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Because water isotope ratios respond to phase changes during evaporation (E) and precipitation (P), they are candidate fingerprints of changing atmospheric hydrology. Moreover, through preservation in ice cores and other paleoproxies, they provide important insight into the past. Still, there is disagreement over what specific attributes of hydroclimate variability isotopes reveal. Here we argue that variations in zonal mean isotope ratios of water vapor and precipitation are largely a response to geographically shifting patterns of E and P. Differences in the relative importance of local versus remote changes in these moisture variables explain the apparent distinct isotopic sensitivities to temperature and precipitation amount in high and low latitudes, respectively. Not only does our work provide a unified framework for interpreting water isotopic measurements globally, but it also presents a novel approach for diagnosing water cycle changes in a warmer world.

Plain Language Summary Observations that track changes in the water cycle are critical for improving our understanding of the climate system. Particularly important are measurements that can verify whether imbalances in evaporation and precipitation increase in response to global warming. Because the isotope ratios of hydrogen and oxygen in water vapor and precipitation vary with rates of evaporation and precipitation, they are candidate fingerprints of water cycle changes. Here we use a simple mass balance model to evaluate the isotopic response of water vapor in the atmosphere to spatial variations in evaporation and precipitation. We find that these spatial patterns shape the isotope ratios by changing two critical factors: the efficiency with which precipitation dries the atmosphere and the probability that moisture is transported downwind (rather than rained out). These two factors suggest that isotope ratios are influenced both by local and by remote changes in evaporation and precipitation. Low-latitude isotope ratios respond largely to local imbalances in evaporation and precipitation, while high-latitude isotope ratios depend more on what happens "upstream." These findings provide key guidance for interpreting climate changes of the past and offer a novel approach for diagnosing water cycle changes in a warmer future.