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

=== Atlas.9.4 Assessment and Synthesis of Projections ===

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CMIP5 and CMIP6 surface temperature and precipitation projections over the region are similar, with all regions warming more than the global average, most prominently those in the north (Figure Atlas.26). CMIP6 projects, for all scenarios and time periods, higher temperature changes (Chapter 4), with this contrast more accentuated in the long-term future and at higher global warming levels. The higher warming in the north (Interactive Atlas) is clear when comparing NEN, with increases from 2°C to over 8.5°C on an annual basis for SSP5-8.5 (near term to long term compared to a 1995–2014 baseline), to NCA, where changes range from 1.5°C to 6°C across the same periods. Maps showing changes in temperature and precipitation, and their robustness, are available in the Interactive Atlas. The number of model results (i.e., ensemble size used to generate these figures) differs, and this sample size difference may affect the results, but the patterns and magnitudes of change are generally consistent and thus it is ''very likely'' that temperatures will increase throughout the 21st century in all land areas, with stronger warming in the far north.

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[[File:0f5495015e52f10ea2a8b085346935cc IPCC_AR6_WGI_Atlas_Figure_26.png]]

'''Figure Atlas.26''' '''|''' '''Regional changes over land in annual mean surface air temperature andprecipitation relative to the 1995–2014 baseline for the reference regions in North America (warming since the 1850–1900 pre-industrial baseline is also provided as an offset).''' Bar plots in the left panel of each region triplet show the median (dots) and 10th–90th percentile range (bars) across each model ensemble for annual mean temperature changes for four datasets (CMIP5 in intermediate colours; a subset of CMIP5 used to drive CORDEX in light colours; CORDEX overlying the CMIP5 subset with dashed bars; and CMIP6 in solid colours); the first six groups of bars represent the regional warming over two time periods (near-term 2021–2040 and long-term 2081–2100) for three scenarios (SSP1-2.6/RCP2.6, SSP2-4.5/RCP4.5 and SSP5-8.5/RCP8.5), and the remaining bars correspond to four global warming levels (GWLs: 1.5°C, 2°C, 3°C and 4°C). The scatter diagrams of temperature against precipitation changes display the median (dots) and 10th–90th percentile ranges for the above four warming levels for December–January–February (DJF; middle panel) and June–July–August (JJA; right panel), respectively; for the CMIP5 subset only the percentile range of temperature is shown, and only for 3°C and 4°C GWLs. Changes are absolute for temperature (in °C) and relative (as %) for precipitation. See [[#Atlas.1.3|Atlas.1.3]] for more details on reference regions ( [[#Iturbide--2020|Iturbide et al., 2020]] ) and [[#Atlas.1.4|Atlas.1.4]] for details on model data selection and processing. The script used to generate this figure is available online ( [[#Iturbide--2021|Iturbide et al., 2021]] ) and similar results can be generated in the Interactive Atlas for flexibly defined seasonal periods. Further details on data sources and processing are available in the chapter data table (Table Atlas.SM.15).

CMIP5 results have been analysed extensively (e.g., [[#Maloney--2014|Maloney et al., 2014]] ) and used in major climate change assessments. The most recent US National Climate Assessment analysis of CMIP5 focusing on RCP4.5 and RCP8.5 for two future time periods stated that the USA would continue to warm regardless of the scenario, but is ''likely'' to be higher with higher-emissions scenarios (e.g., RCP8.5). Projected changes in precipitation are somewhat complex, but increased precipitation dominates in winter and spring, whereas in summer changes are more variable and uncertain. Canada’s Changing Climate Report (Bush and Lemmen, 2019) presents changes in temperature and precipitation, as well as other variables, such as snow, for future periods in Canada using results from CMIP5. It indicates that annual and winter precipitation is projected to increase everywhere in Canada over the 21st century with larger percentage increases in the north. Temperature is also projected to increase, regardless of the scenario, and with larger changes occurring in the north.

To provide the basis for generating additional information compared to that derived from CMIP5 the NA-CORDEX experiments were designed to involve a GCM-RCM matrix which included multiple GCMs that sampled the full range of climate sensitivity, multiple RCMs, at two different spatial resolutions (25 and 50 km) and a range of emissions scenarios (in most cases RCP4.5 and RCP8.5; [[#Mearns--2017|Mearns et al., 2017]] ). [[#Karmalkar--2018|Karmalkar (2018)]] noted that the NA-CORDEX models cover sub-regional ranges of temperature change from the CMIP5 GCMs better than NARCCAP did for the CMIP3 models. This structural design shift provides greater confidence in the NA-CORDEX results in terms of sampling the uncertainty across the CMIP5 models (Figure Atlas.27; [[#Bukovsky--2020|Bukovsky and Mearns, 2020]] ). The pattern of warming is as seen in CMIP5 and CMIP6, which also builds confidence that the RCMs generate high-resolution results consistent with CMIP5 on large scales whilst providing added value over regions such as the complex topography of the Rocky Mountains in the western USA, which are not well resolved in the GCMs. There is ''high confidence'' that downscaling a subset of CMIP models that spans the range of climate sensitivities in the full ensemble is critical for producing a representative range of dynamically downscaled projections.

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[[File:148f532049c9b5cbf83f229a952736e2 IPCC_AR6_WGI_Atlas_Figure_27.png]]

'''Figure Atlas.27''' '''|''' '''Changes (2070–2099 relative to 1970–1999) in the annual mean surface air temperature by three GCMs (GFDL-ESM2M, MPI-ESM-LR, HadGEM2-ES) and two RCMs (WRF and RegCM4) nested in the GCMs, for the RCP8.5 scenario over North America (after [[#Bukovsky--2020|Bukovsky and Mearns, 2020]] ).'''

