[Show abstract][Hide abstract] ABSTRACT: Combustion of fuels in the residential sector for cooking and heating, results in the emission of aerosol and aerosol precursors impacting air quality, human health and climate. Residential emissions are dominated by the combustion of solid fuels. We use a global aerosol microphysics model to simulate the uncertainties in the impact of residential fuel combustion on atmospheric aerosol. The model underestimates black carbon (BC) and organic carbon (OC) mass concentrations observed over Asia, Eastern Europe and Africa, with better prediction when carbonaceous emissions from the residential sector are doubled. Observed seasonal variability of BC and OC concentrations are better simulated when residential emissions include a seasonal cycle. The largest contributions of residential emissions to annual surface mean particulate matter (PM 2.5) concentrations are simulated for East Asia, South Asia and Eastern Europe. We use a concentration response function to estimate the health impact due to long-term exposure to ambient PM 2.5 from residential emissions. We estimate global annual excess adult (> 30 years of age) premature mortality of 308,000 (113,300 – 497,000, 5th to 95th percentile uncertainty range) for monthly varying residential emissions and 517,000 (192,000 – 827,000) when residential carbonaceous emissions are doubled. Mortality due to residential emissions is greatest in Asia, with China and India accounting for 50% of simulated global excess mortality. Using an offline radiative transfer model we estimate that residential emissions exert a global annual mean direct radiative effect of between-66 mW m-2 and +21 mW m-2 , with sensitivity to the residential emission flux and the assumed ratio of BC, OC and SO 2 emissions. Residential emissions exert a global annual mean first aerosol indirect effect of between-52 mW m-2 and-16 mWm-2, which is sensitive
to the assumed size distribution of carbonaceous emissions. Overall, our results 25 demonstrate that reducing residential combustion emissions would have substantial benefits for human health through reductions in ambient PM2.5 concentrations.
[Show abstract][Hide abstract] ABSTRACT: Aviation emissions impact both air quality and climate. Using a coupled tropospheric chemistry-aerosol microphysics model we investigate the effects of varying aviation fuel sulfur content (FSC) on premature mortality from long-term exposure to aviation-sourced PM2.5 (particulate matter with a dry diameter of < 2.5 μm) and on the global radiation budget due to changes in aerosol and tropospheric ozone. We estimate that present-day non-CO2 aviation emissions with a typical FSC of 600 ppm result in 3597 (95 % CI: 1307–5888) annual mortalities globally due to increases in cases of cardiopulmonary disease and lung cancer, resulting from increased surface PM2.5 concentrations. We quantify the global annual mean combined radiative effect (REcomb) of non-CO2 aviation emissions as −13.3 mW m−2; from increases in aerosols (direct radiative effect and cloud albedo effect) and tropospheric ozone. Ultra-low sulfur jet fuel (ULSJ; FSC =15 ppm) has been proposed as an option to reduce the adverse health impacts of aviation-induced PM2.5. We calculate that swapping the global aviation fleet to ULSJ fuel would reduce the global aviation-induced mortality rate by 624 (95 % CI: 227–1021) mortalities a−1 and increase REcomb by +7.0 mW m−2. We explore the impact of varying aviation FSC between 0–6000 ppm. Increasing FSC increases annual mortality, while enhancing climate cooling through increasing the aerosol cloud albedo effect (aCAE). We explore the relationship between the injection altitude of aviation emissions and the resulting climate and air quality impacts. Compared to the standard aviation emissions distribution, releasing aviation emissions at the ground increases global aviation-induced mortality and produces a net warming effect, primarily through a reduced aCAE. Aviation emissions injected at the surface are 5 times less effective at forming cloud condensation nuclei, reducing the aviation-induced aCAE by a factor of 10. Applying high FSCs at aviation cruise altitudes combined with ULSJ fuel at lower altitudes result in reduced aviation-induced mortality and increased negative RE compared to the baseline aviation scenario.
