Thresholds of aridity
Increasing aridity due to climate change is expected to affect multiple ecosystem structural and functional attributes in global drylands, which cover ∼45% of the terrestrial globe. Berdugo et al. show that increasing aridity promotes thresholds on the structure and functioning of drylands (see the Perspective by Hirota and Oliveira). Their database includes 20 variables summarizing multiple aspects and levels of ecological organization. They found evidence for a series of abrupt ecological events occurring sequentially in three phases, culminating with a shift to low-cover ecosystems that are nutrient- and species-poor at high aridity values. They estimate that more than 20% of land surface will cross at least one of the thresholds by 2100, which can potentially lead to widespread land degradation and desertification worldwide.

Aridity, which is increasing worldwide because of climate change, affects the structure and functioning of dryland ecosystems. Whether aridification leads to gradual (versus abrupt) and systemic (versus specific) ecosystem changes is largely unknown. We investigated how 20 structural and functional ecosystem attributes respond to aridity in global drylands. Aridification led to systemic and abrupt changes in multiple ecosystem attributes. These changes occurred sequentially in three phases characterized by abrupt decays in plant productivity, soil fertility, and plant cover and richness at aridity values of 0.54, 0.7, and 0.8, respectively. More than 20% of the terrestrial surface will cross one or several of these thresholds by 2100, which calls for immediate actions to minimize the negative impacts of aridification on essential ecosystem services for the more than 2 billion people living in drylands.

Our results, which are based on analyzing the most comprehensive empirical evidence available so far, show that the responses of multiple functional and structural ecosystem attributes to increases in aridity follow a series of sequential thresholds. Our work goes beyond current knowledge by identifying, for the first time, three phases of abrupt ecosystem changes characterized by consecutive aridity thresholds. Along with recent studies dealing with multiscale regime shifts (12), our study provides a well-defined framework for sequential shifts that can inspire a new generation of multiscale models to explore ecosystem responses to climate change. Our findings also set the stage for future studies exploring temporal changes in the ecosystem variables investigated, particularly in areas likely to cross the aridity thresholds identified in the future, and put the focus on identifying potential catastrophic shifts and early warning indicators for them. Finally, the framework introduced here can be used to identify those attributes for which the responses to aridity are more sensitive to buffering, and for establishing effective adaptation and mitigation actions aimed at preserving the capacity of drylands to supply essential ecosystem services needed to sustain a growing human population