Challenges in Quantifying Changes in the Global Water Cycle

TLDR

Understanding observed changes to the global water cycle is essential for predicting future climate impacts, yet datasets—though covering key variables such as precipitation, salinity, runoff, and humidity—are uncertain, sparse in many regions, and lack long‑term stability. The study aims to assess the robustness of observed water‑cycle changes by comparing related variables and to highlight the need for improved in‑situ and satellite observing networks, which face funding threats. The authors use ocean salinity, interpreted through ocean processes, to cross‑validate precipitation changes. Evidence of human influence on the water cycle is emerging, yet uncertainties from internal variability, observational errors, and aerosol effects limit confidence in attributing observed changes.

Abstract

Abstract Understanding observed changes to the global water cycle is key to predicting future climate changes and their impacts. While many datasets document crucial variables such as precipitation, ocean salinity, runoff, and humidity, most are uncertain for determining long-term changes. In situ networks provide long time series over land, but are sparse in many regions, particularly the tropics. Satellite and reanalysis datasets provide global coverage, but their long-term stability is lacking. However, comparisons of changes among related variables can give insights into the robustness of observed changes. For example, ocean salinity, interpreted with an understanding of ocean processes, can help cross-validate precipitation. Observational evidence for human influences on the water cycle is emerging, but uncertainties resulting from internal variability and observational errors are too large to determine whether the observed and simulated changes are consistent. Improvements to the in situ and satellite observing networks that monitor the changing water cycle are required, yet continued data coverage is threatened by funding reductions. Uncertainty both in the role of anthropogenic aerosols and because of the large climate variability presently limits confidence in attribution of observed changes.