Threshold
The level of magnitude in an ecological, economic or other system at which process at which sudden or rapid change occurs and new properties emerge, altering relationships or predictions that apply at lower levels.
In detail
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 1.1–3.8°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, 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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The ability to explore potential changes in the frequency in particular thresholds being exceeded is an important aspect of the UKCP09 probabilistic projections. In addition, the Threshold Detector allows thresholds associated with daily climate outputs from the UKCP09 Weather Generator to be investigated.
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