Mineral aerosol or desert dust is thought to impact climate and biogeochemistry through several different mechanisms: direct radiative forcing, indirect radiative forcing, ocean biogeochemistry, terrestrial biogeochemistry and atmospheric chemistry. Mahowald's group is trying to understand both the causes of variability in mineral aerosols on short and long time scales, as well as the impacts of this variability on climate and biogeochemistry.
Future Dust?
We know that mineral aerosols are highly sensitive to climate and can vary globally about a factor of 3-4 between glacial and interglacial climates (e.g. Mahowald et al., 1999), and regionally by a factor of 4 (e.g. Mahowald et al., 2002). What will happen in the future? That depends if land use is important (e.g. Mahowald et al., 2002; Luo et al., 2003) and if carbon dioxide fertilization is important (e.g. Mahowald et al., 1999). Unfortunately, looking at the current climate has not allowed us to eliminate land use as a source of dust (e.g. Mahowald et al., 2002; Luo et al., 2003).
Results using the NCAR Climate System Model and MATCH/DEAD suggest that the future holds less dust than the current climate. However, this is very sensitive to how important land use, climate change and carbon dioxide fertilization is to modulating the global dust cycle. Results from our study are shown here:
For future simulations, our modeling study predicts 20-60% lower dust loading in 2090-2100. Relative to the pre-industrial climate, humans have contributed up to 60% of the dust loading in the current climate, or have reduced dust by 20%. Thus our predictions of the human contribution to dust are somewhat sensitive to climate changes predicted by the model (~20%), but are very sensitive to how important we think carbon dioxide fertilization of arid species and land use are. For more details, see Mahowald and Luo, 2003 (download)
Anthropogenic Dust
The contribution of humans to anthropogenic dust is very uncertain. There are several processes by which humans could impact dust:
Human land use Small scale studies (e.g. Gillette and Passi, 1988 or Neff et al., 2005) suggest that cultivation or pasture use makes soils more easily erodible. Large scale studies have suggested these sources aren’t important (e.g. Prospero et al., 2002 or Goudie and Middleton, 2001), but they are based on TOMS AAI, which is biased to not see sources at the edges of deserts (Mahowald and Dufresne, 2003), so so far, no best numbers are known. Every attempt has been inhibited by the inate uncertainty in dust sources and meteorology (e.g. Mahowald et al., 2002; Luo et al., 2003; Tegen et al., 2004; Mahowald et al., 2004). In order to better constrain globally the human land use impact on dust, we will need regional studies (Yoshioka et al., 2005 or Xuan and Sokolik, 2003) or better yet field studies.
Human water use Again, small scale studies suggest that drying up lake beds should make regions better sources, but there no good estimates of the global impact.
Climate change We know that dust is very sensitive to climate, and estimates with our model suggest that changes in climate from preindustrial to present could be important in changing dust (see figure above)
Carbon dioxide fertilization Plants may become more able to deal with water stress under higher carbon dioxide levels, thus deserts may get smaller as carbon dioxide levels increase. This impact is thought to be quite important.
Only one study has looked at the combined impact of human land use, climate change and carbon dioxide fertilization to look at human impacts on dust (Mahowald and Luo, 2003), the results of which are shown above. Based on this study, humans could have increased dust by 60% or decreased it about 20% since preindustrial times. We cannot use ice core or other data to constrain this, unfortunately, at this point (Mahowald and Luo, 2003)
Climate change impacts on dust
CCSM3 simulations (with dust online in the model, but vegetation changes done asynchronously), suggest large changes in dust due to climate change, especially source changes (see Mahowald et al., 2006).
Abstract:
Desert dust simulations generated by the
This paper includes new estimates of terrestrial sediment records, which we use to constrain the amount of glaciogenic dust.
Here’s comparisons using a vegetation only source, and a source which is based on geological data about where glaciers might be producing large amounts of sediment that is easily erodible:
Impact of dust onto climate
Simulations in CCSM3 suggest that dust could be playing a role in the
extended Sahel drought (Yoshioka et al., submitted):
Simulation
with dust direct radiative forcing-without dust shows a shift of precip to
farther south.
Consistent with
other studies, within the CCSM3, sea surface temperatures are the most
important forcing for Sahel precip. But dust forcing could be responsible
for up to 30% of the observed trend in Sahel
precip, and is more important in our simulations than changes in vegetation.
Climate response to dust under different climates (Mahowald et al submitted).
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.
Collaborations
Datasets available (please email Mahowald@ucar.edu, web pages not yet
ready).
Dust deposition
Current climate:
· Dust deposition datasets for current climate (1979-2004 daily averaged, monthly averaged or climatology available from Luo et al., 2003; Mahowald et al., 2003).
· Composite of Ginoux et al., 2004, Tegen et al., 2004 and Luo et al., 2003 as a monthly average available (described in Mahowald et al., in press, atmospheric iron review paper).
