Anthropogenic Climate Forcing Research Articles

Tracing human influence on rising surface air temperature in Venezuela

The rise in surface air temperature (SAT) in Venezuela, leading to the loss of all its glaciers, underscores the urgency of understanding human contributions to this phenomenon. This study investigates the impact of anthropogenic climate forcings on SAT across Venezuela, employing observational data, multi-model simulations, and optimal fingerprinting method. Anthropogenic forcings have driven a 0.40–0.85 ∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{\\circ }$$\\end{document}C SAT rise during the industrial era, with land use (LU) emerging as a significant driver (0.36–0.68 ∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{\\circ }$$\\end{document}C), surpassing greenhouse gases (GHGs) (0.10–0.62 ∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{\\circ }$$\\end{document}C). Conversely, anthropogenic aerosols (Aaer) exhibit a cooling effect (− 0.93 to − 0.25 ∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{\\circ }$$\\end{document}C) on SAT. Projections under Representative Concentration Pathways 4.5 indicate substantial SAT increases by the 21st century’s end, underscoring human-induced SAT rise. Effective management of regional Aaer and LU changes in Venezuela holds the potential for mitigating current and future warming and its subsequent impacts on the fragile ecosystem of this region.

Ecosystem damage by increasing tropical cyclones

Climate change is driving an ongoing increase in tropical cyclone (TC) activity. While global economic losses are projected to double by 2100, there are no comparable predictions for TC impacts to coastal ecosystems that protect and sustain human lives and livelihoods. Here, rising North Atlantic TC (NATC) activity from 1970 to 2019, influenced by anthropogenic and natural climate forcing, is used to study the ecosystem impacts of intensifying TCs, potentially indicative of broader future climate change scenarios. Analysis of 97 NATC landfalls revealed 891 immediate post-storm impacts on ecosystems, with particularly detrimental effects on mangrove forests. Specifically, NATCs reduced the performance of individual species. Additionally, they altered community structure and processes through impacts on foundation species and their associated organisms. The severity of impacts was directly correlated with NATC landfall intensity (wind speed) for mangroves, whereas changes to waves, surge, sediments, and salinity caused most impacts on coral reefs, salt marshes, seagrass meadows, and oyster reefs (respectively), indicating complex intensity-damage interactions for many ecosystems. The analyses also revealed a positive correlation between very intense NATC activity and ecosystem damages. The research highlights a concerning trend of escalating impacts on coastal ecosystems under rising storm intensities, with the potential to challenge ecosystem resilience.

Segregated Supply of Sustainable Aviation Fuel to reduce Contrail Energy Forcing – Demonstration and Potentials

