Atmospheric concentrations of black carbon are substantially higher in spring than summer in the Arctic

A key driving factor behind rapid Arctic climate change is black carbon, the atmospheric aerosol that most efficiently absorbs sunlight. Our knowledge about black carbon in the Arctic is scarce, mainly limited to long-term measurements of a few ground stations and snap-shots by aircraft observations. Here, we combine observations from aircraft campaigns performed over nine years, and present vertically resolved average black carbon properties. A factor of four higher black carbon mass concentration (21.6 ng m$^{–3}$ average, 14.3 ng m$^{–3}$ median) was found in spring, compared to summer (4.7 ng m$^{–3}$ average, 3.9 ng m$^{–3}$ median). In spring, much higher inter-annual and geographic variability prevailed compared to the stable situation in summer. The shape of the black carbon size distributions remained constant between seasons with an average mass mean diameter of 202 nm in spring and 210 nm in summer. Comparison between observations and concentrations simulated by a global model shows notable discrepancies, highlighting the need for further model developments and intensified measurements

Global and Regional Trends of Atmospheric Sulfur

The profound changes in global SO[subscript 2] emissions over the last decades have affected atmospheric composition on a regional and global scale with large impact on air quality, atmospheric deposition and the radiative forcing of sulfate aerosols. Reproduction of historical atmospheric pollution levels based on global aerosol models and emission changes is crucial to prove that such models are able to predict future scenarios. Here, we analyze consistency of trends in observations of sulfur components in air and precipitation from major regional networks and estimates from six different global aerosol models from 1990 until 2015. There are large interregional differences in the sulfur trends consistently captured by the models and observations, especially for North America and europe. europe had the largest reductions in sulfur emissions in the first part of the period while the highest reduction came later in North America and east Asia. the uncertainties in both the emissions and the representativity of the observations are larger in Asia. However, emissions from East Asia clearly increased from 2000 to 2005 followed by a decrease, while in India a steady increase over the whole period has been observed and modelled. the agreement between a bottom-up approach, which uses emissions and process-based chemical transport models, with independent observations gives an improved confidence in the understanding of the atmospheric sulfur budget

Reduced Complexity Model Intercomparison Project Phase 1: introduction and evaluation of global-mean temperature response

Reduced-complexity climate models (RCMs) are critical in the policy and decision making space, and are directly used within multiple Intergovernmental Panel on Climate Change (IPCC) reports to complement the results of more comprehensive Earth system models. To date, evaluation of RCMs has been limited to a few independent studies. Here we introduce a systematic evaluation of RCMs in the form of the Reduced Complexity Model Intercomparison Project (RCMIP). We expect RCMIP will extend over multiple phases, with Phase 1 being the first. In Phase 1, we focus on the RCMs' global-mean temperature responses, comparing them to observations, exploring the extent to which they emulate more complex models and considering how the relationship between temperature and cumulative emissions of CO2 varies across the RCMs. Our work uses experiments which mirror those found in the Coupled Model Intercomparison Project (CMIP), which focuses on complex Earth system and atmosphere–ocean general circulation models. Using both scenario-based and idealised experiments, we examine RCMs' global-mean temperature response under a range of forcings. We find that the RCMs can all reproduce the approximately 1 ∘C of warming since pre-industrial times, with varying representations of natural variability, volcanic eruptions and aerosols. We also find that RCMs can emulate the global-mean temperature response of CMIP models to within a root-mean-square error of 0.2 ∘C over a range of experiments. Furthermore, we find that, for the Representative Concentration Pathway (RCP) and Shared Socioeconomic Pathway (SSP)-based scenario pairs that share the same IPCC Fifth Assessment Report (AR5)-consistent stratospheric-adjusted radiative forcing, the RCMs indicate higher effective radiative forcings for the SSP-based scenarios and correspondingly higher temperatures when run with the same climate settings. In our idealised setup of RCMs with a climate sensitivity of 3 ∘C, the difference for the ssp585–rcp85 pair by 2100 is around 0.23∘C(±0.12 ∘C) due to a difference in effective radiative forcings between the two scenarios. Phase 1 demonstrates the utility of RCMIP's open-source infrastructure, paving the way for further phases of RCMIP to build on the research presented here and deepen our understanding of RCMs

Cloudy sky contributions to the direct aerosol effect

The radiative forcing of the aerosol-radiation interaction can be decomposed into clear sky and cloudy sky portions. Two sets of multi-model simulations within AeroCom, combined with observational methods, and the time evolution of aerosol emissions over the industrial era show that the contribution from cloudy sky regions is likely weak. A mean of the simulations considered is 0.01 ± 0.1 Wm-2. Multivariate data analysis of results from AeroCom Phase II shows that many factors influence the strength of the cloudy sky contribution to the forcing of the aerosol-radiation interaction. Overall, single scattering albedo of anthropogenic aerosols and the interaction of aerosols with the shortwave cloud radiative effects are found to be important factors. A more dedicated focus on the contribution from the cloud free and cloud covered sky fraction respectively to the aerosol-radiation interaction will benefit the quantification of the radiative forcing and its uncertainty range

Tracking uncertainties in the causal chain from human activities to climate

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95537/1/grl25489.pd

Multi-model simulations of aerosol and ozone radiative forcing for the period 1990-2015

