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

== Frequently Asked Questions ==

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=== FAQ 12.1 | What Is a Climatic Impact-driver (CID)? ===

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A climatic impact-driver is a physical climate condition that directly affects society or ecosystems. Climatic impact-drivers may represent a long-term average condition (such as the average winter temperatures that affect indoor heating requirements), a common event (such as a frost that kills off warm-season plants), or an extreme event (such as a coastal flood that destroys homes). A single climatic impact-driver may lead to detrimental effects for one part of society while benefiting another, while others are not affected at all. A climatic impact-driver (or its change caused by climate change) is therefore not universally hazardous or beneficial, but we refer to it as a ‘hazard’ when experts determine it is detrimental to a specific system.

Climate change can alter many aspects of the climate system, but efforts to identify impacts and risks usually focus on a smaller set of changes known to affect, or potentially affect, things that society cares about. These climatic impact-drivers (CIDs) are formally defined in this Report as ‘physical climate system conditions (e.g., means, events, extremes) that affect an element of society or ecosystems. Depending on system tolerance, CIDs and their changes can be detrimental, beneficial, neutral, or a mixture of each across interacting system elements and regions’. Because people, infrastructure and ecosystems interact directly with their immediate environment, climate experts assess CIDs locally and regionally. CIDs may relate to temperature, the water cycle, wind and storms, snow and ice, oceanic and coastal processes or the chemistry and energy balance of the climate system. Future impacts and risk may also be directly affected by factors unrelated to the climate (such as socio-economic development, population growth, or a viral outbreak) that may also alter the vulnerability or exposure of systems.

CIDs capture important characteristics of the average climate and both common and extreme events that shape society and nature (see FAQ 12.2). Some CIDs focus on aspects of the average climate (such as the seasonal progression of temperature and precipitation, average winds or the chemistry of the ocean) that determine, for example, species distribution, farming systems, the location of tourist resorts, the availability of water resources and the expected heating and cooling needs for buildings in an average year. CIDs also include common episodic events that are particularly important to systems, such as thaw events that can trigger the development of plants in spring, cold spells that are important for fruit crop chill requirements, or frost events that eliminate summer vegetation as winter sets in. Finally, CIDs include many extreme events connected to impacts such as hailstorms that damage vehicles, coastal floods that destroy shoreline property, tornadoes that damage infrastructure, droughts that increase competition for water resources, and heatwaves that can strain the health of outdoor labourers.

Many aspects of our daily lives, businesses and natural systems depend on weather and climate, and there is great interest in anticipating the impacts of climate change on the things we care about. To meet these needs, scientists engage with companies and authorities to provide climate services – meaningful and possibly actionable climate information designed to assist decision-making. Climate science and services can focus on CIDs that substantially disrupt systems to support broader risk management approaches. A single CID change can have dramatically different implications for different sectors or even elements of the same sector, so engagement between climate scientists and stakeholders is important to contextualize the climate changes that will come. Climate services responding to planning and optimization of an activity can focus on more gradual changes in operating climate conditions.

FAQ 12.1, Figure 1 tracks example outcomes of seasonal snow cover changes that connect climate science to the need for mitigation, adaptation and regional risk management. The length of the season with snow on the ground is just one of many regional climate conditions that may change in the future, and it becomes a CID because there are many elements of society and ecosystems that rely on an expected seasonality of snow cover. Climate scientists and climate service providers examining human-driven climate change may identify different regions where the length of the season with snow cover could increase, decrease, or stay relatively unaffected. In each region, change in seasonal snow cover may affect different systems in beneficial or detrimental ways (in the latter case, changing seasonal snow cover would be a ‘hazard’), although systems such as coastal aquaculture remain relatively unaffected. The changing profile of benefits and hazards connected to these changes in the seasonal snow cover CID affects the profile of impacts, risks and benefits that stakeholders in the region will grapple with in response to climate change.

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'''FAQ 12.1, Figure 1 |''' '''A single climatic impact-driver can affect ecosystems and society in different ways.''' A variety of impacts from the same climatic impact-driver change, illustrated with the example of regional seasonal snow cover.

