Pre-industrial Research Articles

Importance of tropospheric volcanic aerosol for indirect radiative forcing of climate

Abstract. Observations and models have shown that continuously degassing volcanoes have a potentially large effect on the natural background aerosol loading and the radiative state of the atmosphere. We use a global aerosol microphysics model to quantify the impact of these volcanic 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, volcanic 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 radiative state. Assuming a −50/+100% uncertainty range in the volcanic sulphur flux, we estimate the annual mean anthropogenic cloud albedo forcing to lie between −1.16 W m−2 and −0.86 W m−2. Therefore, the volcanically induced uncertainty in the PI baseline cloud radiative state substantially adds to the already large uncertainty in the magnitude of the indirect radiative forcing of climate.

Dependence of the effect of aerosols on cirrus clouds on background vertical velocity

Cirrus clouds cover approximately 20–25% of the globe and thus play an important role in the Earth's radiation budget. This important role in the radiation budget played by cirrus clouds indicates that aerosol effects on cirrus clouds can have a substantial impact on the variation of global radiative forcing if the ice–water path (IWP) changes. This study examines the aerosol indirect effect (AIE) through changes in the IWP for cirrus cloud cases. This study also examines the dependence of aerosol–cloud interactions in cirrus clouds on the large-scale vertical motion. We use a cloud-system resolving model (CSRM) coupled with a double-moment representation of cloud microphysics.Intensified interactions among the cloud ice number concentration (CINC), deposition and dynamics play a critical role in the IWP increases due to aerosol increases from the preindustrial (PI) level to the present-day (PD) level with a low large-scale vertical velocity. Increased aerosols lead to an increased CINC, providing an increased surface area for water vapor deposition. The increased surface area leads to increased deposition despite decreased supersaturation with increasing aerosols. The increased deposition causes an increased depositional heating which produces stronger updrafts, and these stronger updrafts lead to the increased IWP. However, with a high large-scale vertical velocity, the effect of increased CINC on deposition was not able to offset the effect of decreasing supersaturation with increasing aerosols. The effect of decreasing supersaturation on deposition dominant over that of increasing CINC leads to smaller deposition and IWP at high aerosol with the PD aerosol than at low aerosol with the PI aerosol.The conversion of ice crystals to aggregates through autoconversion and accretion plays a negligible role in the IWP responses to aerosols, as does the sedimentation of aggregates. The sedimentation of ice crystals plays a more important role in the IWP response to aerosol increases than the sedimentation of aggregates, but, not more important than the interactions among the CINC, supersaturation, deposition and dynamics. These interactions not only determine the effect of aerosols on IWP but also control how this effect varies with varying large-scale vertical velocities.

Reply to Quaas et al.: Can satellites be used to estimate indirect climate forcing by aerosols?

We welcome the comments by Quaas et al. (1). In our paper (2), we used a model to show that the methods used to estimate indirect aerosol forcing using satellite data, especially those based on relating the slope of present-day (PD) drop number (Nc) to aerosol optical depth (AOD), underestimate the forcing calculated when both PD and preindustrial (PI) data are available. [↵][1]1To whom correspondence should be addressed. E-mail: penner{at}umich.edu. [1]: #xref-corresp-1-1

Satellite methods underestimate indirect climate forcing by aerosols

Satellite-based estimates of the aerosol indirect effect (AIE) are consistently smaller than the estimates from global aerosol models, and, partly as a result of these differences, the assessment of this climate forcing includes large uncertainties. Satellite estimates typically use the present-day (PD) relationship between observed cloud drop number concentrations (N(c)) and aerosol optical depths (AODs) to determine the preindustrial (PI) values of N(c). These values are then used to determine the PD and PI cloud albedos and, thus, the effect of anthropogenic aerosols on top of the atmosphere radiative fluxes. Here, we use a model with realistic aerosol and cloud processes to show that empirical relationships for ln(N(c)) versus ln(AOD) derived from PD results do not represent the atmospheric perturbation caused by the addition of anthropogenic aerosols to the preindustrial atmosphere. As a result, the model estimates based on satellite methods of the AIE are between a factor of 3 to more than a factor of 6 smaller than model estimates based on actual PD and PI values for N(c). Using ln(N(c)) versus ln(AI) (Aerosol Index, or the optical depth times angstrom exponent) to estimate preindustrial values for N(c) provides estimates for N(c) and forcing that are closer to the values predicted by the model. Nevertheless, the AIE using ln(N(c)) versus ln(AI) may be substantially incorrect on a regional basis and may underestimate or overestimate the global average forcing by 25 to 35%.

