Black Carbon in Snow:Role in Arctic Climate, Uncertainties, and

Future ScenariosMark Flanner1

Charlie Zender2

Collaborators:Tami Bond, Jim Randerson, Phil Rasch

1 National Center for Atmospheric Research, Boulder CO

2 University of California - Irvine

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Talk Outline

Positive feedback mechanisms Arctic climate response and efficacy Uncertainties and research possibilities Arctic shipping Arctic warming mitigation possibilities

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Aerosol-induced Snow Heating:Multiple Positive Feedbacks

Snow/IceCover

Albedo

R_net

(-)

(+)

(-)+

SnowGrainSize

(-)(+)

+ G

Soot(-)

(+)

+

(+) ?

Concentration of hydrophobic and large impurities at the surface during melting?

(+)

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Importance of Snow Aging

Radiative intensity diminishes more slowly with depth in larger-grained snow because it is less extinctive and scatters more strongly in the forward direction. Hence photons tend to travel through a greater optical depth of (homogeneously-interspersed) BC.

Albedo perturbation caused by a given mass of BC varies more than three-fold for a reasonable range of effective grain size.

snow density = 150 kg m-3

zenith angle = 60 degreeswavelength = 550 nm

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Measurements and Model Estimates of BC in Snow

Flanner et al. (2007) Present day climate forcing and response from black carbon in snow, J. Geophys. Res.

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Forcing Pattern of BC in Snow

Forcing operates mostly in local springtime, when and where there is large snow cover exposed to intense insolation, coincidentally with peak snowmelt. Hence, it is a strong trigger of snow-albedo feedback, which is maximal in spring (Hall and Qu, 2006).

Forcing is dominated by FF+BF sources, but strong biomass burning events can have significant impact on Arctic

Flanner et al. (2007) Present day climate forcing and response from black carbon in snow, J. Geophys. Res.

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Climate Response from BC in Snow

Earlier snowmelt Reduced surface albedo Surface air warming

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Arctic BC+OC forcing From policy perspective, interested in net effect of

an anthropogenic activity on Arctic climate. Aerosol direct effects (atm. and snow), indirect effects

on cloud LW and SW forcing, and greenhouse gases Focus on BC+OC emitted from fossil fuel+biofuel

burning. Direct Arctic effects: Increase in solar energy absorbed at TOA

Bright surfaces (clouds, snow) allow moderately absorbing aerosols to darken the atmosphere-surface column

High BC:OC emission ratio from FF/BF combustion produces a relatively dark aerosol mixture

Decrease in solar energy absorbed by surface from atmospheric aerosols

Increase in solar energy absorbed by surface from snow darkening

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Arctic-wide FF+BF BC+OC forcing Atmospheric BC+OC, TOA: +0.30 W/m2 (GISS=+0.53) Atmospheric BC+OC, SFC: - 0.08 W/m2 (GISS=-0.25) Snow BC, SFC: +0.23 W/m2

Small surface forcing from atmospheric soot because of high surface reflectance

Surface darkening from soot exceeds surface dimming from atmospheric soot Globally, not the case, but when averaged over only snow,

darkening is greater than surface dimming by factor 2-3. Atmospheric heating, in spite of negative surface forcing,

ultimately transfers thermal energy to surface – why TOA or “adjusted” forcing has been used in past. Quinn et al. Report -∆Ts before thermal transfer (?) True for moderate aerosol burdens, but with very high

optical thickness the atmosphere becomes decoupled from surface (Cess, 1983)

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Effective Arctic Forcings

Eatm

= 0.8 (Hansen et al., 2005) E

snow=3.1 (Flanner et al., 2007)

TOA atmospheric aerosol forcing: +0.30 W/m2 * Eatm = +0.24 W/m2

TOA snow forcing: +0.21 W/m2 * Esnow = +0.65 W/m2

Combined BC+OC Arctic effective forcing: +0.89 W/m2 (> half of CO

2)

Dominant forcing season: spring Effect: earlier snowmelt

Caution: These are global efficacy factors. How should we define “Arctic Efficacy”?

Hansen, et al. (2005) The efficacy of climate forcings, J. Geophys. Res.

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Uncertainties

Range in Global Forcing (Scalar)

1) BC Emissions 0.54 – 2.002) Snow Aging 0.58 – 1.583) Melt Scavenging 0.69 – 1.084) BC Optical Properties 0.88 – 1.125) Snow Cover Fraction 0.83 – 1.086) Brown carbon (e.g., Sun et. al., 2007) ???

(Unaccounted absorption, Sato et. al., 2003)

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Constraining Direct Forcing Snow Aging

Need widespread observations of surface snow effective grain size (SSA) to constrain spatial and temporal variability

New measurement techniques (Domine et al., 2007) are providing useful data

Is there hope for remotely-sensed effective grain size (Painter et al., 2003 with AVIRIS)?

In theory, yes. However, snow BRDF is highly variable in direction. Potential problem: For apriori BRDF, current MODIS snow product assumes a globally-uniform grain size.

