Global Mean Radiative Forcing Research Articles

Aeolian dust experiment on climate impact: An overview of Japan–China joint project ADEC

The Aeolian Dust Experiment on Climate Impact (ADEC) was initiated in April 2000 as a joint five-year Japan–China project. The goal was to understand the impact of aeolian dust on climate via radiative forcing (RF). Field experiments and numerical simulations were conducted from the source regions in northwestern China to the downwind region in Japan in order to understand wind erosion processes temporal and spatial distribution of dust during their long-range transportation chemical, physical, and optical properties of dust and the direct effect of radiative forcing due to dust. For this, three intensive observation periods (IOP) were conducted from April 2002 to April 2004. The in situ and network observation results are summarized as follows: (1) In situ observations of the wind erosion process revealed that the vertical profile of moving sand has a clear size dependency with height and saltation flux and that threshold wind velocity is dependent on soil moisture. Results also demonstrated that saltation flux is strongly dependent on the parent soil size distribution of the desert surface. (2) Both lidar observations and model simulations revealed a multiple dust layer in East Asia. A numerical simulation of a chemical transport model, CFORS, illustrated the elevated dust layer from the Taklimakan Desert and the lower dust layer from the Gobi Desert. The global-scale dust model, MASINGAR, also simulated the dust layer in the middle to upper free troposphere in East Asia, which originated from North Africa and the Middle East during a dust storm in March 2003. Raman lidar observations at Tsukuba, Japan, found the ice cloud associated with the dust layer at an altitude of 6 to 9 km. Analysis from lidar and the radio-sonde observation suggested that the Asian dust acted as ice nuclei at the ice-saturated region. These results suggest the importance of dust's climate impact via the indirect effect of radiative forcing due to the activation of dust into ice nuclei. (3) Studies on the aerosol concentration indicated that size distributions of aerosols in downwind regions have bimodal peaks. One peak was in the submicron range and the other in the supermicron range. The main soluble components of the supermicron peak were Na +, Ca 2+, NO 3 −, and Cl −. In the downwind region in Japan, the dust, sea salt, and a mixture of the two were found to be dominant in coarse particles in the mixed boundary layer. (4) Observation of the optical properties of dust by sky-radiometer, particle shoot absorption photometer (PSAP), and Nephelometer indicated that unpolluted dust at source region has a weaker absorption than originally believed. A sensitivity experiment of direct RF by dust indicated that single scattering albedo is the most important of the optical properties of dust and that the sensitivity of instantaneous RF in the shortwave region at the top of the atmosphere to the refractive index strongly depends on surface albedo. A global scale dust model, MASINGAR, was used for evaluation of direct RF due to dust. The results indicated the global mean RF at the top and the bottom of the atmosphere were − 0.46 and − 2.13 W m − 2 with cloud and were almost half of the RF with cloud-free condition.

Sensitivity of global warming to the pattern of tropical ocean warming

The current generations of climate models are in substantial disagreement as to the projected patterns of sea surface temperatures (SSTs) in the Tropics over the next several decades. We show that the spatial patterns of tropical ocean temperature trends have a strong influence on global mean temperature and precipitation and on global mean radiative forcing. We identify the SST patterns with the greatest influence on the global mean climate and find very different, and often opposing, sensitivities to SST changes in the tropical Indian and West Pacific Oceans. Our work stresses the need to reduce climate model biases in these sensitive regions, as they not only affect the regional climates of the nearby densely populated continents, but also have a disproportionately large effect on the global climate.

