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GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L20705, doi:10.1029/2006GL026126, 2006
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Climate response and radiative forcing from mineral aerosols during the last glacial maximum, pre-industrial, current and doubled-carbon dioxide climates Natalie M. Mahowald,1,2 Masaru Yoshioka,1,2 William D. Collins,1 Andrew J. Conley,1 David W. Fillmore,1 and Danielle B. Coleman1 Received 6 March 2006; revised 10 July 2006; accepted 9 August 2006; published 27 October 2006.
[1] Mineral aerosol impacts on climate through radiative forcing by natural dust sources are examined in the current, last glacial maximum, pre-industrial and doubled-carbon dioxide climate. Modeled globally averaged dust loadings change by +88%, +31% and 60% in the last glacial maximum, pre-industrial and future climates, respectively, relative to the current climate. Model results show globally averaged dust radiative forcing at the top of atmosphere is 1.0, 0.4 and +0.14 W/m2 for the last glacial maximum, pre-industrial and doubled-carbon dioxide climates, respectively, relative to the current climate. Globally averaged surface temperature changed by 0.85, 0.22, and +0.06 °C relative to the current climate in the last glacial maximum, pre-industrial and doubled carbon dioxide climates, respectively, due solely to the dust radiative forcing changes simulated here. These simulations only include natural dust source response to climate change, and neglect possible impacts by human land and water use. Citation: Mahowald, N. M., M. Yoshioka, W. D. Collins, A. J. Conley, D. W. Fillmore, and D. B. Coleman (2006), Climate response and radiative forcing from mineral aerosols during the last glacial maximum, pre-industrial, current and doubled-carbon dioxide climates, Geophys. Res. Lett., 33, L20705, doi:10.1029/2006GL026126.
1. Introduction [2] Mineral aerosols both absorb and scatter shortwave and long wave radiation, and thus are thought to modify climate [e.g., Miller and Tegen, 1998]. Additionally, mineral aerosol deposition to high latitude ice cores are 2 –20 times higher during cold periods, such as the last glacial maximum, than warm periods like today [e.g., Petit et al., 1999]; and marine sediment records suggest that globally dust deposition is 3 – 4 times higher during cold periods than warm periods [Rea, 1994]. Thus, the large expected changes in dust deposition in response to climate can play an important role in modulating climate. [3] Although climate models can include the impact of mineral aerosols on climate [e.g., Miller and Tegen, 1998], there have not been published simulations of the climate response of mineral aerosols under different climate regimes using realistic dust distributions. Additionally, there have 1
National Center for Atmospheric Research, Boulder, Colorado, USA. Also at Institute of Computational Earth System Science, University of California, Santa Barbara, Santa Barbara, California, USA. 2
Copyright 2006 by the American Geophysical Union. 0094-8276/06/2006GL026126$05.00
been limited estimates of radiative forcing under different climates [Claquin et al., 2002; Woodward et al., 2005]. Here, we include both estimates of radiative forcing for four different time periods, as well as the climate response to those dust radiative forcings in the National Center for Atmospheric Research’s (NCAR) Community Climate System Model (CCSM3) [Collins et al., 2006a]. We include only the climate response to ‘natural’ sources of desert dust, because current estimates of the land use or water use impacts are not well constrained (see Mahowald et al. [2005] for review).
2. Modeling Methodology [4] Climate models simulations are conducted in the Community Atmospheric Model (CAM3) of the CCSM [Collins et al., 2006b]. The dust source, transport, and deposition parameterizations used for these simulations are described briefly in Text S1 in the auxiliary material and in detail by Mahowald et al. [2006] and are based on the Dust Entrainment and Deposition module [Zender et al., 2003].1 Radiative forcings and climate response are calculated as described by Yoshioka et al. [2006]; a brief description is given Text S1. We use the slab ocean model version of the model to allow dust feedbacks onto the sea surface temperature, but this does not allow for changes in the ocean circulation from the dust. The simulations in this paper have exactly the same boundary conditions and radiative forcing (except for dust) as the slab ocean model simulations they are an extension of, which are described in more detail by Kiehl et al. [2006] and Otto-Bliesner et al. [2006]. Please note that for the last glacial maximum, the slab ocean model heat fluxes are based on a fully coupled simulation of these climates [Otto-Bliesner et al., 2006]. Simulations for the current and doubled-carbon dioxide climate are based on the ocean heat fluxes of the fully coupled system in the current climate [Kiehl et al., 2006]. [5] The radiative forcing of dust is calculated online using just one year of simulations (interannual variability in loading is less than 10% in this simulation), as described in more detail by Yoshioka et al. [2006]. The default version of the CAM3 includes a prescribed dust climatology for shortwave radiative forcing [Collins et al., 2006b]. For this paper, climate response is calculated relative to the case where this forcing is turned off, described by Yoshioka et al. [2006]. Longwave and shortwave forcings from dust are included in these results. The radiative forcings reported in 1 Auxiliary material data sets are available at ftp://ftp.agu.org/apend/gl/ 2006gl026126. Other auxiliary material files are in the HTML.
