Aug 24, 2011 07:37 GMT  ·  By
The red line in this image shows the long-term increase in global sea level since satellite altimeters began measuring it in the early 1990s
   The red line in this image shows the long-term increase in global sea level since satellite altimeters began measuring it in the early 1990s

For the past few decades, sea levels have been continuously rising due to global warming, but at times the constant expansion comes to a standstill for a period of time. For 2011, the rise appears to have hit a pothole, with sea levels dropping by a quarter of an inch. Experts now explain why this is happening.

The best analogy to explain the behavior of the world's oceans in response to global warming and climate change is to compare it with the behavior of mercury inside a thermometer. Whenever things hit up, both expand upwards.

In the case of seas, temperature is not the only thing driving the expansion. Other factors include the destabilization of ices and the changing of wind patterns, the melting of Arctic, Antarctic and mountaintop glaciers and so on.

Over the long term, rising sea levels are unavoidable, unless concrete measures are taken to reduce pollution and global warming. For nearly two decades, the US/French Jason-1, Jason-2 and Topex/Poseidon satellites have been collecting data on how ocean levels vary.

What these data indicate is that oceans are increasing their levels quited steadily. However, every once in a while there is a gap in the data, where the water expansion speed dips. Between the summers of 2010 and 2011, water levels not only stopped growing, but also retreated a little bit.

Now, experts at the NASA Jet Propulsion Laboratory (JPL) say that this change in pattern was caused by the effects of the ocean-atmosphere-coupled El Niño and La Niña events in the Pacific Ocean.

JPL climate scientist Josh Willis explains that the sizable El Niño that manifested itself in early 2010 was quickly and abruptly replaced with the most intense La Niña events in recorded history.

This impressive shift not only affected global sea level rise, but also modified precipitation patterns around the globe, brought drought to regions in the southern United States, and produced massive floods in the Amazon Basin of South America, and in Australia.

In early 2011, a lot of water was therefore piled up on the continents and stored there, which is the main reason why global sea levels dropped by a small margin. This was inferred from readings collected by the NASA/DLR twin Gravity Recovery and Climate Experiment (Grace) spacecraft.

“By detecting where water is on the continents, Grace shows us how water moves around the planet,” University of Colorado in Boulder sea level scientist Steve Nerem explains. These results were presented at the annual Grace Science Team Meeting in Austin, Texas, earlier this year.

“This year, the continents got an extra dose of rain, so much so that global sea levels actually fell over most of the last year,” JPL oceanographer and climate scientist Carmen Boening goes on to say.

“We're heating up the planet, and in the end that means more sea level rise. But El Niño and La Niña always take us on a rainfall rollercoaster, and in years like this they give us sea-level whiplash,” Willis concludes.