[Show abstract][Hide abstract] ABSTRACT: Regional patterns of aerosol radiative forcing are important for understanding climate change on decadal time scales. Uncertainty in aerosol forcing is likely to vary regionally and seasonally because of the short aerosol lifetime and heterogeneous emissions. Here the sensitivity of regional aerosol cloud albedo effect (CAE) forcing to 31 aerosol process parameters and emission fluxes is quantified between 1978 and 2008. The effects of parametric uncertainties on calculations of the balance of incoming and outgoing radiation are found to be spatially and temporally dependent. Regional uncertainty contributions of opposite sign cancel in global-mean forcing calculations, masking the regional importance of some parameters. Parameters that contribute little to uncertainty in Earth's global energy balance during recent decades make significant contributions to regional forcing variance. Aerosol forcing sensitivities are quantified within 11 climatically important regions, where surface temperatures are thought to influence large-scale climate effects. Substantial simulated uncertainty in CAE forcing in the eastern Pacific leaves open the possibility that apparent shifts in the mean ENSO state may result from a forced aerosol signal on multidecadal time scales. A likely negative aerosol CAE forcing in the tropical North Atlantic calls into question the relationship between Northern Hemisphere aerosol emission reductions and CAE forcing of sea surface temperatures in the main Atlantic hurricane development region on decadal time scales. Simulated CAE forcing uncertainty is large in the North Pacific, suggesting that the role of the CAE in altering Pacific tropical storm frequency and intensity is also highly uncertain.
Journal of Climate 05/2015; 28(17):150526130735008. DOI:10.1175/JCLI-D-15-0127.1 · 4.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Atmospheric aerosol scatters solar radiation increasing the fraction of diffuse radiation and the efficiency of photosynthesis. We quantify the impacts of biomass burning aerosol (BBA) on diffuse radiation and plant photosynthesis across Amazonia during 1998–2007. Evaluation against observed aerosol optical depth allows us to provide lower and upper BBA emissions estimates. BBA increases Amazon basin annual mean diffuse radiation by 3.4–6.8% and net primary production (NPP) by 1.4–2.8%, with quoted ranges driven by uncertainty in BBA emissions. The enhancement of Amazon basin NPP by 78–156 Tg C a�1 is equivalent to 33–65% of the annual regional carbon emissions from biomass burning. This NPP increase occurs during the dry season and acts to counteract some of the observed effect of drought on tropical production. We estimate that 30–60 Tg C a�1 of this NPP enhancement is within woody tissue, accounting for 8–16% of the observed carbon sink across mature Amazonian forests.
Geophysical Research Letters 05/2015; 42(11):n/a-n/a. DOI:10.1002/2015GL063719 · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Tropospheric ozone directly affects the radiative balance of the Earth through interaction with shortwave and longwave radiation. Here we use measurements of tropospheric ozone from the Tropospheric Emission Spectrometer satellite instrument, together with chemical transport and radiative transfer models to produce a first estimate of the stratospherically adjusted annual radiative effect (RE) of tropospheric ozone. We show that differences between modelled and observed ozone concentrations have little impact on the RE, indicating that our present-day tropospheric ozone RE estimate of 1.17±0.03 W m−2 is robust. The RE normalised by column ozone decreased between the pre-industrial and the present-day. Using a simulation with historical biomass burning and no anthropogenic emissions, we calculate a radiative forcing of 0.32 W m−2 for tropospheric ozone, within the current best estimate range. We propose a radiative kernel approach as an efficient and accurate tool for calculating ozone REs in simulations with similar ozone abundances.
Geophysical Research Letters 05/2015; DOI:10.1002/2015GL064037 · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Emissions of biogenic volatile organic compounds (BVOCs) have changed in the past millennium due to changes in land use, temperature, and CO2 concentrations. Recent reconstructions of BVOC emissions have predicted that global isoprene emissions have decreased, while monoterpene and sesquiterpene emissions have increased; however, all three show regional variability due to competition between the various influencing factors. In this work, we use two modeled estimates of BVOC emissions from the years 1000 to 2000 to test the effect of anthropogenic changes to BVOC emissions on secondary organic aerosol (SOA) formation, global aerosol size distributions, and radiative effects using the GEOS-Chem-TOMAS (Goddard Earth Observing System; TwO-Moment Aerosol Sectional) global aerosol microphysics model. With anthropogenic emissions (e.g., SO2, NOx, primary aerosols) turned off and BVOC emissions changed from year 1000 to year 2000 values, decreases in the number concentration of particles of size D-p > 80 nm (N80) of > 25% in year 2000 relative to year 1000 were predicted in regions with extensive land-use changes since year 1000 which led to regional increases in the combined aerosol radiative effect (direct and indirect) of > 0.5W m(-2) in these regions. We test the sensitivity of our results to BVOC emissions inventory, SOA yields, and the presence of anthropogenic emissions; however, the qualitative response of the model to historic BVOC changes remains the same in all cases. Accounting for these uncertainties, we estimate millennial changes in BVOC emissions cause a global mean direct effect of between +0.022 and +0.163W m(-2) and the global mean cloud-albedo aerosol indirect effect of between -0.008 and -0.056 W m(-2). This change in aerosols, and the associated radiative forcing, could be a largely overlooked and important anthropogenic aerosol effect on regional climates.
