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Threshold

A 'threshold' refers to the point in a system (be it ecological, economic, or climatic) at which sudden or rapid change occurs and new properties emerge, potentially altering processes and relationships with other systems.

One example of a threshold in a system is the level of pollutant that a lake ecosystem can sustain before the fish die.

In detail

Exceeding ecosystem thresholds is often serious as many are irreversible on time-scales relevant to human society. Biodiversity loss through extinction, disruption of species' ecological interactions, and major changes in ecosystem structure and disturbance regimes (especially wildfire and insects) are all good examples of this.

Thresholds exist with regards to sea level rise. Sea level rise has substantial inertia (it takes time for sea levels to react to the various things that influence them). This means that sea levels will continue to rise 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.

Vulnerabilities of industry and society are often related to:

1. 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

2. Limited access to resources (including financial, human and institutional resources).

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 are associated with normative thresholds defined by stakeholders or decision-makers (such as 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.

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 UKCP09 User Interface allows thresholds associated with daily climate outputs from the UKCP09 Weather Generator to be investigated.

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