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

== Executive Summary ==

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'''Climate change information is increasingly available and robust at regional scale for impacts and risk assessment.''' Climate services and vulnerability, impacts and adaptation studies require regional scale multi-decadal climate observations and projections. Since the IPCC Fifth Assessment Report (AR5), the increased availability of coordinated ensemble regional climate model projections and improvements in the level of sophistication and resolution of global and regional climate models, completed by attribution and sectoral vulnerability studies, have enabled the investigation of past and future evolution of a range of climatic quantities that are relevant to socio-economic sectors and natural systems. [https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-12 Chapter 12] consolidates core physical knowledge from preceding AR6 Working Group I (WGI) chapters and post-AR5 climate impact assessment literature to assess the spatio-temporal evolution of the climatic conditions that may lead to regional scale impacts and risks (following the sectoral classes adopted by AR6 WGII) in the world’s regions (presented in Chapter 1). {12.1}

'''The climatic impact-driver (CID) framework adopted in [https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-12 Chapter 12] allows for assessment of changing climate conditions that are relevant for regional impacts and risk assessment.''' CIDs are physical climate system conditions (e.g., means, events, extremes) that affect an element of society or ecosystems and are thus a priority for climate information provision. Depending on system tolerance, CIDs and their changes can be detrimental, beneficial, neutral or a mixture of each across interacting system elements, regions and society sectors. Each sector is affected by multiple CIDs and each CID affects multiple sectors. A CID can be measured by indices to represent related tolerance thresholds. {12.1–12.3}

'''The current climate in most regions is already different from the climate of the early or mid-20th century with respect to several CIDs. Climate change has already altered CID profiles and resulted in shifts in the magnitude, frequency, duration, seasonality and spatial extent of associated indices''' ( ''high confidence'' ''').''' Changes in temperature-related CIDs such as mean temperatures, growing season length, extreme heat and frost have already occurred and many of these changes have been attributed to human activities ( ''medium confidence'' ). Mean temperatures and heat extremes have emerged above natural variability in all land regions ( ''high confidence'' ). In tropical regions, recent past temperature distributions have already shifted to a range different to that of the early 20th century ( ''high confidence'' ). Ocean acidification and deoxygenation have already emerged over most of the global open ocean, as has reduction in Arctic sea ice ( ''high confidence'' ). Using CID index distributions and event probabilities accurately in both current and future risk assessments requires accounting for the climate change-induced shifts in distributions that have already occurred. {12.4, 12.5}

'''Several impact-relevant changes have not yet emerged from the natural variability but will emerge sooner or later in this century depending on the emissions scenario''' ( ''high confidence'' ''').''' Increasing precipitation is projected to emerge before the middle of the century in the high latitudes of the Northern Hemisphere ( ''high confidence'' ). Decreasing precipitation will emerge in a very few regions (Mediterranean, Southern Africa, south-western Australia) ( ''medium confidence'' ) by mid-century ( ''medium confidence'' ). The anthropogenic forced signal in near-coast relative sea level rise will emerge by mid-century RCP8.5 in all regions with coasts, except in the West Antarctic region where emergence is projected to occur before 2100 ( ''medium confidence'' ). The signal of ocean acidification in the surface ocean is projected to emerge before 2050 in every ocean basin ( ''high confidence'' ). However, there is ''limited evidence'' of drought trends emerging above natural variability in the 21st century. {12.5}

'''Every region of the world will experience concurrent changes in multiple CIDs by mid-century''' ( ''high confidence'' '''), challenging the resilience and adaptation capacity of the region.''' Changes in heat, cold, snow and ice, coastal, oceanic, and CO <sub>2</sub> at surface CIDs are projected with ''high confidence'' in most regions, indicating worldwide challenges, while additional region-specific changes are projected in other CIDs that may lead to more regional challenges. ''High confidence'' increases in some of the drought, aridity and fire weather CIDs will challenge, for example, agriculture, forestry, water systems, health and ecosystems in Southern Africa, the Mediterranean, North Central America, Western North America, the Amazon regions, South-Western South America, and Australia. ''High confidence'' changes in snow, ice and pluvial or river flooding will pose challenges for, for example, energy production, river transportation, ecosystems, infrastructure and winter tourism in North America, Arctic regions, Andes regions, Europe, Siberia, Central, South and East Asia, Southern Australia and New Zealand. Only a few CIDs are projected to change with ''high confidence'' in the Sahara, Madagascar, Arabian Peninsula, Western Africa and Small Islands; however, the ''lower confidence'' levels for CID changes in these regions can originate from knowledge gaps or model uncertainties, and does not necessarily mean that these regions have relatively low risk. {12.5}