Aviation contributes about 4 % to net anthropogenic climate forcing, with contrails being the largest individual contributor to radiative forcing from aviation. One option to mitigate contrail-related climate impacts is using kerosene containing fewer or no aromatic components and thus showing a higher hydrogen content compared to conventional kerosene (i.e., fossil fuel-based). Such “low contrail” kerosene can be provided as a blend of conventional (crude oil-based) and synthetic kerosene or from hydroprocessing conventional kerosene.Low contrail kerosene reduces contrail lifetime and optical thickness and thus the magnitude of contrail climate forcing. However, market shares of such kerosene are presently very low. Simultaneously, a small fraction (< 10 %) of all flights globally accounts for the majority (> 80 %) of global warming contrail climate forcing. Hence, the targeted use of low contrail kerosene on those flights appears promising. But, such an approach would require additional operational efforts, such as a duplication of supply lines and storage tanks.This study evaluates the feasibility and operational efforts of a segregated supply of a 35 m-% SAF-blend (14.34 m-% hydrogen content) to 84 winter time demonstration flights to reduce contrail climate forcing. Between 17th January 2023 and 2nd February 2023, low contrail kerosene was supplied to commercial A320 type aircraft flights on the route between Stockholm and Copenhagen in northern Europe. The operational feasibility and related efforts to target flights with the highest contrail energy forcing as well as a large-scale application are described. The evolution of contrails is tracked using data from the Meteosat Second Generation (MSG) satellite. The contrail energy forcing is calculated for the corresponding flight trajectories assuming another, well-validated engine model (CFM56-5B4 for the simulations instead of LEAP1A-26 for the demonstration flights) using the Contrail Cirrus Prediction (CoCiP) model with meteorological input fields from European Reanalysis data (ERA5).For the first time, the experiment demonstrates the operational feasibility for a segregated supply of low contrail kerosene to medium range aircraft at Stockholm airport. The segregated supply of low contrail kerosene can be realized for short to medium range flights, which can be fueled by a refueller truck. Targeting individual flights via single line hydrant fueling systems seems impractical as of now. Operational efforts to target single flights with highest contrail energy forcing are almost identical to the efforts in this demonstration experiment.Simulations estimate that the segregated supply of the medium blend kerosene (14.34 m-% hydrogen content) can reduce contrail energy forcing by about 11 % assuming the use of a “Rich-Quench-Lean” (RQL) engine (CFM56-5B4). The contrail climate benefit increases to >20 % for a 50 % blend ratio (14.7 m-% hydrogen content). Also, the location and evolution of the demonstration flights’ 28 contrails calculated with CoCiP was tracked with satellite data.The uncertainty of absolute contrail climate forcing estimates is mainly limited due to meteorological data input and also by lacking information on fuel composition in terms of cycloalkane, mono- and polycyclic aromatics content. Contrarily, the uncertainty of relative changes in contrail climate forcing is subject to low uncertainty, since it compares the use of different fuels for an identical fleet and identical weather conditions.

Cloud water adjustments to aerosol perturbations are buffered by solar heating in non-precipitating marine stratocumuli

Abstract. Marine low-level clouds are key to the Earth's energy budget due to their expansive coverage over global oceans and their high reflectance of incoming solar radiation. Their responses to anthropogenic aerosol perturbations remain the largest source of uncertainty in estimating the anthropogenic radiative forcing of climate. A major challenge is the quantification of the cloud water response to aerosol perturbations. In particular, the presence of feedbacks through microphysical, dynamical, and thermodynamical pathways at various spatial and temporal scales could augment or weaken the response. Central to this problem is the temporal evolution in cloud adjustment, governed by entangled feedback mechanisms. We apply an innovative conditional Monte Carlo subsampling approach to a large ensemble of diurnal large-eddy simulation of non-precipitating marine stratocumulus to study the role of solar heating in governing the evolution in the relationship between droplet number and cloud water. We find a persistent negative trend in this relationship at night, confirming that the role of microphysically enhanced cloud-top entrainment. After sunrise, the evolution in this relationship appears buffered and converges to ∼-0.2 in the late afternoon. This buffering effect is attributed to a strong dependence of cloud-layer shortwave absorption on cloud liquid water path. These diurnal cycle characteristics further demonstrate a tight connection between cloud brightening potential and the relationship between cloud water and droplet number at sunrise, which has implications for the impact of the timing of advertent aerosol perturbations.

Earth’s atmosphere protects the biosphere from nearby supernovae

Geological evidence indicates that a supernova within 100 parsecs of Earth occurs around once per million years. Such nearby supernovas can produce an intense gamma-ray burst and a 100-fold increase of cosmic rays, lasting several centuries. We find that the effect of a short burst of gamma rays is small since they are strongly attenuated before reaching the lower stratosphere. Intense cosmic radiation affects stratospheric ozone but, due to compensating effects in catalytic chemical cycles, ozone depletion is moderate and comparable to that from current anthropogenic emissions. This also holds for the low-oxygen atmosphere during early evolution of terrestrial life. We estimate the increase in aerosol and clouds from a 100-fold increase of cosmic rays exerts a radiative forcing comparable in magnitude but opposite in sign to current anthropogenic climate forcing. We conclude that Earth’s atmosphere is effective at shielding the biosphere from nearby supernovae.