Different climate regimes
· LGM vs. Current (Mahowald et al., 1999)
· Current, pre-industrial and 2100, using 6 different scenarios (Mahowald and Luo, 2003)
· LGM, preindustrial, current and doubled CO2 climates (Mahowald et al., submitted)
We have conducted studies of 1979-2002, and 1980s vs 1960s dust distributions using NCEP-reanalysis, MATCH and Dust Entrainment and Deposition module , as well as the older glacial/interglacial study. In addition, we have available the pre-industrial, current climate and future studies discussed above. The results from these analysis are available for other scientists use (please contact mahowald@ucar.edu for information).
Our research on mineral aerosols is funded through NASA-IDS, NASA-NIP and NSF-Biocomplexity.
Review paper showing state of science
and new composite dust deposition
Mahowald, N.; Baker, A.;
Bergametti, G.; Brooks, N.; Duce, R.; Jickells, T.; Kubilay, N.; Prospero, J.;
Tegen, I.Atmospheric global dust cycle
and iron inputs to the ocean Global Biogeochem. Cycles, Vol. 19, No. 4, GB4025,10.1029/2004GB002402,
2005. text
and color figures, online
supplement. AGU copyright version.
Papers on dust variability
Mahowald, N. M., D.
R. Muhs, S. Levis, P. J Rasch, M. Yoshioka, C. S. Zender, and C. Luo (2006),
Change in atmospheric mineral aerosols in response to climate: Last glacial
period, preindustrial, modern, and doubled carbon dioxide climates, J. Geophys.
Res., 111, D10202, doi:10.1029/2005JD006653. AGU copyright.
Text + figures, online supplement
C. Luo, N. Mahowald, N. Meskhidze, Y. Chen, R.
Siefert, A. Baker, A. Johansen, Estimation
of iron solubility from observations and a global aerosol model, J. Geophys.
Res., 110, D23307, doi:10.1029/2005JD006059 ,
text and figures (tar file)
M. Yoshioka, N. Mahowald, J.-L. Dufresne, and C.
Luo, Simulation of absorbing aerosol
indices for African dust, JGR-atmospheres, 110, D18S17,
doi:10.1029/2004JD005276, 2005 Text
and b/w and color figures , copyright AGU.
T.D. Jickells, An Z. S. , K. K. Andersen , A.R. Baker, G. Bergametti , N. Brooks , Cao J.J. , P. W. Boyd , R.A. Duce , K.A. Hunter , H. Kawahata , N. Kubilay , J. laRoche, P.S. Liss , N. Mahowald , J. M. Prospero , A.J. Ridgwell , I. Tegen , R. Torres, Global Iron Connections Between Desert Dust, Ocean Biogeochemistry and Climate, Science, 308, p67-71, 2005.
E. Boyle, B. Bergquist, R. Kayser, N. Mahowald, Iron, Manganese and Lead at Hawaii Ocean Time-Series Station ALOHA: Temporal Variability and an Intermediate Water Hydrothermal Plume, Geochimica Cosmochimica Acta. 69 (4), Doi: 10.1016/j.gca.2004.07.034, 933-952,2005.
Mahowald,
N., G. Rivera, C. Luo, Comment on “Relative importance of climate and
land use in determining present and future global soil dust emission” by
I. Tegen et al. Geophys. Res. Lett., Vol. 31, No. 24, L24105,
10.1029/2004GL021272, 30 December 2004, Text copyright AGU, 2004.
Simulation of absorbing aerosol indices for African dust, M. Yoshioka, N. Mahowald, J.-L. Dufresne, and C. Luo, submitted to JGR-Atmospheres, Text and b/w and color figures
C. Luo, N. Mahowald and C. Jones,Temporal variability of dust mobilization and concentration in source regions , in press JGR-Atmospheres, Text and b/w and color figures
N. Mahowald, C. Luo, A less dusty future?, Geophysical Research Letters, vol 30, no 17, 1903 doi:10.1029/2003GL017880,2003. download
N. Mahowald, J.-L. Dufresne, Sensitivity of TOMS AI to PBLH: Implications for detection of mineral aerosol sources, GRL vol31,no.3., 10.1029/2003GL018865 Text and b/w and color figures copy right AGU 2004
C. Luo, N. Mahowald, J. del Corral, Sensitivity study of meteorological parameters on mineral aerosol mobilization, transport and distribution, J. Geophys. Res., 108, D15, 4447, 10.1029/2003JD0003483, August 2, 2003.
C. Jones, N. Mahowald, C. Luo, The role of easterly waves in African desert dust transport, J. Climate. vol. 16, p. 3617-3628, 2003. Text and b/w figures
N. Mahowald, C. Luo, J. del Corral, C. Zender, Interannual variability in atmospheric mineral aerosols from a 22-year model simulation and observational data, JGR, 108 (D12), 10.1029/2002JD002821, 2003.
N. Mahowald, R. Bryant, J. del Corral, L. Steinberger, Ephemeral lakes and desert dust sources, GRL, vol 30, no 2,10.1029/2002GL016041,January, 2003.
