In 2020, motivated by improving air quality in major ports and shipping lanes, the International Maritime Organization imposed strict new regulations on the sulfur content of shipping fuel. This led to a rapid reduction in the number of observed ship tracks (linear tracks of clouds brightened by aerosol perturbations; Watson-Parris et al. 2022), and presumably a commensurate reduction in anthropogenic aerosol forcing. The magnitude of this forcing, and the resulting temperature change, are uncertain however. The recent confirmation that 2023 was the hottest year on record can only partly be explained by the onset of the El Niño phase of the El Niño-Southern Oscillation (ENSO). Such warming, in addition to the sizable warming in NH ocean basins- geographically collocated with shipping- raise the question of how much shipping emissions changes might have contributed to this signal, and any extreme weather events associated with it. In this study we aim to answer this question by utilizing a large ensemble of fully-coupled Community Earth System Model version 2 (CESM2) simulations with and without the shipping emissions changes. We leverage the CESM2 large ensemble and choose 20 simulations with varying ENSO conditions from which to branch off with shipping emissions reduced to 20% of their baseline value. These are integrated forward for another 20 years, while non-shipping emissions follow the SSP3-7.0 scenario, in order to robustly explore the transient climate response.In this talk we will highlight the forced climate response, focusing on temperature (T), precipitation (P), and atmospheric circulation, both globally and in key regions such as the North Atlantic. Given the change in ENSO phase during 2023, we will also describe how this climate response is modulated by different ENSO conditions, the Atlantic Multidecadal Variability and other modes of climate variability. The underlying relevant climate processes, including cloud dynamics, radiative imbalances at the top of the atmosphere, and daily variability will be summarized to link our single model study to observed changes.References:[1] Watson-Parris, D., Christensen, M., Laurenson, A., Clewley, D., Gryspeerdt, E., Stier, P. “Shipping regulations lead to large reduction in cloud perturbations”. PNAS 119 (41) e2206885119: https://doi.org/10.1073/pnas.2206885119 (2022)

Heat waves across Europe have intensified in recent decades, posing significant risks to human health and environmental systems. This study investigates patterns and projections of heat waves and extreme heat days across continental Europe using the Weather Research and Forecasting (WRF) model for dynamical downscaling of the Community Earth System Model version 2 global simulations under Coupled Model Intercomparison Project phase 6 projection scenarios. We analyze historical simulations (1980–2014) and present-to-future projections (2015–2049) under three scenarios (SSP1-2.6, SSP2-4.5, and SSP3-7.0), evaluating heat wave duration (WSDI), magnitude (HWMId), and various heat stress indices. During 1980–2014, WRF simulations detect statistically significant increasing trends in heat wave duration (2.0–3.4 d/decade), magnitude (0.3–0.9/decade), and extreme heat days (0.9–2.6 d/decade) across European climate zones, aligning with observational data. These trends are projected to intensify toward 2049, even under ambitious mitigation scenarios. Substantial spatial heterogeneity exists across Europe, with Southern Europe showing the most pronounced changes. Heat waves in southern Europe are projected to increase at 6.3–7.8 d/decade in duration and 1.2–1.5/decade in magnitude, with extreme heat days increasing at 3.0–4.5 d/decade during 1980–2049. The pace of change in Southern Europe is 30%–150% greater than historical trends in the same region and 30%–80% greater than the average future increase across Europe, highlighting the particular vulnerability to intensifying heat extremes in this region.

