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

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, 10100x regionally
Mahowald et al., 1999
• 1960s/1980s Barbados:
4x change
Data courtesy of J. Prospero and D. Savoie
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)
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]
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?
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])
Mineral aerosol effective
source areas
TIMIND
BASE
BASECO2
CULT+TIMIND
CULT+BASE
CULT+BASECO2
Effective Source Area (m2)
4.50E+00
4.00E+00
3.50E+00
3.00E+00
2.50E+00
2.00E+00
1.50E+00
1.00E+00
5.00E-01
0.00E+00
1880s
1990s
2090s
• 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)
Source/Deposition
Mineral aerosol deposition
2.50E+03
TIMIND
BASE
BASECO2
CULT+TIMIND
CULT+BASE
CULT+BASECO2
Deposition (Tg/year)
2.00E+03
1.50E+03
1.00E+03
5.00E+02
0.00E+00
1880s
1990s
2090s
• 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
Mineral aerosol loading
TIMIND
BASE
BASECO2
CULT+TIMIND
CULT+BASE
CULT+BASECO2
3.00E+01
Loading (Tg/m2)
2.50E+01
2.00E+01
1.50E+01
1.00E+01
5.00E+00
0.00E+00
1880s
1990s
2090s
• 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
Comparison with ice core data for preindustrial/
current climate
(use ratio of deposition or concentration for comparison)
Obs. vs. Model Ratios
Ratios 1880s/1990s
2.000
1.500
TIMIND
BASE
BASECO2
1.000
CULT+TIMIND
CULT+BASE
0.500
OBS
Da
su
op
u*
G
IS
Hu
P
as
ca
ra
Ki
n*
lim
an
ja
ro
*
Ne
wa
ll*
Pe
nn
Q
y*
ue
lcc
ay
a*
av
Si
pl
er
e*
ag
e
av
An
er
de
ag
s
e
S.
Po
le
0.000
CULT+BASECO2
•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
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