Global distribution and radiative forcing of soil dust aerosols in the Last Glacial Maximum simulated by the aerosol climate model

Global distribution and radiative forcing of soil dust aerosols in the Last Glacial Maximum simulated by the aerosol climate model T. Takemura, M. Egashira, K. Matsuzawa, H. Ichijo, R. O’ishi, and A. Abe-Ouchi Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan Interdisciplinary Graduate School of Engineering Sciences, Kyushu Univ., Fukuoka, Japan Center for Climate System Research, University of Tokyo, Chiba, Japan Received: 12 September 2008 – Accepted: 15 October 2008 – Published: 9 December 2008 Correspondence to: T. Takemura (toshi@riam.kyushu-u.ac.jp) Published by Copernicus Publications on behalf of the European Geosciences Union.


Introduction
The air temperature during the glacial and interglacial periods has been analyzed with ice cores, marine sediments, and pollens.It is assumed that the annual mean surface air temperature is about 3 and 10 K lower in the Last Glacial Maximum (LGM), which is about twenty-one thousands years ago, than in the present climate over the tropics and southern Europe, respectively, and 3 to 6 K lower on the global mean (Jouzel et al., 1993;Kucera et al., 2005;Masson-Delmotte et al., 2005;Wu et al., 2007).It is thought to be impossible to quantitatively explain the remarkable low temperature and extended the lower greenhouse gas concentrations, extended ice sheets, and feedback mechanisms within and among the atmosphere, ocean, cryosphere, and vegetation play important roles in the LGM climate.Jansen et al. (2007) reported based on the Paleoclimate Modeling Intercomparison Project 2 (PMIP-2) (Braconnot et al., 2007) that the radiative perturbation in the LGM, which do not include the effects of vegetation and aerosol changes, is estimated to be −4 to −7 W m −2 due to lower greenhouse gas concentrations, extended continental ice, and lower sea level than the present climate, and that the scientific understanding of the radiative perturbation due to atmospheric mineral dust is very low.
From analyses of ice cores and sediments, it has been found that the deposition flux of atmospheric soil dust aerosols varies between the glacial and interglacial periods.Soil dust aerosols are considered as one of the factors inducing climate change principally through two effects.One is the direct effect in which they scatter and absorb the solar and thermal radiation.The other is the indirect effect in which they alter the microphysical and optical properties of cloud droplets and ice crystals acting as cloud condensation nuclei (CCN) and ice nuclei (IN).Soil dust is one of the essential aerosols for IN although it has less effect for CCN because of hydrophobicity in the external mixing with other aerosols.The aerosol semi-direct effect is also discussed, in which aerosols that absorb the solar and/or thermal radiation, such as soil dust, warm the surrounding atmosphere, resulting in changes in the atmospheric stability and cloud production.Therefore the difference in the concentration of soil dust aerosols between the glacial and interglacial periods may contribute to large difference in the climate condition between them.Petit et al. (1999) indicated that the concentration of soil dust aerosols in the ice core from Vostok, Antarctica is order-of-magnitude larger in the glacial than in the interglacial periods, especially in the LGM.The data of marine sediments also show that the dust fluxes from Africa transported to the tropical and subtropical Atlantic in the LGM are three to five times higher than the present-day (Harrison et al., 2001).The increase of the soil dust concentration in the LGM may assign the less precipitation, expansion of land areas, and strong wind.Introduction

