Understanding the costs and benefits of climate change mitigation and adaptation options is crucial to justify and prioritise future decarbonisation pathways to achieve net zero. Here, we quantified the co-benefits of decarbonisation for air quality and public health under scenarios that aim to limit end-of-century warming to 2ºC and 1.5ºC. We estimated the mortality burden attributable to ambient PM2.5 exposure using population attributable fractions of relative risk, incorporating projected changes in population demographics. We found that implementation of decarbonisation scenarios could produce substantial global reductions in population exposure to PM2.5 pollution and associated premature mortality, with maximum health benefits achieved in Asia around mid-century. The stringent 1.5ºC-compliant decarbonisation scenario (SSP1-1.9) could reduce the PM2.5-attributable mortality burden by 29% in 2050 relative to the middle-of-the-road scenario (SSP2-4.5), averting around 2.9M annual deaths worldwide. While all income groups were found to benefit from improved air quality through decarbonisation, the smallest health benefits are experienced by the low-income population. The disparity in PM2.5 exposure across income groups is projected to reduce by 2100, but a 30% disparity between high- and low- income groups persists even in the strongest mitigation scenario. Further, without additional and targeted air quality measures, low- and lower-middle-income populations (predominantly in Africa and Asia) will continue to experience PM2.5 exposures that are over three times the World Health Organization (WHO) Air Quality Guideline.

Effective radiative forcing (ERF) is evaluated in the ACCESS1.0 General Circulation Model (GCM) with fixed land and sea-surface-temperatures as well as sea-ice. The 4xCO2 ERF is 8.0 Wm-2. In contrast, a typical ERF experiment with only fixed sea-surface-temperatures (SST) and sea-ice gives rise to an ERF of only 7.0 Wm-2. This difference arises due to the influence of land warming in the commonly used fixed-SST ERF experimental design, which results in: (i) increased emission of longwave radiation to space from the land surface (-0.45 Wm-2) and troposphere (-0.90 Wm-2), (ii) reduced land snow-cover and albedo (+0.17 Wm-2), (iii) increased water-vapour (+0.49 Wm-2), and (iv) a cloud adjustment (-0.26 Wm-2) due to reduced stability and cloudiness over land (positive ERF) counteracted by increased lower tropospheric stability and marine cloudiness over oceans (negative ERF) . The sum of these radiative adjustments to land warming is to reduce the 4xCO2 ERF in fixed-SST experiments by ~1.0 Wm-2. CO2 stomatal effects are quantified and found to contribute just over half of the land warming effect and adjustments in the fixed-SST ERF experimental design in this model. The basic physical mechanisms in response to land warming are confirmed in a solar ERF experiment. We test various methods that have been proposed to account for land warming in fixed-SST ERFs against our GCM results and discuss their strengths and weaknesses.

An observationally-constrained time series of historical aerosol effective radiative forcing (ERF) from 1750 to 2019 is developed in this paper. We find that the time history of aerosol ERFs diagnosed in CMIP6 models exhibits considerable variation and explore how the time history of aerosol forcing influences the probability distributions of present-day aerosol forcing and emergent metrics such as climate sensitivity. Using a simple energy balance model, trained on CMIP6 climate models and constrained by observed near-surface warming and ocean heat uptake, we derive estimates for the historical aerosol forcing. We find 2005-2014 mean aerosol ERF to be -1.1 (-1.8 to -0.5) W m-2 relative to 1750. Assuming recently published historical emissions from fossil fuel and industrial sectors and biomass burning emissions from SSP2-4.5, aerosol ERF in 2019 is -0.9 (-1.5 to -0.4) W m-2. There is a modest recovery in aerosol forcing (+0.025 W m-2 decade-1) between 1980 and 2014. This analysis also gives a 5-95% range of equilibrium climate sensitivity (ECS) of 1.8-5.1C (best estimate 3.1C) with a transient climate response (TCR) of 1.2-2.6C (best estimate 1.8C).

The IPCC’s scientific assessment of the timing of net-zero emissions and 2030 emission reduction targets consistent with limiting warming to 1.5C or 2C rests on large scenario databases. Updates to this assessment, such as between the IPCC’s Special Report on Global Warming of 1.5C (SR1.5) of warming and the Sixth Assessment Report (AR6), are the result of intertwined, sometimes opaque, factors. Here we isolate one factor: the Earth System Model emulators used to estimate the global warming implications of scenarios. We show that warming projections using AR6-calibrated emulators are consistent, to within around 0.1C, with projections made by the emulators used in SR1.5. The consistency is due to two almost compensating changes: the increase in assessed historical warming between the IPCC’s Fifth Assessment Report (AR5) and AR6, and a reduction in projected warming due to improved agreement between the emulators’ response to emissions and the underlying assessment.