Comparisons of atmosphere–ocean simulations of greenhouse gas-induced climate change for pre-industrial and hypothetical ‘no-anthropogenic’ radiative forcing, relative to present day

We compare climate simulations for Present-Day (PD), Pre-Industrial (PI) time, and a hypothetical (inferred) state termed No-Anthropogenic (NA) based upon the low greenhouse gas (GHG) levels of the late stages of previous interglacials that are comparable in time (orbital configuration) to the present interglacial. We use a fully coupled dynamical atmosphere–ocean model, the CCSM3. We find a consistent trend toward colder climate (lower surface temperature, more snow and sea-ice cover, lower ocean temperature, and modified ocean circulation) as the net change in GHG radiative forcing trends more negative from PD to PI to NA. The climatic response of these variables becomes larger relative to the changed GHG forcing for each step toward a colder climate state (PD to PI to NA). This amplification is significantly enhanced using the dynamical atmosphere–ocean model compared with our previous results with an atmosphere–slab ocean model, a result that conforms to earlier idealized GHG forcing experiments. However, in our case this amplification is not an idealized result, but instead helps frame important questions concerning aspects of Holocene climate change. This enhanced amplification effect leads to an increase in our estimate of the climate’s response to inferred early anthropogenic CO2 increases (NA to PI) relative to the response to industrial-era CO2 increases (PI to PD). Although observations of the climate for the hypothetical NA (inferred from observations of previous interglacials) and for PI have significant uncertainties, our new results using CCSM3 are in better agreement with these observations than our previous results from an atmospheric model coupled to a static slab ocean. The results support more strongly inferences by Ruddiman concerning indirect effects of ocean solubility/sea-ice/deep ocean ventilation feedbacks that may have contributed to a further increase in late-Holocene atmospheric CO2 beyond that caused by early anthropogenic emissions alone.

ENERGETICS OF CLIMATE MODELS: NET ENERGY BALANCE AND MERIDIONAL ENTHALPY TRANSPORT

We analyze the publicly released outputs of the simulations performed by climate models (CMs) in preindustrial (PI) and Special Report on Emissions Scenarios A1B (SRESA1B) conditions. In the PI simulations, most CMs feature biases of the order of 1 W m−2 for the net global and the net atmospheric, oceanic, and land energy balances. This does not result from transient effects but depends on the imperfect closure of the energy cycle in the fluid components and on inconsistencies over land. Thus, the planetary emission temperature is underestimated, which may explain the CMs' cold bias. In the PI scenario, CMs agree on the meridional atmospheric enthalpy transport's peak location (around 40°N/S), while discrepancies of ∼20% exist on the intensity. Disagreements on the oceanic transport peaks' location and intensity amount to ∼10° and ∼50%, respectively. In the SRESA1B runs, the atmospheric transport's peak shifts poleward, and its intensity increases up to ∼10% in both hemispheres. In most CMs, the Northern Hemispheric oceanic transport decreases, and the peaks shift equatorward in both hemispheres. The Bjerknes compensation mechanism is active both on climatological and interannual time scales. The total meridional transport peaks around 35° in both hemispheres and scenarios, whereas disagreements on the intensity reach ∼20%. With increased CO2 concentration, the total transport increases up to ∼10%, thus contributing to polar amplification of global warming. Advances are needed for achieving a self‐consistent representation of climate as a nonequilibrium thermodynamical system. This is crucial for improving the CMs' skill in representing past and future climate changes.

