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

=== Atlas.8.2 Assessment and Synthesis ofObservations, Trends and Attribution ===

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To support climatological analyses and model evaluation, national meteorological and hydrological services are increasingly making available high spatial and temporal resolution gridded and in situ homogenized and quality-checked datasets ( [[#Déqué--2008|Déqué and Somot, 2008]] ; [[#Vidal--2010|Vidal et al., 2010]] ; [[#Rauthe--2013|Rauthe et al., 2013]] ; [[#Noël--2015|Noël et al., 2015]] ; [[#Spinoni--2015b|Spinoni et al., 2015b]] ; [[#Ruti--2016|Ruti et al., 2016]] ; [[#Fantini--2018|Fantini et al., 2018]] ; [[#Lussana--2018|Lussana et al., 2018]] ; [[#Herrera--2019|Herrera et al., 2019]] ; [[#Skrynyk--2020|Skrynyk et al., 2020]] ). The inclusion of additional station data and data rescue activities lead to a better representation of extreme precipitation statistics than the global- or continental-scale datasets ( [[#Atlas.1.4.1|Atlas.1.4.1]] ). Recent gridded products merging radar and station data allow higher spatial and temporal resolutions to be reached ( [[#Haiden--2011|Haiden et al., 2011]] ; [[#Tabary--2012|Tabary et al., 2012]] ; [[#Berg--2016|Berg et al., 2016]] ; [[#Fumière--2020|Fumière et al., 2020]] ). A number of regional reanalysis products has become available for the European region ( [[#Bollmeyer--2015|Bollmeyer et al., 2015]] ; [[#Bach--2016|Bach et al., 2016]] ; [[#Dahlgren--2016|Dahlgren et al., 2016]] ; [[#Landelius--2016|Landelius et al., 2016]] ). A European ensemble of regional reanalyses from 1961 to 2019 is shown to add accuracy and reliability in comparison to global reanalysis products, but also introduces additional uncertainties, especially for threshold-based climate indices ( [[#Kaiser-Weiss--2019|Kaiser-Weiss et al., 2019]] ). However, gridded European datasets are unreliable over data-sparse regions. Also, many datasets employ different approaches to interpolation and gridding, which adds to their uncertainty and complicates comparative evaluations ( [[#Fantini--2018|Fantini et al., 2018]] ; [[#Kotlarski--2019|Kotlarski et al., 2019]] ; [[#Berthou--2020|Berthou et al., 2020]] ). For some sub-regions and performance metrics, differences between datasets have been shown to be of the same magnitude as errors in regional climate models ( [[#Prein--2016|Prein et al., 2016]] ; [[#Prein--2017|Prein and Gobiet, 2017]] ; [[#Fantini--2018|Fantini et al., 2018]] ), but observational uncertainty is substantially reduced when datasets of similar nature and representativeness are used ( [[#Kotlarski--2019|Kotlarski et al., 2019]] ).

In addition to the global display of observed temperature and precipitation trends in Figure Atlas.11, annual mean temperature and precipitation trends between 1980 and 2015 calculated from the gridded ensemble E-OBS dataset ( [[#Cornes--2018|Cornes et al., 2018]] ) are shown in Figure Atlas.23, together with time series of temperature and precipitation anomalies relative to the 1980–2015 mean value from E-OBS, CRU, EWEMBI and Berkeley for temperature, and E-OBS, CRU, GPCC and GPCP for precipitation (see also Figure 2.11 for global mean values, and [[#Atlas.1.4.1|Atlas.1.4.1]] for description of global datasets).

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[[File:d16110155935d9d2ed66213584ddae69 IPCC_AR6_WGI_Atlas_Figure_23.png]]

'''Figure Atlas.23''' '''|''' '''(a) Mean 1980–2015 trend ofannual mean surface air temperature (°C per decade) from E-OBS''' ( [[#Cornes--2018|Cornes et al., 2018]] ). Data for non-European countries in the MED area are masked out. '''(b)''' Time series of mean annual temperature anomaly relative to the 1980–2015 period (shown with grey shading) aggregated for the land area in each of the four European sub-regions, from E-OBS, CRU, Berkeley and ERA5 (see [[#Atlas.1.4.1|Atlas.1.4.1]] for description of global datasets). Mean trends for 1901–2015, 1961–2015 and 1980–2015 are shown for each dataset in corresponding colours in the same units as panel (a) (see legend in upper panel). '''(c)''' As panel (a) for annual mean precipitation (mm day <sup>–1</sup> per decade). '''(d)''' As panel (b) for annual mean precipitation, from datasets E-OBS, CRU, GPCC and GPCP. Note that E-OBS data are not shown in panels (b) and (d) for the region EEU. For the MED region data are aggregated over the European countries alone. Trends have been calculated using ordinary least squares regression and the crosses indicate non-significant trend values (at the 0.1 level) following the method of [[#Santer--2008|Santer et al. (2008)]] to account for serial correlation. Further details on data sources and processing are available in the chapter data table (Table Atlas.SM.15).