'''Worldwide changes in heat, cold, snow and ice, coastal, oceanic and CO''' <sub>2</sub> '''-related CIDs will continue over the 21st century, albeit with regionally varying rates of change, regardless of the climate scenario''' ( ''high confidence'' ''').''' In all regions there is ''high confidence'' that, by 2050, mean temperature and heat extremes will increase, and there is ''high confidence'' that sea surface temperature will increase in all oceanic regions except the North Atlantic. Apart from a few regions with substantial land uplift, relative sea level rise is ''very likely'' to ''virtually certain'' (depending on the region) to continue in the 21st century, contributing to increased coastal flooding in most low-lying coastal areas ( ''high confidence'' ) and coastal erosion along most sandy coasts ( ''high confidence'' ), while ocean acidification is ''virtually certain'' to increase. It is ''virtually certain'' that atmospheric CO <sub>2</sub> at the surface will increase in all emissions scenarios until net zero emissions are achieved. Glaciers will continue to shrink and permafrost to thaw in all regions where they are present ( ''high confidence'' ). These changes will lead to climate states with no recent analogue that are of particular importance for specific regions such as tropical forests or biodiversity hotspots. {12.4}

'''A wide range of region-specific CID changes relative to recent past are expected with''' ''high'' '''or''' ''medium confidence'' ''', by 2050 and beyond.''' Most of these changes concern CIDs related to the water cycle and storms. Agricultural and ecological drought changes are generally of ''higher confidence'' than hydrological drought changes, with increases projected in North and Southern Africa, Madagascar, Southern and Eastern Australia, some regions of Central and South America, Mediterranean Europe, Western North America and North Central America ( ''medium'' to ''high confidence'' ). Fire weather conditions will increase by 2050 under RCP4.5 or above in several regions in Africa, Australia, several regions of South America, Mediterranean Europe, and North America ( ''medium'' to ''high confidence'' ). Extreme precipitation and pluvial flooding will increase in many regions around the world ( ''high confidence'' ). Increases in river flooding are also expected in Western and Central Europe and in polar regions ( ''high confidence'' ), most of Asia, Australasia, North America, the South American Monsoon region and South-Eastern South America ( ''medium confidence'' ). Mean winds are projected to slightly decrease by 2050 over much of Europe, Asia and Western North America, and increase in many parts of South America except Patagonia, West and South Africa and the eastern Mediterranean ( ''medium confidence'' ). Extratropical storms are expected to have a decreasing frequency but increasing intensity over the Mediterranean, increasing intensity over most of North America, and an increase in both intensity and frequency over most of Europe ( ''medium confidence'' ). Enhanced convective conditions are expected in North America ( ''medium confidence'' ). Tropical cyclones are expected to increase in intensity despite a decrease in frequency in most tropical regions ( ''medium confidence'' ). Climate change will modify multiple CIDs over Small Islands in all ocean basins, most notably those related to heat, aridity and droughts, tropical cyclones and coastal impacts. {12.4}