Seasonally asymmetric transition of the Asian monsoon in response to ice age boundary conditions

Modulation of a monsoon under glacial forcing is examined using an atmosphere–ocean coupled general circulation model (AOGCM) following the specifications established by Paleoclimate Modelling Intercomparison Project phase 2 (PMIP2) to understand the air–sea–land interaction under different climate forcing. Several sensitivity experiments are performed in response to individual changes in the continental ice sheet, orbital parameters, and sea surface temperature (SST) in the Last Glacial Maximum (LGM: 21 ka) to evaluate the driving mechanisms for the anomalous seasonal evolution of the monsoon. Comparison of the model results in the LGM with the pre-industrial (PI) simulation shows that the Arabian Sea and Bay of Bengal are characterized by enhancement of pre-monsoon convection despite a drop in the SST encompassing the globe, while the rainfall is considerably suppressed in the subsequent monsoon period. In the LGM winter relative to the PI, anomalies in the meridional temperature gradient (MTG) between the Asian continents minus the tropical oceans become positive and are consistent with the intensified pre-monsoon circulation. The enhanced MTG anomalies can be explained by a decrease in the condensation heating relevant to the suppressed tropical convection as well as positive insolation anomalies in the higher latitude, showing an opposing view to a warmer future climate. It is also evident that a latitudinal gradient in the SST across the equator plays an important role in the enhancement of pre-monsoon rainfall. As for the summer, the sensitivity experiments imply that two ice sheets over the northern hemisphere cools the air temperature over the Asian continent, which is consistent with the reduction of MTG involved in the attenuated monsoon. The surplus pre-monsoon convection causes a decrease in the SST through increased heat loss from the ocean surface; in other words, negative ocean feedback is also responsible for the subsequent weakening of summer convection.

Arctic Oscillation during the Mid-Holocene and Last Glacial Maximum from PMIP2 Coupled Model Simulations

Abstract Changes in the Arctic Oscillation (AO) during the mid-Holocene and the Last Glacial Maximum were compared to preindustrial (PI) simulations using four coupled ocean–atmosphere models [i.e., Community Climate System Model (CCSM), third climate configuration of the Met Office Unified Model (HadCM3) Met Office Surface Exchanges Scheme, version 2 (MOSES2), L’Institut Pierre-Simon Laplace (IPSL), and Model for Interdisciplinary Research on Climate 3.2 (MIROC3.2)] from the second phase of the Paleoclimate Modeling Intercomparison Project. Results show that the amplitude of the simulated AO during the mid-Holocene is a little smaller than that of the preindustrial simulation. Although the AO pattern and vertical structures are similar to those in the preindustrial simulation, the polar westerlies are slightly weakened and displaced downward to the lower stratosphere, accompanied by weakening of the polar vortex and warming of the cold polar cap region. During the Last Glacial Maximum, when the Northern Hemisphere experiences severe cooling, the intensity of the AO decreases substantially compared to the mid-Holocene, with smaller standard deviations of the AO indices in all models. Furthermore, the magnitude of positive and negative centers of the AO spatial pattern decreases and the strength of the polar vortex and westerlies weakens further with the center of westerlies displaced into the midlatitude upper troposphere. The polar cap region becomes anomalously warm in the stratosphere, whereas it remains cold in the troposphere. The AO appears to be sensitive to background climate state. Upward-propagating stationary Rossby waves are found to be stronger during the mid-Holocene and Last Glacial Maximum than in the preindustrial simulation. This increase in planetary wave activity might be responsible for the simulated weakening of the AO during the mid-Holocene and Last Glacial Maximum. Recent studies have shown that there is a significant correlation between Eurasian fall snow cover and the winter AO. The upward propagation of Rossby waves was further proposed to explain the physical process linking the AO with the snow depth. It is suggested that a large increase in fall snow depth during the Last Glacial Maximum strengthens the upward-propagating stationary Rossby waves relative to the PI.