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Interactive Discussion To simulate and analyze the soil dust emission, concentration, and deposition in the LGM, several modeling studies have proceeded.Joussaume (1993) demonstrated the desert dust cycle in the LGM with an atmospheric general circulation model (AGCM), however only a weak increase was simulated.Andersen et al. (1998) simulated larger dust amounts almost everywhere over the globe in the LGM than in the present-day considering variation of the soil moisture, but they were still lower than values reported from ice cores.They indicated that inclusion of more realistic boundary conditions for the dust emission (e.g., vegetation) could also improve the LGM dust cycle.Mahowald et al. (1999) showed that the simulated dust deposition flux in the LGM were roughly in agreement with observed data from marine sediments and ice cores by including a difference in vegetation between the glacial and interglacial periods.They suggested that changes in source areas are required to predict any substantial increase in the dust deposition over the polar region.Werner et al. (2002) suggest from their simulation that one third of an increase in the total global dust emission flux in the LGM is related to source-region changes, while two thirds is caused by glacial wind speed changes over modern dust emission regions.Mahowald et al. (2006) indicated that the impact of CO 2 fertilization on arid vegetation and the glaciogenic dust sources are sensitive to dust sources.Mentioned above, the previous studies have improved the reproductivity of the atmospheric dust condition in the LGM by the simulations.
Soil dust aerosols affect the radiation budget through the direct and indirect effects as mentioned above, so that they may have important potential to compound the climate system in the glacial period.The PMIP-2, however, did not treat the dust effects on the climate system as well as the coupled ocean-atmosphere system.Claquin et al. (2003) simulated the direct radiative forcing of dust based on Mahowald et al. (1999).In this study, the radiative forcings both of the direct and indirect effects by soil dust aerosols in the LGM are calculated with a global aerosol climate model, Spectral Radiation-Transport Model for Aerosol Species (SPRINTARS), which is accompanied by the simulation of global distributions for main tropospheric aerosols, using the prescribed sea surface temperature (SST) and sea ice by the ocean-atmosphere coupled Introduction

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Interactive Discussion general circulation model.One of the significant features of this study is inclusion of the interaction between ice crystals and aerosol particles in order to calculate the aerosol indirect effect.The model description is given in Sect. 2. Section 3 shows the simulated emission, distribution, and deposition of soil dust aerosols as well as sea salt aerosols both in the LGM and present-day.The simulated deposition fluxes of soil dust aerosols are compared with their estimations from ice cores and marine sediments in order to understand reproductivity and problems of the present model.Section 4 presents the calculated direct and indirect radiative forcings of soil dust aerosols and discuss their effects on the climate system in the LGM.This study is concluded in Sect. 5.

Model description
In this study, global distributions and radiative forcings of aerosol particles in the LGM climate condition are simulated by SPRINTARS (Takemura et al., 2000(Takemura et al., , 2002(Takemura et al., , 2005)) (Sitch et al., 2003;Gerten et al., 2004).The potential vegetation types for dust emission are wooded c4 grassland, bare ground/shrub, tundra, and warm grassland/shrub in this study (Fig. 1).The bare ground/shrub in the LGM expands over the inland Asia, Middle and Near East, and southern Sahara in comparison with the present condition.The tundra also increases in the LGM, which is partly due to expansion of land because of sea level falling, although there are largescale ice sheets over the North America and Northern Europe.The prescribed vegetation distribution in the LGM is confirmed to be in general agreement with reconstruction data (Harrison and Prentice, 2003).Figure 2 shows the annual mean LAI distribution, and it is possible to emit soil dust aerosols if the LAI is less than about 0.575 according to Eqs. ( A3)-(A4).The other natural emissions included in this study is sea salt aerosols (see Appendix B), BC, OC, and SO 2 from biomass burning, OC from the gasto-particle conversion of terpene (Guenther et al., 1995), and dimethylsulfide (DMS) from oceanic phytoplankton and land.The global distribution of emission for biomass burning BC is based on the monthly mean data of the Global Fire Emissions Database version 2 (GFEDv2) from the year 1997 to 2006 (Randerson et al., 2005), and the natural sources are assumed to be 10% of the total GFEDv2 emission (Andreae, 1991).
The natural emissions of BC and OC mentioned above are assumed to be the same between the simulations for the LGM and a present climate condition.A scheme of the DMS emission from oceanic phytoplankton which is a function of the downward surface solar radiation is the same as Takemura et al. (2000) and that from land vegetation and soil which is function of LAI, solar zenith angle, and temperature is according to Spiro et al. (1992).The other transport processes are according to Takemura et al. (2000Takemura et al. ( , 2002Takemura et al. ( , 2005)).
The radiation scheme in the CCSR/NIES/FRCGC AGCM, which adopts the twostream discrete ordinate and adding method (Nakajima et al., 2000), includes the calculation of the aerosol direct effect.The refractive indices depending on wavelengths, size distributions, and hygroscopic growth are considered for each kind of aerosol.The Introduction