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Table 1. Dust Impacts on Climate SOM
SOMB
SOMLGMC
SOMLGMT
10880 0.096 0.96 1.28
14020 0.099 1.47 1.59
8040 0.069 0.87 1.03
1930 0.021 0.29 0.36
1.04 0.091
0.41 0.049
0.13 0.017
0.43 0.47 0.22
0.14 0.20 0.06
Dust source (Tg/year) Dust AOD (500 nm) TOA RF (W/m2) Sfc RF (W/m2)
4650 0.039 0.46 0.60
5320 0.053 0.43 0.56
Sfc. T (°K) Precip (mm/day)
0.20 0.035
Change From Dust Radiative Feedbacks 0.19 1.00 0.033 0.095
TOA RF (W/m2) Sfc RF (W/m2) Sfc. T (°K)
Change Relative to Current From Dust Radiative Feedbacks 0.53 1.04 0.72 1.02 0.81 0.85
this paper are estimated as instantaneous net forcings from both long wave and short wave without stratospheric adjustment. [6] The sources of dust are allowed to change with climate based on changes in vegetation calculated in the BIOME3 equilibrium vegetation model [Haxeltine and Prentice, 1996], following the methodology of Mahowald et al. [1999]. We include the impact of carbon dioxide on vegetation in modifying the sources for the simulations included here (see Mahowald et al. [2006] for more cases). These cases are called SOMB, SOMBLGMC, SOMBPIC and SOMBDC for the BIOME3 vegetation based simulations for the current climate (SOMB, B meaning using the BIOME3 output), last glacial maximum (LGM), preindustrial (PI) and doubled-carbon dioxide climates (D), respectively (the C at the end of the acronym means that the carbon dioxide fertilization effect is included in the vegetation change). We also include a case where the radiative forcing was examined in more detail, compared to other models and observations [Yoshioka et al., 2006], using the default vegetation in the model (SOM). For the last glacial maximum, we include a case where glaciogenic sources are included and tuned to match observations (SOMBLGMT). We deduced the size of these sources using an optimization algorithm and available observations (terrestrial sediment, ice and marine sediment cores), and use those source areas for this study. These simulations, including comparisons to observations (concentration, deposition and aerosol optical depth) are described in more detail by Mahowald et al. [2006].
