Editing IPCC:AR6/WGII/Chapter-16 (section)

=== 16.2.1 Definitions ===

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The section adopts the general definition of '''detection''' as ‘demonstration that a considered system has changed without providing reasons for the change’ and '''attribution''' as ‘identifying the causes of the observed long-term change in an impact indicator or of the change in the temporal or spatial extent, the intensity or frequency of a specific event’ (see Glossary (Annex II)).

Based on these general definitions and following the approach applied in WGII AR5 [[IPCC:Wg2:Chapter:Chapter-18|Chapter 18]] ( [[#Cramer--2014|Cramer et al., 2014]] ), we define an '''observed impact''' as the difference between the observed state of a '''natural, human or managed system''' and a counterfactual baseline that characterises the system’s state in the absence of changes in the '''climate-related systems''' , defined here as climate system including the ocean and the cryosphere as physical or chemical systems.

The difference between the observed and the counterfactual baseline state is considered the change in the natural, human or managed system that is attributed to the changes in the climate-related systems ( '''impact attribution''' ). The counterfactual baseline may be stationary or may change over time, for example due to direct human influences such as changes in land use patterns and agricultural or water management affecting exposure and vulnerability to climate-related hazards (see [[#16.2.3|Section 16.2.3]] for methods on how to construct the counterfactual baseline).

In line with the AR5 definition, ‘changes in climate-related systems’ here refer to any long-term trend, irrespective of the underlying causes; thus, an observed impact is not necessarily an observed impact of anthropogenic climate forcing. For example, in this section, sea level rise is defined as relative sea level rise measured against a land-based reference frame (tide gauge measurements), meaning that it is driven not only by thermal expansion and loss of land ice influenced by anthropogenic climate forcing, but also by vertical land movements. As attribution of coastal damages to sea level rise does not distinguish between these components, it does not imply attribution to anthropogenic forcing. Where the literature does allow attribution of changes in natural, human or managed systems to anthropogenic climate forcing (‘joint attribution’, [[#Rosenzweig--2007|Rosenzweig et al., 2007]] ), this is highlighted in the assessment. Often the attribution of changes in the natural, human or managed systems to anthropogenic forcing can be done in a two-step approach where (i) an observed change in a climate-related system is attributed to anthropogenic climate forcing (‘climate attribution’) and (ii) changes in natural, human or managed systems are attributed to this change in the climate-related system (‘impact attribution’).

For climate attribution, the main challenge is the separation of externally human forced changes in the climate-related systems from their internal variability, while for impact attribution it often is the separation of the effects of other external forcings (i.e., direct human influences or natural disturbances) from the impacts of the changes in the climate-related systems. Direct influences not related to changes in the climate-related systems could, for example, be pollution and land use changes amplifying biodiversity losses, intensification of fishing reducing fish stocks, and increasing protection reducing losses due to river floods. The direct human or natural influences may counter the impacts of climate change (e.g., climate change may have reduced flood hazards, but exposure may have increased as people have moved to flood-prone areas, resulting in no change in observed damages). Given the definition of impact attribution, this means that there may be an observed impact of climate change without the detection of a change in the natural, human or managed system. This is different from ‘climate attribution’, where detection and attribution usually are consecutive steps.

Changes in climate-related systems can certainly also affect natural, human and managed systems through indirect effects on land use, pollution or exposure. However, these indirect effects are barely addressed in existing studies.

In addition to impact attribution, there is research on the identification of natural, human or managed systems’ response to short-term (typically daily, monthly or annual) weather fluctuations or individual ''extreme weather events'' . As different from impact attribution, we separately define:

'''‘Identification of weather sensitivity’''' refers to the attribution of the response of a system to fluctuations in weather and short-term changes in the climate-related systems including individual ''extreme weather events'' (e.g., a heatwave or storm surge).

Typical questions addressed include: ‘How much of the observed variability of crop yields is due to variations in weather conditions compared to contributions from management changes?’ (e.g., [[#Ray--2015|Ray et al., 2015]] ; [[#Müller--2017|Müller et al., 2017]] ) and ‘Can weather fluctuations explain part of the observed variability in annual national economic growth rates?’ (e.g., [[#Burke--2015|Burke et al., 2015]] ). Identification of weather sensitivity may also address the effects of individual ''climate extremes'' , for example asking, ‘Was the observed outbreak of cholera triggered by an associated flood event?’ (e.g., [[#Rinaldo--2012|Rinaldo et al., 2012]] ; [[#Moore--2017b|Moore et al., 2017b]] ). It is important to note that sensitivity could be described in diverse ways and that, for example, the fraction of the observed variability in a system explained by weather variability differs from the strength of the systems’ response to a specific change in a weather variable. Nevertheless, all these different measures are integrated in the ‘identification of weather sensitivity’ assessment, where ‘sensitivity’ should not be considered a quantitative one-dimensional mathematical measure.

In this chapter, we explicitly distinguish between assessment statements related to ‘climate attribution’ (listed in Table SM16.21), ‘impact attribution’ (listed in Table SM16.22) and ‘identification of weather sensitivity’ (listed in Table SM16.23). The identification of ‘weather sensitivity’ does not necessarily imply that there also is an impact of long-term changes in the climate-related systems on the considered system. However, if the probability or intensity of an ''extreme weather event'' has increased due to anthropogenic forcing (‘climate attribution’) ( [[#NASEM--2016|NASEM, 2016]] ; WGI AR6 [[IPCC:Wg2:Chapter:Chapter-11|Chapter 11]] [[#Seneviratne--2021|Seneviratne et al., 2021]] ) and the event is also identified as an important driver of an observed fluctuation in a natural, human or managed system (‘identification of weather sensitivity’), then the observed fluctuation is considered (partly) attributed to long-term climate change (‘impact attribution’) and even to anthropogenic forcing.

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