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Interactive Discussion refractive indices of dry aerosols and water are according to Deepak and Gerber (1983) and d 'Almeida et al. (1991), respectively, except the imaginary part of soil dust aerosols which is a quarter of values in Deepak and Gerber (1983) because their weaker absorption of the solar radiation has been recently reported (e.g., Kaufman et al., 2001).
The detailed description of the aerosol direct effect in SPRINTARS is in Takemura et al. (2002Takemura et al. ( , 2005)).
The aerosol indirect effect is also included both for water and ice clouds in SPRINT-ARS.The cloud droplet and ice crystal number concentrations are prognostic variables.The nucleation of the cloud droplets depends not only on the aerosol particle number concentrations but also on the size distributions, curvature effect, and solute effect of each kind of aerosol, and the updraft velocity.The nucleation of the ice crystal number concentration includes both the homogeneous and heterogeneous processes (see Appendix C).The growth and collision processes for cloud droplets and ice crystals are described also in Appendix C. Changes in the cloud droplet and ice crystal number concentrations induce changes in the cloud droplet and ice crystal effective radii, respectively, that is the first indirect effect, which result in a change in the radiation budget.The precipitation rate for warm rain is according to the Berry's parameterization depending on the cloud droplet number concentration, that is the second indirect effect.The detailed description of the aerosol indirect effect for water coluds in SPRINTARS is in Takemura et al. (2005).Note that a change in the precipitation rate for cold rain is erence simulation SPRINTARS also calculates global distributions and radiative forcings of aerosols in the pre-industrial era as a present climate condition (PRE).The HadISST (Rayner et al., 2003) averaged from the year 1870 to 1879 is used as the monthly SST and sea ice in the PRE simulation.The solar radiative flux at the top of the atmosphere is calculated with the eccentricity, obliquity of the earth's axis, and longitude of perihelion (Table 1).The atmospheric CO 2 , N 2 O, and CH 4 concentration are set to be constant globally and annually (Table 1).Each experiment is integrated for 6 years and analyzed for the last 5 years.

Aerosol emission, distribution, and deposition in the LGM
Figure 3a and b shows simulated global distributions of the annual dust emission in the LGM and PRE, respectively.In the PRE, the main emission sources are distributed between 50 • N to 40 • S; the Sahara, Middle East, Central Asia, Kalahari Desert, Patagonia, and Australia.In the LGM, they expand into south over the Sahara and into north over the Eastern Europe and Central Asia due to extended arid regions (Fig. 1).The other sources appear over the northern Siberia during the glacial period because of the sea level falling.As well as expansion of emission sources, the dust emission flux is also larger over the Sahara, Middle and Near East, Asia, and Europe in the LGM than in the PRE (Table 2).The global total flux in the LGM is calculated to be 6200 Tg/yr, about 2.4 times as large as that in the PRE.Werner et al. ( 2002) estimated 2.2-fold higher dust emission flux in the LGM, which is close to this study, though their estimation of the total flux was smaller (2383 Tg/yr).The dust emission flux under the present climate condition was estimated to be 2594 Tg/yr in this study, which is in agreement with most past studies, ranging about from 1500 to 3000 Tg/yr (e.g., Tegen and Fung, 1994;Dentener et al., 1996;Chin et al., 2002;Tanaka and Chiba, 2005), while Werner et al. ( 2002) estimated it to be 1060 Tg/yr.
Reasons for the high dust emission flux in the LGM are assumed to be mainly due to not only expansion of emission sources but also strong wind in the LGM.In order to Introduction