[Show abstract][Hide abstract] ABSTRACT: The oxidation of biogenic volatile organic compounds (BVOCs) gives a range of products, from semi-volatile to extremely low-volatility compounds. To treat the interaction of these secondary organic vapours with the particle phase, global aerosol microphysics models generally use either a thermodynamic partitioning approach (assuming instant equilibrium between semi-volatile oxidation products and the particle phase) or a kinetic approach (accounting for the size-dependence of condensation). We show that model treatment of the partitioning of biogenic organic vapours into the particle phase, and consequent distribution of material across the size distribution, controls the magnitude of the first aerosol indirect effect (AIE) due to biogenic secondary organic aerosol (SOA). With a kinetic partitioning approach, SOA is distributed according to the existing condensation sink, enhancing the growth of the smallest particles, i.e., those in the nucleation mode. This process tends to increase cloud droplet number concentrations in the presence of biogenic SOA. By contrast, a thermodynamic approach distributes SOA according to pre-existing organic mass, restricting the growth of the smallest particles, limiting the number that are able to form cloud droplets. With an organically medicated new particle formation mechanism, applying a thermodynamic rather than a kinetic approach reduces our calculated global mean AIE due to biogenic SOA by 24%. Our results suggest that the mechanisms driving organic partitioning need to be fully understood in order to accurately describe the climatic effects of SOA.
[Show abstract][Hide abstract] ABSTRACT: Aerosols and their effect on the radiative properties of clouds are one of the largest sources of uncertainty in calculations of the Earth's energy budget. Here the sensitivity of aerosol cloud-albedo effect forcing to 31 aerosol parameters is quantified. Sensitivities are compared over three periods; 1850-2008, 1978-2008 and 1998-2008. Despite declining global anthropogenic SO2 emissions during 1978-2008, a cancellation of regional positive and negative forcings leads to a near-zero global mean cloud-albedo effect forcing. In contrast to existing negative estimates, our results suggest that the aerosol cloud-albedo effect was likely positive (0.006 to 0.028 Wm-2) in the recent decade, making it harder to explain the temperature hiatus as a forced response. Proportional contributions to forcing variance from aerosol processes and natural and anthropogenic emissions are found to be period dependent. To better constrain forcing estimates, the processes that dominate uncertainty on the timescale of interest must be better understood.
[Show abstract][Hide abstract] ABSTRACT: We use a global aerosol microphysics model in combination with an offline radiative transfer model to
quantify the radiative effect of biogenic secondary organic aerosol (SOA) in the present-day atmosphere. Through its role in particle growth and ageing, the presence of biogenic SOA increases the global annual mean concentration of cloud condensation nuclei (CCN; at 0.2% supersaturation) by 3.6–21.1 %, depending upon the yield of SOA production from biogenic volatile organic compounds (BVOCs),
and the nature and treatment of concurrent primary carbonaceous emissions. This increase in CCN causes a rise in global annual mean cloud droplet number concentration (CDNC) of 1.9–5.2%, and a global mean first aerosol indirect effect (AIE) of between +0.01Wm−2 and −0.12Wm−2. The radiative
impact of biogenic SOA is far greater when biogenic oxidation products also contribute to the very early stages of new particle formation; using two organically mediated mechanisms for new particle formation, we simulate global annual mean first AIEs of −0.22Wm−2 and −0.77Wm−2. The inclusion of biogenic SOA substantially improves the simulated seasonal cycle in the concentration of CCN-sized particles observed at three forested sites. The best correlation is found when the organically mediated nucleation mechanisms are applied, suggesting that the first AIE of biogenic SOA could be as large as −0.77Wm−2. The radiative impact of SOA is sensitive to the presence of anthropogenic emissions. Lower background aerosol concentrations simulated with anthropogenic emissions from 1750 give rise to a greater fractional CCN increase and a more substantial first AIE from biogenic SOA. Consequently, the anthropogenic
indirect radiative forcing between 1750 and the present day is sensitive to assumptions about the amount and role of biogenic SOA. We also calculate an annual global mean direct radiative effect of between −0.08Wm−2 and −0.78Wm−2 in the present day, with uncertainty in the amount of SOA
produced from the oxidation of BVOCs accounting for most of this range.