Scientific issues in the design of metrics for inclusion of oxides of nitrogen in global climate agreements

The Kyoto Protocol seeks to limit emissions of various greenhouse gases but excludes short-lived species and their precursors even though they cause a significant climate forcing. We explore the difficulties that are faced when designing metrics to compare the climate impact of emissions of oxides of nitrogen (NO(x)) with other emissions. There are two dimensions to this difficulty. The first concerns the definition of a metric that satisfactorily accounts for its climate impact. NO(x) emissions increase tropospheric ozone, but this increase and the resulting climate forcing depend strongly on the location of the emissions, with low-latitude emissions having a larger impact. NO(x) emissions also decrease methane concentrations, causing a global-mean radiative forcing similar in size but opposite in sign to the ozone forcing. The second dimension of difficulty concerns the intermodel differences in the values of computed metrics. We explore the use of indicators that could lead to metrics that, instead of using global-mean inputs, are computed locally and then averaged globally. These local metrics may depend less on cancellation in the global mean; the possibilities presented here seem more robust to model uncertainty, although their applicability depends on the poorly known relationship between local climate change and its societal/ecological impact. If it becomes a political imperative to include NO(x) emissions in future climate agreements, policy makers will be faced with difficult choices in selecting an appropriate metric.

A simulation of the effect of climate change–induced desertification on mineral dust aerosol

Vegetation and climate fields from a coupled carbon‐cycle – climate model integration, which included the feedback of vegetation on climate, have been used to drive the HadAM3 AGCM incorporating the Hadley Centre mineral dust scheme in experiments to investigate future dust concentration and forcing. Comparison of 2000 with 2100 simulations shows the global annual mean atmospheric dust load increases from 4 × 104 to 1.3 × 105 mg m−2, due to the combination of desertification and climate change. The global mean radiative forcing due to dust increases from 0.04 to 0.21 Wm−2 at the top of the atmosphere and from −0.74 to −1.82 Wm−2 at the surface.

An upgraded estimate of the radiative forcing of cryoplane contrails

The radiative forcing of contrails is quantified for a hypothetical fleet of cryoplanes in comparison with a conventional aircraft fleet. The differences in bulk optical properties between conventional and cryoplane contrails are determined by numerical simulations of the microphysical evolution of conventional and cryoplane contrails, under several ambient conditions. Both types of contrails contain about the same ice mass, but the mean effective particle radius is found to be smaller by about a factor of 0.3 in conventional contrails than in cryoplane ones. Hence, in case of cryoplanes the contrail optical depth is lower, which counteracts (with respect to radiative forcing) the effect of increased contrail cover due to the higher specific emission of water vapour. If the information gained from the microphysical simulations is translated to the framework of a global climate model, the global mean radiative forcing of cryoplane contrails is simulated to be between about 30% lower and 30% higher compared to the radiative forcing of conventional contrails, depending on the quantitative assumptions made for the mean particle properties and also depending on the time slice considered. Our results indicate that the effect of decreased optical depth is about the same magnitude as the effect of increased contrail cover. Current state of knowledge does not allow a conclusive assessment whether the net radiative impact of cryoplane contrails will be smaller or larger than that of conventional contrails. Uncertainty with respect to radiative forcing arises mainly from insufficient knowledge regarding the mean effective ice crystal radius for both conventional and, especially, cryoplane contrails.

Response of climate to regional emissions of ozone precursors: sensitivities and warming potentials

The response of climate to ozone perturbations caused by regional emissions of NOx or CO has been studied through a sequence of model simulations. Changes in O3 and OH concentrations due to emission perturbations in Europe and southeast Asia have been calculated with two global 3-D chemical tracer models(CTMs; LMDzINCA and Oslo-CTM2). The radiative transfer codes of three general circulation models (GCMs; ECHAM4, UREAD and LMD) have been used to calculate the radiative forcing of the O3 perturbations, and for a subset of the cases full GCM simulations have been performed with ECHAM4 and UREAD. The results have been aggregated to a global number in two ways: first, through integrating the global-mean radiative forcing of a sustained step change in emissions, and second through a modified concept (SGWP*) which includes possible differences in the climate sensitivity of O3, CH4 and CO2 changes. In terms of change in global tropospheric O3 burden the two CTMs differ by less than 30%. Both CTMs show a higher north/south gradient in the sensitivity to changes in NOx emission than for CO. We are not able to conclude whether real O3 perturbations in general have a different climate sensitivity from CO2. However, in both GCMs high-latitude emission perturbations lead to climate perturbations with higher (10–30%) climate sensitivities. The calculated SGWP*, for a 100 yr time horizon, are negative for three of the four CTM/GCM combinations for European emissions (−9.6 to +6.9), while for the Asian emissions the SGWP* (H= 100) is always positive (+2.9 to +25) indicating a warming. For CO the SGWP* values (3.8 and 4.4 for European and Asian emissions respectively, with only the Oslo-CTM2/ECHAM4 model combination) are less regionally dependent. Our results support the view that for NOx, regionally different weighting factors for the emissions are necessary. For CO the results are more robust and one global number may be acceptable.