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Full separate the influence of extended arid regions from the other meteorological factors on the dust emission flux, an additional experiment (LGMfv) is done with the presentday vegetation distribution as shown in Fig. 1b and with the same conditions as LGM for others.Table 2 shows that the large emission flux of soil dust aerosols in the LGM is more affected by the meteorological condition than the vegetation over the Sahara and Middle and Near East, and vice versa over Asia and Europe.The simulation suggests that about 60% of the increase in the global total dust emission in the LGM relative to PRE is due to a difference in the meteorological condition, especially the strong wind, and the other is due to a difference in vegetation.These rations of contributing to an increase in the LGM dust emission are close to those in Werner et al. (2002).
Figure 3c and d shows simulated global distributions of the annual dust column loading in the LGM and PRE, respectively, and Table 3 shows zonal column loading and deposition flux of soil dust aerosols.Those emitted from the Sahara are transported to west by the trade winds and from Asia to east by the westerlies.The atmospheric dust loading is larger in the LGM than in the present all over the Northern Hemisphere, especially in the high latitudes mainly due to expansion of the emission sources.The distribution of the dust deposition flux is similar to that of the mass column loading (not shown).The zonal mean mass column loading and deposition flux in the LGM are about 7 and 20 times, respectively, as large as those in the present between 90 • N and 60 • N, and 2 to 3 times between 60 • N to 0 • .The ratio of the dry deposition to the total deposition flux is high near the source regions, and the wet deposition becomes a predominant deposition process with being away from the sources.One of the simulated characteristics in the LGM is a high ratio of the dry deposition in the high latitudes of the Northern Hemisphere.Ratio of the dry deposition is also larger in the LGM in the low-and mid-latitude of the Northern Hemisphere than in the PRE because of the drier atmospheric condition in the LGM as well as the larger mass column loading.The simulated dust deposition fluxes both in the LGM and PRE are compared with DIRTMAP (Dust Indicators and Records of Terrestrial and Marine Palaeoenvironments) database (Kohfeld and Harrison, 2001) which is estimated mainly from ice cores and Introduction

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Full Screen / Esc Printer-friendly Version Interactive Discussion marine sediments.It is best database for understand the simulated results, though there are some uncertainty factors, for example, that the fluxes from ice cores rely on estimates of ice accumulation rate in the cores and that ocean currents can advect dust substantially before it is deposited (Kohfeld and Harrison, 2001;Mahowald et al., 2006).It has been suggested from DIRTMAP that the dust deposition fluxes during the LGM are 2 to 5 times as large as those during interglacial periods in the tropical and mid-latitudes, and over 20 times in the polar regions.Figure 4 shows comparisons of the annual dust deposition fluxes between DIRTMAP and the simulation by SPRINT-ARS in the LGM and PRE.They are in general agreement in the PRE, though there is a little underestimation over the Indian Ocean.Also in the LGM, the simulation generally captures the five order of magnitude range in the deposition fluxes except the Antarctic region.The isotopic measurement has suggested that the ice-core dust in the Antarctic during glacial periods have a primary origin from Patagonia with a small amount (10 to 15%) from southern Africa and/or Australia (Basile et al., 1997).There are three possible reasons for underestimating the dust emission from Patagonia in the LGM simulated in this study as shown in Fig. 3 and Table 2.The first is inappropriate vegetation indices and underestimation of emerging land due to the sea level falling because there is a large difference in the emission flux from the South America between the LGM and LGMfv.The second is underestimation of wind speed because the emission flux in the LGMfv experiment is smaller than in the PRE, in spite of vice versa in the Northern Hemisphere.The other additional reason is underestimation of precipitation, because the deposition flux over the high latitudes of the Southern Hemisphere in the LGM is a half of that in the present though the mass column loading in the LGM is a little larger than that in the present and because the wet deposition is a primary process in the total dust deposition (Table 3).
The other principal natural aerosol is sea salt.present in the tropics and mid-latitudes, though the emission flux in the LGM is slightly larger due to the stronger wind.On the other hand, the emission is prevented by extended sea ice in the LGM in the high latitudes.This study cannot analyze differences in natural carbonaceous and sulfate aerosols between the LGM and PRE because their setup is almost same both in the simulations.

Dust radiative forcing in the LGM
The direct radiative forcing of soil dust aerosols is calculated as a difference in the radiative budget between inclusion and exclusion of soil dust aerosol in the radiative process within one simulation.Figure 6 shows the annual mean distributions of the direct radiative forcing due to soil dust aerosols under the all-sky condition.The forcing at the tropopause is basically negative over oceanic and forest areas, while it is positive over arid regions due to high surface albedo.Even over the ocean where the surface albedo is low, it is partly positive, in spite of small values, because soil dust aerosols multiply absorb scattered solar radiation enhanced by the lower cloud layer than the aerosol layer.It has been known that the aerosol direct radiative forcing at the tropopause and top of the atmosphere is much sensitive to the vertical structure of cloud layers for aerosol species absorbing the solar radiation (Haywood and Ramaswamy, 1998;Takemura et al., 2002).A significant characteristic in the LGM is the strong positive forcing over the northern Europe and Arctic Ocean due to extended and continuous ice sheets and sea ice through the year.At the surface, the direct radiative forcing is negative almost over the globe because the solar radiation is attenuated at the surface whether soil dust aerosols scatter or absorb it.The negative forcing is generally in correspondence to the column loading (Fig. 3c and d) and larger in the LGM than in the present.
Table 5 shows the annual global mean direct forcing both under the clear-sky and all-sky conditions.The negative shortwave forcing and positive longwave forcing under the all-sky condition are smaller than the clear-sky due to absorbing the multi-scattered Introduction

