The Impact of the GRACE Mission on Glaciology and Climate Science

Science and technology have come a long way. We continue to learn more and more about our planet and its complex dynamics each and every day, and much of this new data is attained through cutting-edge tools and Earth monitoring systems.

One of the most revolutionary advances for physical sciences in recent decades is GRACE, (short for Gravity Recovery and Climate Experiment) mission. Launched by NASA and the German Aerospace Center in 2002, GRACE was a satellite mission aimed towards better understanding the mass changes of the planet’s hydrosphere and cryosphere.

Byron D. Tapley and colleagues recently published a review of the GRACE mission in Nature Climate Change. The researchers examined the contributions of GRACE to our current observations and understanding of mass transport of water, whether liquid, solid ice, or vapor in the atmosphere.

Some additions from this mission include observations of terrestrial water cycles, ice sheet melting and glacier retreat, and a first look at groundwater resources from up above. Check out this video below on GRACE and it’s effects by the NASA Jet Propulsion Laboratory.

An Overview of GRACE

Unlike previous single satellite approaches, the GRACE mission utilized two satellites orbiting one behind the other. As they orbit, they shift a miniscule The measurements are produced by tracking of the distance between the satellites, which varies depending on the gravitational attractions as they circle the globe. Measurements are collected after each month and estimates of the mass balance of the Earth’s surface are then composed through changes in its gravity field.

Battery failures resulted in the end of the GRACE mission on October 15, 2017. However the over 15 years of data collected has been monumental in perceiving quantifiable changes on the Earth’s surface.

“For the first time, GRACE enabled the quantification of mass trends and mass fluctuations of terrestrial water storage, continental aquifers, and glaciers and ice sheets, and enlightened our view of large-scale mass redistribution associated with glacial-isostatic adjustment and earthquakes” the authors state. GRACE was able to measure global and regional changes, and also capture both natural variability and anthropogenic influences on the planet’s water storage.

A diagram of satellite separation (Source: NASA Jet Propulsion Laboratory)

Major Contributions to Glaciology and Climate Science

One way in which GRACE was different from previous satellite observations was  that it provided direct measurements of the net mass change of ice sheets and glaciers. The measurements include precipitation, evaporation, runoff, and ice discharge. Without GRACE, satellite altimetry is limited to just surface mass change, and it is also limited by sampling errors and multi-annual trends. GRACE has fewer sampling errors for ice sheet measurements, which are obtained monthly. This makes the data obtained through GRACE relatively more robust.

According to the authors, GRACE was able to reveal a clear signal of ice-mass loss in Greenland and Antarctica after just two years from the launch date. Throughout the lifespan of GRACE, ice-mass loss encompassed the entire ice sheet in Greenland, while in Antarctica the mass lost came mostly from the Amundsen Sea Embayment, which was found to be influenced by ocean conditions. The satellites continued to build more robust mass trends over time, as well as develop higher quality gravity field solutions.

The GRACE mission has also been impactful in providing a robust survey of terrestrial water storage, groundwater and the anthropogenic influences on depletion, and also sea level rise and ocean dynamic changes. It has been able to produce annual zonal mean plots of terrestrial water storage and groundwater variability, which can be representative of such events as floods and droughts. It’s identified hot spots for water loss among some of the world’s major aquifer systems, in which studies confirmed excessive groundwater extraction.

Artist’s concept of GRACE (Source: NASA Jet Propulsion Laboratory)

Scientists continue to produce analyses of global sea levels with data from GRACE, altimetry readings, and Argo floats, which drift on the surface of the oceans to measure temperature and salinity. Respectively, these different tools provide measurements of total sea-level trend, mass inflow, and thermal expansion.The combined use of GRACE and temperature measurements from Argo also produced reliable measurements on ocean heat content. Although Argo floats are unable to measure temperatures 2,000 meters below sea level, other observations are applied for an indirect approach to the oceans’ heat budget.  

As a follow-on to GRACE, NASA recently launched the GRACE-FO mission on May 22, 2018. This mission will continue to monitor the planet’s water storage, and the authors are hopeful that this project will bring us one step closer to achieving a multi-decadal record of mass variability on Earth’s hydrological systems.

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Satellites Detect Both Steady and Accelerated Ice Loss

A new study published in Geophysical Research Letters reports the findings of a pair of satellites that measure gravity to get a clearer picture of the continued ice mass loss in Greenland, the Gulf of Alaska, and the Canadian Arctic Archipelago. The study found accelerated ice loss in the Arctic, and steady loss in Alaska, which will have significant implications for sea level rise globally.

Depiction of the GRACE satellites. (Source: NASA Jet Propulsion Laboratory)
Depiction of the GRACE satellites. (Source: NASA Jet Propulsion Laboratory)

The researchers, Christopher Harig and Frederik J. Simmons, both of Princeton University, analyzed data from the two satellites, called the Gravity Recovery and Climate Experiment (GRACE), in order to not only find the current state of ice mass within glaciers and ice sheets, but the changes in mass since 2003.

GRACE’s dual satellites circle the Earth together, and minute fluctuations in their orbit serve as a basis for measuring the Earth’s gravitational field. The two are separated by approximately 137 miles, and as they fluctuate with the changing gravitational pull, the distance between the two varies slightly. (The two satellites are nicknamed Tom and Jerry, a reference to the cartoon cat and mouse.)

Coupling the differing distances with precise GPS locations, GRACE is able to provide a view of the Earth’s gravity with “unprecedented accuracy” as NASA says. This level of detail allows researchers to easily find even minute trends in mass changes.

GRACE is more commonly used over large areas, such as ice sheets, but in this research the authors studied areas “near the [lower] limit that can be resolved by GRACE data.” After thermal expansion, mountain glaciers and ice caps are the second highest contributor to sea level rise, making accurate and efficient study of the mass loss from smaller areas critical for future sea level projections.

The researchers found that the glacial ice on the north region of the Gulf of Alaska was decreasing at a faster rate than the south region. GRACE detected an unexpectedly large ice loss in 2009 which the authors attribute to a lowered albedo after the eruption of Mount Redoubt.

NASA image of Eureka Sound on Ellesmere Island. (Source: Stuart Rankin/Flickr)
NASA image of Eureka Sound on Ellesmere Island. (Source: Stuart Rankin/Flickr)

The Canadian Archipelago as a whole has been losing ice mass steadily. Within it, the Ellesmere Island region was stable in 2003, when the data was first collected, but mass loss has been accelerating since. In 2013, the researchers found that the mass loss within the Ellesmere Island region had dramatically accelerated, but has since continued closer to average. Baffin Island, the second area studied within the Archipelago, also saw significant ice loss but not at the same rate as Ellesmere.

Greenland saw “an order of a magnitude” more total volume ice loss than Baffin and Ellesmere. Partially due to its sheer size, ice loss there is significant; in the previous decade the largest land-based contributor to sea level rise has been Greenland.

As ice mass loss continues in these regions due to natural variability and climate change, it will be important to have accurate and localized data to better prepare for the corresponding sea level rise.  

Visual depiction of sea level rise. (Source: go_greener_oz/Flickr)
Visual depiction of sea level rise. (Source: go_greener_oz/Flickr)

“Worldwide, on the order of 500 million people could be directly impacted by rising sea level by the end of this century. The human impact is combined with a large financial impact as well. So regardless of where people live, I think the impacts of ice loss and sea level rise will be easily seen in the future,” co-author Christopher Harig said in an email to GlacierHub.

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