N. Mahowald, C. Zender, C. Luo, D. Savoie, O. Torres, J. del Corral, Understanding the 30-year Barbados desert dust record, JGR, 10.1029/2002JD002097, 2002, D21, CN:4561.
N. Mahowald, K. Kohfeld, M. Hansson, Y. Balkanski, S. Harrison, C. Prentice, M. Schulz, H. Rodhe, Dust sources and deposition during the last g lacial maximum and current climate: A comparison of model results with paleodata from ice cores and marine sediments. Journal of Geophysical Research, 104, 1589 5-15916, 1999.
C. Luo, N. Mahowald, C. Zender, J. del Corral, Mineral Aerosol Climatology from a 22-year Simulation, unpublished manuscript Text and Figures (b/w) and Color Plates.
Papers on dust impacts
C. Luo, N. Mahowald, N. Meskhidze, Y. Chen, R.
Siefert, A. Baker, A. Johansen, Estimation
of iron solubility from observations and a global aerosol model, J. Geophys.
Res., 110, D23307, doi:10.1029/2005JD006059 ,
text and figures (tar file)
M. Yoshioka, N. Mahowald, J.-L. Dufresne, and C.
Luo, Simulation of absorbing aerosol
indices for African dust, JGR-atmospheres, 110, D18S17,
doi:10.1029/2004JD005276, 2005 Text
and b/w and color figures , copyright AGU.
T.D. Jickells, An Z. S. , K. K. Andersen , A.R. Baker, G. Bergametti , N. Brooks , Cao J.J. , P. W. Boyd , R.A. Duce , K.A. Hunter , H. Kawahata , N. Kubilay , J. laRoche, P.S. Liss , N. Mahowald , J. M. Prospero , A.J. Ridgwell , I. Tegen , R. Torres, Global Iron Connections Between Desert Dust, Ocean Biogeochemistry and Climate, Science, 308, p67-71, 2005.
E. Boyle, B. Bergquist, R. Kayser, N. Mahowald, Iron, Manganese and Lead at Hawaii Ocean Time-Series Station ALOHA: Temporal Variability and an Intermediate Water Hydrothermal Plume, Geochimica Cosmochimica Acta. 69 (4), Doi: 10.1016/j.gca.2004.07.034, 933-952,2005.
J.L. Hand, N. Mahowald, Y. Chen, R. Siefert, C. Luo, A.
Subramaniam, I. Fung, Estimates of soluble iron from observations and a global mineral
aerosol model: Biogeochemical implications., JGR-Atmospheres,
109, No. D17, D17205, 10.1029/2004JD004574,
Text and b/w and color figures copyright AGU 2004.
C. Jones, N. Mahowald, C. Luo, Observational evidence of African desert dust intensification of easterly waves, GRL, 31, L17208, doi:10.1029/2004GL020107, 2004., Text and figures copyright AGU 2004.
G. Okin, N. Mahowald, O. Chadwick, P. Artaxo, The impact of desert dust on the biogeochemistry of phosphorus in terrestrial ecosystems, Global Biogeochemical Cycles,vol 18, GB2005,doi: 10.1029/2003GB002145,2004. Text and b/w and color figures, copyright AGU 2004
E. Boyle, B. Bergquist, R. Kayser, N. Mahowald, Iron, Manganese and Lead at Hawaii Ocean Time-Series Station ALOHA: Temporal Variability and an Intermediate Water Hydrothermal Plume, in press, Geochimica Cosmochimica Acta.
G. Okin, N. Mahowald, O. Chadwick, P. Artaxo, The impact of desert dust on the biogeochemistry of phosphorus in terrestrial ecosystems, Global Biogeochemical Cycles,vol 18, GB2005,doi: 10.1029/2003GB002145,2004. Text and b/w and color figures, copyright AGU 2004
N. Mahowald, L. Kiehl, Mineral aerosols and cloud interactions, Geophys. Res. Let., 30, No. 9, 10.109/2002GL016762, 2003.
Tanguy Claquin. C. Roelandt, K. Kohfeld, S. Harrison, I. Tegen, I. C. Prentice,Y. Balkanski, G. Bergametti, M. Hansson, N. Mahowald, H. Rodhe, M. Schulz, Radiative forcing of climate by ice-age dust, Climate Dynamics, 2003, 20:193-202; DOI 10.1007/s00382-002-0269-1.
C. Mahaffrey, R. G. Williams, G. A. Wolff, N. Mahowald, W. Anderson, Isotopic signals of nitrogen fixation over the eastern North Atlantic, GRL, vol, 40, No. 6, 10.1019/2002/GL016542, March, 2003.
D. Archer, A. Winguth, D. Lea, N Mahowald, What caused the glacial/interglaci al atmospheric pCO2 cycles? Reviews of Geophysics, 38 (2): 159-189, 2000.
Movies
(These sometimes play and sometimes don't--please don't ask me why):
TOMS AI for 1998 http://toms.gsfc.nasa.gov/aerosols/aerosols/html