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The world has changed: You can feel it in the air as temperatures rise, you can feel it in the water as precipitation patterns change. Much of what has once been our daily weather is lost. It began with humankind emitting greenhouse gases and aerosols. However, there is a growing resistance that wants to limit theses emissions. Daily variability can be described by probability density functions (PDF). Change can manifest as changes of the mean properties of weather-related variables, and/or changes in the chape of their PDFs. In this study, we examine how regional PDF shapes change due to increasing temperature, driven primarily by greenhouse gas emissions, and due to emissions of different aerosols species (black carbon and sulfate). Our main questions are: (1) How do shapes of regional daily PDFs evolve with global warming? (2) How do these changes differ in response to aerosol and greenhouse gas emissions? (3) And which aerosol-related teleconnections induce these changes in PDF shapes? As changes in shape affect low and high extremes differently, we aim to link changes in PDF shape to changes in extreme events of daily temperature and precipitation by using parameters describing PDF width and asymmetry.We use data from PDRMIP single forcer climate model simulations to examine how changes in regional and global aerosol concentrations change the PDF shapes. We also use three CMIP6 large ensembles (MPI-ESM1-2-LR, CanESM5 and ACCESS-ESM1-5) to examine changes in PDF shape at five different levels of global warming, from 1°C to 4 °C. Our main questions are how the shapes of regional daily PDFs evolve with globalwarming, how their changes differ between aerosol and greenhouse gas induced changes, and what teleconnections due to regional aerosol changes induce in PDF shapes.For the time will soon come when aerosols will shape the near future of our weather.

Mineral dust is an important aerosol specie in the atmosphere that impacts the Earth’s climate system through its interactions with radiation, clouds, hydrology, atmospheric chemistry, and biogeochemistry. Because dust sizes span more than three orders of magnitude in diameter and dust properties are size-dependent, most previous studies separate dust particles into different classes – broadly defined as fine and coarse dust – which could produce distinct impacts on the Earth system. However, there are general inconsistencies in the terminology, the diameter boundaries, and diameter ranges currently attributed to dust size classes across the literature. As part of a comprehensive review of coarse dust recently completed, we propose, with justification, a new uniform classification that defines coarse and super-coarse dust as particles between 2.5 - 10 µm and 10 - 62.5 µm in diameter, respectively. In addition, we will show several lines of observational evidence that indicate coarse and super-coarse dust particles are transported much farther than previously expected and that the abundance of these particles is substantially underestimated in current global models. Despite the limitations of representing coarse and super-coarse dust aerosols in models, we will highlight their unique impacts on several aspects of the Earth's climate system.

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.

Desert dust accounts for a large fraction of shortwave radiation absorbed by aerosols, which adds to the climate warming produced by greenhouse gases. However, it remains uncertain exactly how much shortwave radiation dust absorbs. We leverage in-situ measurements of dust single-scattering albedo to constrain absorption at mid-visible wavelength by North African dust, which accounts for approximately half of the world's dust. We find that models overestimate North African dust absorption aerosol optical depth (AAOD) by up to a factor of two. This occurs primarily because models overestimate the dust imaginary refractive index, the effect of which is partially masked by an underestimation of large dust particles. Additionally, similar factors contribute to an overestimation of AAOD retrieved by the ground-based Aerosol Robotic Network over North Africa. We conclude that the overestimation of simulated and retrieved dust absorption suggests substantial biases in current estimates of dust impacts on the Earth system, including a warm bias in dust radiative effects.

The uncertainty in aerosol radiative forcing is currently the largest source of uncertainty in estimates of the magnitude of the total anthropogenic forcing on climate, and changes in aerosol emissions are likely important for regional climate over the next few decades. This is especially the case for Africa and Asia where large aerosol emission changes are anticipated, and where aerosol has played an important role in historical changes. Uncertainty in near-term projections due to the substantial spread in aerosol (or their precursor) emissions pathways is compounded by uncertainty in the simulated response to these emissions, so a multi-model framework is needed to identify robust changes. Several earlier studies have explored the climate response to regional aerosol perturbations, with interesting, but not always consistent, results. Using these studies to inform our understanding of the potential role of aerosol in near-future changes is not straightforward. Many are based around equilibrium experiments that are challenging to use to interpret transient simulations, and the effects of different experimental designs are difficult to separate from the effects of structural differences between the models. In Regional Aerosol MIP, we will perform a set of transient experiments based on emissions from the Shared Socioeconomic Pathways. Regional Aerosol MIP will better enable us to assess the potential contribution of aerosol to near-future climate change, to describe the robust features of the response to regional aerosol changes, and to identify where the key uncertainties lie. In this presentation we will introduce the experiment design, alongside some early analysis.