This study assesses the effective climate sensitivity (EffCS) and transient climate response (TCR) derived from global energy budget constraints within historical simulations of 8 CMIP6 global climate models (GCMs). These calculations are enabled by use of the Radiative Forcing Model Intercomparison Project (RFMIP) simulations, which permit accurate quantification of the historical effective radiative forcing. We find that long-term historical energy budget constraints generally underestimate EffCS from CO2 quadrupling and TCR from CO2 ramping, both by 12%, owing to changes in radiative feedbacks and changes in ocean heat uptake efficiency. Atmospheric GCMs forced by observed warming patterns produce lower values of EffCS that are more in line with those inferred from observed historical energy budget constraints. Understanding the discrepancies between modeled and observed historical surface warming patterns remains critical for constraining EffCS and TCR from the historical record.

The COVID19 pandemic led to dramatic changes in economic activity in 2020. We use estimates of emissions changes for 2020 in two Earth System Models (ESMs) to simulate the impacts of the COVID19 economic changes. Ensembles of nudged simulations are used to separate small signals from meteorological variability. Reductions in aerosol and precursor emissions, chiefly Black Carbon (BC) and sulfate (SO$_4$), led to reductions in total anthropogenic aerosol cooling through aerosol-cloud interactions. The average overall Effective Radiative Forcing (ERF) peaks at +0.29$\pm$0.15 Wm$^{-2}$ in spring 2020. Changes in cloud properties are smaller than observed changes during 2020. Impacts of these changes on regional land surface temperature range up to +0.3K. The peak impact of these aerosol changes on global surface temperature is very small (+0.03K). However, the aerosol changes are the largest contribution to COVID19 emissions induced radiative forcing and temperature changes, dominating ozone, CO$_2$ and contrail effects.

Climate model emulators are widely used to generate temperature projections for climate scenarios, including in the recent IPCC Sixth Assessment Report. Here we evaluate the performance of a two-layer energy balance model in emulating historical and future temperature projections from CMIP6 models. We find that prediction errors can be large (greater than 0.5oC in a given year) and differ markedly between climate models, forcing scenarios and time periods. Errors arise in emulating the near-surface temperature response to both greenhouse gas and aerosol forcing; in some periods the errors due to these forcings oppose one another, giving the spurious impression of better emulator performance. Time-varying and state-dependent feedbacks may contribute to prediction errors. Close emulations can be produced for a given period but, crucially, this does not guarantee reliable emulations of other scenarios and periods. Therefore, rigorous out-of-sample evaluation is necessary to characterize emulator performance.

Science has been written into the Glasgow Climate Pact. Four factors helped bring science to the fore: 1) the August 2021 publication of the Intergovernmental Panel on Climate Change (IPCC) Working Group One report; 2) the focus on science within the COP26 programme; 3) targeted synthetic independent advice from a range of national bodies; 4) main streaming of climate science stories by the media. We should capitalize on this success to make our science even more useful going forward.

Changes in atmospheric composition, such as increasing greenhouse gases, cause an initial radiative imbalance to the climate system, quantified as the instantaneous radiative forcing. This fundamental metric has not been directly observed globally and previous estimates have come from models. In part, this is because current space‐based instruments cannot distinguish the instantaneous radiative forcing from the climate’s radiative response. We apply radiative kernels to satellite observations to disentangle these components and find all‐sky instantaneous radiative forcing has increased 0.53±0.11 W/m2 from 2003 through 2018, accounting for positive trends in the total planetary radiative imbalance. This increase has been due to a combination of rising concentrations of well‐mixed greenhouse gases and recent reductions in aerosol emissions. These results highlight distinct fingerprints of anthropogenic activity in Earth’s changing energy budget, which we find observations can detect within 4 years.

Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth’s radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This poster presents the outcome of an international workshop and subsequent review paper, which quantify the likely range of aerosol radiative forcing over the industrial era based on multiple lines of evidence, including modelling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes narrow the range of the forcing from aerosol-radiation interactions compared to the latest assessment by the Intergovernmental Panel on Climate Change (IPCC). A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol-driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction and on mixed-phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to total aerosol radiative forcing ranges that are of similar width to the last IPCC assessment but more clearly based on physical arguments.