Comparison of a global-climate model to a cloud-system resolving model for the long-term response of thin stratocumulus clouds to preindustrial and present-day aerosol conditions

Abstract. The response of a case of thin, warm marine-boundary-layer (MBL) clouds to preindustrial (PI) and present-day (PD) conditions is simulated by a cloud-system resolving model (CSRM). Here, both the aerosol conditions and environmental conditions match those of a general circulation model (GCM). The environmental conditions are characterized by the initial condition and the large-scale forcings of humidity and temperature, as well as the surface fluxes. The response of the CSRM is compared to that simulated by the GCM. The percentage increase of liquid-water path (LWP) due to a change from the PI to PD conditions is ~3 times larger in the CSRM than that in the GCM due to the formation of cumulus clouds. The formation of cumulus clouds is controlled by a larger increase in the surface latent-heat (LH) flux in the PD environment than in the PI environment rather than by the change in aerosols. However, the aerosol increase from the PI to PD level determines the LWP response in the stratocumulus clouds, while the impacts of changes in environmental conditions are negligible for stratocumulus clouds. The conversion of cloud liquid to rain through autoconversion and accretion plays a negligible role in the CSRM in the response to aerosols, whereas it plays a role that is as important as condensation in the GCM. Also, it is notable that the explicit simulation of microphysics in the CSRM leads to a smaller LWP in the CSRM than that in the GCM using heavily parameterized microphysics for stratocumulus clouds. The smaller LWP in the CSRM is closer to an observed LWP than the LWP in the GCM for stratocumulus clouds. Supplementary simulations show that increasing aerosols increase the sensitivity of the cloud responses to the PI and PD environmental conditions. They also show that aerosol effects on clouds depend on the cloud type. The LWP of warm cumulus clouds is more sensitive to aerosols than that of stratocumulus clouds.

Increased greenhouse gases enhance regional climate response to a Maunder Minimum

The climate responses to the Maunder Minimum‐type total solar irradiance (TSI) decrease in the pre‐industrial (PI) era and IPCC B1 global warming scenario are examined using the NCAR CAM3 coupled with a mixed‐layer slab ocean model. The TSI reduction shifts the AO/NAO state to a more negative index, resulting in regional surface air temperature (SAT) change in the model. In addition, the positive sea ice‐solar radiation feedback amplifies the surface cooling due to the TSI decrease. The global annual average cooling effect induced by the TSI decrease is reduced to 0.254°C in the B1 scenario from 0.347°C in the PI epoch, this difference being due to both the suppressed sea ice‐solar radiation feedback and the stronger greenhouse effect associated with the increase of greenhouse gases. However, regional SAT changes in the B1 scenario are strengthened markedly due to the enhanced negative AO/NAO. The warming in the western Greenland, Central Asia and East Asia will be enhanced in the B1 global warming scenario when the TSI decreases. These differences in response between the B1 and PI scenarios illustrate that the impact of multi‐decadal solar variation on climate depends on the atmospheric background trace gas composition. A future solar grand minimum does not offset the CO2‐induced warming in the B1 scenario, but through the enhanced negative AO/NAO may indeed amplify the warming in certain regions.

Impacts of aerosol-cloud interactions on past and future changes in tropospheric composition