This dataset contains the data displayed in the figures or the article "High-resolution projections of ambient heat for major European cities using different heat metrics". The different files contain: Data_Fig1_DeltaTXx_EURO-CORDEX_1981-2010_to_3K-European-warming_RCP85.nc: Change of yearly maximum temperature in Europe between 1981-2010 and 3 °C European warming relative to 1981-2010. Data_Fig2_timeseries-GSAT-ESAT_EURO-CORDEX_CMIP5_CMIP6_1971-2100_RCP85_SSP585.xlsx: Time series of global mean surface air temperature (GSAT) for CMIP5 and CMIP6 models, and for European mean surface air temperature (ESAT) for EURO-CORDEX, CMIP5, and CMIP6 models for the period 1971-2100. Data_Fig3_TX-distribution_distance-from-city-centre_E-OBS_1981-2010.xlsx: Distribution of average daily maximum temperature in summer (June, July, August) in 1981-2010 for E-OBS for all investigated cities. Temperature data are indicated as a function of the distance to the city centre. Data_Fig3_TX-distribution_distance-from-city-centre_ERA5-Land_1981-2010.xlsx: Distribution of average daily maximum temperature in summer (June, July, August) in 1981-2010 for ERA5-Land for all investigated cities. Temperature data are indicated as a function of the distance to the city centre. Data_Fig3_TX-distribution_distance-from-city-centre_EURO-CORDEX_1981-2010.xlsx: Distribution of average daily maximum temperature in summer (June, July, August) in 1981-2010 for the EURO-CORDEX models for all investigated cities. Temperature data are indicated as a function of the distance to the city centre. Data_Fig3_TX-distribution_distance-from-city-centre_weather-stations_1981-2010.xlsx: Distribution of average daily maximum temperature in summer (June, July, August) in 1981-2010 for GSOD and ECA&D stations for all investigated cities. Temperature data are indicated as a function of the distance to the city centre. Data_Fig4_TX-ambient-heat_EURO-CORDEX_3K-European-warming.xlsx: Daytime heat metrics for the investigated cities: HWMId-TX at 3 °C European warming relative to 1981-2010, TX exceedances above 30 °C at 3 °C European warming relative to 1981-2010, and TXx change between 1981-2010 and 3 °C European warming relative to 1981-2010 for EURO-CORDEX models. Data_Fig5_Contribution-of-explanatory-variables-to-total-explained-variance.xlsx: Contribution of different explanatory variables (climate and location factors) to the total explained variance of spatial patterns of heat metrics. Data_Fig6_TN-ambient-heat_EURO-CORDEX_3K-European-warming.xlsx: Nighttime heat metrics for the investigated cities: HWMId-TN at 3 °C European warming relative to 1981-2010, TN exceedances above 20 °C at 3 °C European warming relative to 1981-2010, and TNx change between 1981-2010 and 3 °C European warming relative to 1981-2010 for EURO-CORDEX models. Data_Fig7_TX-ambient-heat_CMIP5_3K-European-warming.xlsx: Daytime heat metrics for the investigated cities: HWMId-TX at 3 °C European warming relative to 1981-2010, TX exceedances above 30 °C at 3 °C European warming relative to 1981-2010, and TXx change between 1981-2010 and 3 °C European warming relative to 1981-2010 for CMIP5 models. Data_Fig7_TX-ambient-heat_CMIP6_3K-European-warming.xlsx: Daytime heat metrics for the investigated cities: HWMId-TX at 3 °C European warming relative to 1981-2010, TX exceedances above 30 °C at 3 °C European warming relative to 1981-2010, and TXx change between 1981-2010 and 3 °C European warming relative to 1981-2010 for CMIP6 models. Data_Fig8_GCM-RCM-matrix_ambient-heat_3K-European-warming.xlsx: GCM-RCM matrices for the three heat metrics.

Global warming is rapidly shifting climate conditions away from what societies and ecosystems are adapted to. While the magnitude of changes in mean and extreme climate are broadly studied, regional rates of change, a key driver of climate risk, have received less attention. Here we show, using large ensembles of climate model simulations, that large parts of the tropics and subtropics, encompassing 70% of current global population, are expected to experience strong (>2 s.d.) joint rates of change in temperature and precipitation extremes combined over the next 20 years, under a high-emissions scenario, dropping to 20% under strong emissions mitigation. This is dominated by temperature extremes, with most of the world experiencing unusual (>1 s.d.) rates relative to the pre-industrial period, but unusual changes also occur for precipitation extremes in northern high latitudes, southern and eastern Asia and equatorial Africa. However, internal variability is high for 20 year trends, meaning that in the near term, trends of the opposite sign are still likely for precipitation extremes, and rare but not impossible for temperature extremes. We also find that rapid clean-up of aerosol emissions, mostly over Asia, leads to accelerated co-located increases in warm extremes and influences the Asian summer monsoons.

