As I noted here earlier in August, human-caused global warming drove sea level to its highest value seen in the satellite record in 2012 — after having fallen very dramatically during an 18-month period starting in 2010.
Now, researchers have uncovered new, significant details about why that happened, along with the key role that Australia played.
To understand what happened, first have a look at the animated gif above.
I put it together using two images acquired by NASA’s Aqua satellite. The first, captured in September of 2009, shows an inland delta during a relatively dry period in a region of Australia called the Channel Country. The second, acquired in March 2011, shows what happened when three atmospheric patterns came together to drive a gargantuan amount of rain into Australia’s interior. The blue colors represent water, with darker tones indicating deeper water.
What does that have to do with sea level? Here’s where things get interesting.
Researchers had previously tied the drop in sea level to La Niña conditions, which caused an increase in precipitation over some land areas and a concomitant decrease over the oceans. The result was a shift in water from the oceans to the land, and thus a drop in sea level.
In the new research, to be published next month in the journal Geophysical Research Letters, the same scientists have shown that two other semi-cyclic climatic phenomena teamed up with La Niña to drive a huge amount of water from the oceans to the land, and particularly to Australia.
Here’s how it all came together according to the researchers, led by John T. Fasullo of the National Center for Atmospheric Research here in Boulder:
First, La Niña shifted moisture to the western side of the Pacific — toward Australia. Then the Southern Annular Mode helped push that moisture into the interior of Australia, as rain. This resulted in massive flooding. Later, yet more moisture poured into Australia’s interior from the climatic phenomenon known as the Indian Ocean Dipole.
But wouldn’t all that moisture simply have run off into rivers and out into the ocean, resulting in little to no change in sea level? Surprisingly, the answer is no.
That’s because much of Australia’s interior comprises one of our planet’s largest internal drainage systems. And that means when rainfall comes in, water does not go out. At least not out into rivers that drain into the sea. After the 2010 and 2011 floods, some of the water evaporated, and much of it slowly percolated into the desert sands.
To bring things full circle, Fasullo and his colleagues used a variety of tools to piece this complex picture together, including data from NASA’s Grace satellites, which enabled them to track how mass in the form of water shifted from the oceans to Australia.
In their paper, the researchers conclude with this:
Lastly, the current global sea level anomaly is an interesting counterpoint to the 2011 drop, with strongly positive anomalies in mid-2013 accompanying ENSO-neutral conditions.
Translation: Now that La Niña has subsided, and the flooding is long gone, sea level has continued to go up again.
That long-term trend is driven by thermal expansion of ocean waters, and meltwater from Earth’s glaciers and ice sheets rushing into the seas — all a product of a warming world.
One moral of the story is this: If the rise in sea level takes a break again at some point in the future, don’t accept at face value claims that it disproves anthropogenic global warming. Wait for scientists to piece together the complex picture of what’s really happening.