Abstract. The development of effective emissions control policies that are beneficial to both climate and air quality requires a detailed understanding of all the feedbacks in the atmospheric composition and climate system. We perform sensitivity studies with a global atmospheric composition-climate model to assess the impact of aerosols on tropospheric chemistry through their modification on clouds, aerosol-cloud interactions (ACI). The model includes coupling between both tropospheric gas-phase and aerosol chemistry and aerosols and liquid-phase clouds. We investigate past impacts from preindustrial (PI) to present day (PD) and future impacts from PD to 2050 (for the moderate IPCC A1B scenario) that embrace a wide spectrum of precursor emission changes and consequential ACI. The aerosol indirect effect (AIE) is estimated to be −2.0 Wm−2 for PD-PI and −0.6 Wm−2 for 2050-PD, at the high end of current estimates. Inclusion of ACI substantially impacts changes in global mean methane lifetime across both time periods, enhancing the past and future increases by 10% and 30%, respectively. In regions where pollution emissions increase, inclusion of ACI leads to 20% enhancements in in-cloud sulfate production and ~10% enhancements in sulfate wet deposition that is displaced away from the immediate source regions. The enhanced in-cloud sulfate formation leads to larger increases in surface sulfate across polluted regions (~10–30%). Nitric acid wet deposition is dampened by 15–20% across the industrialized regions due to ACI allowing additional re-release of reactive nitrogen that contributes to 1–2 ppbv increases in surface ozone in outflow regions. Our model findings indicate that ACI must be considered in studies of methane trends and projections of future changes to particulate matter air quality.

Corrigendum to "Influence of Giant CCN on warm rain processes in the ECHAM5 GCM" published in Atmos. Chem. Phys., 8, 2949–2963, 2008

In the simulations for the paper “Influence of Giant CCN on warm rain processes in the ECHAM5 GCM” by Rebekka Posselt and Ulrike Lohmann (Atmos. Chem. Phys., 8, 2949‐2963, 2008) we discovered that the pre-industrial (PI) simulations included only the effect of pre-industrial sulphate. All other emissions were by mistake present-day (PD). Thus, the reported values for the aerosol indirect effect (AIE) have to be corrected and the conclusions regarding the effect of the giant cloud condensation nuclei (GCCN) on the AIE change slightly. The present-day simulations are correct and thus the conclusions regarding this part are not affected. The corrected figures (Figs. 11, 12, 13) and table (Table 5) appear below. In most cases only the numbers change but the explanation remain the same. Cases where the effect of GCCN on anthropogenic aerosols also changes are discussed below. The changes of the zonal mean GCCN concentration as well as GCCN fraction from the pre-industrial to present-day climate are shown in Fig. 11. The GCCN concentration increases between 45 S and 60 N. Below and above these latitudes the GCCN concentrations are decreasing. These changes are mostly caused by the changes in the 10 m wind speed from pre-industrial to present-day climate as shown in Fig. 12. Most obvious changes are visible in the Southern Hemisphere, where the 10 m wind speed increases between 30 S and 45 S but decreases between 45 S and 60 S. In the Northern Hemisphere the 10m wind speed is slightly increasing. The changes in the annual global and zonal means from pre-industrial to present-day climate are shown in Table 5 and Fig. 13. The increase in total cloud cover (TCC) from PI to PD decreases with an increasing amount of GCCN from 0.43 for CTL30 to 0.4 and 0.33 for simulations GCCN10 and GCCN5, respectively. Main changes are visible in the Northern Hemisphere in analogy to the changes in total water path (TWP) and the cloud drop number concentration (Nl) of the original paper. The decreasing TCC differences results in reduced changes in the shortwave (SW) radiation budget depending on whether and how many GCCN are present. The implementation of the prognostic rain scheme reduces the SW total anthropogenic effect by 0.29 W m 2 in CTL30 and by 0.39 W m 2 in CTL10 as compared to 2 W m 2 reported by Lohmann et al. (2007) for the standard ECHAM5-HAM. In case of the simulation GCCN10 the GCCN number is not yet large enough to cause considerable changes in the SW radiation globally but in simulation GCCN5 the GCCN lead to further reduction in the SW radiation by 0.28 W m 2 . As in

Differences in the non‐stationary influence of the North Atlantic Oscillation on European precipitation under different scenarios of greenhouse gas concentrations