In NEU continued warming has been observed, particularly during spring. An annual mean temperature increase of 0.4°C per decade was reported between 1970 and 2008 ( [[#Rutgersson--2015|Rutgersson et al., 2015]] ). In WCE temperature increases since the mid-20th century have been documented for Poland ( [[#Degirmendžić--2004|Degirmendžić et al., 2004]] ) and Ukraine ( [[#Boychenko--2016|Boychenko et al., 2016]] ; [[#Balabukh--2017|Balabukh and Malitskaya, 2017]] ). Land-only observations indicate a rapid increase in summer (JJA) mean surface air temperature since the mid-1990s ( [[#Dong--2017|Dong et al., 2017]] ). In Eastern Europe no significant trend in winter mean air temperatures was found between 1881 and 2016 in Belarus ( [[#Loginov--2018|Loginov et al., 2018]] ). In parts of the European area of the MED, spring and summer temperatures are reported to increase faster than in the other seasons (see the Mediterranean case study in [[IPCC:Wg1:Chapter:Chapter-10#10.6.4|Section 10.6.4]] and Figure 10.18; [[#Brunetti--2006|Brunetti et al., 2006]] ; [[#Homar--2009|Homar et al., 2009]] ; [[#Lionello--2012|Lionello et al., 2012]] ; [[#Philandras--2015|Philandras et al., 2015]] ; [[#Gonzalez-Hidalgo--2016|Gonzalez-Hidalgo et al., 2016]] ; [[#Vicente-Serrano--2017|Vicente-Serrano et al., 2017]] ). Figure Atlas.23 shows that since 1980 in each European region all datasets show a consistent warming of annual mean temperature of 0.04°C yr <sup>–1</sup> to 0.05°C yr <sup>–1</sup> . Trends in European land temperature cannot be explained without accounting for anthropogenic warming offset by anthropogenic aerosol emissions ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.1.1|Section 3.3.1.1]] and Figure 3.9). It is ''virtually certain'' that annual mean temperature continues to increase in each European subdomain.

Multi-decadal trends in mean precipitation are generally small and non-significant. Apart from difficulties related to observational coverage ( [[#Prein--2017|Prein and Gobiet, 2017]] ), gauge undercatch (e.g., [[#Murphy--2020|Murphy et al., 2020]] ) and data inhomogeneity (e.g., [[#Camuffo--2013|Camuffo et al., 2013]] ), strong interannual and multi-decadal variability is dominant over at least the last two centuries. However, significant precipitation trends have been recorded for recent periods, for example in south-western Europe between 1960 and 2000 ( [[#Peña-Angulo--2020|Peña-Angulo et al., 2020]] ), and between 1961 and 2015 in NEU (Interactive Atlas). Also, some studies suggest that in the MED precipitation has declined and more frequent and severe meteorological droughts have occurred between 1960 and 2000 ( [[#Spinoni--2015a|Spinoni et al., 2015a]] ; [[#Gudmundsson--2016|Gudmundsson and Seneviratne, 2016]] ), and in some regions cannot be explained without anthropogenic forcing ( [[IPCC:Wg1:Chapter:Chapter-10#10.4.1.2|Section 10.4.1.2]] ; [[#Knutson--2018|Knutson and Zeng, 2018]] ). Other studies suggest that this trend can be seen as an expression of multi-decadal internal variability driven mainly by the North Atlantic Oscillation (Table Atlas.1; [[#Kelley--2012|Kelley et al., 2012]] ; [[#Zittis--2018|Zittis, 2018]] ). Global dimming and brightening also are reported to affect precipitation trends in the Mediterranean region ( [[IPCC:Wg1:Chapter:Chapter-8#8.3.1.6|Section 8.3.1.6]] and Figure 8.7).

