Illuminating water cycle modifications and Earth system resilience in the Anthropocene

Fresh water—the bloodstream of the biosphere—is at the center of the planetary drama of the
Anthropocene. Water fluxes and stores regulate the Earth's climate and are essential for thriving aquatic
and terrestrial ecosystems, as well as water, food, and energy security. But the water cycle is also being
modified by humans at an unprecedented scale and rate. A holistic understanding of freshwater's role for
Earth system resilience and the detection and monitoring of anthropogenic water cycle modifications across
scales is urgent, yet existing methods and frameworks are not well suited for this. In this paper we
highlight four core Earth system functions of water (hydroclimatic regulation, hydroecological regulation,
storage, and transport) and key related processes. Building on systems and resilience theory, we review the
evidence of regional‐scale regime shifts and disruptions of the Earth system functions of water. We then
propose a framework for detecting, monitoring, and establishing safe limits to water cycle modifications and
identify four possible spatially explicit methods for their quantification. In sum, this paper presents an
ambitious scientific and policy grand challenge that could substantially improve our understanding of the
role of water in the Earth system and cross‐scale management of water cycle modifications that would
be a complementary approach to existing water management tools.