Aerosol‐Cloud Interactions From Aviation Soot Emissions

AbstractCurrent models estimate global aviation contributes approximately 5% to the total anthropogenic climate forcing, with aerosol‐cloud interactions having the greatest effect. However, radiative forcing estimates from aviation aerosol‐cloud interactions remain undetermined. There is an expected significant increase in aircraft emissions with aviation demand expected to rise by over 4% per year. Soot may play an important role in the ice nucleation of aircraft‐induced cirrus formation due to a high emission rate, but the ice nucleating properties are poorly constrained. Understanding the microphysical processes leading to atmospheric ice crystal formation is crucial for the reliable parameterization of aerosol‐cloud interactions in climate models due to their impact on precipitation and cloud radiative properties. Ice nucleation of aircraft‐emitted soot is potentially affected by particle morphology with condensation of supercooled water occurring in pores followed by ice nucleation. However, soot has heterogeneous properties and undergoes atmospheric aging and oxidation that could change surface properties and contribute to complex ice nucleation processes. This review synthesizes current knowledge of ice nucleation catalyzed by aviation in the cirrus regime and its effects on global radiative forcing. Further research is required to determine the ice nucleation and microphysical processes of cirrus cloud formation from aviation emissions in both controlled laboratory and field investigations to inform models for more accurate climate predictions and to provide efficient mitigation strategies.

Anthropogenic climate change has influenced global river flow seasonality.

Riverine ecosystems have adapted to natural discharge variations across seasons. However, evidence suggesting that climate change has already impacted magnitudes of river flow seasonality is limited to local studies, mainly focusing on changes of mean or extreme flows. This study introduces the use of apportionment entropy as a robust measure to assess flow-volume nonuniformity across seasons, enabling a global analysis. We found that ~21% of long-term river gauging stations exhibit significant alterations in seasonal flow distributions, but two-thirds of these are unrelated to trends in annual mean discharge. By combining a data-driven runoff reconstruction with state-of-the-art hydrological simulations, we identified a discernible weakening of river flow seasonality in northern high latitudes (above 50°N), a phenomenon directly linked to anthropogenic climate forcing.

Predicting tail risks and the evolution of temperatures

This paper explores a range of simple models to study the relationship between global temperature anomalies and climate forcings. In particular, we consider quantile regression models with potentially time-varying parameters (TVP), implemented by Bayesian methods. In its most general specification, this approach is flexible in that it models distinct regions of distribution of global temperature anomalies, while also allowing us to investigate changes in the relationship between (natural and anthropogenic) climate forcings and temperatures. Our results indicate that there is indeed considerable variation over time in the relationship between temperatures and its drivers, and that these effects may be heterogeneous across different quantiles. We then perform a long-range forecasting exercise for temperatures, which suggests that incorporating TVP or explicitly modelling quantile levels or the combination of both features can improve prediction for different parts of the temperature distribution. In addition, we produce forecasts for 2030 considering the intermediate RCP 4.5 scenario: given that no single specification dominates, we account for model uncertainty by considering forecast averaging across all specifications. Our approach allows us to make statements about the probability of temperature levels — for instance, we find that a scenario of +1.8 °C will occur with a non-negligible probability under RCP 4.5.

Effect of a severe cold spell on overwintering survival of an invasive forest insect pest

Cold temperatures can play a significant role in the range and impact of pest insects. Severe cold events can reduce the size of insect outbreaks and perhaps even cause outbreaks to end. Measuring the precise impact of cold events, however, can be difficult because estimates of insect mortality are often made at the end of the winter season. In late January 2023 long-term climate models predicted a significant cold event to occur over eastern North America. We used this event to evaluate the immediate impact on hemlock woolly adelgid (Adelges tsugae Annand) overwintering mortality at four sites on the northern edge of the insects invaded range in eastern North America. We observed complete mortality, partial mortality and no effects on hemlock woolly adelgid mortality that correlated with the location of populations and strength of the cold event. Our data showed support for preconditioning of overwintering adelgids having an impact on their overwintering survival following this severe cold event. Finally, we compared the climatic conditions at our sites to historical weather data and previous observations of mortality in Nova Scotia. The cold event observed in February 2023 resulted in the coldest temperatures observed at these sites, including the period within which hemlock woolly adelgid invaded, suggesting cold conditions, especially under anthropogenic climate forcing, may not be a limiting factor in determining the ultimate northern range of hemlock woolly adelgid in eastern North America.

