The Budyko framework consists of a curvilinear relationship between the evaporative ratio (i.e., actual evaporation over precipitation) and the aridity index (i.e., potential evaporation over ...precipitation) and defines evaporation's water and energy limits. A basin's movement within the Budyko space illustrates its hydroclimatic change and helps identify the main drivers of change. On the one hand, long‐term aridity changes drive evaporative ratio changes, moving basins along their Budyko curves. On the other hand, historical human development can cause river basins to deviate from their curves. The question is if basins will deviate or follow their Budyko curves under the future effects of global warming and related human developments. To answer this, we quantify the movement in the Budyko space of 405 river basins from 1901–1950 to 2051–2100 based on the outputs of seven models from the Coupled Model Intercomparison Project ‐ Phase 6 (CMIP6). We account for the implications of using different potential evaporation models and study low‐ and high‐emissions scenarios. We find considerable differences of movement in Budyko space regarding direction and intensity when using the two estimates of potential evaporation. However, regardless of the potential evaporation estimate and the scenario used, most river basins will not follow their reference Budyko curves (>72%). Furthermore, the number of basins not following their curves increases under high greenhouse gas emissions and fossil‐fueled development SP585 and across dry and wet basin groups. We elaborate on the possible explanations for a large number of basins not following their Budyko curves.
Plain Language Summary
The Budyko framework relates aridity in the Earth's surface with the partitioning of rain into evaporation and runoff. The framework finds that all basins on earth fall on a curvilinear relationship between these two variables. It is argued that river basins will move along this curve if aridity is the only variable driving these changes in these partitioning. However, historical human development may cause river basins to deviate from these curves. The question is if basins will deviate or follow their Budyko curves under future global warming. To answer this, we quantify the movement in the Budyko space in 405 river basins from 1901 to 2100 based on the outputs of seven models from the Coupled Model Intercomparison Project ‐ Phase 6 (CMIP6). We account for the implications of using different potential evaporation models and study low‐ and high‐emissions scenarios. We find that regardless of the potential evaporation estimate and the scenario used most river basins will not follow their curves. Furthermore, the number of basins that do not follow their curves increases under the high greenhouse gas emissions and fossil‐fueled development scenario; SP585. We elaborate on the possible explanations for this large number of basins not following their Budyko curves.
Key Points
We quantify CMIP6‐simulated movements in Budyko space of 405 river basins from 1901 to 2100
Most river basins will not follow their original Budyko‐curve trajectories
The number of basins that do not follow their Budyko trajectories increases with the effects of global warming
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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.
Plain language summary
Freshwater is crucially important for all life on Earth. There is abundant research and evidence on how different processes within the water cycle regulate climate and support ecosystems, and by extension, human societies. Humans are also a major force disturbing those processes and modifying the water cycle. These modifications include, for instance, surface water withdrawals, groundwater pumping, deforestation and other land cover change, and ice melt due to warming climate. As most previous research on human–water interactions focuses on understanding systems at smaller scales, such as a watershed or a nation, comprehensive understanding of what human modifications of the water cycle mean for the stability of the planet is still lacking. In this paper we propose a new framework for analysing and establishing limits to a variety of human modifications of the water cycle, to ensure that the stability of the Earth would not be compromised. We see this as an important and urgent scientific challenge that has the potential to substantially improve our understanding of the functioning of the Earth system and to inform local and global policy toward a more sustainable future.
Key Points
Earth system resilience depends on an improved understanding and management of water cycle modifications
We identify four key functions of freshwater in the Earth system and evidence of regional to global regime shifts and disruptions
The water planetary boundary is a compelling framework to improve our understanding and management of water cycle modifications in the Earth system
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Forestation efforts are accelerating across the globe in the fight against global climate change, in order to restore biodiversity, and to improve local livelihoods. Yet, so far the non‐local effects ...of forestation on rainfall have largely remained a blind spot. Here we build upon emerging work to propose that targeted rainfall enhancement may also be considered in the prioritization of forestation. We show that the tools to achieve this are rapidly becoming available, but we also identify drawbacks and discuss which further developments are still needed to realize robust assessments of the rainfall effects of forestation in the face of climate change. Forestation programs may then mitigate not only global climate change itself but also its adverse effects in the form of drying.
