can have different threshold and tipping point characteristics. Understanding these thresholds and tipping points, and the mechanisms controlling them, is among the most important challenges in Earth system science (NRC, 2007).

There is a great deal of complexity in Earth system science. The principal components of the Earth system may be defined and bounded differently, depending on the object of study (e.g., the climate system, biogeochemical cycles, ecosystems, and local to global-scale economies). Some Earth system components are defined more clearly than others; for example, ocean and atmospheric circulation is a relatively well-known system, whereas the climate system is a less-well-understood example. Additionally, system components interact according to rules that may or may not be able to be defined adequately. A principal property of systems is feedback, in which reciprocal interaction of components may be self-limiting (negative feedback) or reinforcing (positive feedback).

A principal tool for studying systems in general and the Earth system in particular is numerical simulation modeling. Models may focus on any particular subcomponent, for example, a polar coastal system including subsistence-based human communities, the Northern or Southern Annular Modes, and the Greenland or West Antarctic Ice Sheets. At higher levels of organization, a reduced-complexity model might include simplified parameterizations of each of these subcomponents in a model of the “full” Polar System. There are many different approaches to simulation modeling involving different strategies for defining parameters and interactions, but in general they all follow the systems concept, concentrating on defined systems of interacting components.

PLENARY PRESENTATIONS: INSIGHTS IN POLAR ECOSYSTEM SCIENCE

The following sections summarize plenary presentations from the workshop; these presentations were designed to set the stage for what is already known about climate change and polar ecosystems (see Appendix A for the agenda and Appendix B for plenary speakers and abstracts). Illustrative examples from both the Arctic and Antarctic terrestrial and marine ecosystems highlight climate change impacts currently observed in these regions. This is not intended to be an exhaustive list of impacts in the polar regions, but it is representative of the issues and climate-related changes discussed by workshop participants and speakers.

During the opening presentation of the workshop, Dr. Jeffrey Severinghaus addressed some of the differences between Arctic and Antarctic ecosystems based on current evidence of polar climate changes and atmospheric composition from ice core records. These records reveal that



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