'''The level of confidence in the projected direction of change in CIDs and the intensity of the signal depend on mitigation efforts over the 21st century, as reflected by the differences between end-century projections for different climate scenarios.''' Dangerous humid heat thresholds, such as the US National Oceanic and Atmospheric Administration Heat Index (NOAA HI) of 41°C, will be exceeded much more frequently under the SSP5-8.5 scenario than under SSP1-2.6 and will affect many more regions ( ''high confidence'' ). In many tropical regions, the number of days per year where an HI of 41°C is exceeded will increase by more than 100 days relative to the recent past under SSP5-8.5, while this increase will be limited to less than 50 days under SSP1-2.6 ( ''high confidence'' ). The number of days per year where temperature exceeds 35°C will increase by more than 150 days in many tropical areas, such as the Amazon basin and South East Asia under SSP5-8.5, while it is expected to increase by less than two months in these areas under SSP1-2.6 (except for the Amazon Basin). There is ''high confidence'' that sandy shorelines will retreat in most regions of the world, in the absence of additional sediment sources or physical barriers to shoreline retreat. The total length of sandy shorelines around the world that are projected to retreat by more than 100 m by the end of the century is about 35% greater under RCP8.5 (about 130,000 km) compared to that under RCP 4.5 (about 95,000 km). The frequency of the present-day 1-in-100 year extreme sea level event (represented here by extreme total water level) is also projected to increase substantially in most regions ( ''high confidence'' ). In a globally averaged sense, the 1-in-100 year extreme sea level is projected to become an event that occurs multiple times per year under RCP8.5, while under RCP4.5 it is projected to become a one-in-five-year event, representing at least a five fold increase from RCP4.5 to RCP8.5. {12.4, 12.5}

'''There is''' ''low confidence'' '''in past and future changes of several CIDs.''' In nearly all regions there is ''low confidence'' in changes in hail, ice storms, severe storms, dust storms, heavy snowfall and avalanches, although this does not indicate that these CIDs will not be affected by climate change. For such CIDs, observations are short term or lack homogeneity, and models often do not have sufficient resolution or accurate parametrization to adequately simulate them over climate change time scales. {12.4}

'''Many global- and regional-scale CIDs have a direct relation to global warming levels (GWLs) and can thus inform the hazard component of ‘Representative Key Risks’ and ‘Reasons for Concern’ assessed by AR6 WGII.''' These include both mean and extreme heat, cold, wet and dry hazards; cryospheric hazards (snow cover, ice extent, permafrost); and oceanic hazards (marine heatwaves) ( ''high confidence'' ) ''.'' For some of these, a quantitative relation can be drawn ( ''high confidence'' ). For example, with each degree of global surface air temperature (GSAT) warming, the magnitude and intensity of many heat extremes show a linear change, while some changes in frequency of threshold exceedances are exponential: Arctic temperatures warm about twice as fast as GSAT; global sea surface temperatures increase by about 80% of GSAT change; Northern Hemisphere spring snow cover decreases by about 8% <sup></sup> per 1°C. For other hazards (e.g., ice-sheet behaviour, glacier mass loss, global mean sea level rise, coastal floods and coastal erosion) the time and/or scenario dimensions remain critical and a simple relation with GWLs cannot be drawn ( ''high confidence'' ), but still quantitative estimates assuming specific time frames and/or stabilized GWLs can be derived ( ''medium confidence'' ). Model uncertainty challenges the link between specific GWLs and tipping points and irreversible behaviour, but their occurrence cannot be excluded and their chances increase with warming levels ( ''medium confidence'' ). {CCB 12.1}

'''Since AR5, climate change information produced in climate service contexts has increased significantly due to scientific, technological advancements and growing user demand''' ( ''very high confidence'' ''').''' Climate services involve the provision of climate information in such a way as to assist decision-making. These services include appropriate engagement from users and providers, are based on scientifically credible information and expertise, have an effective access mechanism, and respond to user needs. Climate services are being developed across regions, sectors, time scales and target users. {12.6}

'''Climate services are growing rapidly and are highly diverse in their practices and products''' ( ''very high confidence'' ''').''' The decision-making context, level of user engagement and co-production between scientists, practitioners and intended users are important determinants of the type of climate service developed and its utility supporting adaptation, mitigation and risk management decisions. User needs and decision-making contexts are very diverse and there is no universal approach to climate services. {12.6}

'''Realization of the full potential of climate services is often hindered by limited resources for the co-design and c''' '''o-product''' '''ion process, including sustained engagement between scientists, service providers and users''' ( ''high confidence'' ''').''' Further challenges relate to climate services development, provision of climate services, generation of climate service products, communication with users, and evaluation of the quality and socio-economic value of climate services. The development of climate services often uncovers and presents new research challenges to the scientific community. {12.6}

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