Abstract. In 2020 the International Maritime Organization (IMO) implemented strict new regulations on the emissions of sulphate aerosol from the world's shipping fleet. This can be expected to lead to a reduction in aerosol-driven cooling, unmasking a portion of greenhouse gas warming. The magnitude of the effect is uncertain, however, due to the large remaining uncertainties in the climate response to aerosols. Here, we investigate this question using an 18-member ensemble of fully coupled climate simulations evenly sampling key modes of climate variability with the NCAR CESM2 model. We show that while there is a clear physical response of the climate system to the IMO regulations, including a surface temperature increase, we do not find global mean temperature influence that is significantly different from zero. The 20-year average global mean warming for 2020–2040 is +0.03 °C, with a 5–95 % confidence range of [-0.09, 0.19], reflecting the weakness of the perturbation relative to internal variability. We do, however, find a robust, non-zero regional temperature response in part of the North Atlantic. We also find that the maximum annual-mean ensemble-mean warming occurs around a decade after the perturbation in 2029, which means that the IMO regulations have likely had very limited influence on observed global warming to date. We further discuss our results in light of other, recent publications that have reached different conclusions. Overall, while the IMO regulations may contribute up to at 0.16 °C [-0.17, 0.52] to the global mean surface temperature in individual years during this decade, consistent with some early studies, such a response is unlikely to have been discernible above internal variability by the end of 2023 and is in fact consistent with zero throughout the 2020–2040 period.

<p>Compound extreme events describe the simultaneous occurrence of two or more individual extreme weather or climate events that often have a significant impact on environment, society or economy. Many studies have investigated such events, often using different spatiotemporal scales for the same event, depending on e.g., the country or region of interest. Although appropriate from an impact point of view, this practice might lead to conflicting or inconsistent results. It is therefore necessary to find objective definitions of extreme events for attribution studies or to investigate how likelihoods of certain extreme events change over time.</p> <p>Building on previous work for single extreme events, we propose a roadmap for obtaining objective compound event definitions, especially with regards to their spatiotemporal characteristics, by estimating multivariate probability distributions via copulas and then maximizing the rarity of the event across several scales. We present applications to past compound extreme events with considerable impact on e.g., human health and agriculture, such as the European heat wave/high ozone event in summer 2003, and also investigate how probabilities of these events change under different emission scenarios.</p>

In May 2014, the Balkan Region experienced exceptionally heavy rainfall. Between May 14 and May 19, 2014, there was a devastating flood in Serbia, Croatia, and Bosnia & Herzegovina. The event shattered a number of historical records and seriously endangered economies across the region. The close proximity of human settlements, infrastructure (houses, buildings, bridges), and agricultural land to flood plains further amplified the destructive effects. Although atmospheric thermodynamic and dynamic processes were used to describe this exceptional rainfall event, there was no mention of how climate change may have contributed to it. We show that the probability of this brief and powerful event occurring without human-caused climate change were incredibly low. Our research aims to demonstrate how climate change may have affected the likelihood that this extreme rainfall event will occur as well as to outline the difficulties in doing so. This was accomplished using the methods recommended by the World Weather Attribution (WWA) group. We examine whether and how much human-caused climate change has affected the likelihood and intensity of the rainfall over the Balkans as well as the peak 5-day precipitation in order to achieve this. We consider both historical weather data and climate models with and without anthropogenic forcing. The findings suggested that one of the key elements in determining event likelihood calculations is domain selection. Given the current situation and the possibility for further excessive rainfall over the Balkans, it is critical to enhance water management and lessen vulnerability to extreme rainfall. Acknowledgement:This research was supported by ExtremeClimTwin project, which has received funding from theEuropean Union’s Horizon 2020 research and innovation programme under grant agreement No 952384.