Comments on “Contrails, Cirrus Trends, and Climate”

Minnis et al. (2004, hereafter MAPP) estimate that surface temperature trends over the United States due to aircraft-induced contrails and cirrus are in “good agreement” with corresponding observations over the period 1973 to 1994. MAPP reach this conclusion based on a simple model [their Eq. (3)] calibrated using general circulation model (GCM) results from Rind et al. (2000, hereafter RLS). Depending on the assumed cirrus optical depth, MAPP estimate an annual-mean surface warming trend over the United States of between 0.16 and 0.27 K decade , compared with Angell’s (1999) observed trend of 0.27 K decade . I believe that MAPP significantly overestimate the climate impact of aircraft-induced clouds. A useful first-order estimate for the equilibrium global-mean surface temperature change Ts comes from applying the approximate equation Ts F, where is a climate sensitivity parameter and F is the radiative forcing (e.g., Houghton et al. 2001). The range of values of found in current GCMs is about 0.4 to 1.2 K (W m ) , corresponding to an equilibrium surface warming due to a doubling of carbon dioxide from about 1.5 to 4.5 K (e.g., Houghton et al. 2001); RLS’s GCM has a climate sensitivity toward the upper end of this range. MAPP estimate a maximum global-mean radiative forcing due to aircraft-induced clouds (for 1992) of 0.0255 W m . With these values, the above equation yields a global-mean warming of 0.01 to 0.03 K, or about 0.003 to 0.01 K decade , assuming that the forcing grew quasi-linearly over the previous three decades. Hence, this estimate is one to two orders of magnitude lower than that given by MAPP. The realized warming is likely to be 25%–50% smaller than this because of the thermal inertia of the climate system. MAPP’s estimate is for the United States, rather than for the global mean, but evidence for significant localization of the climate response resulting from a geographically limited forcing, beyond a broad hemispheric scale effect, is scant. Plate 3 of RLS shows that their modeled surface warming due to aircraft-induced clouds over the United States as a whole differs by no more than 50% from the global mean. More importantly, RLS compare two experiments, both with a global-mean increase in aircraft-induced clouds of about 1%. In one experiment (which they label “1/99”), the spatial pattern of cloud change is related to flight path occurrence; in the other (labeled “scaled”), the spatial pattern is related to flight path density, yielding a far more inhomogeneous change in cloudiness (see RLS, Plate 1). The scaled experiment yields a cloud change over the eastern United States that reaches about a factor of 2 larger than 1/99—despite this, the pattern of surface temperature response in the two experiments is very similar and RLS conclude that “there is no obvious geographical bias in the warming” despite the localization of the forcing. Other GCM studies on the impact of inhomogeneous patterns of forcing reach similar conclusions. For example, Boer and Yu (2003) conclude that “temperature response patterns are at best a weak reflection of their parent forcing pattern.” Certainly the regional nature of the aircraft-induced cloud forcing cannot explain the order(s) of magnitude difference in surface temperature change between the estimate derived here and that derived by MAPP. I believe that the main cause of MAPP’s overestimate is that their use of Eq. (3) implicitly assumes that there is a 1% cloud cover change present globally, or at least hemispherically, rather than just over the United States. Indeed, RLS (their Table 3a) show that a 1% Corresponding author address: Dr. Keith P. Shine, Dept. of Meteorology, University of Reading, Earley Gate, Reading RG6 6BB, United Kingdom. E-mail: k.p.shine@reading.ac.uk 15 JULY 2005 N O T E S A N D C O R R E S P O N D E N C E 2781

