and thresholds within polar regions to ultimately develop strategies to minimize and/or adapt to the impact of climate change on ecosystem services and processes.

A tipping point describes a critical threshold reached in a nonlinear system, where a small perturbation to the system can cause a shift from one stable state to another (see Box 1.1). The global climate system is a nonlinear system and there are several possible tipping points that could potentially be reached this century as a result of human-induced activities. These have been referred to as “policy-relevant” tipping points (Lenton et al., 2008; see Figure 2.1 for examples). Abrupt climate change can be considered a sub-type of tipping point, where a climate system response is faster than the cause itself (NRC, 2002). Lenton et al. (2008) describes “tipping elements” as large-scale components of the Earth system (at least subcontinental in scale) that may pass a tipping point. The transition of the tipping element in response to forcing can be faster, slower, or no different in rate than the cause, and can be either reversible or irreversible. Although variable in nature, the inherent common property of these tipping elements is that they exhibit “threshold-type behavior in response to anthropogenic climate forcing, where a small perturbation at a critical point qualitatively alters the future fate of the system” (Lenton et al., 2008). A large proportion of defined tipping elements have direct relevance to polar regions, not only because these areas are warming more rapidly than any other place on Earth, but also because these tipping elements typically involve amplifying ice-albedo and greenhouse gas feedbacks that are specific to high-latitude regions.

Declining seasonal sea ice and the disappearance of the Arctic perennial sea ice pack, as well as the shrinking Greenland and West Antarctic ice sheets are processes of particular concern to the workshop participants because of their inevitability and/or severity of impacts and the potential for tipping points to be reached. Additional processes with potential tipping points of concern include dieback of the boreal forest, a northward shifting treeline into tundra regions, CO2 and CH4 release from carbon-rich permafrost soils, and release of marine methane hydrates from subsea permafrost. Recent work has been put forth advancing the ability to anticipate and forecast an approaching tipping point in the Earth’s climate system, where an initial slowing down in response to a perturbation is commonly experienced (e.g., Dakos et al., 2008). Advances in modeling and forecasting an approaching tipping element may enable us to further understand whether these critical thresholds and their repercussions can be avoided (i.e., mitigation) and/or whether they can be tolerated (i.e., adaptation).

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