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Full radiation with lower cloud layer as mentioned above and due to absorption of the terrestrial radiation by clouds, respectively.A difference in the global mean direct forcing between the LGM and PRE both in the shortwave and longwave radiation is approximately proportional to a difference in the emission flux and column loading (Tables 2  and 3).In the LGM, the direct radiative forcing of soil dust aerosols is close to zero at the tropopause and −0.4 W m −2 at the surface.
The indirect radiative forcing of soil dust aerosols in the LGM is also estimated in this study.Soil dust aerosols play an important role as ice nuclei in forming ice crystals through the heterogeneous nucleation (Lohmann and Feichter, 2005), though they are not much effective in forming water cloud droplets.Appendix C shows the microphysical treatment for the relationship between ice crystals and aerosol particles in the model.The indirect radiative forcing is calculated as a difference in the cloud radiative forcing between inclusion and exclusion of the dust emission.Figure 7 shows the annual mean distribution of the indirect radiative forcing due to soil dust aerosols both in the LGM and PRE.Inclusion of soil dust aerosol as ice nuclei results in smaller ice crystals, which leads to absorption of the longwave radiation more effectively than scattering of the shortwave radiation.Therefore the indirect forcing is positive at the tropopause almost over the globe.At the surface, on the other hand, attenuation of the shortwave radiation due to the smaller ice crystals is prominent.The indirect radiative forcing is large where the ice water content is large.The obvious difference between the LGM and PRE is the large positive forcing at the tropopause and the large negative forcing at the surface in the PRE, especially over the tropics because of the reduced ice crystal concentration due to inactive convection in the LGM.
The global mean indirect forcing is larger in the PRE at the tropopause both in the negative shortwave and positive longwave radiations than in the LGM though the dust loading is larger in the LGM (Table 6).It might be because the ice and liquid water contents are generally less in the LGM due to the low temperature.The simulated ice water path on the global mean in the LGM is 11% less than that in the present.The global mean positive value of the net indirect radiative forcing due to soil dust aerosols Introduction

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Full ) is smaller than that in the PRE (+2.9 W m −2 ), therefore atmospheric dust might contribute to the cold climate during the glacial periods through the indirect effect as well as the direct effect relative to the interglacial periods.