3. Results [7] A comparison of the results of this study to available observations is given by Mahowald et al. [2006]. The climate response of dust to these different climates includes the impact of changes in vegetation (from carbon dioxide fertilization, precipitation, temperature and insolation changes), surface winds, precipitation and soil moisture. The dust response to source area is much stronger than to surface winds or precipitation in this model, as described by Mahowald et al. [2006]. The globally averaged dust source changes by +105%, +164%, +51% and 64% relative to the current climate (SOMB) for the SOMBLGMC, SOMBLGMT, SOMBPIC, and SOMBDC cases, respectively, when dust direct radiative feedback is included in the models (Table 1). The latitudinal distribution of the sources
SOMBPIC
SOMBDC
expands to northern high latitudes under last glacial maximum conditions, similar to previous studies [e.g., Mahowald et al., 1999]. Notice that adding the impact of the glaciogenic sources on dust (and tuning the model to best match the observations) increases our dust source and loading in the extra-tropics in the last glacial maximum in our simulations (SOMBLGMT vs. SOMBLGMC). Atmospheric desert dust aerosol optical depth (500 nm) changes dramatically during the different climates due to the changes in source strength (Figure 1a). Globally averaged dust aerosol optical depth (AOD) change by +81%, +88%, +31% and 60% in the SOMBLGMC, SOMBLGMT, SOMBPIC and SOMBDC cases relative to the SOMB case, respectively (Table 1). The response of dust sources and deposition to climate change is discussed in more detail and compared to available observations from Mahowald et al. [2006]. Note that the dust source strength and loadings in this study will differ slightly from that by Mahowald et al. [2006] because we are reporting the results from simulations including the radiative feedbacks of prognostic dust, while those results were without radiative feedback of predicted dust. [8] Dust radiative forcing changes with climate and the amount of dust, and the radiative forcings are highest at the latitudes with the most dust, although the relationship is not strictly linear (Figures 1a, 1b, and 1c). The globally averaged largest radiative forcings are seen in climates with the largest change in aerosol optical depth (Table 1). Our top of atmosphere radiative forcing tend to have larger longwave and smaller short wave components than previous modeling studies, although these results are consistent with available observations, as discussed in detail by Yoshioka et al. [2006], and briefly in Text S1. The case where we consider the impact of glaciogenic sources in the LGM has higher dust loading than without these sources (case SOMBLGMT vs. SOMBLGMC in Figure 1a) and a larger magnitude of radiative forcing (Figures 1c and 1d). If we look in more detail at the radiative forcings, we see that this increase is a balance between large negative radiative forcings over ocean, small negative forcings over land and positive radiative forcings over the ice sheets (see Figures S1– S4). Even though adding glaciogenic sources increases our dust over ice sheets, it increases the dust loading more over oceans, and thus is more negative (SOMLGMT vs. SOMLGMC—see Figures S1– S4). More discussion on the comparison of our radiative forcing results to published results is in Text S1.
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Figure 1. Zonally averaged values for the SOM, SOMB, SOMBLGMC, SOMBLGMT, SOMBPIC and SOMBDC cases for (a) dust optical depth (500 nm), (b) dust net radiative forcing at the top of atmosphere (TOA), (c) dust net radiative forcing at the surface (SFC), (d) surface temperature (radiative) response (only values significant at 95% are shown) and (e) precipitation response (only values significant at 95% are shown). [9] The climate response to dust can occur at different latitudes than the radiative forcing. Here we show the difference in surface temperature (radiative) between the case where dust radiative forcing is included in the climate simulation minus the case where there is no radiative forcing, for each climate. The response of surface temperature to dust radiative forcing tends to be between 0.1 and 2.1 °C cooling, and these values are statistically significant across a broad range of latitudes (Figure 1d). The strongest cooling occurs in northern midlatitudes (Figure 1d), even when the dust optical depth and radiative forcing has a larger magnitude in the tropics (Figures 1a, 1b, and 1c). The strongest response in temperature occurs at 40– 90 °N in the last glacial maximum cases, and is approximately 2 °C. It is unclear why the response should be largest at northern midlatitudes, but it could be due to the lower heat capacity of land versus ocean, and the large portion of land at this latitude. [10] The climate response of precipitation to the dust radiative forcing tends to be a shift in the precipitation to the hemisphere with less dust (Figure 1e) and a decrease in the globally averaged precipitation (Table 1). Our results suggest that further study of the importance of climate feedbacks of desert dust aerosols is warranted, and suggests some patterns that are robust across different climates in our simulations.