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=== 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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=== FAQ 12.3 | How Will Climate Change Affect the Regional Characteristics of a Climate Hazard? ===

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Human-driven climate change can alter the regional characteristics of a climate hazard by changing the magnitude or intensity of the climate hazard, the frequency with which it occurs, the duration that hazardous conditions persist, the timing when the hazard occurs, or the spatial extent threatened by the hazard. By examining each of these aspects of a hazard’s profile change, climate services may provide climate risk information that allows decision makers to better tailor adaptation, mitigation and risk management strategies.

A climate hazard is a climate condition with the potential to harm natural systems or society. Examples include heatwaves, droughts, heavy snowfall events and sea level rise. Climate scientists look for patterns in climatic impact-drivers to detect the signature of changing hazards that may influence stakeholder planning (FAQ 12.1). Climate service providers work with stakeholders and impacts experts to identify key system responses and tolerance thresholds (FAQ 12.2) and then examine historical observations and future climate projections to identify associated changes to the characteristics of a regional hazard’s profile. Climate change can alter at least five different characteristics of the hazard profile of a region (FAQ 12.3, Figure 1):

Magnitude or intensity is the raw value of a climate hazard, such as an increase in the maximum yearly temperature or in the height of flooding that results from a coastal storm with a 1% change of occurring each year.

Frequency is the number of times that a climate hazard reaches or surpasses a threshold over a given period. For example, increases to the number of heavy snowfall events, tornadoes, or floods experienced in a year or in a decade.

Duration is the length of time over which hazardous conditions persist beyond a threshold, such as an increase in the number of consecutive days where maximum air temperature exceeds 35°C, the number of consecutive months of drought conditions, or the number of days that a tropical cyclone affects a location.

Timing captures the occurrence of a hazardous event in relation to the course of a day, season, year, or other period in which sectoral elements are evolving or co-dependent (such as the time of year when migrating animals expect to find a seasonal food supply). Examples include a shift towards an earlier day of the year when the last spring frost occurs or a delay in the typical arrival date for the first seasonal rains, the length of the winter period when the ground is typically covered by snow, or a reduction in the typical time needed for soil moisture to move from normal to drought conditions.

Spatial extent is the region in which a hazardous condition is expected, such as the area currently threatened by tropical cyclones, geographical areas where the coldest day of the year restricts a particular pest or pathogen, terrain where permafrost is present, the area that would flood following a common storm, zones where climate conditions are conducive to outdoor labour, or the size of a marine heatwave.

Hazard profile changes are often intertwined or stem from related physical changes to the climate system. For example, changes in the frequency and magnitude of extreme events are often directly related to each other as a result of atmospheric dynamics and chemical processes. In many cases, one aspect of hazard change is more apparent than others, which may provide a first emergent signal indicating a larger set of changes to come (FAQ 1.2).

Information about how a hazard has changed or will change helps stakeholders prioritize more robust adaptation, mitigation and risk management strategies. For example, allocation of limited disaster relief resources may be designed to recognize that tropical cyclones are projected to become more intense even as the frequency of those storms may not change. Planning may also factor in the fact that even heatwaves that are not record-breaking in their intensity can still be problematic for vulnerable populations when they persist over a long period. Likewise, firefighters recognize new logistical challenges in the lengthening of the fire weather season and an expansion of fire conditions into parts of the world where fires were not previously a great concern. Strong engagement between climate scientists and stakeholders therefore helps climate services tailor and communicate clear information about the types of changing climate hazards to be addressed in resilience efforts.

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'''FAQ 12.3, Figure 1 |''' '''Types of changes to a region’s hazard profile.''' The first five panels illustrate how climate changes can alter a hazard’s intensity (or magnitude), frequency, duration, and timing (by seasonality and speed of onset) in relation to a hazard threshold (horizontal grey line, marked ‘H’). The difference between the historical climate (blue) and future climate (red) shows the changing aspects of climate change that stakeholders will have to manage. The bottom right-hand panel shows how a given climate hazard (such as a current once-in-100-year river flood, geographic extent in blue) may reach new geographical areas under a future climate change (extended area in red).

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