There is a large range of future aerosol emissions scenarios explored in the Shared Socioeconomic Pathways (SSPs), with plausible pathways spanning a range of possibilities from large global ...reductions in emissions by 2050 to moderate global increases over the same period. Diversity in emissions across the pathways is particularly large over Asia. Rapid reductions in anthropogenic aerosol and precursor emissions between the present day and the 2050s lead to enhanced increases in global and Asian summer monsoon precipitation relative to scenarios with weak air quality policies. However, the effects of aerosol reductions do not persist to the end of the 21st century for precipitation, when instead the response to greenhouse gases dominates differences across the SSPs. The relative magnitude and spatial distribution of aerosol changes are particularly important for South Asian summer monsoon precipitation changes. Precipitation increases here are initially suppressed in SSPs 2-4.5, 3-7.0, and 5-8.5 relative to SSP1-1.9 when the impact of remote emission decreases is counteracted by continued increases in South Asian emissions.
Abstract Uncertainty in climate sensitivity has been shown to warrant early-on mitigation to limit global warming while anticipating future carbon dioxide removal creates mitigation deterrence. Here ...we use an integrated assessment model to quantify the impacts of under- or overestimating the cost and availability (feasibility) of carbon dioxide removal when limiting warming to 1.5 °C by 2100 under uncertain climate sensitivity. If climate sensitivity uncertainty is disregarded, initial assumptions on the feasibility have only minor effects on mitigation costs. However, the climate sensitivity risk compounds the impact of prior assumptions. Wrong assumptions on carbon dioxide removal feasibility can lead to lower costs under extreme realizations of climate sensitivity. Moreover, scenarios considering uncertainty in climate sensitivity rely less on carbon dioxide removal. A prudential strategy assuming low feasibility for carbon dioxide removal reduces the “double whammy” risk of overestimating carbon dioxide removal in combination with a realization of high climate sensitivity.
We carry out molecular Monte Carlo simulations of clusters in an imperfect vapor. We show that down to very small cluster sizes, classical nucleation theory built on the liquid drop model can be used ...very accurately to describe the work required to add a monomer to the cluster. However, the error made in modeling the smallest of clusters as liquid drops results in an erroneous absolute value for the cluster work of formation throughout the size range. We calculate factors needed to correct the cluster formation work given by the liquid drop model. The corrected work of formation results in nucleation rates in good agreement with recent nucleation experiments on argon and water.
We use a global aerosol microphysics model to predict the contribution of boundary layer (BL) particle formation to regional and global distributions of cloud condensation nuclei (CCN). Including an ...observationally derived particle formation scheme, where the formation rate of molecular clusters is proportional to gas‐phase sulfuric acid to the power one, improves modeled particle size distribution and total particle number concentration at three continental sites in Europe. Particle formation increases springtime BL global mean CCN (0.2% supersaturation) concentrations by 3–20% and CCN (1%) by 5–50%. Uncertainties in particle formation and growth rates must be reduced before the accuracy of these predictions can be improved. These results demonstrate the potential importance of BL particle formation as a global source of CCN.
Geoengineering methods are intended to reduce climate change, which is already having demonstrable effects on ecosystem structure and functioning in some regions. Two types of geoengineering ...activities that have been proposed are: carbon dioxide (CO2) removal (CDR), which removes CO2 from the atmosphere, and solar radiation management (SRM, or sunlight reflection methods), which reflects a small percentage of sunlight back into space to offset warming from greenhouse gases (GHGs). Current research suggests that SRM or CDR might diminish the impacts of climate change on ecosystems by reducing changes in temperature and precipitation. However, sudden cessation of SRM would exacerbate the climate effects on ecosystems, and some CDR might interfere with oceanic and terrestrial ecosystem processes. The many risks and uncertainties associated with these new kinds of purposeful perturbations to the Earth system are not well understood and require cautious and comprehensive research.
We have developed new parameterizations of electrically neutral homogeneous and ion‐induced sulfuric acid‐water particle formation for large ranges of environmental conditions, based on an improved ...model that has been validated against a particle formation rate data set produced by Cosmics Leaving OUtdoor Droplets (CLOUD) experiments at European Organization for Nuclear Research (CERN). The model uses a thermodynamically consistent version of the Classical Nucleation Theory normalized using quantum chemical data. Unlike the earlier parameterizations for H2SO4‐H2O nucleation, the model is applicable to extreme dry conditions where the one‐component sulfuric acid limit is approached. Parameterizations are presented for the critical cluster sulfuric acid mole fraction, the critical cluster radius, the total number of molecules in the critical cluster, and the particle formation rate. If the critical cluster contains only one sulfuric acid molecule, a simple formula for kinetic particle formation can be used: this threshold has also been parameterized. The parameterization for electrically neutral particle formation is valid for the following ranges: temperatures 165–400 K, sulfuric acid concentrations 104–1013 cm−3, and relative humidities 0.001–100%. The ion‐induced particle formation parameterization is valid for temperatures 195–400 K, sulfuric acid concentrations 104–1016 cm−3, and relative humidities 10−5–100%. The new parameterizations are thus applicable for the full range of conditions in the Earth's atmosphere relevant for binary sulfuric acid‐water particle formation, including both tropospheric and stratospheric conditions. They are also suitable for describing particle formation in the atmosphere of Venus.
