Overview of Dust in the Earth System - National Academiessites.nationalacademies.org/.../pgasite/documents/webpage/pga_06… · Role of dust in the earth system Modified from Jickells

AAAS Symposium ‐ 1

Dr. Karen E. KohfeldSchool of Resource and Environmental Management, Simon Fraser University,

CANADA

Overview of Dust in the Earth System


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)


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


‘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


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


LAND – dust emissions


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


Controls on Dust Emissions

• LAND SURFACE– Soil moisture & properties, availability

• VEGETATION COVER– Temperature, precipitation, CO2

• SURFACE WINDS

LAND USE‐Cultivation‐Rangeland‐Infrastructure


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


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).


Future Changes?

• Natural climate changes – (and resultant changes in land surface)

• Changes in land use

• Human‐induced changes in climate

Klein Goldewijk, 2001


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


AIR – radiative forcing

Dust over dark ocean:“Cooling”

Dust over white clouds:“Warming”


Ice age – Top of Atmosphere Radiative Forcing

‐10 ‐5 ‐2 ‐1 1 2 5 10Wm‐2

Claquin et al. (2003)


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


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)


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


+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


SEA – dust as a nutrient


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


A driver of lower atmospheric pCO2, Last Ice Age

Enhanced Ice Age Dust Deposition

Bopp et al. (2003); Kohfeld and Ridgwell (2009)


Increased Carbon Export, Lower Atm CO2

PaleoceanographicExport Production Data

LGM increase

LGM decrease

no change

‐40‐80 0 40 80

gC/m2/y


A moderate driver of ice‐age CO2 change

Iron fertilizationfrom dust

Kohfeld and Ridgwell (2009)


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


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


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


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)



Extra slides


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)


Modeled response of desert area to future climate change

(Mahowald, 2007)

% Change in Desert Area with Time also model dependent


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


Anthropogenic perturbations of land surface

(IPCC, 2007, Ch 2)


Global dust deposition today

Jickells et al. (2005)g/m2/year


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


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

Documents

Overview of Dust in the Earth System - National Academiessites.nationalacademies.org/.../pgasite/documents/webpage/pga_06… · Role of dust in the earth system Modified from Jickells