As the ocean warms, it expands, and that thermal expansion is the prime contributor to changes in global sea levels, which in recent years have been rising by an average of about 3 mm a year. (Regionally and locally, changes may be greater or lower, affected not only by thermal expansion but factors ranging from local wind patterns to the mining of groundwater aquifers.) Even as the underlying trend clearly slopes upward, there are of course seasonal and annual variations. But, as the image above shows clearly, from mid-2010 to mid-2011, there was a sharp deviation of 6 mm – downward.

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Climate change skeptics jumped all over this, but NASA researchers have an explanation for what they call “a pothole on the road to higher seas.”

That explanation? It rained. A lot. The water that provided that rain, of course, came from the ocean, but whereas most of the time the bulk of a year’s rainfall takes place over the ocean, immediately depositing that water back where it came from, this year a disproportionate amount fell on land.

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According to climate scientist Josh Willis of NASA’s Jet Propulsion Laboratory, 2010 saw a transition from a strong El Niño to “one of the strongest La Niñas in recent memory.” This sudden shift in the Pacific “changed rainfall patterns all across the globe, bringing massive floods to places like Australia and the Amazon basin,” according to a JPL press release.

Data from NASA/German Aerospace Center’s twin Gravity Recovery and Climate Experiment (Grace) spacecraft showed clearly how water was redistributed around the planet as a consequence of this huge amount of rainfall.

However, over the long term, says NASA, the trend remains clear:

Water flows downhill, and the extra rain will eventually find its way back to the sea. When it does, global sea level will rise again. “We’re heating up the planet, and in the end that means more sea level rise,” says Willis. “But El Niño and La Niña always take us on a rainfall roller-coaster, and in years like this they give us sea-level whiplash.”

Images courtesy of NASA Jet Propulsion Laboratory.