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

=== FAQ 16.3 | How do climate scientists differentiate between impacts of climate change and changes in natural or human systems that occur for other reasons? ===

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''We can already observe many impacts of climate change today. The large body of climatic impact data and research confirms this. To decide whether an observed change in a natural or human system is at least partly an impact of climate change, we systematically compare the observed situation with a theoretical situation without observed levels of climate change. This is detection and attribution research.''

Global mean temperature has already risen by more than 1°C, and that also means that the impacts of climate change become more visible. Many natural and human systems are sensitive to weather conditions. Crop yields, river floods and associated damages, ecosystems such as coral reefs, or the extent of wildfires are affected by temperatures and precipitation changes. Other factors also come into play. So, for example, crop yields around the world have increased over the last decades because of increasing fertilizer input, improved management and varieties. How do we detect the effect of climate change itself on these systems, when the other factors are excluded? This question is central for impact attribution. ‘Impact of climate change’ is defined as the difference between the observed state of the system (e.g., level of crop yields, damage induced by a river flood, coral bleaching) and the state of the system assuming the same observed levels of non-climate-related drivers (e.g., fertilizer input, land use patterns or settlement structures) but no climate change.

So:

‘Impact of climate change’ is defined as the difference between the observed state of the system and the state of the system assuming the same observed levels of non-climate-related drivers but no climate change. For example, we can compare the level of crop yields, damage induced by a river flood, and coral bleaching with differences in fertilizer input, land use patterns or settlement structures, without climate change and with climate change occurring.

While this definition is quite clear, there certainly is the problem that, in real life, we do not have a ‘no climate change world’ to compare with. We use model simulations where the influence of climate change can be eliminated to estimate what might have happened without climate change. In a situation where the influence of other non-climate-related drivers is known to be minor (e.g., in very remote locations), the non-climate-change situation can also be approximated by observation from an early period where climate change was still minor. Often, a combination of different approaches increases our confidence in the quantification of the impact of climate change.

Impacts of climate change have been identified in a wide range of natural, human and managed systems. For example, climate change is the major driver of observed widespread shifts in the timing of events in the annual cycle of marine and terrestrial species, and climate change has increased the extent of areas burned by wildfires in certain regions, increased heat-related mortality, and had an impact on the expansion of vector-borne diseases.

In some other cases, research has made considerable progress in identifying the sensitivity of certain processes to weather conditions without yet attributing observed changes to long-term climate change. Two examples of weather sensitivity without attribution are observed crop price fluctuations and waterborne diseases.

Finally, it is important to note that ‘attribution to climate change’ does not necessarily mean ‘attribution to anthropogenic climate change’. Instead, according to the IPCC definition, climate change means any long-term change in the climate system, no matter where it comes from.

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