overview of dust in the earth system - national...
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AAAS Symposium ‐ 1
Dr. Karen E. KohfeldSchool of Resource and Environmental Management, Simon Fraser University,
CANADA
Overview of Dust in the Earth System
AAAS Symposium ‐ 2
What is dust?
• Soil mineral fragments– Quartz, feldspars, carbonate, gypsum
• clay minerals– Kaolinite, illite, montmorillonite, chlorite, iron oxides
• Size range– ~0.1 – ~100 μm
Dust storm approaching Stratford, Texas. (Credit: NOAA Photo Library, Historic NWS collection)
AAAS Symposium ‐ 3
Data: Jouzel et el.,2007; Siegenthaler et al., 2005; Lambert et al., 2008; Figure: Kohfeld and Ridgwell (2009).
Dust through the ice ages ‐ Antarctic Ice
AAAS Symposium ‐ 4
‘Recent’ changes
Porphyry LakeWestern Interior USA
Log age (yr before present)
(Neff et al., 2008; McConnell et al., 2007)
Antarctic Peninsula
Last 10,000 years Last two centuries
AAAS Symposium ‐ 5
Role of dust in the earth system
Modified from Jickells et al. (2005)
N O and CH2 4
ocean CO sequestration2
oceananoxia
halocarbon, alkylnitrate, & DMSemissions to atmosphere
sinking dustparticles
temperature, precipitation
climate ‘sea’ (marineproductivity)
‘land’ (surface propertiesand dust availability)
‘air’ (atmospheric aerosol loading)
wind speed
aeolian iron supply to the open ocean
cloud cover, sea-ice, SSTs,ocean circulation
CO fe
rtiliza
tion
2
precipitation
optic
al p
rope
rties
ecosystemcomposition and
CaCO production3
anthropogenicland-usechange
aeolian P supplyto terrestrial ecosystems
LAND
AIR
SEACLIMATE
AAAS Symposium ‐ 6
LAND – dust emissions
AAAS Symposium ‐ 7
Dust Emission “hot spots” today
Engelstaedter and Washington (2007)
Hot spots associated with lake, fluvial, and dune deposits.
>60% found in North Africa, Middle East.
Globally, almost 2 billion tonnesare removed from surface and transported by wind within the atmosphere
AAAS Symposium ‐ 8
Controls on Dust Emissions
• LAND SURFACE– Soil moisture & properties, availability
• VEGETATION COVER– Temperature, precipitation, CO2
• SURFACE WINDS
LAND USE‐Cultivation‐Rangeland‐Infrastructure
AAAS Symposium ‐ 9
Dust Emissions, Last Ice Age (20 ka)
• Emissions controls different during Last Ice Age:– Temperature
– Precipitation
– CO2
– Surface winds
– Vegetation
– Availability: Glacial grinding
Glaciogenic dust
http://earthobservatory.nasa.gov
AAAS Symposium ‐ 10
Last Ice Age
Mahowald et al. (2006)
Dust Source areas expanded due to changes in vegetation cover, winds, and glacial sources.
Inferred glaciogenic sources increase ice age emission by just over 55%.
LGM Source areas
Global datasets and models suggest that Ice Age deposition rates were ~3 times greater than today (globally).
AAAS Symposium ‐ 11
Future Changes?
• Natural climate changes – (and resultant changes in land surface)
• Changes in land use
• Human‐induced changes in climate
Klein Goldewijk, 2001
AAAS Symposium ‐ 12
Dust emissions in future?
MODEL YEAR CHANGE SOURCE
NCAR CSM 2090 ‐20 to ‐60% Mahowald and Luo (2003)
HADCM3 2050 ‐19% Tegen et al. (2004)
ECHAM4 2050 +9% Tegen et al. (2004)
HADAM3 2090 +200% Woodward et al. (2005)
Global changes in dust emissions are model dependent
AAAS Symposium ‐ 13
AIR – radiative forcing
Dust over dark ocean:“Cooling”
Dust over white clouds:“Warming”
AAAS Symposium ‐ 14
Ice age – Top of Atmosphere Radiative Forcing
‐10 ‐5 ‐2 ‐1 1 2 5 10Wm‐2
Claquin et al. (2003)
Mahowald et al. (2006)
Important spatial differences, but both simulations show:
Globally averaged Cooling of about ‐1 W m‐2
Largest impact observed in tropics
Wm‐2
Average Surface T cooled by 0.85°CDue to dust
AAAS Symposium ‐ 15
Direct Radiative Forcing by Total (Natural + Anthropogenic) Mineral Dust
IPCC, 2001: IPCC 2007:
‐1.2 Wm‐2 to +0.8 Wm‐2 ‐1.5 Wm‐2 to +0.5 Wm‐2
Main reasons for large uncertainty range:1. Dust optical properties uncertain
2. Dust distribution: Uncertainties in emission location and fluxes, vertical transport, deposition…
Uncertainty range remains at 2 Wm‐2
(global annual mean, re‐computed for natural and anthropogenic dust sources)
AAAS Symposium ‐ 16
Radiative Impact of dust in the Future?
