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Human greenhouse gas emissions have created an Earth energy imbalance (EEI), with Earth absorbing more energy than it emits. This imbalance is small compared to total energy flows, making it hard to measure precisely. Since the ocean stores about 91% of the excess energy, Ocean Heat Content (OHC) is a key metric for monitoring climate change and the effectiveness of mitigation policies. Current OHC and EEI estimates combine satellite observations (altimetry, gravimetry) with in-situ measurements. However, uncertainties—mainly from GRACE and GRACE-FO gravity data—remain too high to meet the accuracy needed for operational monitoring (< 0.1 W/m²).
This study evaluates how future gravity missions—GRACE-C, NGGM, and MAGIC—can reduce errors in OHC and EEI estimates across different spatial and temporal scales.
Two main approaches are used:
Error realisation comparison – Quantify how errors in simulated OHC and EEI vary between the three mission configurations and compare them with real-world estimates (MOHeaCAN product).
Uncertainty propagation – Use covariance matrices from each mission scenario to assess the reduction in uncertainty relative to a GRACE-C-like baseline.
Key results:
At regional scale, NGGM significantly reduces errors in Ocean Heat Content (OHC) recovery when compared to GRACE-C
In the Southern Ocean, Southern Pacific and Southern Atlantic, OHC errors estimated from NGGM simulations are up to 80% lower than those derived from GRACE-C simulations.
Due to the use of a stronger filter, NGGM simulations exhibit lower errors than MAGIC. Without filtering, OHC errors are lower with MAGIC than NGGM.
Figure: Differences in the variance between OHC from NGGM simulations and OHC from (a) GRACE-C and (b) MAGIC simulations. Percentage differences are relatice to NGGM data.
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