Earth's land masses have stored increasing amounts of water in the last decade, slowing the pace of sea level rise. Image credit: U.S. National Park Service
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New measurements
from a NASA satellite have allowed researchers to identify and quantify,
for the first time, how climate-driven increases of liquid water
storage on land have affected the rate of sea level rise.
A new study by scientists at NASA's Jet Propulsion Laboratory (JPL)
in Pasadena, California, and the University of California, Irvine, shows
that while ice sheets and glaciers continue to melt, changes in weather
and climate over the past decade have caused Earth’s continents to soak
up and store an extra 3.2 trillion tons of water in soils, lakes and
underground aquifers, temporarily slowing the rate of sea level rise by
about 20 percent.
The water gains over land were spread globally, but taken together
they equal the volume of Lake Huron, the world’s seventh largest lake.
The study is published in the Feb. 12 issue of the journal Science.
Each year, a large amount of water evaporates from the oceans, falls
over land as rain or snow, and returns to the oceans through runoff and
river flows. This is known as the global hydrologic, or water, cycle.
Scientists have long known small changes in the hydrologic cycle -- by
persistent regional changes in soil moisture or lake levels, for
instance -- could change the rate of sea level rise from what we would
expect based on ice sheet and glacier melt rates. However, they did not
know how large the land storage effect would be because there were no
instruments that could accurately measure global changes in liquid water
on land.
"We always assumed that people’s increased reliance on groundwater
for irrigation and consumption was resulting in a net transfer of water
from the land to the ocean,” said lead author J.T. Reager of JPL, who
began work on the study as a graduate student at UC Irvine. "What we
didn’t realize until now is that over the past decade, changes in the
global water cycle more than offset the losses that occurred from
groundwater pumping, causing the land to act like a sponge -- at least
temporarily. These new data are vital for understanding decadal
variations in sea level change. The information will be a critical
complement to future long-term projections of sea level rise, which
depend on melting ice and warming oceans.”
The 2002 launch of NASA’s Gravity Recovery and Climate Experiment
(GRACE) twin satellites provided the first tool capable of quantifying
land liquid water storage trends. By measuring the distance between the
two GRACE satellites to within the width of a strand of human hair as
they orbit Earth, researchers can detect changes in Earth’s
gravitational pull that result from regional changes in the amount of
water across Earth’s surface. With careful analysis of these data, JPL
scientists were able to measure the change in liquid water storage on
the continents, as well as the changes in ice sheets and glaciers.
“These results will lead to a refinement of global sea level budgets,
such as those presented in the Intergovernmental Panel on Climate
Change (IPCC) reports, which acknowledge the importance of
climate-driven changes in hydrology, but have been unable to include any
reliable estimate of their contribution to sea level changes,” said JPL
senior water scientist Jay Famiglietti, senior author of the paper and a
professor at the University of California, Irvine.
Famiglietti also noted the study is the first to observe global
patterns of changes in land water storage, with wet regions getting more
wet and dry areas getting drier.
“These patterns are consistent with earlier observations of changing
precipitation over both land and oceans, and with IPCC projections of
changing precipitation under a warming climate,” he said. “But we’ll
need a much longer data record to fully understand the underlying cause
of the patterns and whether they will persist.”
NASA uses the vantage point of space to increase our understanding of
our home planet, improve lives and safeguard our future. NASA develops
new ways to observe and study Earth's interconnected natural systems
with long-term data records. The agency freely shares this unique
knowledge and works with institutions around the world to gain new
insights into how our planet is changing.
For more on NASA's sea level rise research:
https://sealevel.nasa.gov/