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Threshold
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| The level of magnitude of a system process at which sudden or rapid change occurs. A point or level at which new properties emerge in an ecological, economic or other system, invalidating predictions based on mathematical relationships that apply at lower levels.
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In detail
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Exceedance of ecosystem resilience is very likely to be characterised by threshold-type responses, many irreversible on time-scales relevant to human society, such as biodiversity loss through extinction, disruption of species’ ecological interactions, and major changes in ecosystem structure and disturbance regimes (especially wildfire and insects).
Sea-level rise has substantial inertia and will continue beyond 2100 for many centuries. Breakdown of the West Antarctic and/or Greenland ice sheets would make this long-term rise significantly larger. For Greenland, the temperature threshold for breakdown is estimated to be about 1.1 to 3.8°C above today’s global average temperature. This is likely to happen by 2100 under the medium emissions scenario.
Key vulnerabilities of industry, settlements and society are most often related to (i) climate phenomena that exceed thresholds for adaptation, related to the rate and magnitude of climate change, particularly extreme weather events and/or abrupt climate change, and (ii) limited access to resources (financial, human, institutional) to cope, rooted in issues of development context.
Key vulnerabilities may be linked to systemic thresholds where non-linear processes cause a system to shift from one major state to another (such as a hypothetical sudden change in the Asian monsoon or disintegration of the West Antarctic ice sheet or positive feedbacks from ecosystems switching from a sink to a source of CO2). Other key vulnerabilities can be associated with normative thresholds defined by stakeholders or decision-makers (e.g. a magnitude of sea-level rise no longer considered acceptable by low-lying coastal dwellers).
Climate threshold refers to the point at which external forcing of the climate system, such as the increasing atmospheric concentration of greenhouse gases, triggers a significant climatic or environmental event which is considered unalterable, or recoverable only on very long time-scales, such as widespread bleaching of corals or a collapse of oceanic circulation systems.
With very high confidence, no temperature threshold associated with any subjective judgment of what might constitute dangerous climate change can be guaranteed to be avoided by anything but the most stringent of mitigation interventions.
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