of avian flu? What are the health impacts of an expanded ozone hole (Figure 2.1) that could result from a cooling of the stratosphere, which would be associated with climate change?
Extreme events, including severe storms, heat waves, earthquakes, and volcanic eruptions. Will tropical cyclones and heat waves become more frequent and more intense? Are major fault systems nearing the release of stress via strong earthquakes?
While past investments in Earth remote sensing have provided spectacular advances, such as in the accuracy of weather predictions, the above list of challenges highlights the class of new, interrelated questions being asked by the public and policy makers as they seek to understand new risks and the vulner-abilities of a rapidly evolving Earth system. Additional issues and questions will emerge from continued system-level study of Earth. The next 20 years must bring a new level of integration in the understanding of Earth-system components.
The Earth observation plan presented here distills into a core set of space-based missions many of the key measurements needed across the range of interrelated societal challenges and Earth science areas shown in Figure 2.2. Building on lessons learned from the development and use of the current fleet of space-based instruments, the committee’s recommended set of well-defined and justified missions will