The changing spatial patterns of the North Atlantic Oscillation (NAO) dipole and the changing correlations between the NAO and European precipitation were analyzed using the AOGCM CGCM3.1(T63) model provided by the Canadian Centre for Climate Modelling and Analysis (CCCma). Four output scenarios were considered: the IPCC pre‐industrial (PI) scenario, the 20th century (20C) scenario, and the B1 and A2 emission scenarios for the 21st century. We detected some differences in the spatial patterns of the NAO‐precipitation relationship among the four scenarios. Moreover, there were large differences in the temporal stability of the NAO pressure pattern, and also in the NAO‐precipitation relationship among the four scenarios. Under increased greenhouse gas emissions the stability of the NAO‐precipitation correlations is enhanced due to the lower variability in the pattern of the NAO dipole.

Simulations of preindustrial, present-day, and 2100 conditions in the NASA GISS composition and climate model G-PUCCINI

Abstract. A model of atmospheric composition and climate has been developed at the NASA Goddard Institute for Space Studies (GISS) that includes composition seamlessly from the surface to the lower mesosphere. The model is able to capture many features of the observed magnitude, distribution, and seasonal cycle of trace species. The simulation is especially realistic in the troposphere. In the stratosphere, high latitude regions show substantial biases during period when transport governs the distribution as meridional mixing is too rapid in this model version. In other regions, including the extrapolar tropopause region that dominates radiative forcing (RF) by ozone, stratospheric gases are generally well-simulated. The model's stratosphere-troposphere exchange (STE) agrees well with values inferred from observations for both the global mean flux and the ratio of Northern (NH) to Southern Hemisphere (SH) downward fluxes. Simulations of preindustrial (PI) to present-day (PD) changes show tropospheric ozone burden increases of 11% while the stratospheric burden decreases by 18%. The resulting tropopause RF values are −0.06 W/m2 from stratospheric ozone and 0.40 W/m2 from tropospheric ozone. Global mean mass-weighted OH decreases by 16% from the PI to the PD. STE of ozone also decreased substantially during this time, by 14%. Comparison of the PD with a simulation using 1979 pre-ozone hole conditions for the stratosphere shows a much larger downward flux of ozone into the troposphere in 1979, resulting in a substantially greater tropospheric ozone burden than that seen in the PD run. This implies that reduced STE due to stratospheric ozone depletion may have offset as much as 2/3 of the tropospheric ozone burden increase from PI to PD. However, the model overestimates the downward flux of ozone at high Southern latitudes, so this estimate is likely an upper limit. In the future, the tropospheric ozone burden increases by 101% in 2100 for the A2 scenario including both emissions and climate changes. The primary reason is enhanced STE, which increases by 124% (168% in the SH extratropics, and 114% in the NH extratropics). Climate plays a minimal role in the SH increases, but contributes 38% in the NH. Chemistry and dry deposition both change so as to reduce tropospheric ozone, partially in compensation for the enhanced STE, but the increased ozone influx dominates the burden changes. The net RF due to projected ozone changes is 0.8 W/m2 for A2. The influence of climate change alone is −0.2 W/m2, making it a substantial contributor to the net RF. The tropospheric oxidation capacity increases seven percent in the full A2 simulation, and 36% due to A2 climate change alone.

Last Glacial Maximum and Holocene Climate in CCSM3

Abstract The climate sensitivity of the Community Climate System Model version 3 (CCSM3) is studied for two past climate forcings, the Last Glacial Maximum (LGM) and the mid-Holocene. The LGM, approximately 21 000 yr ago, is a glacial period with large changes in the greenhouse gases, sea level, and ice sheets. The mid-Holocene, approximately 6000 yr ago, occurred during the current interglacial with primary changes in the seasonal solar irradiance. The LGM CCSM3 simulation has a global cooling of 4.5°C compared to preindustrial (PI) conditions with amplification of this cooling at high latitudes and over the continental ice sheets present at LGM. Tropical sea surface temperature (SST) cools by 1.7°C and tropical land temperature cools by 2.6°C on average. Simulations with the CCSM3 slab ocean model suggest that about half of the global cooling is explained by the reduced LGM concentration of atmospheric CO2 (∼50% of present-day concentrations). There is an increase in the Antarctic Circumpolar Current and Antarctic Bottom Water formation, and with increased ocean stratification, somewhat weaker and much shallower North Atlantic Deep Water. The mid-Holocene CCSM3 simulation has a global, annual cooling of less than 0.1°C compared to the PI simulation. Much larger and significant changes occur regionally and seasonally, including a more intense northern African summer monsoon, reduced Arctic sea ice in all months, and weaker ENSO variability.