Line-by-line calculations of thermal infrared radiation representative for global condition: CFC-12 as an example

We estimate a current direct radiative forcing due to CFC-12 of 0.18 Wm - 2 , which is likely to be the peak radiative forcing for CFC-12. Global measurements of CFC-12 show at present an almost negligible trend for CFC-12 and measurement in an industrialized region show evidence that the peak concentration is reached. It is expected that concentration of CFC-12 in industrialized regions begins to decline 1–3 years before the global concentration. Our radiative forcing calculations are based on a line-by-line model appropriate for simulation of global mean radiative forcing, including clouds and stratospheric temperature adjustment. The radiative forcing of 0.33 Wm - 2 /ppbv is close to earlier published results for this compound. New spectroscopic measurements for CFC-12 are performed and compared to previously published results.

Simulation of climate response to aerosol direct and indirect effects with aerosol transport‐radiation model

With a global aerosol transport‐radiation model coupled to a general circulation model, changes in the meteorological parameters of clouds, precipitation, and temperature caused by the direct and indirect effects of aerosols are simulated, and its radiative forcing are calculated. A microphysical parameterization diagnosing the cloud droplet number concentration based on the Köhler theory is introduced into the model, which depends not only on the aerosol particle number concentration but also on the updraft velocity, size distributions, and chemical properties of each aerosol species and saturation condition of the water vapor. The simulated cloud droplet effective radius, cloud radiative forcing, and precipitation rate, which relate to the aerosol indirect effect, are in reasonable agreement with satellite observations. The model results indicate that a decrease in the cloud droplet effective radius by anthropogenic aerosols occurs globally, while changes in the cloud water and precipitation are strongly affected by a variation of the dynamical hydrological cycle with a temperature change by the aerosol direct and first indirect effects rather than the second indirect effect itself. However, the cloud water can increase and the precipitation can simultaneously decrease in regions where a large amount of anthropogenic aerosols and cloud water exist, which is a strong signal of the second indirect effect. The global mean radiative forcings of the direct and indirect effects at the tropopause by anthropogenic aerosols are calculated to be −0.1 and −0.9 W m−2, respectively. It is suggested that aerosol particles approximately reduce 40% of the increase in the surface air temperature by anthropogenic greenhouse gases on the global mean.

Response of climate to regional emissions of ozone precursors: sensitivities and warming potentials

The response of climate to ozone perturbations caused by regional emissions of NOx or CO has been studied through a sequence of model simulations. Changes in O3 and OH concentrations due to emission perturbations in Europe and southeast Asia have been calculated with two global 3-D chemical tracer models(CTMs; LMDzINCA and Oslo-CTM2). The radiative transfer codes of three general circulation models (GCMs; ECHAM4, UREAD and LMD) have been used to calculate the radiative forcing of the O3 perturbations, and for a subset of the cases full GCM simulations have been performed with ECHAM4 and UREAD. The results have been aggregated to a global number in two ways: first, through integrating the global-mean radiative forcing of a sustained step change in emissions, and second through a modified concept (SGWP*) which includes possible differences in the climate sensitivity of O3, CH4 and CO2 changes. In terms of change in global tropospheric O3 burden the two CTMs differ by less than 30%. Both CTMs show a higher north/south gradient in the sensitivity to changes in NOx emission than for CO. We are not able to conclude whether real O3 perturbations in general have a different climate sensitivity from CO2. However, in both GCMs high-latitude emission perturbations lead to climate perturbations with higher (10–30%) climate sensitivities. The calculated SGWP*, for a 100 yr time horizon, are negative for three of the four CTM/GCM combinations for European emissions (−9.6 to +6.9), while for the Asian emissions the SGWP* (H= 100) is always positive (+2.9 to +25) indicating a warming. For CO the SGWP* values (3.8 and 4.4 for European and Asian emissions respectively, with only the Oslo-CTM2/ECHAM4 model combination) are less regionally dependent. Our results support the view that for NOx, regionally different weighting factors for the emissions are necessary. For CO the results are more robust and one global number may be acceptable.