4. Discussion [11] Radiative forcings and climate feedbacks from atmospheric desert dust for last glacial maximum, preindustrial, current and doubled-carbon dioxide climates are calculated for the first time in one single modeling framework. Included in these dust scenarios are changes in dust sources due to changes in vegetation using the BIOME3 equilibrium vegetation model [Haxeltine and Prentice, 1996] and glaciogenic sources in the last glacial maximum. We include in the model only the response due to vegetation changes, soil moisture and surface winds in this simulation,
and ignore possible changes due to human land or water use [e.g., Mahowald and Luo, 2003; Mahowald et al., 2005]; estimates of anthropogenic radiative forcing and climate feedbacks including land use are given in Text S1, based on previous studies. Comparisons of the dust distribution in this study to available observations is conducted in a separate paper [Mahowald et al., 2006]; and shows a good performance of the model. The net instantaneous topof-atmosphere radiative forcing differences due to dust between the last glacial maximum, pre-industrial and doubledcarbon dioxide climates and the current climate are 0.53, 0.43, and +0.14 W/m2, respectively. If we include the impact of glaciogenic sources, the net top-of-atmosphere radiative forcing difference between the last glacial maximum and the current climate increases in magnitude to 1.04 W/m2. In the future we simulate a 0.14 W/m2 increase in radiative forcing because of a reduction in dust from carbon dioxide fertilization of the vegetation. It is uncertain that the carbon dioxide fertilization effect will continue in the future, when other nutrients may become limiting [Smith et al., 2000; Schlesinger and Lichter, 2001]. Indeed other studies include the impact of climate induced desertification and obtain a significant increase in desert dust in the future [Woodward et al., 2005] while others simulate little change [Tegen et al., 2004], suggesting that more studies of vegetation and climate interactions are vital to predicting future dust. [12] There is roughly a linear relationship between annually averaged aerosol optical depth and radiative forcing under different climates in the model simulations (see Table 1 and Figures S1– S4); the correlation coefficients are 0.96 and 0.92 and the slopes are 14.7 and 13.0 W/m2/AOD, for top-of-atmosphere and surface radiative forcing, respectively. Globally averaged surface temperature changed by 0.85, 0.22, and +0.06 °C relative to the current climate in the last glacial maximum, pre-industrial and doubled carbon dioxide climates, respectively, due solely to the dust radia-
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tive forcing changes simulated here. The response in surface temperature or precipitation to dust forcing is approximately linear in this model in these different climates, with correlation coefficients of 0.94 and 0.98, respectively (see Table 1 and Figures S1 – S4). The slope in the global response is 12.7 °C/AOD and 1.0 mm/day/AOD for surface temperature and precipitation, respectively. These values should be considered tentative, due to both the shortness of these simulations, and the dependence of these values on specific model characteristics. However, that the response remains linear while the surface albedo changes so strongly, suggests a robust response of surface temperature and precipitation to aerosol optical depth with this set of optical properties. More analysis of the differences between dust impacts on climate in this model and other models are discussed by Yoshioka et al. [2006]. The sensitivity of dust feedback results to optical properties is explored by Miller et al. [2004]. These results will ignore the impact of dust radiation on changes in ocean circulation, since we use a slab ocean model. We explore the efficacy of dust forcing within our model in Text S1. [13] In the last glacial maximum, our results suggest a radiative forcing relative to the current climate of 1.04 W/m2 at the top of atmosphere, for the case including glaciogenic sources and best matches available deposition observations. Compared to the forcings from carbon dioxide ( 1.7 W/m2) and insolation and albedo ( 5.2 W/m2) [e.g., Hewitt and Mitchell, 1997], the dust forcings are about 15% of the total of other forcings. The surface temperature response from dust is 0.85 °C relative to the current climate, while the changes in carbon dioxide, insolation and albedo in the same simulations cause a change in surface temperature of 5.6 °C. [14] Despite the possible sensitivity of the results to our model specifications, our results suggest some interesting relationships across the different climate studies. Radiative forcing at the top-of-atmosphere and surface is linear with aerosol optical depth, even in different climates. Climate response in surface temperature and precipitation are roughly linear with aerosol optical depth in our model, with a decrease in both surface temperature and precipitation associated with increasing optical depth. Finally, our model predicts statistically significant decreases in temperature at many latitudes (not just close to the dust sources) when dust is added in the different climates, and a shift in precipitation from the northern part of the ITCZ to the southern part of the ITCZ. [15] Acknowledgments. This work was supported by NASA-IDS (NAG5-9671), NSF-Biocomplexity (OCE-9981398) and DOE (DE-FG0202ER-63387). Simulations were conducted at the National Center for Atmospheric Research, which is funded by the National Science Foundation. This manuscript was improved by comments from the anonymous reviewers and Ron Miller.