Climate change is causing a range of weather phenomena to move outside the range to which people and ecosystems are adapted. Much attention has been given to absolute changes, such as average temperatures or changes to the return values of extreme events, with global warming. However, the rate of change, and how that compares to the rates of change experienced in the preindustrial climate, i.e. the amount of change we have previously experienced over a short time period, is also an important determinant of impacts, and yet has not been given as much attention. In particular, as climate extremes are responsible for a disproportionate share of impacts, society can be expected to be particularly vulnerable to high rates of change of extremes – especially when multiple hazards increase at once.Using large ensembles of climate model simulations, we examine rates of change of temperature and precipitation extremes, both separately and combined, over the next twenty years (2021-2040) and compare with 20-year rates of change in the pre-industrial (PI) period. We consider regional scales, due to their increased relevance to the experience of people and ecosystems compared to global mean changes. We find that for many sub-continental-scale regions, the rates of change over the next twenty years will shift substantially away from the distribution of trends simulated in the preindustrial period. In more than a third of the regions studied, ensemble mean combined rates for both extremes types are at least two standard deviations greater than PI variability of trends, and more than one standard deviation greater in almost all regions under a high emissions scenario (SSP5-8.5). Substantial changes are also seen in a scenario with drastically reduced emissions (SSP1-2.6). Low latitude regions are particularly affected due to their small internal variability in temperature extremes trends. These tend to be low-income countries that are particularly vulnerable to the impacts of climate change. Changes in rates are most obvious for temperature extremes, but a number of regions also experience substantial simultaneous changes in rates for precipitation extremes. In low emission scenarios, the rates of change tend to flatten out in subsequent 20-year periods, but accelerate in the highest emissions scenarios.Notably, we find that rapid reductions of anthropogenic aerosols over the next twenty years in low emissions scenarios lead to accelerated increases of both hot and wet extremes over India and parts of China, due to the compound effects of surface warming from greenhouse gas warming and loss of cooling from atmospheric aerosolsThese findings have important implications for climate policy, decision making and near-term adaptation strategies. However, despite the emerging signal of rapid 20-year rates of change, spread amongst ensemble members is nevertheless large, particularly in the mid to high latitudes, meaning that trends of the opposite sign are not impossible in the near term, even if not that probable. This is also an important consideration to take into account when communicating these, and other, results on near-term decadal rates of change.

Changes in mean and extreme precipitation are arguably the most impactful aspects of climate change. Detailed and accurate projections are therefore crucial for climate risk assessments and adaptation strategies. Generally, global mean precipitation increases with surface warming, with a global mean hydrological sensitivty of around 1 to 2 %/K, and stronger increases in rates or extreme precipitation events. Local variations are however very large, and model projections are much more uncertain than for temperature. Absorbing aerosols, notably black carbon (BC), brown carbon (BrC) and mineral dust, are an exception to the rule. Their absorption of shortwave radiation inhibits precipitation formation, through rapid adjstments that overwhelm their influence on surface temperature. The hydrological sensitivty to black carbon emissions is therefore around -4 %/K, again with large regional variations, and with a very high spread between models ( -1 to -7 %/K) In this talk, we discuss the near-term (2015-2045) dependence of precipitation change on the evolution of absorbing aerosols. We show the transient hydrological sensitivty in CMIP6, globally and regionally, and how it is affected by air quality policy (i.e. scenario choice) and model treatment of BC, BrC and dust. We confirm that, globally, while black carbon emissions have a modest impact on surface temperature, their influence on precipitation is outsized, causing a factor of 2 difference in hydrological sensitivty beween future scenarios with strict (SSP126) and weak (SSP370) air quality control measures. Further, for highly populated regions close to, or downwind from, major emission sources - notably India, China and northern Brazil - we find very high sensitivity of precipitation evolution to the levels of absorbing aerosol emissions. Several of these regions are therefore set for a "double whammy" of precipitation increase from global warming and a removal of short wave absorbing air pollution. We also discuss the rates of change of extreme precipitation events, and how they relate to absorbing aerosols in different regions. Our key message is that changes in absorbing aerosols over the coming decades is a key uncertainty in near term precipitation and extreme event evolution, and therefore a burning knowledge gap for the aerosol-climate community.

