ocean bound, might have ended up.
Now he believes he has one. His paper explaining the sea-level drop was recently accepted into the journal Geophysical Research Letters.
Fasullo worked to determine where the water might be, if it wasn't contributing to sea-level rise. In an earlier paper, the researcher and other scientists concluded it had probably gotten stored on the land somewhere. They also thought La Niña might have something to do with this.
Where did the water go?
In most cases, though, water that falls on land eventually drains into the ocean. Even if a whole lot of rain fell in South America's Amazon, for example, it could slow sea-level rise for only about a couple of months, as it slowly made its way to the sea.
So in order to make sea levels fall, the water had to be stored in a place where it didn't reach the ocean for a long while. That place, it turns out, was Australia.
"Australia is really unique," said Fasullo. In the continent's eastern interior, most of the rain that falls runs inland, into a salt lake called Lake Eyre -- never reaching the sea.
Lake Eyre is the lowest point in Australia. It's usually a dry, salty flat. But when it rains heavily, the basin fills, and the lake teems with new life, as long-dormant seeds spring to life and birds flock to the lake.
From 2010 to 2011, enough rain fell on Australia to fill the lower part of the lake almost completely, and the upper portion at least 75 percent. Australia got about a foot of rain more than normal over that period, said Fasullo.
The continent stored that excess water for long enough to change global sea levels.
That the world's smallest continent can affect global sea levels this way is pretty extraordinary, said Fasullo. It's also rare.
There are a number of climate factors that have to be in place for this to happen. La Niña is not enough; when Fasullo looked back at other La Niña years he did not find the same response in sea levels.
In addition to La Niña, other climate variations also played a role. Together, they forced the extraordinary rainfall and water storage in the Australian continent.
Collision of climate variants
One, called the negative phase of the Indian Ocean Dipole, is an atmospheric circulation in the tropical Indian Ocean that transports water from west to east. At the same time, La Niña in the eastern Pacific Ocean transports water east to west.
"So these things collide in the western Pacific Ocean," Fasullo said.
Then another climate phenomenon, called the Southern Annular Mode, drew that moisture south, into Australia. The last time Fasullo thinks this may have happened was 1973 and 1974, based on an examination of Australian rainfall records.
It's important for climate scientists to understand why sea levels, which have been steadily rising, might periodically fall, or rise at faster rates, said Fasullo.
"Monitoring climate is intimately tied to monitoring sea level. So understanding why you would have this brief hiatus in sea-level rise is really key to our understanding of the climate system and being able to monitor the system," he said.
In the last two years, he added, researchers have noticed a sharper-than-normal increase in sea-level rise, from the 3 mm yearly to 10 mm.
"It's never gone up that quickly in our observed record," he said.
That's yet another curveball that Fasullo and others are now working to explain.
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