2000‐2100
Δ (W m‐2)
Process Study
+0.14 less‐negative TOA forcing, due to reduced dust emissions
Mahowald et al. (2006)
+0.17 5X increase in positive TOA forcing, due to increased dust emissions
Woodward et al. (2005)
Similar, small change predicted, entirely different reasons, large (and differing!) regional effects
AAAS Symposium ‐ 17
SEA – dust as a nutrient
AAAS Symposium ‐ 18
Dust as a nutrient
141 E
chl (mg m )a -30.02 0.2 0.7 2 4 8 40200.07
Iron fertilization in Southern Ocean
(NASA SeaWiFS project)
0 5 10 15 20 25 30
Surface Ocean Nitrate
Conkright et al. (2004)
Source of Fe, Si, P, N
AAAS Symposium ‐ 19
A driver of lower atmospheric pCO2, Last Ice Age
Enhanced Ice Age Dust Deposition
Bopp et al. (2003); Kohfeld and Ridgwell (2009)
Mahowald et al. (2006)
Increased Carbon Export, Lower Atm CO2
PaleoceanographicExport Production Data
LGM increase
LGM decrease
no change
‐40‐80 0 40 80
gC/m2/y
AAAS Symposium ‐ 20
A moderate driver of ice‐age CO2 change
Iron fertilizationfrom dust
Kohfeld and Ridgwell (2009)
AAAS Symposium ‐ 21
Dust as an ocean fertilizer in the future?
(modified from Parekh et al., 2006)
5‐fold increase ‐8 ppm
50% decrease +14 ppm
Sensitivity Study using ocean Iron cycle model
AAAS Symposium ‐ 22
Still open questions
• Iron in dust:– Solubility varies by orders of
magnitude (but not in models yet!)
– Chemical interactions with combustion products will change iron solubility?
• Fe cycle in ocean models– Complexation with ligands poorly
understood
– surface vs subsurface Fe contributions not well constrained
• Regional ecosystem impacts could be very important Schroth et al. (2009)
Variable Iron Solubility
AAAS Symposium ‐ 23
Summary
• Dust has varied in the past and will vary in the future!– And will contribute to changes in radiative forcing and ocean biogeochemistry
• Knowledge of past changes in dust provided some first‐order answers to questions about dust cycle, but many challenges remain:– emissions, transport, dust removal– Spatial (and temporal) gaps in data– Radiative properties of dust– Chemical characteristics of dust– Impacts on biogeochemistry
• The future is wide open:– Natural climate variability and associated land‐surface changes– Human‐induced changes in climate– Human‐induced changes in the land‐surface
AAAS Symposium ‐ 24
Thanks!
Organizers: Art Bettis, Paul Bertsch, Nick Lancaster, Ester Sztein
Funding: Canadian NSERC Discovery, Canada Research Chair, and Canadian Fund for Infrastructure Programmes, Simon Fraser University
Contributions: N. Mahowald (Cornell U.); I. Tegen(Leibniz Laboratory for Tropospheric Research); A. Ridgwell (Bristol U.); G. Winckler (LDEO)
AAAS Symposium ‐ 25
AAAS Symposium ‐ 26
Extra slides
AAAS Symposium ‐ 27
Dust Accumulation in the Central North Pacific Oceanfor the last 20 Million Years
Age (1000 years)
0 5000 10000 15000 20000
Terri
geno
us A
ccum
ulat
ion
Rat
e (g
/m2/
y)
0.0
0.5
1.0
1.5
2.0
2.5
ODP Hole 145-885AODP Site 145-886
The last 20 Ma ‐ Asia
(Rea et al., 1998; An et al.; 2001)
AAAS Symposium ‐ 28
Modeled response of desert area to future climate change
(Mahowald, 2007)
% Change in Desert Area with Time also model dependent
AAAS Symposium ‐ 29
Global Emissions / Transport
Kohfeld and Tegen (2007) (adapted from Livingstone and Warren, 1996)
Globally, almost 2 billion tonnes are removed from surface and transported by wind within the atmosphere
AAAS Symposium ‐ 30
Anthropogenic perturbations of land surface
(IPCC, 2007, Ch 2)
AAAS Symposium ‐ 31
Global dust deposition today
Jickells et al. (2005)g/m2/year
AAAS Symposium ‐ 32
IPCC AR4: Radiative Forcing of Anthropogenic Mineral Dust
• Backscattering by aerosols partly offsets greenhouse gas warming
• Soil dust aerosols major part of atmospheric aerosol load
• Direct radiative effect of dust is negative
• Aerosol forcing remains large uncertainty
IPCC, 2007, Ch 2
AAAS Symposium ‐ 33
Direct radiative forcing of anthropogenic mineral dust
IPCC, 2001:‐0.6 Wm‐2 to +0.4 Wm‐2
Assumption: Maximally 50% from anthropogenic sources
IPCC 2007: ‐0.3 Wm‐2 to +0.1 Wm‐2
Assumption: Maximally 20% from anthopogenic sources
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