Are GRACE-Era Terrestrial Water Trends Driven by Anthropogenic Climate Change
To provide context for observed trends in terrestrial water storage (TWS) during the GRACE era (2003-2014), the observed trends are compared to trends and variability in TWS in the CESM1-CAM5 Large Ensemble (LE). Motivated in part by the anomalous nature of climate variability during the record, the characteristics of both forced change and internal modes of variability are quantified and their relative roles in influencing observed trends are estimated.
It is found that trends during the GRACE era in the LE are generally dominated by internal variability rather than by the forced response, with major TWS anomalies in much of the Americas, eastern Australia, Africa, and southwestern Eurasia attributable to climatic conditions associated with the negative phases the Pacific Decadal (PDO) and Atlantic Multi-decadal Oscillations (AMO).
While many similarities between observed trends and the model-inferred forced response also exist, it is nonetheless likely inappropriate to attribute such trends mainly to anthropogenic forcing. For several key river basins, trends in the mean state and interannual variability, along with the time of emergence of the forced response from background variability, are also estimated while aspects of global mean TWS, including changes in its annual amplitude and decadal trends, are explored. The findings highlight the challenges of detecting anthropogenically driven changes in temporally finite satellite datasets and underscores the benefits of utilizing models to aid in the interpretation of the observed record.