There are striking contrasts in the seasonal results for precipitation for the sub-regions (Figure Atlas.26). The northern regions and ENA all show steady increases with the global warming levels ( ''very high confidence'' ). For example, the projected increases in the NEN region range from 7% in the near term to 40% at the end of the 21st century for the SSP5-8.5 scenario. In contrast, projected changes for NCA are for significant decreases both on an annual basis (Interactive Atlas) and in winter, and which become greater as warming increases ( [[#Akinsanola--2020b|Akinsanola et al., 2020b]] ; [[#Almazroui--2021|Almazroui et al., 2021]] ). The other two regions (WNA and CNA) exhibit mainly increases in winter. In summer, distributions are in general less uniform except for NWN and NEN, which display steady increases with global warming levels (but smaller than in winter). WNA and CNA mainly show decreases (based on the median values) but with some models projecting increases. Projections from the NA-CORDEX ensemble are consistent with those from the GCMs whilst providing greater detail of precipitation changes over the mountains and along the coasts (Interactive Atlas; [[#Bukovsky--2020|Bukovsky and Mearns, 2020]] ). Similar results are found in other analyses of RCM projections ( [[#Wang--2015|Wang and Kotamarthi, 2015]] ; [[#Ashfaq--2016|Ashfaq et al., 2016]] ; [[#Teichmann--2021|Teichmann et al., 2021]] ). Also, further analysis of the NA-CORDEX projections showed substantial changes in weather types related to increased monsoonal flow frequency and drying of the northern Great Plains in summer ( [[#Prein--2019|Prein et al., 2019]] ).

In summary, NEN, NWN and most of ENA will ''very likely'' experience increased annual mean precipitation, with greater increases at higher levels of warming ( ''very high confidence'' ). In NCA decreases predominate on an annual basis and particularly in winter ( ''high confidence'' ). Projected changes in summer are highly uncertain throughout other regions apart from the far northern parts of NEN and NWN which will ''likely'' experience increases ( ''high confidence'' ).

As discussed in [[IPCC:Wg1:Chapter:Chapter-10#10.3.3.4|Section 10.3.3.4]] , an important advance in regional modelling over the past decade or so is the use of convection-permitting regional models (CPMs; [[#Prein--2015|Prein et al., 2015]] , [[#Prein--2017a|Prein et al., 2017a]] ). There have been a number of experiments using CPMs over North America (e.g., [[#Rasmussen--2014|Rasmussen et al., 2014]] ; [[#Prein--2015|Prein et al., 2015]] , [[#Prein--2019|Prein et al., 2019]] ; [[#Liu--2017|Liu et al., 2017]] ; [[#Komurcu--2018|Komurcu et al., 2018]] ). A CPM study over North America that investigated changes in Mesoscale Convective Systems projected that by the end of the century, assuming an RCP8.5 scenario, their frequency more than tripled and associated precipitation increased by 80% ( [[#Prein--2017b|Prein et al., 2017b]] ). A multiple nesting of WRF over the north-eastern USA, downscaling to 3 km a CESM GCM climate projection assuming an RCP8.5 scenario, found a different pattern of precipitation change of mixed increases and decreases compared to the GCM projection of increases every month ( [[#Komurcu--2018|Komurcu et al., 2018]] ). These investigations demonstrate the potential of very-high-resolution simulations to add important dimensions to our understanding of regional climate change, though not necessarily to reduce uncertainty ( ''high confidence'' ).

It is ''virtually certain'' that snow cover will experience a general decline across North America during the 21st century, in terms of extent, annual duration and SWE, based on CMIP5 ( [[#Maloney--2014|Maloney et al., 2014]] ), CMIP6 ( [[#Mudryk--2020|Mudryk et al., 2020]] ), NA-CORDEX ( [[#Mahoney--2021|Mahoney et al., 2021]] ) and NARCCAP (e.g., [[#McCrary--2019|McCrary and Mearns, 2019]] ) simulations. For some regions the decline could be discernible over the next few decades, for example in the western USA ( [[#Fyfe--2017|Fyfe et al., 2017]] ). It is, however, ''likely'' that some high-latitude regions will rather experience an increase in certain winter snow cover properties ( [[#Mudryk--2018|Mudryk et al., 2018]] ; [[#McCrary--2019|McCrary and Mearns, 2019]] ), due to snowfall increase ( [[#Krasting--2013|Krasting et al., 2013]] ) prevailing over the warming effect. Discussion of changes in snow in the future is also covered in [[IPCC:Wg1:Chapter:Chapter-9#9.5.3|Section 9.5.3]] , but for larger regions.

The fraction of precipitation falling as snow is projected to decrease practically everywhere over North America, including over the western USA and south-western Canada ( [[#Mahoney--2021|Mahoney et al., 2021]] ), and in the Great Lakes basin where lake-effect precipitation is important ( [[#Suriano--2016|Suriano and Leathers, 2016]] ). In this basin, the frequency of heavy lake-effect snowstorms is expected to decrease during the 21st century, except for a possible temporary increase around Lake Superior by mid-century, if local air temperatures remain low enough ( [[#Notaro--2015|Notaro et al., 2015]] ). CMIP5 simulations of the periods 1981–2000 and 2081–2100 over the central and eastern USA suggest a northward shift in the transition zone between rain-dominated and snow-dominated areas, by about 2° latitude under the RCP4.5 scenario and 4° latitude under the RCP8.5 scenario ( [[#Ning--2015|Ning and Bradley, 2015]] ). Rain-on-snow event properties over North America should also evolve during the 21st century, with non-trivial dependencies on the positioning relative to the freezing line ( [[#Jeong--2018|Jeong and Sushama, 2018]] ) and on elevation ( [[#Musselman--2018|Musselman et al., 2018]] ).

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