Forestation efforts are accelerating across the globe to mitigate climate change, but its effects on regional rainfall are often overlooked. This article proposes the concept of “targeted rainfall enhancement”, which may be included in decision making on forestation priorities. Forestation programs may then mitigate not only global climate change itself but also its adverse effects in the form of drying.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Reduced rainfall increases the risk of forest dieback, while in return forest loss might intensify regional droughts. The consequences of this vegetation-atmosphere feedback for the stability of the ...Amazon forest are still unclear. Here we show that the risk of self-amplified Amazon forest loss increases nonlinearly with dry-season intensification. We apply a novel complex-network approach, in which Amazon forest patches are linked by observation-based atmospheric water fluxes. Our results suggest that the risk of self-amplified forest loss is reduced with increasing heterogeneity in the response of forest patches to reduced rainfall. Under dry-season Amazonian rainfall reductions, comparable to Last Glacial Maximum conditions, additional forest loss due to self-amplified effects occurs in 10-13% of the Amazon basin. Although our findings do not indicate that the projected rainfall changes for the end of the twenty-first century will lead to complete Amazon dieback, they suggest that frequent extreme drought events have the potential to destabilize large parts of the Amazon forest.
The planetary boundaries framework defines the “safe operating space for humanity” represented by nine global processes that can destabilize the Earth System if perturbed. The water planetary ...boundary attempts to provide a global limit to anthropogenic water cycle modifications, but it has been challenging to translate and apply it to the regional and local scales at which water problems and management typically occur. We develop a cross‐scale approach by which the water planetary boundary could guide sustainable water management and governance at subglobal contexts defined by physical features (e.g., watershed or aquifer), political borders (e.g., city, nation, or group of nations), or commercial entities (e.g., corporation, trade group, or financial institution). The application of the water planetary boundary at these subglobal contexts occurs via two approaches: (i) calculating fair shares, in which local water cycle modifications are compared to that context's allocation of the global safe operating space, taking into account biophysical, socioeconomic, and ethical considerations; and (ii) defining a local safe operating space, in which interactions between water stores and Earth System components are used to define local boundaries required for sustaining the local water system in stable conditions, which we demonstrate with a case study of the Cienaga Grande de Santa Marta wetlands in Colombia. By harmonizing these two approaches, the water planetary boundary can ensure that water cycle modifications remain within both local and global boundaries and complement existing water management and governance approaches.
Key Points
Local and regional water management responds to and is influenced by global water cycle processes
The planetary boundaries framework is useful for reconciling local and global water sustainability goals
The framework can be applied at scales including nations, watersheds, aquifers, or commercial entities
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
An ecosystem service is a benefit derived by humanity that can be traced back to an ecological process. Although ecosystem services related to surface water have been thoroughly described, the ...relationship between atmospheric water and ecosystem services has been mostly neglected, and perhaps misunderstood. Recent advances in land-atmosphere modeling have revealed the importance of terrestrial ecosystems for moisture recycling. In this paper, we analyze the extent to which vegetation sustains the supply of atmospheric moisture and precipitation for downwind beneficiaries, globally. We simulate land-surface evaporation with a global hydrology model and track changes to moisture recycling using an atmospheric moisture budget model, and we define vegetation-regulated moisture recycling as the difference in moisture recycling between current vegetation and a hypothetical desert world. Our results show that nearly a fifth of annual average precipitation falling on land is from vegetation-regulated moisture recycling, but the global variability is large, with many places receiving nearly half their precipitation from this ecosystem service. The largest potential impacts for changes to this ecosystem service are land-use changes across temperate regions in North America and Russia. Likewise, in semi-arid regions reliant on rainfed agricultural production, land-use change that even modestly reduces evaporation and subsequent precipitation, could significantly affect human well-being. We also present a regional case study in the Mato Grosso region of Brazil, where we identify the specific moisture recycling ecosystem services associated with the vegetation in Mato Grosso. We find that Mato Grosso vegetation regulates some internal precipitation, with a diffuse region of benefit downwind, primarily to the south and east, including the La Plata River basin and the megacities of Sao Paulo and Rio de Janeiro. We synthesize our global and regional results into a generalized framework for describing moisture recycling as an ecosystem service. We conclude that future work ought to disentangle whether and how this vegetation-regulated moisture recycling interacts with other ecosystem services, so that trade-offs can be assessed in a comprehensive and sustainable manner.