[Show abstract][Hide abstract] ABSTRACT: The effect of anthropogenic aerosols on cloud droplet concentrations and radiative properties is the source of one of the largest uncertainties in the radiative forcing of climate over the industrial period. This uncertainty affects our ability to estimate how sensitive the climate is to greenhouse gas emissions. Here we perform a sensitivity analysis on a global model to quantify the uncertainty in cloud radiative forcing over the industrial period caused by uncertainties in aerosol emissions and processes. Our results show that 45 per cent of the variance of aerosol forcing since about 1750 arises from uncertainties in natural emissions of volcanic sulphur dioxide, marine dimethylsulphide, biogenic volatile organic carbon, biomass burning and sea spray. Only 34 per cent of the variance is associated with anthropogenic emissions. The results point to the importance of understanding pristine pre-industrial-like environments, with natural aerosols only, and suggest that improved measurements and evaluation of simulated aerosols in polluted present-day conditions will not necessarily result in commensurate reductions in the uncertainty of forcing estimates.
[Show abstract][Hide abstract] ABSTRACT: Thirty years of balloon-borne measurements over Boulder (40°N, 105°W) are used
to investigate the water vapor trend in the tropopause region. This analysis extends
previously published trends, usually focusing on altitudes greater than 16 km, to lower
altitudes. Two new concepts are applied: (1) Trends are presented in a thermal tropopause
(TP) relative coordinate system from –2 km below to 10 km above the TP, and (2) sonde
profiles are selected according to TP height. Tropical (TPz > 14 km), extratropical
(TPz < 12 km), and transitional air mass types (12 km < TPz < 14 km) reveal three
different water vapor reservoirs. The analysis based on these concepts reduces the
dynamically induced water vapor variability at the TP and principally favors refined
water vapor trend studies in the upper troposphere and lower stratosphere. Nonetheless,
this study shows how uncertain trends are at altitudes –2 to +4 km around the TP. This
uncertainty in turn has an influence on the uncertainty and interpretation of water vapor
radiative effects at the TP, which are locally estimated for the 30 year period to be of
uncertain sign. The much discussed decrease in water vapor at the beginning of 2001 is
not detectable between –2 and 2 km around the TP. On lower stratospheric isentropes, the
water vapor change at the beginning of 2001 is more intense for extratropical than for
tropical air mass types. This suggests a possible link with changing dynamics above the
jet stream such as changes in the shallow branch of the Brewer-Dobson circulation.
[Show abstract][Hide abstract] ABSTRACT: natural environment is an important source of atmospheric aerosol such
as dust, sea spray, and wildfire smoke. Climate controls many of these
natural aerosol sources, which, in turn, can alter climate through
changing the properties of clouds and the Earth's radiative balance.
However, the Earth's atmosphere is now heavily modified by anthropogenic
pollution aerosol, but how this pollution may alter these natural
aerosol-climate feedbacks has not been previously explored. Here we use
a global aerosol microphysics model to analyze how anthropogenic aerosol
alters one link within these feedbacks, namely, the sensitivity of cloud
albedo to changes in natural aerosol. We demonstrate that anthropogenic
aerosol in the Northern Hemisphere has halved the hemispheric mean cloud
albedo radiative effect that occurs due to changes in natural aerosol
emissions. Such a suppression has not occurred in the more pristine
Geophysical Research Letters 10/2013; 40(19):5316-5319. DOI:10.1002/2013GL057966 · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Natural aerosol plays a significant role in the Earth's system due to its ability to alter the radiative balance of the Earth. Here we use a global aerosol microphysics model together with a radiative transfer model to estimate radiative effects for five natural aerosol sources in the present‐day atmosphere: dimethyl sulfide (DMS), sea‐salt, volcanoes, monoterpenes, and wildfires. We calculate large annual global mean aerosol direct and cloud albedo effects especially for DMS‐derived sulfate (–0.23 Wm–2 and –0.76 Wm–2, respectively), volcanic sulfate (–0.21 Wm–2 and –0.61 Wm–2) and sea‐salt (–0.44 Wm–2 and –0.04 Wm–2). The cloud albedo effect responds nonlinearly to changes in emission source strengths. The natural sources have both markedly different radiative efficiencies and indirect/direct radiative effect ratios. Aerosol sources that contribute a large number of small particles (DMS‐derived and volcanic sulfate) are highly effective at influencing cloud albedo per unit of aerosol mass burden.
