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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Glacial Retreat and Water Impacts Around the World

The availability of water under ever-increasing climate stress has never been more important. Nowhere is this more apparent than in glacial mountain regions where runoff from glaciers provides water in times of drought or low river flows. As glaciers retreat due to climate change, the water supplied to these basins will diminish. To better understand these hydrological changes, a recent study published in Nature Climate Change examined the world’s largest glacierized drainage basins under future climate change scenarios.

Photo of a glacier in the Pamir Mountains
A glacier in the Pamir Mountains of Central Asia where some of the largest runoff changes are projected to occur (Source: ‏Nozomu Takeuchi‏/Twitter).

Expansive in scale, the study differentiates itself from previous research that assessed the hydro-glacier issue at more localized scales like specific mountain ranges, for example. This study analyzes 56 glacierized drainage basins on four continents excluding Antarctica and Greenland. The basins examined were selected based on their size: they needed to be bigger than 5,000 km2, in addition to having at least 30 km2 of ice cover and greater than 0.01 percent of total glacier cover during the chosen base period of 1981 to 2010.

The motivation behind the study’s global scale, the first ever completed, according to Regine Hock, one of the study’s authors, is that “at a local scale you can only cover a fraction of the glaciers/catchments that may be relevant.” She told GlacierHub that while there are advantages to local studies because they can be more detailed and accurate, the advantage of a global study is that spatial patterns across regions can be identified and analyzed.

In order to calculate changes in glacial mass and accompanying runoff, defined as water that leaves a glacierized area, the authors utilized the Global Glacier Evolution Model to simulate relevant glacial processes including mass accumulation and loss, changes in glacial extent, and glacier elevation. The glacier model was driven by three of the IPCC’s Representative Concentration Pathways (RCP). These are future greenhouse gas (GHG) concentration scenarios based on different socio-economic pathways. The RCP’s chosen by the authors were the 2.6 scenario, which they note is the most similar to the 2015 Paris agreement, the 4.5 scenario where GHG concentrations stabilize by 2100, and the 8.5 or “business-as-usual” scenario where GHG concentrations continue to increase past 2100.

Aerial photo of the Susitna Glacier of south-central Alaska.
The Susitna Glacier of south-central Alaska, which feeds the Susitna river basin, is not expected to reach peak water until the end of the 21st century. The vegetation appears red due to the wavelengths used by the satellite (Source: NASA Goddard Space Flight Center/Creative Commons).

How do these three scenarios impact glacial volume in the study’s glacierized basins? After running the glacier model, total volume was projected to decrease in all three with a decrease of 43±14 percent for the 2.6, 58±13 percent for the 4.5, and 74±11 percent for the 8.5, respectively.

A decrease in glacial volume will in the short term mean an increase in water for a basin as runoff increases, that is until the point of “peak water,” where the amount of glacial runoff begins to decrease as glacier volume declines. Distressingly, peak water has already been reached in 45 percent of the basins examined in the study including most of the Andes, Alps, and Rocky Mountains.

Three factors— total glacial area, ice cover as a fraction of the basin, and the basin’s latitude— influence the timing of peak water occurrence in a basin. Basins with many large glaciers at higher latitudes like in coastal Alaska were projected to reach peak water near the end of the century whereas basins closer to the equator with small glaciers like the Peruvian Andes have already experienced or will soon experience peak water. Furthermore, the Himalayas are projected to experience peak water around mid-century as their high elevation tempers the effect of their relatively low latitude.

Map of peak water occurrence across all studied basins.
Time of peak water occurrence in all of the studied basins (Source: Huss & Hock).

The study also examined changes to glacial runoff on a monthly timescale for the years 2050 and 2100, focusing specifically on the melt season from June to October in the Northern Hemisphere and December to April in the Southern Hemisphere. The monthly results showed spatial consistency, which surprised the authors, according to Hock, with runoff increasing in almost all basins at the beginning of the melt season (June/December) and decreasing toward the end (August and September/February and March). Another unexpected finding was the significant reduction in overall runoff, up to a 10 percent decrease by 2100 in at least one month, in basins with very low glacial cover, a phenomenon that was observed in a third of the basins, Hock added.

It is important to remember that these changes in basin runoff mean more than just changing numbers and statistics: there are people and communities that rely on water provided by glaciers. The authors note that 26 percent of the Earth’s land surface is covered by glacierized drainage basins, impacting a third of the world’s population.

Photo of a glacier in the Cordillera Blanca of Peru.
A glacier in the Cordillera Blanca of Peru. Basins of the Peruvian Andes are especially at risk to climate change as many have already reached peak water (Source: Dharamvir Tanwar‏/Twitter).

The ramifications of glacier retreat will not be felt equally across the basins observed in this study. When asked what regions are most at risk, Hock identified both the Andes and Central Asia as places of concern. In the Andes, runoff is decreasing in almost all basins. This is of particular concern due to the limited water resources of the South American west coast. In Central Asia, glaciers contribute to basin runoff in all months, leading to potential problems if runoff is significantly reduced.

These regions, along with other glacier reliant places, face an uncertain and atypical water future, one that will likely see an increase in glacial runoff, followed by a sharp decline.To prepare for these forthcoming challenges, further study is needed, particularly with a focus on the human dimensions of glacial retreat.

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