The large-scale spatial patterns of the E-OBS annual mean precipitation trend between 1980 and 2015 shown in Figure Atlas.23 is broadly consistent with trends derived from CRU, GPCP and GPCC (Figure Atlas.11) but with more explicit spatial detail. Trends calculated for regional averages are sensitive to the selection of the time window: for 1980–2015 annual mean precipitation averaged over the regions shows a positive trend (not significant at p = 0.05), while for CRU and GPCC the trend calculated over 1901–2015 is positive for NEU, EEU and WCE, and non-significant for MED. Precipitation trends in the MED are significant only in selected areas ( [[#Lionello--2012|Lionello et al., 2012]] ; [[#MedECC--2020|MedECC, 2020]] ). Also the NEU trends show large spatial variability and are subject to decadal variability related to NAO ( [[#Heikkilä--2012|Heikkilä and Sorteberg, 2012]] ), but are generally positive over the 20th century (Figure Atlas.23). There is ''medium confidence'' that annual mean precipitation in NEU, WCE and EEU has increased since the early 20th century. In the European Mediterranean, observed land precipitation trends show pronounced variability within the region, with magnitude and sign of trend in the past century depending on time period and exact study region ( ''medium confidence'' ).

Trends in snowfall and snowmelt are related to seasonal changes in both temperature and precipitation. In EEU, melt onset dates have advanced by one to two weeks in the 1979–2012 period ( [[#Mioduszewski--2015|Mioduszewski et al., 2015]] ). Over Eurasia, trends in spring and early summer snow cover extent increased over the 1971–2014 period ( [[#Hernández-Henríquez--2015|Hernández-Henríquez et al., 2015]] ). Between 1966 and 2012, averaged over entire Eurasia, monthly mean snow depth decreased in autumn and increased in winter and spring ( [[#Zhong--2018|Zhong et al., 2018]] ), while the snow cover extent was reported to have decreased during the past 40 years ( [[#Bulygina--2011|Bulygina et al., 2011]] ). In NEU late winter and early spring snow depth and snow cover decreases since the early 1960s are reported over Finland ( [[#Luomaranta--2019|Luomaranta et al., 2019]] ) and Norway ( [[#Rizzi--2018|Rizzi et al., 2018]] ) with a dependence on altitude ( [[#Skaugen--2012|Skaugen et al., 2012]] ), while winter snow depth increased in northern Sweden ( [[#Kohler--2006|Kohler et al., 2006]] ). It is ''very likely'' that since the early 1980s in snow-dominant areas in NEU and EEU the length of the snowfall season is reduced with regional warming, and the melt onset dates have advanced.

The increasing trend in surface shortwave radiation, documented in AR5 ( [[#Hartmann--2013|Hartmann et al., 2013]] ) to have occurred since the 1980s and referred to as a brightening effect, is substantiated over Europe and the Mediterranean region ( [[#Nabat--2014|Nabat et al., 2014]] ; [[#Sanchez-Lorenzo--2015|Sanchez-Lorenzo et al., 2015]] ; [[#Cherif--2020|Cherif et al., 2020]] ). This increasing trend has been attributed to the decrease in anthropogenic sulphate aerosols over the 1980–2012 period ( [[#Nabat--2014|Nabat et al., 2014]] ). In model sensitivity experiments, the aerosol trend has been quantified to explain 81 ± 16% of the European surface shortwave trend and 23 ± 5% of the European surface temperature warming. It is ''likely'' that trends in anthropogenic aerosols in Europe have generated positive trends in shortwave radiation and surface temperature since the 1980s (Sections 6.3.3.1, 8.3.1.6 and 10.6.4).

Assessments of observed European trends in meteorological extremes and CIDs are reported elsewhere in this report. [[IPCC:Wg1:Chapter:Chapter-11#11.3.5|Section 11.3.5]] documents and attributes an increase in the frequency and extent of heatwaves and daily maximum temperatures, and [[IPCC:Wg1:Chapter:Chapter-11#11.6.2|Section 11.6.2]] discusses the uncertainty concerning the detection of trends in meteorological droughts, and the role of increasing atmospheric evaporative demand on hydrological and ecological/agricultural droughts. [[IPCC:Wg1:Chapter:Chapter-8#8.3.1|Section 8.3.1.8]] reports on increasing aridity trends in the Mediterranean related to soil moisture declines and increases in atmospheric water vapor demand. [[IPCC:Wg1:Chapter:Chapter-11#11.4.2|Section 11.4.2]] reports on the increased likelihood and intensity of daily precipitation extremes, while Sections 11.5.2 and 12.4.5.2 discuss implications for peak streamflow. [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.5|Section 12.4.5.5]] discusses the increased likelihood of wildfires, while [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.3|Section 12.4.5.3]] discusses the substantial decadal variability in mean wind speed and the trends in wind storms and gusts. The acceleration of sea level rise in the Atlantic and European seas has been discussed in [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.5|Section 12.4.5.5]] .

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