Conclusions
Global dust distribution and radiative forcings both of the direct and indirect effect in the LGM were simulated by the global aerosol climate model, SPRINTARS, in this study.
The global total dust flux in the LGM is calculated to be about 2.4 times as large as that in the present climate mainly because of the extended arid regions and strong wind.The simulated dust deposition flux was in general agreement with estimations from ice core and marine sediment samplings.Further studies are, however, needed to solve underestimation of the simulated dust deposition over the Antarctic through, for example, detailed sensitivity experiments for analyzing what important factors are to determine the emission, atmospheric loading, and deposition of soil dust aerosols.The global mean negative value of the direct radiative forcing due to soil dust aerosols at the tropopause in the LGM was simulated to be larger than that in the present climate, and the positive value of the indirect effect in the LGM was smaller.Therefore this study suggested that they contribute to the lower temperature during the glacial periods relative to the interglacial periods.
The detailed and progressive studies with atmosphere-ocean coupled general circulation models or earth system models will be important to quantitatively analyze the effects of atmospheric dust on the climate system including feedback processes in the glacial periods.homogeneous freezing of supercooled aerosols, N f rc is the contact freezing, N f ri is the immersion/condensation freezing, N sel is the self-collection of cloud droplets, N agg is the aggregation of ice crystals, Q aut is the autoconversion of cloud droplets, Q arl and Q asl are the accretion of cloud droplets by rain and snow, respectively, and Q asi is the accretion of ice crystals by snow.The nucleation of cloud droplets N nuc is according to Eqs. ( 1)-( 4) in Takemura et al. (2005).The homogeneous freezing N f rh is based on K ärcher and Lohmann (2002).BC and soil dust aerosols act as IN for the heterogeneous freezing including the contact and immersion/condensation processes.Ratios of activated IN to the total number concentration of BC and dust for the contact freezing f f rc and the immersion/condensation freezing f f ri are based on Fig. 1 in Lohmann and Diehl (2006): where T is the temperature in K, a c , b c , T i 0 are constants depending on aerosol species (Table 1), and T i = 3 K.The terms of N f rc and N f ri are according to Lohmann and Diehl (2006) and Diehl et al. (2006).The deposition freezing is neglected in this study because it generally takes place at lower temperatures and higher supersaturation than the other heterogeneous freezing processes (Lohmann and Diehl, 2006).The selfcollection of cloud droplets N sel and the aggregation of ice crystals N agg are following Lohmann et al. (1999) and Levkov et al. (1992), respectively.The Berry's parameterization (Berry, 1967) is adopted as the autoconversion of cloud droplets Q aut (Eq.( 6) in Takemura et al., 2005).The accretion of cloud droplets by rain and snow is calculated as: where F p is the in-cloud flux of rain or snow and E l is a constant, 1.0 for rain and 0.5 by snow.The accretion of ice crystals by snow is also calculated as: where E i is 0.05.
Wu, H., Guiot, J., Brewer, S., and Guo, Z.: Climatic changes in Eurasia and Africa at the last glacial maximum and mid-Holocene: reconstruction from pollen data using inverse vegetation modelling, Clim. Dynam., 29, 211-229, 2007 ice sheets in the LGM only with the change in the insolation.It is a general opinion that not included in the present version of SPRINTARS.The initial condition and the climatological monthly mean data of SST and sea ice in the LGM are prescribed with the simulated result by the CCSR/NIES/FRCGC atmosphere-ocean coupled general circulation model (AOGCM), MIROC (Model for Interdisciplinary Research on Climate) which is based on PMIP-2 (Yanase and Abe-Ouchi, 2007).The model result shows about 2.5 to 3 K SST change in the low latitude and about 5 K global change, which is in the range of uncertainty of reconstructed data of different proxies and also comparable performance to other coupled AOGCMs in PMIP2 experiments (Masson-Delmotteet al., 2005;Kageyama et al., 2006).As a ref- Figure 5 shows simulated distributions of the annual emission flux and annual mean mass column loading of sea salt aerosols, and Table 4 presents their zonal and global total emission fluxes in the LGM and PRE.Both the emission flux and atmospheric loading in the LGM is similar to those in the To understand the climate in the glacial period when it is extremely different in the present are closely related to analyze the essential climate formation and the modern climate change.

Fig. 4 .
Fig. 4. Comparisons of annual total deposition flux of soil dust aerosols between the DIRTMAP database and SPRINTARS simulation in the (a) LGM and (b) PRE.Dots and crosses show comparisons with DIRTMAP from ice cores and marine sediments, respectively.

Table 1 .
. 20464 Yanase, W. and Abe-Ouchi, A.: The LGM surface climate and atmospheric circulation over East Asia and the North Pacic in the PMIP2 coupled model simulations, Clim.Past, 3, 439-451, Introduction Earth's planetary constants and main greenhouse gas concentrations in the LGM and PRE.

Table 2 .
Regional and global total annual emission fluxes of soil dust aerosols in the LGM, PRE, and LGMfv and their ratio of LGM to PRE.

Table 3 .
Zonal and global total mass column loading and deposition fluxes of soil dust aerosols in the LGM and PRE and their ratio of LGM to PRE.Numbers in parentheses in the deposition fluxes are percentages of the wet deposition fluxes to the total ones.

Table 4 .
Zonal and global total annual emission fluxes of sea salt aerosols in the LGM and PRE, and their ratio of LGM to PRE.

Table 5 .
Global and annual mean radiative forcing for the direct effect of soil dust aerosols at the tropopause and surface under clear-sky and all-sky conditions in the LGM and PRE.SW, LW, and SW+LW are shortwave, longwave, and shortwave plus longwave radiations, respectively.