<p>In frame of the H2020-EXHAUSTION project, we present estimated heat indicators from WRF (Weather Research and Forecasting model) downscaling simulations for the periods of 1980-2014 and 2015-2049 at 20 km horizontal resolution over the European domain. WRF simulations are forced by the CESM2 global model simulations, using three shared socio-economic pathways (SSP) future scenarios from the Coupled Model Intercomparison Project Phase 6 (CMIP6): SSP1-2.6, SSP2-4.5 and SSP3-7.0, addressing different levels of mitigation and adaptation. For the period of 1980-2014, another WRF simulation forced by ERA5 is used as comparison in model validation. These near-past simulations have been rigorously evaluated with observations and reanalysis data including European Climate Assessment & Dataset (ECA&D), E-OBS, and ERA5-land for the surface air temperatures. The dynamical downscaling showed clear added value on spatial distribution related to the important coastal or orographic aspects widely present over Europe. Two heat wave indicators, the Warm Spell Duration Index (WSDI) and the Heat Wave magnitude Index daily (HWMId), and four extreme heat indicators, annual maximum temperature (TX­x), NOAA heat index (HIx), wet-bulb globe temperature (WBGTx), and universal thermal climate index (UTCIx), are used to study the heat extremes trends in Europe. During the past 35 years, TXx has been estimated to increase 2.5 °C in WRF_CESM2 and 1.4 °C in WRF_ERA5; the increasing trend is estimated to remain or slow down slightly in the next 35 years with estimated smaller increase of 1.5-2.5 °C in three scenarios. The trends of other extreme heat indicators showed very similar trends with TXx. However, future heat wave duration and magnitude present a contrasted pattern. Heat waves have been estimated to increase 11.2 days of duration, and 2.1 of magnitude during 1980-2014, very similar to the observed increase of 9.1-11 days and 1.8-2.1. Whereas in 2015-2049, heat waves duration and magnitude are estimated to increase 12.3-13 days and 2.5-4.6, respectively. These heat wave changes are also not uniform from a spatial point of view. Heat wave duration and magnitude in Southern Europe are both estimated to increase significantly faster than other zones, with rates at 1.4-2.9 times of which for the whole of Europe. Heat wave indicators in future scenarios also showed much larger interannual variations compared with the past, whereas there are no distinct differences among three mitigation scenarios for all heat indicators. In summary, these results suggested that even though the future increase of air temperatures and heat extreme indicators showed a slowing down sign compared with the near-past, whereas the severity of heat waves are estimated to increase even faster than the past under different levels of mitigation. Southern Europe is expected to be the region that needs the most attention in terms of severe future heat waves.</p>

The African coast contains heritage sites of ‘Outstanding Universal Value’ that face increasing risk from anthropogenic climate change. Here, we generated a database of 213 natural and 71 cultural African heritage sites to assess exposure to coastal flooding and erosion under moderate (RCP 4.5) and high (RCP 8.5) greenhouse gas emission scenarios. Currently, 56 sites (20%) are at risk from a 1-in-100-year coastal extreme event, including the iconic ruins of Tipasa (Algeria) and the North Sinai Archaeological Sites Zone (Egypt). By 2050, the number of exposed sites is projected to more than triple, reaching almost 200 sites under high emissions. Emissions mitigation from RCP 8.5 to RCP 4.5 reduces the number of very highly exposed sites by 25%. These findings highlight the urgent need for increased climate change adaptation for heritage sites in Africa, including governance and management approaches, site-specific vulnerability assessments, exposure monitoring, and protection strategies.