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Water is indispensable for Earth resilience and sustainable development. The capacity of social-ecological systems to deal with shocks, adapting to changing conditions and transforming in situations ...of crisis are fundamentally dependent on the functions of water to e.g., regulate the Earth's climate, support biomass production, and supply water resources for human societies. However, massive, inter-connected, human interference involving climate forcing, water withdrawal, dam constructions, and land-use change have significantly disturbed these water functions and induced regime shifts in social-ecological systems. In many cases, changes in core water functions have pushed systems beyond tipping points and led to fundamental shifts in system feedback. Examples of such transgressions, where water has played a critical role, are collapse of aquatic systems beyond water quality and quantity thresholds, desertification due to soil and ecosystem degradation, and tropical forest dieback associated with self-amplifying moisture and carbon feedbacks. Here, we aggregate the volumes and flows of water involved in water functions globally, and review the evidence of freshwater related linear collapse and non-linear tipping points in ecological and social systems through the lens of resilience theory. Based on the literature review, we synthesize the role of water in mediating different types of ecosystem regime shifts, and generalize the process by which life support systems are at risk of collapsing due to loss of water functions. We conclude that water plays a fundamental role in providing social-ecological resilience, and suggest that further research is needed to understand how the erosion of water resilience at local and regional scale may potentially interact, cascade, or amplify through the complex, globally hyper-connected networks of the Anthropocene.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Despite their importance, wetland ecosystems protected by the Ramsar Convention are under pressure from climate change and human activities. These drivers are altering water availability in these ...wetlands, changing water levels or surface water extent, in some cases, beyond historical variability. Attribution of the effects of human and climate activities is usually focused on changes within the wetlands or their upstream surface and groundwater inputs. However, the reliance of wetland water availability on upwind atmospheric moisture supply is less understood. Here, we assess the vulnerability of 40 Ramsar wetlands to precipitation changes caused by land use and hydroclimatic change occurring in their upwind moisture‐supplying regions. We use moisture flows from a Lagrangian tracking model, atmospheric reanalysis data, and historical land use change (LUC) data to assess and quantify these changes. Our analyses show that historical LUC has decreased precipitation and terrestrial moisture recycling in most wetland hydrological basins, decreasing surface water availability (precipitation minus evaporation). The most substantial effects on wetland water availability occurred in the tropic subtropical regions of Central Europe and Asia. Overall, we found wetlands in Central Asia and South America to be the most vulnerable by a combination of LUC‐driven effects on runoff, high terrestrial precipitation recycling, and recent decreases in surface water availability. This study stresses the need to incorporate upwind effects of land use changes in the restoration, management, and conservation of the world's wetlands.
Plain Language Summary
Wetlands protected by the Ramsar Convention face threats from climate change and human activities, impacting their water availability and altering wetland functions. While past studies often focused on threats from their immediate surroundings, our research looks into the influence of changes in their upwind atmospheric moisture supply. We evaluate the vulnerability of 40 Ramsar wetland basins to precipitation shifts caused by land use and hydroclimatic changes in upwind regions, using the output of an atmospheric moisture tracking model and historical data. The results indicate that historical land use changes have reduced precipitation and moisture recycling, leading to a decrease in water availability in some wetlands, notably affecting tropical and subtropical regions of Central Europe and Asia. We assess that wetlands in Asia and South America face are vulnerable to a combination of land use‐induced runoff impacts, high precipitation recycling, and declining surface water availability. This study highlights the need to incorporate upwind effects of land use changes in wetland restoration, management, and conservation efforts globally, recognizing their crucial role for effective strategies for wetland protection amidst evolving environmental conditions.
Key Points
Land use changes have led to mean annual runoff (P–E) decreases in wetland hydrological basins globally
The most substantial land use‐related P–E changes occurred in tropical and subtropical wetlands across Asia and South America and in Europe
We identify eight wetlands in Asia, South America, and Australia as particularly vulnerable to changes in upwind moisture sources
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Abstract
Tropical forests modify the conditions they depend on through feedbacks at different spatial scales. These feedbacks shape the hysteresis (history-dependence) of tropical forests, thus ...controlling their resilience to deforestation and response to climate change. Here, we determine the emergent hysteresis from local-scale tipping points and regional-scale forest-rainfall feedbacks across the tropics under the recent climate and a severe climate-change scenario. By integrating remote sensing, a global hydrological model, and detailed atmospheric moisture tracking simulations, we find that forest-rainfall feedback expands the geographic range of possible forest distributions, especially in the Amazon. The Amazon forest could partially recover from complete deforestation, but may lose that resilience later this century. The Congo forest currently lacks resilience, but is predicted to gain it under climate change, whereas forests in Australasia are resilient under both current and future climates. Our results show how tropical forests shape their own distributions and create the climatic conditions that enable them.
This planetary boundaries framework update finds that six of the nine boundaries are transgressed, suggesting that Earth is now well outside of the safe operating space for humanity. Ocean ...acidification is close to being breached, while aerosol loading regionally exceeds the boundary. Stratospheric ozone levels have slightly recovered. The transgression level has increased for all boundaries earlier identified as overstepped. As primary production drives Earth system biosphere functions, human appropriation of net primary production is proposed as a control variable for functional biosphere integrity. This boundary is also transgressed. Earth system modeling of different levels of the transgression of the climate and land system change boundaries illustrates that these anthropogenic impacts on Earth system must be considered in a systemic context.
Transgression of planetary boundaries by human activities have now brought humanity well beyond a “safe operating space.”
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