Open access article available here: http://onlinelibrary.wiley.com/doi/10.1002/grl.50441/abstract
[Show abstract][Hide abstract] ABSTRACT: stratospheric ozone depletion has acted to cool the Earth's surface. As
the result of the phase-out of anthropogenic halogenated compounds
emissions, stratospheric ozone is projected to recover and its radiative
forcing (RF-O3 ~ -0.05 W/m2 presently) might
therefore be expected to decay in line with ozone recovery itself. Using
results from chemistry-climate models, we find that, although model
projections using a standard greenhouse gas scenario broadly agree on
the future evolution of global ozone, they strongly disagree on
RF-O3 because of a large model spread in ozone changes in a
narrow (several km thick) layer, in the northern lowermost stratosphere.
Clearly, future changes in global stratospheric ozone cannot be
considered an indicator of its overall RF. The multi-model mean
RF-O3 estimate for 2100 is +0.06 W/m2 but with a
range such that it could remain negative throughout this century or
change sign and reach up to ~0.25 W/m2.
Geophysical Research Letters 06/2013; 40(11):2796-2800. DOI:10.1002/grl.50358 · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To determine the effect of cosmic rays on the Earth's climate via ion-induced nucleation, a parametrisation of inorganic nucleation was formulated based on experiments at the CERN CLOUD experiment. The parametrisation was implemented in the GLOMAP aerosol microphysics model and used to estimate the radiative effect of the change in ionisation experienced over an 11-year solar cycle.
19th International Conference on Nucleation and Atmospheric Aerosols; 05/2013
[Show abstract][Hide abstract] ABSTRACT: The global aerosol microphysics model GLOMAP was used to estimate the
climate effect of trends observed in surface wind speeds over recent
decades. The time evolution of the radiative effect due to changes in
aerosol population and statistical correlations between aerosol fluxes
and populations, and surface wind speeds, were also examined.
[Show abstract][Hide abstract] ABSTRACT: The Mediterranean troposphere exhibits a marked and localised summertime
ozone maximum, which has the potential to strongly impact regional air
quality and radiative forcing. The Mediterranean region can be perturbed
by long-range pollution import from Northern Europe, North America and
Asia, in addition to local emissions, which may all contribute to
regional ozone enhancements. We exploit ozone profile observations from
the Tropospheric Emission Spectrometer (TES) and the Global Ozone
Monitoring Experiment-2 (GOME-2) satellite instruments, and an offline
3-D global chemical transport model (TOMCAT) to investigate the
geographical and vertical structure of the summertime tropospheric ozone
maximum over the Mediterranean region. We show that both TES and GOME-2
are able to detect enhanced levels of ozone in the lower troposphere
over the region during the summer. These observations, together with
surface measurements, are used to evaluate the TOMCAT model's ability to
capture the observed ozone enhancement. The model is used to quantify
sensitivities of the ozone maximum to anthropogenic and natural volatile
organic compound (VOC) emissions, anthropogenic NOx
emissions, wildfire emissions and long-range import of ozone and
precursors. Our results show a dominant sensitivity to natural VOC
emissions in the Mediterranean basin over anthropogenic VOC emissions.
However, local anthropogenic NOx emissions are result in the
overall largest sensitivity in near-surface ozone. We also show that in
the lower troposphere, global VOC emissions account for 40% of the ozone
sensitivity to VOC emissions in the region, whereas, for NOx
the ozone sensitivity to local sources is 9 times greater than that for
global emissions at these altitudes. However, in the mid and upper
troposphere ozone is most sensitive to non-local emission sources. In
terms of radiative effects on regional climate, ozone contributions from
non-local emission sources are more important, as these have a larger
impact on ozone in the upper troposphere where its radiative effects are
larger, with Asian monsoon outflow having the greatest impact. Our
results allow improved understanding of the large-scale processes
controlling air quality and climate in the region of the Mediterranean
[Show abstract][Hide abstract] ABSTRACT: Contrails and especially their evolution into cirrus-like clouds are thought to have very important effects on local and global radiation budgets, though are generally not well represented in global climate models. Lack of contrail parameterisations is due to the limited availability of in situ contrail measurements which are difficult to obtain. Here we present a methodology for successful sampling and interpretation of contrail microphysical and radiative data using both in situ and remote sensing instrumentation on board the FAAM BAe146 UK research aircraft as part of the COntrails Spreading Into Cirrus (COSIC) study.
