A less dusty future

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 lowsvegetation 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 sourcelifetime
• 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
  herecould 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