The climate science and applications communities need a broad and demand-driven concept to assess physical climate conditions that are relevant for impacts on human and natural systems. Here, we augment the description of the "climatic impact-driver" (CID) approach adopted in the Working Group I (WGI) contribution to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report. CIDs are broadly defined as "physical climate system conditions (e.g., means, events, and extremes) that affect an element of society or ecosystems. Depending on system tolerance, CIDs and their changes can be detrimental, beneficial, neutral, or a mixture of each across interacting system elements and regions." We give background information on the IPCC Report process that led to the development of the 7 CID types (heat and cold, wet and dry, wind, snow and ice, coastal, open ocean, and other) and 33 distinct CID categories, each of which may be evaluated using a variety of CID indices. This inventory of CIDs was co-developed with WGII to provide a useful collaboration point between physical climate scientists and impacts/risk experts to assess the specific climatic phenomena driving sectoral responses and identify relevant CID indices within each sector. The CID Framework ensures that a comprehensive set of climatic conditions informs adaptation planning and risk management and may also help prioritize improvements in modeling sectoral dynamics that depend on climatic conditions. CIDs contribute to climate services by increasing coherence and neutrality when identifying and communicating relevant findings from physical climate research to risk assessment and planning activities.

The use of regional climate model (RCM)-based projections for providing regional climate information in a research and climate service contexts is currently expanding very fast. This has been possible thanks to a considerable effort in developing comprehensive ensembles of RCM projections, especially for Europe, in the EURO-CORDEX community (Jacob et al., 2014, 2020). As of end of 2019, EURO-CORDEX has developed a set of 55 historical and scenario projections (RCP8.5) using 8 driving global climate models (GCMs) and 11 RCMs. This article presents the ensemble including its design. We target the analysis to better characterize the quality of the RCMs by providing an evaluation of these RCM simulations over a number of classical climate variables and extreme and impact-oriented indices for the period 1981-2010. For the main variables, the model simulations generally agree with observations and reanalyses. However, several systematic biases are found as well, with shared responsibilities among RCMs and GCMs: Simulations are overall too cold, too wet, and too windy compared to available observations or reanalyses. Some simulations show strong systematic biases on temperature, others on precipitation or dynamical variables, but none of the models/simulations can be defined as the best or the worst on all criteria. The article aims at supporting a proper use of these simulations within a climate services context.

This paper analyzes the ensemble of regional climate model (RCM) projections for Europe completed within the EURO‐CORDEX project. Projections are available for the two greenhouse gas concentration scenarios RCP2.6 (22 members) and RCP8.5 (55 members) at 0.11° resolution from 11 RCMs driven by eight global climate models (GCMs). The RCM ensemble results are compared with the driving CMIP5 global models but also with a subset of available last generation CMIP6 projections. Maximum warming is projected by all ensembles in Northern Europe in winter, along with a maximum precipitation increase there; in summer, maximum warming occurs in the Mediterranean and Southern European regions associated with a maximum precipitation decrease. The CMIP6 ensemble shows the largest signals, both for temperature and precipitation, along with the largest inter‐model spread. There is a high model consensus across the ensembles on an increase of extreme precipitation and drought frequency in the Mediterranean region. Extreme temperature indices show an increase of heat extremes and a decrease of cold extremes, with CMIP6 showing the highest values and EURO‐CORDEX the finest spatial details. This data set of unprecedented size and quality will provide the basis for impact assessment and climate service activities for the European region.

