Gravity Recovery and Climate Experiment – GRACE

Since 2002, the German-American satellite missions GRACE and GRACE-FO have been providing us with data from which we generate high-resolution models of the Earth's gravity field. In this way, mass changes can be recorded on a monthly basis, for instance the melting of the ice masses in the Arctic and Antarctic, changes in the global water cycle or the contribution of meltwater to sea level rise. The years of drought in Europe and even the catastrophic floods in the Ahr valley and the neighbouring regions are clearly visible in the gravity field maps.

So as to prevent an interruption in the globally unique data series, we are now planning the third generation of the successful satellite tandems. In the recent report of the intergovernmental panel of experts on climate change IPCC (IPCC Assessment Report 6 WG-1), GRACE and GRACE-FO are among the three most frequently mentioned satellite missions. German aerospace travel and the scientific community producing and using the gravity field data have, as a result, gained considerable visibility.

Read more about GRACE (2002–2017)
Read more about GRACE-Follow-On (2018)

Due to the continuous, monthly measurement of variations in the Earth's gravitational field, the GRACE satellites provide important information on the impacts of climate change. Two identical satellites circle the Earth at an altitude of about 490 km in an orbit taking them over the poles. Their distance to each other varies depending on the mass beneath them.

With the help of microwaves, the satellites constantly determine this distance with ultra-precisioncontinuously and ultra-precisely: they measure with the an accuracy of a one tenth of a hair's breadth over a distance of around 220 kilometres. On GRACE-FO, a technology demonstrator, measuring instrument developed largely in Hanover  for future gravity field missions, even makes it possible to measure this distance about thirty times more precisely with the help of laser signals.

Please see this video | 4:10min for further information on the measurement method.

In July 2021, an area of heavy rain moved over the west of Germany and neighbouring states and led to disastrous flooding. North Rhine-Westphalia, Rhineland-Palatinate and the neighbouring countries of Holland and Belgium were particularly affected. Using GRACE-FO data, the three maps show the deviations of the complete water reservoir in the months of June, July and August 2021. It can be seen that in June the soil is too dry, in July the soil is oversaturated due to the heavy rain. But already in August the soil moisture decreased drastically again. These data from GRACE-FO underline that during heavy rainfall events a lot of water runs off the surface with hardly any water being actually stored in the soil.

Since 2018, most summers in Europe have been far too dry. This leads to sometimes dramatic groundwater deficits. The maps show the regions where the total water storage (Terrestrial Water Storage; TWS) showed a deficit in May of each of the last three years. The situation in Ukraine is drastic, but large parts of Germany have also been affected by drought and water shortages for years. TWS is a so-called essential climate parameter of the Global Climate Observing System (GCOS) and only the GRACE missions are in a position to determine this value globally.