Globally limited but severe shallow-shelf euxinia during the end-Triassic extinction

One of the most severe extinctions of complex marine life in Earth’s history occurred at the end of the Triassic period (~201.4 million years ago). The marine extinction was initiated by large igneous province volcanism and has tentatively been linked to the spread of anoxic conditions. However, the global-scale pattern of anoxic conditions across the end-Triassic event is not well constrained. Here we use the sedimentary enrichment and isotopic composition of the redox-sensitive element molybdenum to reconstruct global–local marine redox conditions through the extinction interval. Peak δ98Mo values indicate that the global distribution of sulfidic marine conditions was similar to the modern ocean during the extinction interval. Meanwhile, Tethyan shelf sediments record pulsed, positive δ98Mo excursions indicative of locally oxygen-poor, sulfidic conditions. We suggest that pulses of severe marine de-oxygenation were restricted largely to marginal marine environments during the latest Triassic and played a substantial role in shallow-marine extinction phases at that time. Importantly, these results show that global marine biodiversity, and possibly ecosystem stability, were vulnerable to geographically localized anoxic conditions. Expanding present-day marine anoxia in response to anthropogenic marine nutrient supply and climate forcing may therefore have substantial consequences for global biodiversity and wider ecosystem stability.

Rapid saturation of cloud water adjustments to shipping emissions

Abstract. Human aerosol emissions change cloud properties by providing additional cloud condensation nuclei. This increases cloud droplet numbers, which in turn affects other cloud properties like liquid-water content and ultimately cloud albedo. These adjustments are poorly constrained, making aerosol effects the most uncertain part of anthropogenic climate forcing. Here we show that cloud droplet number and water content react differently to changing emission amounts in shipping exhausts. We use information about ship positions and modeled emission amounts together with reanalysis winds and satellite retrievals of cloud properties. The analysis reveals that cloud droplet numbers respond linearly to emission amount over a large range (1–10 kg h−1) before the response saturates. Liquid water increases in raining clouds, and the anomalies are constant over the emission ranges observed. There is evidence that this independence of emissions is due to compensating effects under drier and more humid conditions, consistent with suppression of rain by enhanced aerosol. This has implications for our understanding of cloud processes and may improve the way clouds are represented in climate models, in particular by changing parameterizations of liquid-water responses to aerosol.

The competition between anthropogenic aerosol and greenhouse gas climate forcing is revealed by North Pacific water-mass changes.

Modeled water-mass changes in the North Pacific thermocline, both in the subsurface and at the surface, reveal the impact of the competition between anthropogenic aerosols (AAs) and greenhouse gases (GHGs) over the past 6 decades. The AA effect overwhelms the GHG effect during 1950-1985 in driving salinity changes on density surfaces, while after 1985 the GHG effect dominates. These subsurface water-mass changes are traced back to changes at the surface, of which ~70% stems from the migration of density surface outcrops, equatorward due to regional cooling by AAs and subsequent poleward due to warming by GHGs. Ocean subduction connects these surface outcrop changes to the main thermocline. Both observations and models reveal this transition in climate forcing around 1985 and highlight the important role of AA climate forcing on our oceans' water masses.

