Editing IPCC:AR6/WGI/Chapter-12 (section)

=== FAQ 12.2 | What Are Climatic Thresholds and Why Are They Important? ===

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Climatic thresholds tell us about the tolerance of society and ecosystems so that we can better scrutinize the types of climate changes that are expected to impact things we care about. Many systems have natural or structural thresholds. If conditions exceed those thresholds, the result can be sudden changes or even collapses in health, productivity, utility or behaviour. Adaptation and risk management efforts can change these thresholds, altering the profile of climate conditions that would be problematic and increasing overall system resilience.

Decision makers have long observed that certain weather and climate conditions can be problematic, or hazardous, for things they care about (i.e., things with socio-economic, cultural or intrinsic value). Many elements of society and ecosystems operate in a suitable climate zone selected naturally or by stakeholders considering the expected climate conditions. However, as climate change moves conditions beyond expected ranges, they may cross a climatic ‘threshold’ – a level beyond which there are either gradual changes in system behaviour or abrupt, non-linear and potentially irreversible impacts.

Climatic thresholds can be associated with either natural or structural tolerance levels. Natural thresholds, for instance, include heat and humidity conditions above which humans cannot regulate their internal temperatures through sweat, drought durations that heighten competition between species, and winter temperatures that are lethal for pests or disease-carrying vector species. Structural thresholds include engineered limits of drainage systems, extreme wind speeds that limit wind turbine operation, the height of coastal protection infrastructure, and the locations of irrigation infrastructure or tropical cyclone sheltering facilities.

Thresholds may be defined according to raw values (such as maximum temperature exceeding 35°C) or percentiles (such as the local 99th percentile daily rainfall total). They also often have strong seasonal dependence (see FAQ 12.3). For example, the amount of snowfall that a deciduous tree can withstand depends on whether the snowfall occurs before or after the tree sheds its leaves. Most systems respond to changes in complex ways, and those responses are not determined solely or precisely by specific thresholds of a single climate variable. Nonetheless, thresholds can be useful indicators of system behaviours, and an understanding of these thresholds can help inform risk management decisions.

FAQ 12.2 Figure 1 illustrates how threshold conditions can help us understand climate conditions that are suitable for normal system operation and the thresholds beyond which impacts occur. Crops tend to grow most optimally within a suitable range of daily temperatures that is influenced by the varieties being cultivated and the way the farm is managed. As daily temperatures rise above a ‘critical’ temperature threshold, plants begin to experience heat stress that reduces growth and may lower resulting yields. If temperatures reach a higher ‘limiting’ temperature threshold, crops may suffer leaf loss, pollen sterility, or tissue damage that can lead to crop failure. Farmers typically select a cropping system with some consideration to the probability of extreme temperature events that may occur within a typical season, and so identifying hot temperature thresholds helps farmers select their seed and field management strategies as part of their overall risk management. Climate experts may therefore aim to assist farm planning by providing information about the climate change-induced shifts to the expected frequency of daily heat extremes that exceed crop tolerance thresholds.

Adaptation and other changes in societies and environment can shift climatic thresholds by modifying vulnerability and exposure. For example, adaptation efforts may include breeding new crops with higher heat tolerance levels so that corresponding dangerous thresholds occur less frequently. Likewise, increasing the height of a flood embankment protecting a given community can increase the level of river flow that may be tolerated without flooding, reducing the frequency of damaging floods. Stakeholders therefore benefit from climate services that are based on a co-development process, with scientists identifying system-relevant thresholds and developing tailored climatic impact-driver indices that represent these thresholds (FAQ 12.1). These thresholds help focus the provision of action-relevant climate information for adaptation and risk management.

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'''FAQ 12.2, Figure''' '''1 |''' '''Crop response to maximum temperature thresholds.''' Crop growth rate responds to daily maximum temperature increases, leading to reduced growth and crop failure as temperatures exceed critical and limiting temperature thresholds, respectively. Note that changes in other environmental factors (such as carbon dioxide and water) may increase the tolerance of plants to increasing temperatures.

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