Enhanced emissions of both greenhouse gases and aerosols generate climate responses on a wide range of time scales. An initial radiative response triggers a set of rapid adjustments, which are eventually followed by surface-temperature-driven feedbacks. While a lot happens during the first days and months after a perturbation, the monthly mean data typically used in climate studies are too coarse to show the temporal evolution of responses. In these analyses, we take a closer look at how the climate system responds during the very first hours and days after a sudden increase in carbon dioxide (CO2), in black carbon (BC) or in sulfate (SO4). Five models have performed PDRMIP simulations with hourly output, and we also compare results to monthly PDRMIP and CMIP6 results. We find that the effect of increasing ocean temperatures kicks in after a couple of months. Rapid precipitation reductions are for all three climate perturbations established after just a couple of days, and does for BC not differ much from the full-time response. For CO2 and SO4, the magnitude of the precipitation response gradually increases with surface warming, and for CO2 the sign of the response changes for negative to positive after two years. Rapid cloud adjustments are typically established within the first 24 hours and while the magnitude of cloud feedbacks for CO2 and SO4 increases over time, the latitude-height pattern of the total cloud changes is clearly present after one year. While previously known that climate responses to BC are dominated by rapid adjustments, this work underlines the swiftness of the processes involved.

Understanding the regional surface temperature responses to different anthropogenic climate forcing agents, such as greenhouse gases and aerosols, is crucial for understanding past and future regional climate changes. In modern climate models, the regional temperature responses vary greatly for all major forcing agents, but the causes of this variability are poorly understood. Here, we analyze how changes in atmospheric and oceanic energy fluxes due to perturbations in different anthropogenic climate forcing agents lead to changes in global and regional surface temperatures. We use climate model data on idealized perturbations in four major anthropogenic climate forcing agents (CO2, CH4, sulfate, and black carbon aerosols) from Precipitation Driver Response Model Intercomparison Project (PDRMIP) climate experiments for six climate models (CanESM2, HadGEM2-ES, NCAR-CESM1-CAM4, NorESM1, MIROC-SPRINTARS, GISS-E2). Particularly, we decompose the regional energy budget contributions to the surface temperature responses due to changes in longwave and shortwave fluxes under clear-sky and cloudy conditions, surface albedo changes, and oceanic and atmospheric energy transport. We also analyze the regional model-to-model temperature response spread due to each of these components. The global surface temperature response stems from changes in longwave emissivity for greenhouse gases (CO2 and CH4) and mainly from changes in shortwave clear-sky fluxes for aerosols (sulfate and black carbon). The global surface temperature response normalized by effective radiative forcing is nearly the same for all forcing agents (0.63, 0.54, 0.57, 0.61 K W−1 m2). While the main physical processes driving global temperature responses vary between forcing agents, for all forcing agents the model-to-model spread in temperature responses is dominated by differences in modeled changes in longwave clear-sky emissivity. Furthermore, in polar regions for all forcing agents the differences in surface albedo change is a key contributor to temperature responses and its spread. For black carbon, the modeled differences in temperature response due to shortwave clear-sky radiation are also important in the Arctic. Regional model-to-model differences due to changes in shortwave and longwave cloud radiative effect strongly modulate each other. For aerosols, clouds play a major role in the model spread of regional surface temperature responses. In regions with strong aerosol forcing, the model-to-model differences arise from shortwave clear-sky responses and are strongly modulated by combined temperature responses to oceanic and atmospheric heat transport in the models.

International audience; A correct representation of the spatial distribution of aerosols in atmospheric models is essential for realistic simulations of deposition and calculations of radiative forcing. It has been observed that transport of black carbon (BC) into the Arctic and scavenging is sometimes not captured accurately enough in chemistry transport models (CTM) as well as global circulation models (GCM). In this study we determine the discrepancies between measured equivalent BC (EBC) and modeled BC for several Arctic measurement stations as well as for Arctic aircraft campaigns. For this, we use the output of a set of 5 models based on the same emission dataset (ECLIPSE emissions, see eclipse.nilu.no) and evaluate the simulated concentrations at the measurement locations and times. Emissions are separated for different sources such as biomass burning, domestic heating, gas flaring, industry and the transport sector. We focus on the years 2008 and 2009, where many campaigns took place in the framework of the International Polar Year. Arctic stations like Barrow, Alert, Station Nord in Greenland and Zeppelin show a very pronounced winter/spring maximum in BC. While monthly averaged measured EBC values are around 80 ng/m^3, the models severely underestimate this with some models simulating only a small percentage of the observed values. During summer measured concentrations are a magnitude lower, and still underestimated by almost an order of magnitude in some models. However, the best models are correct within a factor of 2 in winter/spring and give realistic concentrations in summer. In order to get information on the vertical profile we used measurements from aircraft campaigns like ARCTAS, ARCPAC and HIPPO. It is found that BC in latitudes below 60 degrees is better captured by the models than BC at higher latitudes, even though it is overestimated at high altitudes. A systematic analysis of the performance of different models is presented. With the dataset we use we capture remote, polluted and fire-influenced conditions. We estimate the impact of model deficiencies on calculated BC radiative forcing by introducing scaling factors based on the model-measurement comparisons.