An update on the influence of natural climate variability and anthropogenic climate change on tropical cyclones

A substantial number of studies have been published since the Ninth International Workshop on Tropical Cyclones (IWTC-9) in 2018, improving our understanding of the effect of climate change on tropical cyclones (TCs) and associated hazards and risks. These studies have reinforced the robustness of increases in TC intensity and associated TC hazards and risks due to anthropogenic climate change. New modeling and observational studies suggested the potential influence of anthropogenic climate forcings, including greenhouse gases and aerosols, on global and regional TC activity at the decadal and century time scales. However, there are still substantial uncertainties owing to model uncertainty in simulating historical TC decadal variability in the Atlantic, and the limitations of observed TC records. The projected future change in the global number of TCs has become more uncertain since IWTC-9 due to projected increases in TC frequency by a few climate models. A new paradigm, TC seeds, has been proposed, and there is currently a debate on whether seeds can help explain the physical mechanism behind the projected changes in global TC frequency. New studies also highlighted the importance of large-scale environmental fields on TC activity, such as snow cover and air-sea interactions. Future projections on TC translation speed and medicanes are new additional focus topics in our report. Recommendations and future research are proposed relevant to the remaining scientific questions and assisting policymakers.

Interhemispheric Contrasts of Ocean Heat Content Change Reveals Distinct Fingerprints of Anthropogenic Climate Forcings

AbstractDuring recent decades, both greenhouse gases (GHGs) and anthropogenic aerosols (AAs) drove major changes in the Earth's energy imbalance. However, their respective fingerprints in changes to ocean heat content (OHC) have been difficult to isolate and detect when global or hemispheric averages are used. Based on a pattern recognition analysis, we show that AAs drive an interhemispheric asymmetry within the 20°‐35° latitude band in historical OHC change due to the southward shift of the atmospheric and ocean circulation system. This forced pattern is distinct from the GHG‐induced pattern, which dominates the asymmetry in higher latitudes. Moreover, it is found that this significant aerosol‐forced OHC trend pattern can only be captured in analyzed periods of 20 years or longer and including 1975–1990. Using these distinct spatiotemporal characteristics, we show that the fingerprint of aerosol climate forcing in ocean observations can be distinguished from both the stronger GHG‐induced signals and internal variability.

Atmospheric Methane: Comparison Between Methane's Record in 2006–2022 and During Glacial Terminations

AbstractAtmospheric methane's rapid growth from late 2006 is unprecedented in the observational record. Assessment of atmospheric methane data attributes a large fraction of this atmospheric growth to increased natural emissions over the tropics, which appear to be responding to changes in anthropogenic climate forcing. Isotopically lighter measurements of are consistent with the recent atmospheric methane growth being mainly driven by an increase in emissions from microbial sources, particularly wetlands. The global methane budget is currently in disequilibrium and new inputs are as yet poorly quantified. Although microbial emissions from agriculture and waste sources have increased between 2006 and 2022 by perhaps 35 Tg/yr, with wide uncertainty, approximately another 35–45 Tg/yr of the recent net growth in methane emissions may have been driven by natural biogenic processes, especially wetland feedbacks to climate change. A model comparison shows that recent changes may be comparable or greater in scale and speed than methane's growth and isotopic shift during past glacial/interglacial termination events. It remains possible that methane's current growth is within the range of Holocene variability, but it is also possible that methane's recent growth and isotopic shift may indicate a large‐scale reorganization of the natural climate and biosphere is under way.

Overstating the effects of anthropogenic climate change? A critical assessment of attribution methods in climate science

Climate scientists have proposed two methods to link extreme weather events and anthropogenic climate forcing: the probabilistic and the storyline approach. Proponents of the first approach have raised the criticism that the storyline approach could be overstating the role of anthropogenic climate change. This issue has important implications because, in certain contexts, decision-makers might seek to avoid information that overstates the effects of anthropogenic climate change. In this paper, we explore two research questions. First, whether and to what extent the storyline approach overstates the effects of anthropogenic climate change. Second, whether the objections offered against the storyline approach constitute good reasons to prefer the probabilistic approach. Concerning the first question, we show that the storyline approach does not necessarily overstate the effects of climate change, and particularly not for the reasons offered by proponents of the probabilistic approach. Concerning the second question, we show, independently, that the probabilistic approach faces the same or very similar objections to those raised against the storyline approach due to the lack of robustness of climate models and the way events are commonly defined when applying the probabilistic approach. These results suggest that these objections might not constitute good reasons to prefer the probabilistic approach over the storyline approach.

