What can long-term trend estimates from satellite gravimetry contribute to climate research?

The satellite missions GRACE and GRACE Follow-On (GRACE-FO) have substantially improved our understanding of the Earth system by providing monthly snapshots of surface mass change. With a growing data record currently spanning almost 20 years, this data set is well suited to investigate long-term changes in, for example, water storage, ice- and ocean mass. In contrast to other remote sensing techniques, satellite gravimetry is based on geophysical inversion rather than direct imaging. This process introduces high-frequency spatial noise caused by the downward continuation from satellite altitude to Earth’s surface. To reduce the impact of this noise, derived mass change grids are typically constrained in the inversion process, or low-pass filtered in post-processing. Accumulating satellite observations over longer time spans reduces the magnitude of the high-frequency noise and thus requires weaker implicit or explicit filtering. Consequently, the smaller spatial leakage allows to investigate smaller spatial scales at the cost of temporal resolution.

We focus on the extreme case of a single linear trend estimated over the whole GRACE/GRACE-FO observation time span and provide a rigorous uncertainty and leakage analysis in comparison with trend estimates from ITSG and GFZ RL06 monthly solutions. First, we discuss the differences of the two trend estimates on a global scale and then apply the findings to selected regions and aquifers. This enables us to gauge to which degree drying and wetting trends can be resolved and localized and to quantify the different uncertainty levels for specific processes like aquifer depletion.