Key Points
Our H2SO4‐H2O parameterizations reproduce state‐of‐the‐art experimental and theoretical results for large ranges of environmental conditions
We recommend replacing Vehkamäki et al. (2002, 2003) neutral particle formation parameterizations with ours and adding the ion‐induced pathway
Combination of neutral and ion‐induced pathways and the kinetic regime is required to describe H2SO4 particle formation on Earth and Venus
Anthropogenic aerosol emissions are expected to change rapidly over the coming decades, driving strong, spatially complex trends in temperature, hydroclimate, and extreme events both near and far ...from emission sources. Under-resourced, highly populated regions often bear the brunt of aerosols' climate and air quality effects, amplifying risk through heightened exposure and vulnerability. However, many policy-facing evaluations of near-term climate risk, including those in the latest Intergovernmental Panel on Climate Change assessment report, underrepresent aerosols' complex and regionally diverse climate effects, reducing them to a globally averaged offset to greenhouse gas warming. We argue that this constitutes a major missing element in society's ability to prepare for future climate change. We outline a pathway towards progress and call for greater interaction between the aerosol research, impact modeling, scenario development, and risk assessment communities.
We report flow tube measurements of the effective sulfuric acid diffusion coefficient at ranges of different relative humidities (from ∼4 to 70%), temperatures (278, 288 and 298 K) and initial H2SO4 ...concentrations (from 1 × 106 to 1 × 108 molecules·cm−3). The measurements were carried out under laminar flow of humidified air containing trace amounts of impurities such as amines (few ppt), thus representing typical conditions met in Earth’s continental boundary layer. The diffusion coefficients were calculated from the sulfuric acid wall loss rate coefficients that were obtained by measuring H2SO4 concentration continuously at seven different positions along the flow tube with a chemical ionization mass spectrometer (CIMS). The wall loss rate coefficients and laminar flow conditions were verified with additional computational fluid dynamics (CFD) model FLUENT simulations. The determined effective sulfuric acid diffusion coefficients decreased with increasing relative humidity, as also seen in previous experiments, and had a rather strong power dependence with respect to temperature, around ∝ T5.6, which is in disagreement with the expected temperature dependence of ∼T1.75 for pure vapours. Further clustering kinetics simulations using quantum chemical data showed that the effective diffusion coefficient is lowered by the increased diffusion volume of H2SO4 molecules via a temperature-dependent attachment of base impurities like amines. Thus, the measurements and simulations suggest that in the atmosphere the attachment of sulfuric acid molecules with base molecules can lead to a lower than expected effective sulfuric acid diffusion coefficient with a higher than expected temperature dependence.
Satellite data suggest that summertime aerosol optical depth (AOD) over the southeastern USA depends on the air/land surface temperature, but the magnitude of the radiative effects caused by this ...dependence remains unclear. To quantify these radiative effects, we utilized several remote sensing datasets and ECMWF reanalysis data for the years 2005–2011. In addition, the global aerosol–climate model ECHAM-HAMMOZ was used to identify the possible processes affecting aerosol loads and their dependence on temperature over the studied region. The satellite-based observations suggest that changes in the total summertime AOD in the southeastern USA are mainly governed by changes in anthropogenic emissions. In addition, summertime AOD exhibits a dependence on southerly wind speed and land surface temperature (LST). Transport of sea salt and Saharan dust is the likely reason for the wind speed dependence, whereas the temperature-dependent component is linked to temperature-induced changes in the emissions of biogenic volatile organic compounds (BVOCs) over forested regions. The remote sensing datasets indicate that the biogenic contribution increases AOD with increasing temperature by approximately (7 ± 6) × 10−3 K−1 over the southeastern USA. In the model simulations, the increase in summertime AOD due to temperature-enhanced BVOC emissions is of a similar magnitude, i.e., (4 ± 1) × 10−3 K−1. The largest source of BVOC emissions in this region is broadleaf trees, thus if the observed temperature dependence of AOD is caused by biogenic emissions the dependence should be the largest in the vicinity of forests. Consequently, the analysis of the remote sensing data shows that over mixed forests the biogenic contribution increases AOD by approximately (27 ± 13) × 10−3 K−1, which is over four times higher than the value for over the whole domain, while over other land cover types in the study region (woody savannas and cropland/natural mosaic) there is no clear temperature dependence. The corresponding clear-sky direct radiative effect (DRE) of the observation-based biogenic AOD is −0.33 ± 0.29 W/m2/K for the whole domain and −1.3 ± 0.7 W/m2/K over mixed forests only. The model estimate of the regional clear-sky DRE for biogenic aerosols is similar to the observational estimate for the whole domain: −0.29 ± 0.09 W/m2/K. Furthermore, the model simulations showed that biogenic emissions have a significant effective radiative forcing (ERF) in this region: −1.0 ± 0.5 W/m2/K.
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.
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