Sea levels are not rising as rapidly as anticipated despite the accelerated melting of ice sheets and glaciers. In fact, the hot, parched Earth is soaking up more of the water inland before it flows into the oceans, a new study revealed.
NASA scientists examined satellite measurements over the last 10 years. They found, for the first time, that planet Earth's continents absorbed and stored an additional 3.2 trillion tons of water in lakes, soils and underground aquifers.
By doing so, the rate of sea level rise has been temporarily slowed by 20 percent, researchers said.
Thirsty Continents Act Like Sponge
John Reager, lead author of the study and a hydrologist from NASA's Jet Propulsion Laboratory, said they have always assumed that the reliance on groundwater for consumption and irrigation resulted in a net transfer of water from the land to the ocean.
What scientists didn't realize until the study was that the changes in the global water cycle more than compensated for the losses from groundwater pumping. The land began to act like a sponge, albeit temporarily.
Reager said that as the force of gravity depends on mass, the only thing that is mobile enough and heavy enough to affect Earth's gravity is water.
"That movement of water has one of the biggest effects on the earth's gravity field," he said.
The water cycle involves the flow of moisture, from evaporation over the oceans to the fall of precipitation, to rivers and runoff that lead back to the ocean. Earth's continents go through 6 trillion tons of snow, soil moisture, groundwater and surface water. The stores of water are gradually released into the seas, and the process begins again.
Doubling Numbers, But Still Not Fast Enough
The strength of the global water cycle, however, can vary from decade to decade, year to year, because of the changes in weather and climate.
Over the past century, the sea level rise has sped up because of melting ice sheets and glaciers, pouring more water into the ocean. Warming temperatures have caused the sea's volume to expand.
Reager and his colleagues studied data spanning from April 2002 to November 2014, from the Gravity Recovery and Climate Experiment (GRACE). At this time, the sea level rose at an average rate of 2.9 millimeters per year.
According to the Intergovernmental Panel on Climate Change, this is approximately double the average rate during the 20th century. It's fast, but this was not what scientists had anticipated, and so they added a fourth factor to their calculations: water stored inland.
The NASA research team discovered that weather cycles and natural climate brought more snow and rain over land, where it collected in the soils and led to the rise of water tables. With that, the continents absorbed 3.2 trillion tons of water, reducing the rate of sea level rise by 0.71 millimeters annually.
The Pattern May Not Continue
Alex Gardner, a JPL glaciologist, said the land can only store so much water before the continents begin to lose water mass again. They expect the extra absorption of water won't last forever.
Gardner said the only way to reduce the rate of sea level rise is to lessen the amount of energy that the planet soaks up. The only way to do that is by stopping the emissions of greenhouse gases such as carbon dioxide.
"We can't compete with the Earth's system. It's just so huge. Some years are wet, some years are dry. You wait long enough and the climate wins," added Gardner. "Climate always wins."
The findings of the study are featured in the journal Science.
Photo : David Stanley | Flickr