Direct human influence of irrigation on atmospheric water vapour and climate

Human activity increases the atmospheric water vapour content in an indirect way through climate feedbacks. We conclude here that human activity also has a direct influence on the water vapour concentration through irrigation. In idealised simulations we estimate a global mean radiative forcing in the range of 0.03 to +0.1 Wm–2 due to the increase in water vapour from irrigation. However, because the water cycle is embodied in the climate system, irrigation has a more complex influence on climate. We also simulate a change in the temperature vertical profile and a large surface cooling of up to 0.8 K over irrigated land areas. This is of opposite sign than expected from the radiative forcing alone, and this questions the applicability of the radiative forcing concept for such a climatic perturbation. Further, this study shows stronger links than previously recognised between climate change and freshwater scarcity which are environmental issues of paramount importance for the twenty first century.

The radiative effect of the anthropogenic influence on the stratospheric sulfate aerosol layer

Stratospheric sulfate aerosols have a cooling effect on the Earth’s surface. Sulfur aerosols from large volcanic eruptions are often the dominant source, while non-volcanic background stratospheric sulfate aerosols are supposed to mainly originate from carbonyl sulfide (OCS). Several recent studies indicate, however, that this latter source is too small to account for the observed background stratospheric aerosol concentration. Based on model calculations we suggest that most of the lower stratospheric sulfate aerosol concentration is of anthropogenic origin. We estimate a global mean radiative forcing due to the anthropogenic influence on the stratospheric aerosol layer of −0.05 Wm−2. This represents a new climate forcing mechanism and emphasizes anthropogenic sulfur emission as an important cooling mechanism.

Uncertainties in Radiative Forcing due to Surface Albedo Changes Caused by Land-Use Changes

Abstract A radiative transfer model has been used for estimating the radiative forcing due to land-use changes. Five global datasets for current vegetation cover and three datasets of preagriculture vegetation have been adopted. The vegetation datasets have been combined with three datasets for surface albedo values. A distinct feature in all the calculations is the negative radiative forcing at the northern midlatitudes due to the conversion of forest to cropland. Regionally the radiative forcing is likely to be among the strongest of the climate forcing mechanisms. A wider range is estimated for the global mean radiative forcing due to land-use changes than previously reported. The single most important factor yielding the large range in estimated forcing is the cropland surface albedo values. This underlines the importance of characterizing surface albedo correctly.

Design and Analysis of Climate Model Experiments for the Efficient Estimation of Anthropogenic Signals

Abstract Presented herein is an experimental design that allows the effects of several radiative forcing factors on climate to be estimated as precisely as possible from a limited suite of atmosphere-only general circulation model (GCM) integrations. The forcings include the combined effect of observed changes in sea surface temperatures, sea ice extent, stratospheric (volcanic) aerosols, and solar output, plus the individual effects of several anthropogenic forcings. A single linear statistical model is used to estimate the forcing effects, each of which is represented by its global mean radiative forcing. The strong colinearity in time between the various anthropogenic forcings provides a technical problem that is overcome through the design of the experiment. This design uses every combination of anthropogenic forcing rather than having a few highly replicated ensembles, which is more commonly used in climate studies. Not only is this design highly efficient for a given number of integrations, but it also allows the estimation of (nonadditive) interactions between pairs of anthropogenic forcings. The simulated land surface air temperature changes since 1871 have been analyzed. The changes in natural and oceanic forcing, which itself contains some forcing from anthropogenic and natural influences, have the most influence. For the global mean, increasing greenhouse gases and the indirect aerosol effect had the largest anthropogenic effects. It was also found that an interaction between these two anthropogenic effects in the atmosphere-only GCM exists. This interaction is similar in magnitude to the individual effects of changing tropospheric and stratospheric ozone concentrations or to the direct (sulfate) aerosol effect. Various diagnostics are used to evaluate the fit of the statistical model. For the global mean, this shows that the land temperature response is proportional to the global mean radiative forcing, reinforcing the use of radiative forcing as a measure of climate change. The diagnostic tests also show that the linear model was suitable for analyses of land surface air temperature at each GCM grid point. Therefore, the linear model provides precise estimates of the space–time signals for all forcing factors under consideration. For simulated 50-hPa temperatures, results show that tropospheric ozone increases have contributed to stratospheric cooling over the twentieth century almost as much as changes in well-mixed greenhouse gases.