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Collins, W. D., et al. (2006a), The Community Climate System Model: CCSM3, J. Clim., 19(11), 2122 – 2143. Collins, W. D., et al. (2006b), The formulation and atmospheric simulation of the Community Atmosphere Model, CAM3, J. Clim., 19(11), 2144 – 2161. Haxeltine, A., and I. C. Prentice (1996), BIOME3: An equilibrium terrestrial biosphere model based on ecophysiological constraints, resource availability, and competition among plant functional types, Global Biogeochem. Cycles, 10, 693 – 709. Hewitt, C., and J. Mitchell (1997), Radiative forcing and response of a GCM to ice age boundary conditions: Cloud feedback and climate sensitivity, Clim. Dyn., 13, 821 – 834. Kiehl, J., C. Shields, J. Hack, and W. Collins (2006), The climate sensitivity of the Community Climate System Model: CCSM3, J. Clim., 19(11), 2584 – 2596. Mahowald, N. M., and C. Luo (2003), A less dusty future?, Geophys. Res. Lett., 30(17), 1903, doi:10.1029/2003GL017880. Mahowald, N., K. Kohfeld, M. Hansson, Y. Balkanski, S. P. Harrison, I. C. Prentice, M. Schulz, and H. Rodhe (1999), Dust sources and deposition during the last glacial maximum and current climate: A comparison of model results with paleodata from ice cores and marine sediments, J. Geophys. Res., 104, 15,895 – 15,916. Mahowald, N., A. Baker, G. Bergametti, N. Brooks, R. Duce, T. Jickells, N. Kubilay, J. Prospero, and I. Tegen (2005), The atmospheric global dust cycle and iron inputs to the ocean, Global Biogeochem. Cycles, 19, GB4025, doi:10.1029/2004GB002402. Mahowald, N., D. Muhs, S. Levis, P. Rasch, M. Yoshioka, and C. Zender (2006), Change in atmospheric mineral aerosols in response to climate: Last glacial period, pre-industrial, modern and doubled-carbon dioxide climates, J. Geophys. Res., 111, D10202, doi:10.1029/2005JD006653. Miller, R., and I. Tegen (1998), Climate response to soil dust aerosols, J. Clim., 11, 3247 – 3267. Miller, R., I. Tegen, and J. Perlwitz (2004), Surface radiative forcing by soil aerosols and the hydrological cycle, J. Geophys. Res., 109, D04203, doi:10.1029/2003JD004085. Otto-Bliesner, B., E. Brady, G. Clauzet, R. Tomas, S. Levis, and Z. Kothavala (2006), Last glacial maximum and holocene climate in CCSM3, J. Clim., 19(11), 2526 – 2544. Petit, J. R., et al. (1999), Climate and atmospheric history of the past 420,000 years from the Vostok Ice core, Antarctica, Nature, 399, 429 – 436. Rea, D. K. (1994), The paleoclimatic record provided by eolian deposition in the deep sea: The geologic history of wind, Rev. Geophys., 32, 159 – 195. Schlesinger, W. H., and J. Lichter (2001), Limited carbon storage in soil and litter of experimental forest plots under increased atmospheric CO2, Nature, 411, 466 – 469. Smith, S., T. Huxman, S. Zitzer, T. Charlet, D. Housman, J. Coleman, L. Fenstermaker, J. Seemann, and R. Nowak (2000), Elevated CO2 increases productivity and invasive species success in an arid ecosystem, Nature, 408, 79 – 82. Tegen, I., M. Werner, S. P. Harrison, and K. E. Kohfeld (2004), Relative importance of climate and land use in determining present and future global soil dust emission, Geophys. Res. Lett., 31, L05105, doi:10.1029/2003GL019216. Woodward, S., D. Roberts, and R. Betts (2005), A simulation of the effect of climate changed-induced desertification on mineral dust aerosol, Geophys. Res. Lett., 32, L18810, doi:10.1029/2005GL023482. Yoshioka, M., et al. (2006), Impact of dust radiative forcing on Sahel precipitation, J. Clim., in press. (available at www.cgd.ucar.edu/tss/ staff/mahowald) Zender, C., H. Bian, and D. Newman (2003), Mineral Dust Entrainment and Deposition (DEAD) model: Description and 1990s dust climatology, J. Geophys. Res., 108(D14), 4416, doi:10.1029/2002JD002775.
D. B. Coleman, W. D. Collins, A. J. Conley, D. W. Fillmore, N. M. Mahowald, and M. Yoshioka, National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder, CO 80307, USA. (
[email protected])
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