Why does the earth have gravity 2

On March 17, 2002, the German-US satellite duo GRACE (Gravity Recovery and Climate Experiment) started to map the global gravity field more precisely than ever before. After all, the mission lasted a good 15 years - more than three times as long as expected. When the two satellites burned up in the earth's atmosphere in late 2017 and early 2018, they recorded the earth's gravitational field and its changes over time in more than 160 months.

This so-called time-resolved satellite gravimetry makes it possible, among other things, to monitor the terrestrial water cycle, the mass balance of ice sheets and glaciers or the change in sea level and thus to better understand the mechanisms of the global climate system, to evaluate important trends more precisely and to predict possible consequences.

An overview article in the specialist journal Nature Climate Change, in which Frank Flechtner, Christoph Reigber, Christoph Dahle and Henryk Dobslaw from the Helmholtz Center Potsdam Deutsches GeoForschungsZentrum GFZ and Ingo Sasgen from the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research (AWI) were involved, now provides highlights in the area Climate research based on GRACE observations.

Ice sheets and glaciers
GRACE generated the first direct measurement of ice mass loss from ice sheets and glaciers. Before, it was only possible to estimate the masses and their changes using indirect methods. Already within the first two years of the mission it was possible to observe clear signals of the loss of ice mass in Greenland and the Antarctic. From the measurement data it could be deduced that 60 percent of the mass loss in Greenland is due to increased melting due to higher temperatures, while 40 percent can be attributed to the inflow of ice into the oceans. According to the GRACE data, between April 2002 and June 2017 Greenland lost around 260 billion tons of ice a year, and Antarctica around 140 billion tons. In addition to long-term trends, the gravity field data also provide evidence of the direct effects of global climate phenomena such as 'El Niño' on ice sheets and glaciers worldwide.

Earthly water storage
One of the most powerful contributions of the GRACE mission is the unveiling of the earth's changing freshwater landscape. These changes have profound effects on the security of water and food supplies, and thus also on the security of people. Global estimates of the trends disclosed by GRACE indicate increasing water storage in high and low latitudes, while storage in mid-latitudes is decreasing. Although the GRACE records go back only a relatively short time, this observation of far-reaching changes in the global water cycle was an important early confirmation of the changes predicted by climate models for the 21st century.

The GRACE data also help to analyze and assess the sea level more precisely, because the storage of fresh water on land is linked to the sea level via various mechanisms. Analyzes of the GRACE data enabled the first estimates of changes in groundwater levels from space. They confirm excessive abstraction of groundwater from individual aquifers all over the world. The data on terrestrial water storage have also contributed to the review and improvement of various climate models.

Change in sea level and ocean dynamics
Within this century, sea level rise could accelerate to 10 millimeters per year - a value never reached in the past 5000 years and a profound and direct consequence of a warming climate. Although there have been high-precision sea level measurements since the beginning of the 1990s, these only show the absolute change in height of the sea surface. In the 25 years between 1993 and 2017, the sea level rose annually by an average of 3.1 millimeters.

In order to find out how the thermal expansion of the water, melting ice and the inflow of land all affect the sea level, one has to study the mass distribution of the water. With the help of GRACE, it was found that of the average annual sea level rise of 3.8 millimeters between 2005 and 2017, 2.5 millimeters is due to the inflow of water or other mass and 1.1 millimeters to the thermal expansion of the water.

Deciphering this composition is important for predicting sea level height. GRACE data provide an upper limit for the change in ocean mass and thus indirectly also one for the earth's energy balance, which represents a fundamental global measure of climate change. With the help of GRACE, it was possible to show that the majority of the heat released by the rise in temperature remains in the upper 2000 meters of the oceans, which are the most important energy sink for climate change. GRACE also helps to better understand the dynamics and influence of ocean currents and to refine corresponding models, especially for the Arctic Ocean.

Knowledge transfer for climate services
The gravity field data from the GRACE satellites help improve the United States Drought Monitor for forecasting periods of drought and drought in the USA. This helps authorities to react to droughts in a timely and correct manner. With EGSIEM (European Gravity Service for Improved Emergency Management), the European Union has promoted a system with which regional flood risks are to be identified as early as possible. Between April and June 2017, test runs with historical flood data took place, which show that the moisture values ​​determined by GRACE for the solid surface of the earth can improve the predictions, for example for the Mississippi or the Danube. Current data also show that GRACE data can be used to determine the risk of seasonal forest fires more precisely.

The GFZ operated the GRACE mission together with the German Aerospace Center (DLR) and on the US side with the Jet Propulsion Laboratory (JPL) of the space agency NASA. The successor mission GRACE Follow-on (GRACE-FO) started in May 2018. The first monthly gravity field maps should be available to international users at the end of July this year. Unexpected difficulties delayed the start of the measurement. "The cause was the failure of a control unit“, Explains Frank Flechtner from GFZ. "This made it necessary to switch to the replacement unit installed for such scenarios. But now with GRACE-FO a more than two decades long recording of the mass changes in the system earth is within reach.

Original study: Teplay, B. D., Watkins, M.M., Flechtner, F., et al., 2019. Contributions of GRACE to understanding climate change. Nature Climate Change. DOI: 10.1038 / s41558-019-0456-2


The weight of the water
The greater the mass of an object, the greater its gravitational attraction. For example, the Alps are more attractive than the North German Plain. When satellites orbit the earth and fly over a massive region, they accelerate minimally as they approach it and slow down as they fly away.

A tiny fraction of the earth's gravity comes from water on or near the surface in oceans, rivers, lakes, glaciers, and underground. This water reacts to seasons, storms, droughts or other weather effects. GRACE took advantage of the mass displacement of the water by recording its effect on the satellite duo, which orbited our planet 220 kilometers behind one another. Their distance could be measured using microwaves. This changed due to the mass displacement on earth in the course of time. From the data, the researchers then calculated monthly maps of the regional changes in the force of gravity and the causal changes in the masses on the surface.

Additional Information:

Scientific contact:
Prof. Dr. Frank Flechtner
Head of the Global Geomonitoring and Gravity Field Section
Helmholtz Center Potsdam
German Research Center for Geosciences GFZ
c / o DLR Oberpfaffenhofen
Tel .: +49 331 288-1130
Email: [email protected]

Media contact:

Dipl.-Phys. Philipp Hummel
Speaker in press and public relations
Helmholtz Center Potsdam
German Research Center for Geosciences GFZ
14473 Potsdam
Tel .: +49 331 288-1049
Email: [email protected]
Twitter: @GFZ_Potsdam