The importance of thresholds and tipping points

Many impact analyses carried out in recent years have treated potential climate impacts as though they would occur more or less as slowly changing means in the delivery of ecosystem services or natural resources. The MEA (2005) points out the limitations of this assumption, which are evident in the many well-documented instances of ecosystems and natural resources changing rapidly, in a non-linear fashion, or in an irreversible way — i.e., behaving in a manner indicating that there are thresholds in species dynamics, beyond which very different conditions will be exhibited.

We are now beginning to see examples of threshold phenomena in ecosystem responses that are clearly related to climate variability and change. Coral bleaching has already been noted, as have the increases in fire frequency in western North America. But perhaps the most visible example at the present time is the interaction of drought stress, warm winters and pests in lodgepole pine forests of the western and northwestern forests of North America. The mountain pine beetle is the major pest, and, in part, because large areas of lodgepole pine are in drought stress and therefore weakened physiologically, and, in part, because winters have been too warm to kill off over-wintering larvae, infestations of this naturally occurring pest are more than an order of magnitude larger than ever before documented. Projections are for more than 90 per cent mortality of lodgepole pine in British Columbia throughout most of its range, which would make this infestation both an ecological and economic catastrophe for that region.

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