Sound Science Bite: February 19. Earth as Sponge

There is some good news about the rise in sea level. It's not as bad as it could be. In Science, 12 Feb 2016, J.T. Reager et al reported that sea level rise was 0.71 millimeters per year less than it would otherwise be over the past 12 years due to water storage on the land. Since sea-level rise averaged about 3.2 millimeters per year over the past 20 years, we might have seen a rise of nearly 4 millimeters per year recently.

How did these authors find this water? There are a couple of identical satellites orbiting in tandem called the GRACE satellites. (NASA just dearly loves clever acronyms. GRACE equals "Gravity Recovery And Climate Experiment".) The distance between these satellites is measured to a fraction of a human hair by a microwave ranging system and GPS (Global Positioning System). As the lead satellite approaches an increase in mass (a mass concentration or "mascon") near the surface of the Earth, the extra lateral gravitational pull causes it to speed up, lengthening the distance between the two satellites. The opposite happens when the lead satellite moves toward lessening mass concentration.

The GRACE data then needs to be adjusted to remove such things as the effects of tides that occur in the solid Earth, ocean, and atmosphere, etc. What's left are changes in gravity due to the movement of water and ice. The data reveal decreasing water on land in places like Texas, California, and the Middle East due to drought and the pumping of groundwater. Gains in water on land were seen in other locations such as the central US due to heavy rainfall and in the Amazon Basin and Africa due to drought recovery. On average, the gains outstripped the losses, leading to a net storage of liquid water on land, offsetting some of the melting of ice sheets and glaciers. At least some of the positive and negative water storage patterns appear to be related to climate change.