Cities are hotspots of human heat stress due to their large number of inhabitants and the urban heat island effect leading to amplified temperatures. Exposure to heat stress in urban areas is projected to further increase in the future, mainly due to climate change and expected increases in the number of people living in cities. The impacts of climate change in cities have been investigated in numerous studies, but rarely using climate models due to their coarse spatial resolution compared to the typical areal extent of cities. Recent advances in regional climate modelling now give access to an ensemble of high-resolution simulations for Europe, allowing for much more detailed analyses of small-scale features, such as city climate. Focusing on Europe, we compare the evolution of several heat stress indicators for 36 major European cities, based on regional climate model simulations from EURO-CORDEX. The applied EURO-CORDEX ensemble (Vautard et al., 2020) has a spatial resolution of 0.11° (~11 km; comparable to the extent of large cities) and contains over 60 ensemble members, allowing thus for robust multi-model analyses of climate change on city levels. We analyze changes in heat stress both relative to the climatological heat stress variability in each city during 1981-2010 using the Heat Wave Magnitude Index daily (HWMId, Russo et al., 2015) and in absolute terms by counting the yearly number of exceedances of impact-relevant thresholds. Relative and absolute heat stress increase throughout Europe but with distinct patterns. Absolute heat stress increases predominantly in Southern Europe, primarily due to the hotter climate in the South. Relative changes are also highest in Southern Europe but exhibit a secondary maximum in Northern Europe, while being lowest in Central Europe. The main reason for this pattern is that day-to-day variability in heat stress indicators during present climate conditions is highest in Central Europe but lower in Southern and Northern Europe. Large Northern European cities, which are all located at the shore, are further influenced by different heat stress evolutions over land and sea surfaces. As human vulnerability does not only depend on the absolute heat stress but also on what people are adapted to (i.e., the climatological range), the results of this study highlight that cities in all parts of Europe – including in Northern Europe – must prepare for higher heat stress in the future. References: Russo, S., et al. (2015). Top ten European heatwaves since 1950 and their occurrence in the coming decades. Environmental Research Letters, 10(12). doi:10.1088/1748-9326/10/12/124003 Vautard, R., et al. (2020). Evaluation of the large EURO‐CORDEX regional climate model ensemble. Journal of Geophysical Research: Atmospheres. doi:10.1029/2019jd032344

Important heritage sites along the African coast are at risk from the threats associated with rising sea levels. Here, we quantify the exposure of natural and cultural heritage sites in Africa to coastal flooding and erosion in the 21st century. We develop a comprehensive database of 284 coastal African Heritage Sites (AHS), composed of 213 natural and 71 cultural heritage sites, which is then combined with coastal flooding and erosion projections to assess exposure to coastal extreme events for a moderate (RCP4.5) and high (RCP8.5) greenhouse gas emissions scenario. We find that 56 AHS are presently at risk from a 100-year extreme sea-level event, with a total exposed heritage area of 2,222 km 2 . Most of the currently exposed AHS are located in Northern and Western Africa. By mid-century, the number of exposed AHS is projected to increase more than 3 times to reach 191 and 198 under moderate and high emissions respectively. In the second half of the century, the number of exposed sites stabilizes, but the median exposed area increases to 6.6 to 8.5 times the present-day value, under moderate and high emissions, respectively. Mitigation from high to moderate emissions will reduce the end-century median exposed area and number of very highly exposed sites by 20% and 25% respectively.

Many climate extremes, including heatwaves and heavy precipitation events, are projected to worsen 413 under climate change, with important impacts for society. Future projections, required for adaptation, are often based 414 on climate model simulations. Given finite resources, trade-offs must be made concerning model resolution, 415 ensemble size and level of model complexity. Here we focus on the resolution component. A given resolution can be 416 achieved over a region using either global climate models (GCMs) or at lower cost using regional climate models 417 (RCMs) that dynamically downscale coarser GCMs. Both approaches to increasing resolution may better capture 418 small-scale processes and features (downscaling effect), but increased GCM resolution may also improve the 419 representation of large-scale atmospheric circulation (upscaling effect). The size of this upscaling effect is therefore 420 important for deciding modelling strategies. Here we evaluate the benefits of increased model resolution for both 421 global and regional climate models for simulating temperature, precipitation and wind extremes over Europe at 422 resolutions that could currently be realistically used for coordinated sets of climate projections at the pan-European 423 scale. First we examine the benefits of regional downscaling by comparing EURO-CORDEX simulations at 12.5 and 424 50 km resolution to their coarser CMIP5 driving simulations. Secondly, we compare global scale HadGEM3-A 425 simulations at three resolutions (130, 60 and 25 km). Finally, we separate out resolution dependent differences for 426 HadGEM3-A into downscaling and upscaling components using a circulation analogue technique. Results suggest 427 limited benefits of increased resolution for heatwaves, except in reducing hot biases over mountainous regions. 428 Precipitation extremes are sensitive to resolution, particularly over complex orography, with larger totals and heavier 429 tails of the distribution at higher resolution, particularly in the CORDEX vs CMIP5 analysis. CMIP5 models 430 underestimate precipitation extremes, whilst CORDEX simulations overestimate compared to E-OBS, particularly at 431 12.5 km, but results are sensitive to the observational dataset used, with the MESAN reanalysis giving higher totals 432 and heavier tails than E-OBS. Wind extremes are somewhat stronger and heavier tailed at higher resolution, except at 433 coastal regions where large coastal grid boxes spread strong ocean winds further over land. The circulation analogue 434 analysis suggests that differences with resolution for the HadGEM3-A GCM are primarily due to downscaling 435 effects. 436 437

