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A less dusty future?

A less dusty future?. Natalie Mahowald and Chao Luo NCAR/UCSB (submitted to GRL, available at www.cgd.ucar.edu/tss/staff/mahowald. Mineral aerosols highly variable with climate. Glacial/interglacial cycle 3- fold higher deposition globally in LGM, 10-100x regionally.

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A less dusty future?

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  1. A less dusty future? Natalie Mahowald and Chao Luo NCAR/UCSB (submitted to GRL, available at www.cgd.ucar.edu/tss/staff/mahowald

  2. Mineral aerosols highly variable with climate • Glacial/interglacial cycle 3- fold higher deposition globally in LGM, 10-100x regionally • 1960s/1980s Barbados: 4x change Data courtesy of J. Prospero and D. Savoie Mahowald et al., 1999

  3. Atmospheric mineral aerosols/desert dust • Source: unvegetated dry soils with easily erodible soils and strong winds • Sink • wet deposition (precipitation scavenging) • dry deposition (gravitational and turbulent settling)

  4. Mechanisms for variability in desert dust? • Glacial/interglacial • Changes in precipitation in arid source regions • Changes in precipitation along transport pathways [e.g. Yung et al., 1996] • Changes in transport pathways • Changes in CO2 levels in arid source regions impacting vegetation [Mahowald et al., 1999] 50% of source area/loading impacted • Changes in surface winds in source regions (e.g. Rea, 1994; not found in [Mahowald et al., 1999]) • 1960s/1980s in Barbados/North Atlantic • Changes in Precipitation in Sahel [e.g. Prospero and Nees, 1986] • Resulting changes in sources in Sahel • Changes in transport associated with precip changes? • Human land use? [e.g. Prospero and Nees, 1986; Mahowald et al., 2002]

  5. Role of humans • Anthropogenic source of dust? • In situ studies in US (e.g.) [Gillette, 1988] • 50% due to disturbed (natural and anthropogenic) sources postulated by Tegen and Fung [1995] (but model dependent result) • Prospero et al., 2002; Goudie and Middle, 2001, Ginoux et al., 2001 claim sources only natural using TOMS AI, geomorphic arguments and model • Mahowald et al., 2002; Luo et al., 2003; Mahowald et al., 2003 suggest that TOMS AI cannot distinguish between anthropogenic and natural sources • Could be 0-50% of current source??? • Importance of CO2 fertilization?

  6. Past/present/future study • CSM1.0 output: archive meteorology and input into MATCH/DEAD simulation [Zender et al., 2003; Mahowald et al., 2002; Luo et al., 2003; Mahowald et al., 2003] • 1880s, 1990s and 2090s simulated • 6 different scenarios • Time independent source (TIMIND) Ginoux et al., 2001 • Topographic lows+vegetation changes (BASE) • Topographic lows +vegetation changes with CO2 fertilization (BASE-CO2) • 3 above +50% cultivation in desert source (following Mahowald et al., 2002; Luo et al., 2003): desert region* Matthews [1984] land use dataset (“cultivation” includes pasturization) (CULT) • Assume no cultivation in desert in 1880s, similar cultivation in arid regions in 2090s (based on IMAGES1.0 model [Alcamo, 1994])

  7. Source areas increase or decrease between 1880s and 1990s, depending on assumptions • Source areas decrease between present and 2090s • Model is driest during 1990s in desert regions (model/simulation dependent)

  8. Source/Deposition • Source magnitude increase or decrease between 1880s and 1990s, depending on assumptions • Source magnitude decrease between present and 2090s • Model source strongest in 1880s, 1980s then 2090s for TIMIND

  9. Atmospheric loading ~ source*lifetime • Lifetime relatively stable between climates • Loading increase or decrease between 1880s and 1990s, depending on assumptions • Loading decreases between present and 2090s

  10. Comparison with ice core data for preindustrial/ current climate (use ratio of deposition or concentration for comparison) • None of the scenarios does appreciably better or worse than others • Ice cores may not be located in right place to sample • Ice cores give regionally inconsistent signals

  11. Summary/conclusions • Current climate anthropogenic dust (including climate impacts): up to 60% or humans caused decrease of 20% • Future dust 20-60% lower than current climate • Sensitive to model simulations (~20% level here—could be different with other model/simulations) • Sensitive to scenario (role of CO2 fertilization or land use): ~50% level • Could have profound impact on ocean CO2 uptake, radiative forcing, indirect forcing, atmospheric chemistry, terrestrial biosphere • “Natural Aerosol” likely to vary strongly with climate change

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