Renewable fuel options for aviation – A System-Wide comparison of Drop-In and non Drop-In fuel options

The air transport sector’s contribution to anthropogenic climate forcing is estimated at around 4 %. In this respect, alternative fuels are considered as a key measure to reduce greenhouse gas emissions within the air transport system and thus to reduce the contribution of commercial aviation to anthropogenic climate change. Nevertheless, so far the usage-shares of alternative fuels are below 1 %. Thus, the goal of this paper is to analyse a broad selection of aviation fuel options for their greenhouse gas reduction potential based on a set of nine assessment criteria. The fuel options studied are blended synthetic kerosene and neat synthetic kerosene fuels, three alcohols (methanol, ethanol and butanol) and three gases (methane, ammonia and hydrogen). For these fuels a multi-criteria assessment based on a five-step ordinal scale is performed. The assessment results show that the availability of some biogenic feedstock is not sufficient to replace the present overall energy demand of aviation; while the availability of non-biogenic fuels might increase in the near- to mid-term. In general, alternative kerosene-like fuel options are mainly constrained by lacking production capacities, while alcohols and gases considered are constrained by aircraft modifications or development of new aircraft types. All discussed fuel options have the potential to mitigate greenhouse gas emissions of commercial aviation substantially also from a life cycle perspective. Within the next decade, especially alternative kerosene-like options with (intermediate) products experiencing demand from other sectors, such as FT-SPK or AtJ-SPK, show a high development potential. The same holds for hydrogen provided that a hydrogen fueled aircraft will be brought successfully to the market.

Preparation for Denitrification and Phenotypic Diversification at the Cusp of Anoxia: a Purpose for N2O Reductase Vis-à-Vis Multiple Roles of O2.

Adaptation to anoxia by synthesizing a denitrification proteome costs metabolic energy, and the anaerobic respiration conserves less energy per electron than aerobic respiration. This implies a selective advantage of the stringent O2 repression of denitrification gene transcription, which is found in most denitrifying bacteria. In some bacteria, the metabolic burden of adaptation can be minimized further by phenotypic diversification, colloquially termed "bet-hedging," where all cells synthesize the N2O reductase (NosZ) but only a minority synthesize nitrite reductase (NirS), as demonstrated for the model strain Paracoccus denitrificans. We hypothesized that the cells lacking NirS would be entrapped in anoxia but with the possibility of escape if supplied with O2 or N2O. To test this, cells were exposed to gradual O2 depletion or sudden anoxia and subsequent spikes of O2 and N2O. The synthesis of NirS in single cells was monitored by using an mCherry-nirS fusion replacing the native nirS, and their growth was detected as dilution of green, fluorescent fluorescein isothiocyanate (FITC) stain. We demonstrate anoxic entrapment due to e--acceptor deprivation and show that O2 spiking leads to bet-hedging, while N2O spiking promotes NirS synthesis and growth in all cells carrying NosZ. The cells rescued by the N2O spike had much lower respiration rates than those rescued by the O2 spike, however, which could indicate that the well-known autocatalytic synthesis of NirS via NO production requires O2. Our results bring into relief a fitness advantage of pairing restrictive nirS expression with universal NosZ synthesis in energy-limited systems. IMPORTANCE Denitrifying bacteria have evolved elaborate regulatory networks securing their respiratory metabolism in environments with fluctuating oxygen concentrations. Here, we provide new insight regarding their bet-hedging in response to hypoxia, which minimizes their N2O emissions because all cells express NosZ, reducing N2O to N2, while a minority express NirS + Nor, reducing NO2- to N2O. We hypothesized that the cells without Nir were entrapped in anoxia, without energy to synthesize Nir, and that they could be rescued by short spikes of O2 or N2O. We confirm such entrapment and the rescue of all cells by an N2O spike but only a fraction by an O2 spike. The results shed light on the role of O2 repression in bet-hedging and generated a novel hypothesis regarding the autocatalytic nirS expression via NO production. Insight into the regulation of denitrification, including bet-hedging, holds a clue to understanding, and ultimately curbing, the escalating emissions of N2O, which contribute to anthropogenic climate forcing.