Direct radiative forcing of anthropogenic organic aerosol

This study simulates the direct radiative forcing of organic aerosol using the GFDL AM2 GCM. The aerosol climatology is provided by the MOZART chemical transport model (CTM). The approach to calculating aerosol optical properties explicitly considers relative humidity–dependent hygroscopic growth by employing a functional group–based thermodynamic model, and makes use of the size distribution derived from AERONET measurements. The preindustrial (PI) and present‐day (PD) global burdens of organic carbon are 0.17 and 1.36 Tg OC, respectively. The annual global mean total‐sky and clear‐sky top‐of‐the atmosphere (TOA) forcings (PI to PD) are estimated as −0.34 and −0.71 W m−2, respectively. Geographically the radiative cooling largely lies over the source regions, namely part of South America, Central Africa, Europe and South and East Asia. The annual global mean total‐sky and clear‐sky surface forcings are −0.63 and −0.98 W m−2, respectively. A series of sensitivity analyses shows that the treatments of hygroscopic growth and optical properties of organic aerosol are intertwined in the determination of the global organic aerosol forcing. For example, complete deprivation of water uptake by hydrophilic organic particles reduces the standard (total‐sky) and clear‐sky TOA forcing estimates by 18% and 20%, respectively, while the uptake by a highly soluble organic compound (malonic acid) enhances them by 18% and 32%, respectively. Treating particles as non‐absorbing enhances aerosol reflection and increases the total‐sky and clear‐sky TOA forcing by 47% and 18%, respectively, while neglecting the scattering brought about by the water associated with particles reduces them by 24% and 7%, respectively.

Tropical Rainfall Trends and the Indirect Aerosol Effect

An atmospheric global climate model coupled to a mixed layer ocean model is used to study changes in tropical rainfall due to the indirect effects of anthropogenic sulfate aerosol. The model is run to equilibrium for present-day (PD) and preindustrial (PI) sulfur emission scenarios. As in two other recent studies, the model generally gives a southward shift of tropical rainfall in the PD run relative to the PI run. This is largely due to a hemispheric asymmetry in the reduction of sea surface temperature (SST) induced by the perturbation of cloud albedo and lifetime. Observed precipitation trends over land for the period 1900‐98 show a complex pattern in the Tropics, but when zonally averaged, a southward shift similar to (but weaker than) the modeled shift is clearly evident. The zonally averaged tropical trends are significant at the 5% level in several latitude bands. The modeled presentday hemispheric contrast in cloud droplet effective radius (which affects cloud albedo) is well supported by one long-term satellite retrieval, but not by another. A third satellite retrieval, which only covers an 8-month period, does show a marked hemispheric contrast in effective radius. Both in the modeled changes and the observed trends, a prominent feature is the drying of the Sahel in North Africa. Modeled dynamical changes in this region are similar to observed changes that have been associated with Sahelian drought. Previous work has identified a near-global, quasi-hemispheric pattern of contrasting SST anomalies (cool in the Northern Hemisphere and warm in the Southern Hemisphere) associated with dry conditions in the Sahel. The present results, combined with this earlier finding, suggest that the indirect effects of anthropogenic sulfate may have contributed to the Sahelian drying trend. More generally, it is concluded that spatially varying aerosol-related forcing (both direct and indirect) can substantially alter low-latitude circulation and rainfall.