Uncertainties in Radiative Forcing due to Surface Albedo Changes Caused by Land-Use Changes

A radiative transfer model has been used for estimating the radiative forcing due to land-use changes. Five global datasets for current vegetation cover and three datasets of preagriculture vegetation have been adopted. The vegetation datasets have been combined with three datasets for surface albedo values. A distinct feature in all the calculations is the negative radiative forcing at the northern midlatitudes due to the conversion of forest to cropland. Regionally the radiative forcing is likely to be among the strongest of the climate forcing mechanisms. A wider range is estimated for the global mean radiative forcing due to land-use changes than previously reported. The single most important factor yielding the large range in estimated forcing is the cropland surface albedo values. This underlines the importance of characterizing surface albedo correctly.

Climate sensitivity and response

Results from climate change simulations indicate a reasonably robust proportionality between global mean radiative forcing and global mean surface air temperature response. The "constant" of proportionality is a measure of the overall strength of climate feedback processes and hence of global climate sensitivity. Geographically, however, temperature response patterns are generally not proportional to, nor do they resemble, their parent forcing patterns. Temperature response patterns, nevertheless, exhibit a remarkable additivity whereby the sum of response patterns for different forcings closely resembles the response pattern for the sum of the forcings.

Evolution of tropospheric ozone under anthropogenic activities and associated radiative forcing of climate

The budget of ozone and its evolution associated with anthropogenic activities are simulated with the Model for Ozone and Related Chemical Tracers (MOZART) (version 1). We present the changes in tropospheric ozone and its precursors (CH4, NMHCs, CO, NOx) since the preindustrial period. The ozone change at the surface exhibits a maximum increase at midlatitudes in the northern hemisphere reaching more than a factor of 3 over Europe, North America, and Southeast Asia during summer. The calculated preindustrial ozone levels are particularly sensitive to assumptions about natural and biomass burning emissions of precursors. The possible future evolution of ozone to the year 2050 is also simulated, using the Intergovernmental Panel on Climate Change IS92a scenario to estimate the global and geographical changes in surface emissions. The future evolution of ozone stresses the important role played by the tropics and the subtropics. In this case a maximum ozone increase is calculated in the northern subtropical region and is associated with increased emissions in Southeast Asia and Central America. The ozone future evolution also affects the more remote regions of the troposphere, and an increase of 10–20% in background ozone levels is calculated over marine regions in the southern hemisphere. Our best estimate of the global and annual mean radiative forcing associated with tropospheric ozone increase since the preindustrial era is 0.43 W m−2. This value represents about 20% of the forcing associated with well‐mixed greenhouse gases. The normalized tropospheric ozone radiative forcing is 0.048 W m−2 DU−1. An upper estimate on our forcing of 0.77 W m−2 is calculated when a stratospheric tracer is used to approximate background ozone levels. In 2050 an additional ozone forcing of 0.26 W m−2 is calculated, providing a forcing from preindustrial to 2050 of 0.69 W m−2.

