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On 17 March 2002, the GRACE (Gravity Recovery and Climate Experiment) German-American satellite duo was launched to map the global gravitational field with unprecedented accuracy. After all, the mission lasted a good 15 years, more than three times longer than expected. When both satellites burned in the Earth's atmosphere in late 2017 and early 2018, respectively, they recorded Earth's gravitational field and its changes over time, in more than 160 months.
This time-resolved satellite gravimetry makes it possible, among other things, to monitor the Earth's water cycle, the ice and ice mass balance, or the evolution of the sea level, and thus to better understand the mechanisms of the global climate system. to more accurately assess important climate trends and predict possible consequences.
A magazine in the newspaper Nature Climate changeattended by Frank Flechtner, Christoph Reigber, Christoph Dahle and Henryk Dobslaw from the Helmholtz Center in Potsdam, the German Geoscience Research Center (GFZ), and Ingo Sasgen from the Alfred Wegener Institute at the Helmholtz Center for Polar and Marine Research (AWI). the field of climate research based on GRACE observations.
Ice caps and glaciers
GRACE produced the first direct measure of ice mass loss in ice sheets and glaciers. Previously, it was possible to estimate the masses and their modifications only by indirect methods. During the first two years of the mission, it was already possible to observe clear signals of loss of ice mass in Greenland and Antarctica. The measured data showed that 60% of the total mass loss is due to increased melt production in response to warming trends in the Arctic, while 40% is due to an increase in ice flow in l & # 39; ocean. According to GRACE data, between April 2002 and June 2017, Greenland lost about 260 billion tons of ice a year and Antarctic about 140 billion tons. In addition to long-term trends, gravity data also provide evidence of the direct effects of global climate events such as "El Niño" on ice sheets and glaciers around the world.
Land water storage
One of the most outstanding contributions of the GRACE mission has been the unveiling of the changing landscape of freshwater of the Earth, which has profound implications for water, food and human security. Global estimates of GRACE trends suggest an increase in water storage under high and low latitudes, with a decrease in storage at mid-latitudes. Although the GRACE record is relatively short, this observation of large-scale changes in the global hydrological cycle has been an important preliminary confirmation of the changes predicted by climate models in the 21st century.
GRACE data also helps to analyze and assess sea level more accurately, as the storage of freshwater on land is linked to sea level by various mechanisms. The analyzes of the GRACE data allowed the very first estimate of the changes in groundwater storage from space. They confirm the excessive rates of groundwater depletion of individual aquifers worldwide. Terrestrial water storage data also contributed to the validation and calibration of various climate models.
Sea level change and ocean dynamics
During this century, the rise in sea level could reach 10 millimeters per year, a rate unprecedented for 5,000 years and a direct and profound consequence of global warming. Highly accurate sea level measurements have been available since the early 1990s, but they show only absolute sea level change. In the 25 years from 1993 to 2017, sea level has grown on average by 3.1 millimeters a year. To determine the effects of thermal expansion, ice melt and the continental influx of water on sea level, it is necessary to study the mass distribution of water. ;water. GRACE showed that 2.5 millimeters of the average annual sea level rise, which was 3.8 millimeters between 2005 and 2017, resulted from an influx of water or water. another mass and 1.1 millimeters, due to the thermal expansion of the water. Solving this composition is important for sea level projections. GRACE data is a constraint for ocean mass change and therefore indirectly for Earth's energy imbalance, which is a fundamental fundamental measure of climate change. GRACE has shown that most of the warming caused by the rise in temperature occurs in the upper 2,000 meters of the oceans, which are the main energy sink of climate change. GRACE also contributes to a better understanding of the dynamics and impact of ocean currents, especially for the Arctic Ocean.
Climate service applications
GRACE satellite gravity field data help improve the US Drought Monitor. This helps US authorities respond to droughts in a timely and reasonable manner. With EGSIEM (European Severity Service for Improving Emergency Management), the European Union has set up a service designed to identify the regional flood risks most often. early possible. Between April and June 2017, trials with historical flood data took place, showing that the moisture indicators for major river basins determined by GRACE can improve forecasts, for example for the Mississippi or Danube. Current results also show that GRACE data can be used to more accurately predict the risk of seasonal forest fires.
GFZ managed the GRACE mission jointly with the German Aerospace Center (DLR) and, on the US side, with NASA's Jet Propulsion Laboratory (JPL). In May 2018, the GRACE Follow-on monitoring mission (GRACE-FO) was launched. The first monthly Gravity Maps should be available to international users by the end of July this year. Unexpected difficulties delayed the submission of products. "The reason was the failure of a control unit on the second GRACE-FO satellite," says Frank Flechtner of GFZ. "This made it necessary to switch to the installed replacement unit for such scenarios.But now, with GRACE-FO, a record of more than two decades of mass changes in the Earth system is at hand. " (Ph)
Background: the weight of water
The larger the mass of an object, the greater its gravitational attraction. For example, the Alps exert a stronger pull force than the lowlands of northern Germany. When satellites gravitate around the Earth and fly over a vast area, they accelerate minimally on approach and slow down as they fly.
A tiny part of gravitation emanating from the Earth is based on 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 massive displacement of water by recording its effects on the satellite duo that gravitated around our planet 220 kilometers away. Microwaves were used to measure their distance. This distance has changed over time due to mass displacement on Earth. From the data, the researchers then calculated monthly maps of regional changes in Earth's gravitational attraction and changes in the causality of the masses at the surface.
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