Over the past decades, the geographical distribution of emissions of substances that alter the atmospheric energy balance has changed due to economic growth and pollution regulations. Here, we show the resulting changes to aerosol and ozone abundances and their radiative forcing, using recently updated emission data for the period 1990–2015, as simulated by seven global atmospheric composition models. The models broadly reproduce the large-scale changes in surface aerosol and ozone based on observations (e.g., −1 to −3 %/yr in aerosols over US and Europe). The global mean radiative forcing due to ozone and aerosols changes over the 1990–2015 period increased by about +0.2 W m−2, with approximately 1/3 due to ozone. This increase is stronger positive than reported in IPCC AR5. The main reason for the increased positive radiative forcing of aerosols over this period is the substantial reduction of global mean SO2 emissions which is stronger in the new emission inventory compared to the IPCC, and higher black carbon emissions

Indicate separate contributions of long-lived and short-lived greenhouse gases in emission targets

European Union funding: 821205, 821003, 820829. Wellcome Trust: 205212/Z/16/

The IPCC Sixth Assessment Report WGIII climate assessment of mitigation pathways: from emissions to global temperatures

While the Intergovernmental Panel on Climate Change (IPCC) physical science reports usually assess a handful of future scenarios, the Working Group III contribution on climate mitigation to the IPCC's Sixth Assessment Report (AR6 WGIII) assesses hundreds to thousands of future emissions scenarios. A key task in WGIII is to assess the global mean temperature outcomes of these scenarios in a consistent manner, given the challenge that the emissions scenarios from different integrated assessment models (IAMs) come with different sectoral and gas-to-gas coverage and cannot all be assessed consistently by complex Earth system models. In this work, we describe the "climate-assessment"workflow and its methods, including infilling of missing emissions and emissions harmonisation as applied to 1202 mitigation scenarios in AR6 WGIII. We evaluate the global mean temperature projections and effective radiative forcing (ERF) characteristics of climate emulators FaIRv1.6.2 and MAGICCv7.5.3 and use the CICERO simple climate model (CICERO-SCM) for sensitivity analysis. We discuss the implied overshoot severity of the mitigation pathways using overshoot degree years and look at emissions and temperature characteristics of scenarios compatible with one possible interpretation of the Paris Agreement. We find that the lowest class of emissions scenarios that limit global warming to "1.5 ° C (with a probability of greater than 50 %) with no or limited overshoot"includes 97 scenarios for MAGICCv7.5.3 and 203 for FaIRv1.6.2. For the MAGICCv7.5.3 results, "limited overshoot"typically implies exceedance of median temperature projections of up to about 0.1 ° C for up to a few decades before returning to below 1.5 ° C by or before the year 2100. For more than half of the scenarios in this category that comply with three criteria for being "Paris-compatible", including net-zero or net-negative greenhouse gas (GHG) emissions, median temperatures decline by about 0.3-0.4 ° C after peaking at 1.5-1.6 ° C in 2035-2055. We compare the methods applied in AR6 with the methods used for SR1.5 and discuss their implications. This article also introduces a "climate-assessment"Python package which allows for fully reproducing the IPCC AR6 WGIII temperature assessment. This work provides a community tool for assessing the temperature outcomes of emissions pathways and provides a basis for further work such as extending the workflow to include downscaling of climate characteristics to a regional level and calculating impacts

Effective radiative forcing from emissions of reactive gases and aerosols – a multi-model comparison

This paper quantifies the pre-industrial (1850) to present-day (2014) effective radiative forcing (ERF) of anthropogenic emissions of NOX, VOCs (including CO), SO2, NH3, black carbon, organic carbon, and concentrations of methane, N2O and ozone-depleting halocarbons, using CMIP6 models. Concentration and emission changes of reactive species can cause multiple changes in the composition of radiatively active species: tropospheric ozone, stratospheric ozone, stratospheric water vapour, secondary inorganic and organic aerosol and methane. Where possible we break down the ERFs from each emitted species into the contributions from the composition changes. The ERFs are calculated for each of the models that participated in the AerChemMIP experiments as part of the CMIP6 project, where the relevant model output was available. The 1850 to 2014 multi-model mean ERFs (± standard deviations) are -1.03 ± 0.37 Wm-2 for SO2 emissions, -0.25 ± 0.09 Wm-2 for organic carbon (OC), 0.15 ± 0.17 Wm-2 for black carbon (BC) and for NH3 it is -0.07 ± 0.01Wm-2. For the combined aerosols (in the piClim-aer experiment) it is -1.01 ±0.25 Wm-2. The multi-model means for the reactive well-mixed greenhouse gases (including any effects on ozone and aerosol chemistry) are 0.67 ± 0.17 Wm-2 for methane (CH4), 0.26 ± 0.07 Wm-2 for nitrous oxide (N2O) and 0.12 ± 0.2 Wm-2 for ozone-depleting halocarbons (HC). Emissions of the ozone precursors nitrogen oxides (NOx), volatile organic compounds (VOC) and both together (O3) lead to ERFs of 0.14 ± 0.13 Wm-2, 0.09 ± 0.14 Wm-2 and 0.20 ± 0.07 Wm-2 respectively. The differences in ERFs calculated for the different models reflect differences in the complexity of their aerosol and chemistry schemes, especially in the case of methane where tropospheric chemistry captures increased forcing from ozone production