Defining ecological drought for the 21st century

Droughts of the 21st century are characterized by hotter temperatures, longer duration and greater spatial extent, and are increasingly exacerbated by human demands for water. This situation increases the vulnerability of ecosystems to drought, including a rise in drought - driven tree mortality globally and anticipated ecosystem transformations from one state to another, e.g., forest to a shrubland . When a drought drives changes within ecosystems, there can be a ripple effect through human communities that depend on those ecosystems for critical goods and services. We define the term ecological drought as an episodic deficit in water availability that drives ecosystems beyond thresholds of vulnerability, impacts ecosystem services, and triggers feedbacks in natural and /or human systems. We support this definition with a novel, integrated framework for ecological drought that is organized along two dimensions — the components of vulnerability (exposure + sensitivity/adaptive capacity) and a continuum from human to natural factors.