Forecast models were utilised to determine flight regions suitable for contrail formation and sampling; regions that were both free of cloud but showed a high probability of occurrence of air mass being supersaturated with respect to ice. The FAAM research aircraft, fitted with cloud microphysics probes and remote sensing instruments, formed a distinctive spiral-shaped contrail in the predicted area by flying in an orbit over the same ground position as the wind advected the contrails to the east. Parts of these contrails were sampled during the completion of four orbits, with sampled contrail regions being between 7 and 30 min old. Lidar measurements were useful for in-flight determination of the location and spatial extent of the contrails, and also to report extinction values that agreed well with those calculated from the microphysical data. A shortwave spectrometer was also able to detect the contrails, though the signal was weak due to the dispersion and evaporation of the contrails. Post-flight the UK Met Office NAME III dispersion model was successfully used as a tool for modelling the dispersion of the persistent contrail; determining its location and age, and determining when there was interference from other measured aircraft contrails or when cirrus encroached on the area later in the flight. The persistent contrails were found to consist of small (∼10 μm) plate-like crystals where growth of ice crystals to larger sizes (∼100 μm) was typically detected when higher water vapour levels were present. Using the cloud microphysics data, extinction co-efficient values were calculated and found to be 0.01-1 kmĝ̂'1. The contrails formed during the flight (referred to as B587) were found to have a visible lifetime of ∼40 min, and limited water vapour supply was thought to have suppressed ice crystal growth.
[Show abstract][Hide abstract] ABSTRACT: The upper troposphere/lower stratosphere (UTLS) region plays an
important role in the climate system. Changes in the structure and
chemical composition of this region result in particularly large changes
in radiative forcings of the atmosphere. Quantifying the processes that
control UTLS composition (e.g., stratosphere-troposphere exchange)
therefore represents a crucial task. We assess the influence of
uncertainties in the atmospheric mixing strength on global UTLS
distributions of greenhouse gases (water vapor, ozone, methane, and
nitrous oxide) and associated radiative effects. The study is based on
multiannual simulations with the Chemical Lagrangian Model of the
Stratosphere (CLaMS) driven by ERA-Interim meteorological data and on a
state-of-the-art radiance code. Mixing, the irreversible part of
transport, is controlled by the local horizontal strain and vertical
shear of the atmospheric flow. We find that simulated radiative effects
of water vapor and ozone, both characterized by steep gradients in the
UTLS, are particularly sensitive to uncertainties of the atmospheric
mixing strength. Globally averaged radiative effects are about 0.72 and
0.17 W/m2for water vapor and ozone, respectively. For ozone,
the largest impact of mixing uncertainties is observed in the
extra-tropical lower stratosphere.
Journal of Geophysical Research Atmospheres 08/2012; 117(D16305). DOI:10.1029/2012JD017751 · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Observations and models have shown that con-tinuously degassing volcanoes have a potentially large ef-fect on the natural background aerosol loading and the ra-diative state of the atmosphere. We use a global aerosol microphysics model to quantify the impact of these vol-canic emissions on the cloud albedo radiative forcing under pre-industrial (PI) and present-day (PD) conditions. We find that volcanic degassing increases global annual mean cloud droplet number concentrations by 40 % under PI conditions, but by only 10 % under PD conditions. Consequently, vol-canic degassing causes a global annual mean cloud albedo effect of −1.06 W m −2 in the PI era but only −0.56 W m −2 in the PD era. This non-equal effect is explained partly by the lower background aerosol concentrations in the PI era, but also because more aerosol particles are produced per unit of volcanic sulphur emission in the PI atmosphere. The higher sensitivity of the PI atmosphere to volcanic emissions has an important consequence for the anthropogenic cloud radiative forcing because the large uncertainty in volcanic emissions translates into an uncertainty in the PI baseline cloud radia-tive state. Assuming a −50/+100 % uncertainty range in the volcanic sulphur flux, we estimate the annual mean anthro-pogenic cloud albedo forcing to lie between −1.16 W m −2 and −0.86 W m −2 . Therefore, the volcanically induced un-certainty in the PI baseline cloud radiative state substantially adds to the already large uncertainty in the magnitude of the indirect radiative forcing of climate.