Responses of compound daytime and nighttime warm-dry and warm-humid events to individual anthropogenic forcings

Daytime heat is often associated with reduced soil moisture and cloud cover, while nighttime heat is connected to high humidity and increased cloud cover. Due to these differing mechanisms, compound daytime and nighttime heat events may respond differently to major anthropogenic forcings (greenhouse gases, anthropogenic aerosols, land-use and land-cover change). Here, we use GISS ModelE2.1-G historical single-forcing runs from 1955 to 2014 to examine how individual anthropogenic forcings affect compound heat events—specifically warm daytime and nighttime temperatures compounded with dry precipitation or high humidity conditions. We show that greenhouse gases alone amplify the natural frequency of warm-dry events by 1.5–5 times and warm-humid events by 2–9 times in tropical and extratropical latitudes. Conversely, aerosols and land-use/land-cover change reduce the frequency of these events, resulting in more modest increases and in some regions, declines, in the historical ‘all-forcings’ scenario. Individually, aerosol effects are stronger and more widespread compared to land-use, oftentimes reducing the natural frequency of these events by 60%–100%. The responses of these compound events are primarily driven by changes in daytime and nighttime temperatures through large-scale warming via greenhouse gases and cooling from aerosols and land-use/land-cover change. However, changes in warm-dry events are amplified in regions with concurrent precipitation declines (e.g. Central America, Mediterranean regions) and warm-humid events are amplified by global concurrent humidity increases. Additionally, we find differences between daytime and nighttime compound responses in the historical experiment that can be traced back to the individual forcings. In particular, aerosols produce a greater cooling effect on daytime relative to nighttime temperatures, which notably results in a historical reduction of Northern Hemisphere daytime warm-dry events relative to natural conditions. Our analysis provides a more comprehensive understanding of the significant impacts of different anthropogenic climate forcings on daytime and nighttime warm-dry and warm-humid events, informing future risk and impact assessments.

Multi‐model projections of tree species performance in Quebec, Canada under future climate change

Many modelling approaches have been developed to project climate change impacts on forests. By analysing 'comparable' yet distinct variables (e.g. productivity, growth, dominance, biomass, etc.) through different structures, parameterizations and assumptions, models can yield different outcomes to rather similar initial questions. This variability can lead to some confusion for forest managers when developing strategies to adapt forest management to climate change. In this study, we standardized results from seven different models (Habitat suitability, trGam, StandLEAP, Quebec Landscape Dynamics, PICUS, LANDIS-II and LPJ-LMfire) to provide a simple and comprehensive assessment of the uncertainty and consensus in future performance (decline, status quo, improvement) for six tree species in Quebec under two radiative forcing scenarios (RCP 4.5 and RCP 8.5). Despite a large diversity of model types, we found a high level of agreement (73.1%) in projected species' performance across species, regions, scenarios and time periods. Low agreements in model outcomes resulted from small dissensions among models. Model agreement was much higher for cold-tolerant species (up to 99.9%), especially in southernmost forest regions and under RCP 8.5, indicating that these species are especially sensitive to increased climate forcing in the southern part of their distribution range. Lower agreement was found for thermophilous species (sugar maple, yellow birch) in boreal regions under RCP 8.5 mostly as a result of the way the different models are handling natural disturbances (e.g. wildfires) and lags in the response of populations (forest inertia or migration capability) to climate change. Agreement was slightly higher under high anthropogenic climate forcing, suggesting that important thresholds in species-specific performance might be crossed if radiative forcing reach values as high as those projected under RCP 8.5. We expect that strong agreement among models despite their different assumptions, predictors and structure should inspire the development of forest management strategies to be better adapted to climate change.