Process-oriented experiments to quantify effects of: Snow T, dT/dz, density, melt/freeze cycles, LWC, sintering,

aerosol heating

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Detecting Snow Grain Size

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Constraining Direct Forcing Snow forcing – other factors

Closure studies: Measurements and modeling of multi-spectral snow reflectance and vertical absorption profile with known BC concentration, effective grain size – extending work from Grenfell et al., 1994

Scavenging of particles in the snowpack with meltwater (current constraint: 2 orders of magnitude)

Isolation/separation of forcing by BC, OC, dust, algae, other organic material

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Constraining Direct Forcing Atmospheric BC+OC

Significant difference in direct aerosol forcing between GISS and NCAR simulations.

Measurements of column aerosol burdens in Arctic? Barrow, new IPY measurements?

Modeled AOT vs. satellite obs. (MODIS/MISR) Predicted BC in snow vs. observations Offline radiative forcing simulations with GCM column

burdens Effect of surface reflectance on differences in surface forcing Comparison of prescribed optical properties

Emissions Can we constrain forward estimates with inversion

techniques? (e.g., examine AOT and column burdens in models driven with offline winds)

Secondary organic aerosols present major obstacle!

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Arctic Shipping Experiment

Concern because of proximal BC emission source to snow/ice

Experiment:

Emit 0.03/0.08 Tg of BC/OC along Northwest Passage during July, August, Sept. This is order 1% of total PD FF+BF emission, and 25% of 2050 shipping emissions (Eyring et al., 2005a,b)

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Arctic Shipping Experiment: Influence

on Greenland

Mean JAS BC/snow concentration increases from 16.5 to 21.8 ng/g

Surface forcing increases from 0.52 to 0.63 W/m2

Surface air warms by 0.22K

Snow melt rate increases by 7%

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Mitigation Experiments Scenarios:

1. 50% global reduction in FF+BF BC+OC emissions 2. 50% reduction in FF+BF emissions N of 40° 3. 50% reduction from U.S., Canada, and Europe (from T.

Bond)Arctic forcing (W/m2)Snow AtmTOA Sno+atmTOA %Reduction%Rdc_anthro

=================================================================PD ff+bf+bb +0.34 +0.61 +0.92 -- --PD bb +0.11 +0.31 +0.41 -- --DIFF=ANTHRO +0.23 +0.30 +0.51 (55%tot) -- --

50% global +0.23 +0.48 +0.69 -25% -45%50% N40 +0.25 +0.54 +0.77 -16% -29%50% US/CAN/EU+0.28 +0.57 +0.82 -11% -20%

Points:1) reducing global emissions by half reduces forcing by <half because of

non-linear nature of radiative forcing2) Snow forcing is reduced more than atmospheric aerosol forcing. Why?

When considering efficacy, this means potentially greater warming reduction than these numbers would indicate.

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Snow vs. Atmosphere forcing Snow forcing peaks in

May, when snow-coverage is still large, one month earlier than atmospheric forcing

Greatest snow forcing reduction is during May-June, when it has greatest impact.

Reduction in BC deposition during this time is greater than optical depth reduction, partially explaining greater reduction in snow forcing

Forcing1 = Present-day, all sourcesForcing2 = 50% reduction in NH sources

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Conclusions – BC in Snow Snow darkening is a significant component of

carbon aerosol Arctic climate forcing High efficacy because of direct snow-albedo-

feedback trigger, microphysical snow processes Reduced-emission scenarios suggest greater

reduction in snow forcing than atmospheric aerosol forcing – more work needed

Future research should focus on: Constraining snow effective grain size at ground

observatories and perhaps from space Identifying snow aerosol constituents and their relative

contributions to snow heating Constraining the melt scavenging ratio of aerosols in

snow

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Talking Points How to best define “Arctic Efficacy”?

Effectiveness of Arctic forcing agent in changing Arctic climate relative to Arctic CO2

Effectiveness of global forcing agent in changing Arctic climate relative to global CO2

Proposal: Define extrapolar and polar forcing efficacies with global simulations (Shindell, 2007).

Change in Arctic Ts per unit Arctic forcing Change in Arctic Ts per unit extrapolar forcing

Indirect cloud/aerosol forcing is largest non-CO2 forcing term in Arctic (-1.45 W/m2 TOA, Quinn et al, 2007). What portion is due to BC+OC? Cloud emissivity (total~+1W/m2)? Reducing high-latitude BC (only) emissions would likely slow

Arctic warming But reducing BC+OC (more realistic), if a large portion of

indirect effect is from BC+OC, may not

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Measurements of BC in snow (and atm)

Need agreement on, or new definitions of, BC, OC, LAC, brown carbon

If widespread BC/snow measurements are made, a consistent technique should be applied, or methods should be calibrated against each other.

(Utopic view) Wide range of measurement techniques

Filtration/optical (e.g., Grenfell, Warren) Thermal/optical (e.g., Chylek et al.) Chemical (e.g., Slater et al.) LASER/optical (McConnell et al.)