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. 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.