Many climate extremes, including heatwaves and heavy precipitation events, are projected to worsen under climate change, with important impacts for society. Future projections required for adaptation are often based on climate model simulations. Given finite resources, trade-offs must be made concerning model resolution, ensemble size, and level of model complexity. Here we focus on the resolution component. A given resolution can be achieved over a region using either global climate models (GCMs) or at lower cost using regional climate models (RCMs) that dynamically downscale coarser GCMs. Both approaches to increasing resolution may better capture small-scale processes and features (downscaling effect), but increased GCM resolution may also improve the representation of the large-scale atmospheric circulation (upscaling effect). The size of this upscaling effect is therefore important for deciding modelling strategies. Here we evaluate the benefits of increased model resolution for both global and regional climate models for simulating temperature, precipitation, and wind extremes over Europe at resolutions that could currently be realistically used for coordinated sets of climate projections at the pan-European scale. First we examine the benefits of regional downscaling by comparing EURO-CORDEX simulations at 12.5 and 50 km resolution to their coarser CMIP5 driving simulations. Secondly, we compare global-scale HadGEM3-A simulations at three resolutions (130, 60, and 25 km). Finally, we separate out resolution-dependent differences for HadGEM3-A into downscaling and upscaling components using a circulation analogue technique. Results suggest limited benefits of increased resolution for heatwaves, except in reducing hot biases over mountainous regions. Precipitation extremes are sensitive to resolution, particularly over complex orography, with larger totals and heavier tails of the distribution at higher resolution, particularly in the CORDEX vs. CMIP5 analysis. CMIP5 models underestimate precipitation extremes, whilst CORDEX simulations overestimate compared to E-OBS, particularly at 12.5 km, but results are sensitive to the observational dataset used, with the MESAN reanalysis giving higher totals and heavier tails than E-OBS. Wind extremes are somewhat stronger and heavier tailed at higher resolution, except in coastal regions where large coastal grid boxes spread strong ocean winds further over land. The circulation analogue analysis suggests that differences with resolution for the HadGEM3-A GCM are primarily due to downscaling effects.

The summer of 2018 was characterised by prolonged heatwaves over North-Eastern Europe, associated with persistent blocking over Scandinavia, and a jet stream that resided unusually far north on average over this sector. Whilst most event attribution studies tend to focus on the probability or intensity of extreme temperatures themselves, we instead examine whether anthropogenic climate change has affected the likelihood of the circulation pattern that lead to the 2018 hot summer. We examine trends and variability in jet latitude and blocking frequency over the Scandanavian sector in reanalyses, CMIP5 historical simulations, and in two large ensembles of HadGEM3-A simulations with and without anthropogenic forcing. Both the number of blocked days, and the average jet location for last summer were unprecedented in the observational record, and also very rare in climate model simulations. A number of the CMIP5 models examined were able to simulate realistic blocking frequency distributions. Last summer’s circulation did not appear to be part of any systematic increasing trends in blocking frequency or jet latitude in this sector. Instead, this circulation anomaly appears to be explained by a particularly large deviation of natural variability. We will then extend the analysis to examine the western European heatwaves of summer 2019 which were associated with a very different atmospheric circulation pattern –a high pressure ridge which transported warm air northwards from Northern Africa.