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The Variable Influence of Mineral Aerosols on Climate: Insights from Past and Future Scenarios

This study by Natalie Mahowald and Chao Luo analyzes the dynamics of mineral aerosols, particularly desert dust, and their significant variability in relation to climate changes across glacial and interglacial cycles. It highlights that mineral aerosol deposition can increase threefold globally during the Last Glacial Maximum and identifies factors influencing dust variability, such as changes in precipitation and human land use. The findings suggest that anthropogenic activities may account for a substantial fraction of dust sources today, necessitating further investigation into future trends and their implications for ocean CO2 uptake and the broader climate system.

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The Variable Influence of Mineral Aerosols on Climate: Insights from Past and Future Scenarios

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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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