Global sensitivity experiments of the radiative forcing due to mineral aerosols

The radiative effects of mineral dust in the atmosphere are uncertain. Further, the human contribution to the mineral aerosol concentration is difficult to quantify. We have performed several global sensitivity experiments to investigate the radiative forcing due to mineral dust. Two global data sets of mineral aerosol distribution are used. Radiative transfer schemes for thermal infrared and solar radiation are used in the calculations. We have investigated the sensitivity of the global radiative forcing to the spatial distribution of the aerosols, including the altitude, the size distribution, and optical parameters. Our strongest emphasis has been on the size distribution of the mineral aerosol, for which we have found a strong sensitivity. A range of −0.7 Wm−2 to 0.5 Wm−2 is estimated for the human influence on the global radiative forcing due to mineral aerosols. We find it almost as probable with a positive radiative forcing as with a negative forcing. Even if the global mean radiative forcing is small, there are large contributions of different sign in certain regions.

Biogeophysical impacts of land use on present-day climate: near-surface temperature change and radiative forcing

Changes in land cover affect climate through the surface energy and moisture budgets, but these biogeophysical impacts of land use have not yet been included in General Circulation Model (GCM) simulations of 20th century climate change. Here, the importance of these effects was assessed by comparing climate simulations performed with current and potential natural vegetation. The northern mid-latitude agricultural regions were simulated to be approximately 1–2K cooler in winter and spring in comparison with their previously forested state, due to deforestation increasing the surface albedo by approximately 0.1 during periods of snow cover. Some other regions such as the Sahel and India experienced a small warming due to land use. Although the annual mean global temperature is only 0.02K lower in the simulation with present-day land use, the more local temperature changes in some regions are of a similar magnitude to those observed since 1860. The global mean radiative forcing by anthropogenic surface albedo change relative to the natural state is simulated to be −0.2Wm2, which is comparable with the estimated forcings relative to pre-industrial times by changes in stratospheric and tropospheric ozone, N2O, halocarbons, and the direct effect of anthropogenic aerosols. Since over half of global deforestation has occurred since 1860, simulations of climate since that date should include the biogeophysical effects of land use.

Estimates of the direct and indirect radiative forcing due to tropospheric aerosols: A review

This paper reviews the many developments in estimates of the direct and indirect global annual mean radiative forcing due to present‐day concentrations of anthropogenic tropospheric aerosols since Intergovernmental Panel on Climate Change [1996]. The range of estimates of the global mean direct radiative forcing due to six distinct aerosol types is presented. Additionally, the indirect effect is split into two components corresponding to the radiative forcing due to modification of the radiative properties of clouds (cloud albedo effect) and the effects of anthropogenic aerosols upon the lifetime of clouds (cloud lifetime effect). The radiative forcing for anthropogenic sulphate aerosol ranges from −0.26 to −0.82 W m−2. For fossil fuel black carbon the radiative forcing ranges from +0.16 W m−2 for an external mixture to +0.42 W m−2 for where the black carbon is modeled as internally mixed with sulphate aerosol. For fossil fuel organic carbon the two estimates of the likely weakest limit of the direct radiative forcing are −0.02 and −0.04 W m−2. For biomass‐burning sources of black carbon and organic carbon the combined radiative forcing ranges from −0.14 to −0.74 W m−2. Estimates of the radiative forcing due to mineral dust vary widely from +0.09 to −0.46 W m−2; even the sign of the radiative forcing is not well established due to the competing effects of solar and terrestrial radiative forcings. A single study provides a very tentative estimate of the radiative forcing of nitrates to be −0.03 W m−2. Estimates of the cloud albedo indirect radiative forcing range from −0.3 to approximately −1.8 W m−2. Although the cloud lifetime effect is identified as a potentially important climate forcing mechanism, it is difficult to quantify in the context of the present definition of radiative forcing of climate change and current model simulations. This is because its estimation by general circulation models necessarily includes some level of cloud and water vapor feedbacks, which affect the hydrological cycle and the dynamics of the atmosphere. Available models predict that the radiative flux perturbation associated with the cloud lifetime effect is of a magnitude similar to that of the cloud albedo effect.