Impact of Short-Lived Pollutants on Arctic Climate (SPAC)Background and Progress Since January 2007 GISS Meeting

SPAC Workshop, November 5 – 7, 2007Oslo, Norway

Sponsored by NILU, CATF, IGAC, CPC

From NSIDC

Summer 2007

NW passage open for the firsttime in human memory

NE passage closed by a narrowband of sea ice.

1979 - 2000

September sea ice extent from 1979 – 2007

The September rate of sea ice decline is now 10% per decade.Spring melt is occurring earlier and autumn freezing is starting later.

The earlier onset of spring melt is of particular concernas this is the season of maximum snow-albedo feedback.

Snow-Albedo FeedbackAs the area of ice and snow cover decreases, there is less reflectionof solar radiation and more absorption by the darker remaining ocean andland surfaces. This increase in total absorbed radiation contributes tocontinued and accelerated warming.

Impact of Short-Lived Pollutants on Arctic Climate

• Reductions in CO2 are the backbone of any climate forcing mitigation strategy• But reductions in CO2 most likely will not be swift enough to delay Arctic

melting• Targeting short-lived climate forcing agents may be the best strategy for

delaying the onset of spring melt and constraining the length of the melt season

• Particularly important are the species that impose a radiative forcingthat leads to a surface temperature increase and that trigger regionalscale climate feedbacks pertaining to sea ice melting

• Methane• Tropospheric Ozone• Tropospheric Aerosols

Seasonality of Pollutant Transport to the Arctic

Quinn et al., Tellus, 2006.

Monthly averaged values

2.5

2.0

1.5

1.0

0.5

0.0

1/77 1/79 1/81 1/83 1/85 1/87 1/89 1/91 1/93 1/95 1/97 1/99 1/01 1/03 1/05 1/07

302520151050

250

200

150

100

50

Barrow

Barrow

Alert

Scattering, 1/Mm

Absorption, 1/Mm

Black carbon, ng m-3

Arctic Haze

• Arrives late winter/early spring

• Includes troposphericaerosols, ozone, andozone precursors

Barrow

”normal” value

[Stohl et al. (2006): JGR, 111, D22214, doi:10.1029/2006JD007216]

Summertime transport of smokefrom boreal forest fires

• Barrow, Alaska• Summit, Greenland• Zeppelin, Svalbard

Springtime transport of smoke fromagricultural fires in Eastern Europe

Iceland

[Stohl et al. (2006): ACP, 7, 511 - 534.]

Transport from warm, lower latitudespossible because of abnormally warm Arctic temperatures

• Erosion of the Polar Dome

Seasonality of Pollutant Transport to the Arctic

Quinn et al., Tellus, 2006.

Monthly averaged values

2.5

2.0

1.5

1.0

0.5

0.0

1/77 1/79 1/81 1/83 1/85 1/87 1/89 1/91 1/93 1/95 1/97 1/99 1/01 1/03 1/05 1/07

302520151050

250

200

150

100

50

Barrow

Barrow

Alert

Scattering, 1/Mm

Absorption, 1/Mm

Black carbon, ng m-3

• For the short-lived species with lifetimes of days to weeks, the timing of transport to the Arctic can be a factor in the seasonality of the forcing and the corresponding temperature response.

• Methane is an exception due to its longer lifetime of 10 years.

It is globally well-mixed so that transport is not seasonal.

• The next few slides indicate the seasonality of the different forcing mechanisms and emphasize the season of maximum surface temperature response for each forcing agent.

Forcing Mechanism and Season of Maximum Temperature Response

Winter – Enhanced Cloud Longwave Emissivity (ΔT > 0)

Thin, clean cloudPoor insulatorHeat escapes

Thin, polluted cloud.Better insulator. Heat istrapped and re-emitted.

[e.g., Garrett and Zhao, Nature, 2006]

O3 CH4

GHGWarming ΔT > 0

Forcing Mechanism and Season of Maximum Temperature Response

Spring – Black carbon deposition / snow albedo reduction (ΔT > 0)

Forcing Mechanism and Season of Maximum Temperature Response

Summer – Shortwave Aerosol Direct & Indirect Effects (ΔT < 0)

Haze layer:Direct forcing Aerosol Modified Cloud:

Indirect forcing

Enhanced CloudLongwave Emissivity ΔT > 0

SUMMER

O3 CH4

WINTER

Black carbonaerosol /

Snow albedoΔT > 0

SPRING

Aerosol Direct &IndirectEffects ΔT < 0

GHGWarming ΔT > 0

How does each forcing agent impact Arctic Climate?

sign and magnitude of forcing at the surface surface temperature response feedbacks triggered (function of seasonality of the forcing and response)

What sources should be targeted to lessen impact in the Arctic?

most bang for the buck – which species have the largest impact? local or extra-polar sources?

Successful Mitigation Strategy Requires Knowing:

Starting Point (i.e., quick and dirty approach

where you use what you already have)

After the Jan 2007 GISS meeting, output from global climate models was used to obtain:

• Seasonally averaged values of instantaneous forcing andsurface temperature response

• Averaged over 60° to 90°N

Calculated forcing was based on present-day fossil fuel + bio-fuel + biomass burning emissions relative to present-day biomass burning emissions.

The Arctic response was forced globally with changing composition.

Exception was the cloud LW emissivity. Fs was based on measurements of the sensitivity of low-level cloud emissivity to pollution at Barrow. Not a seasonal average as it only includes times when pollution aerosol and clouds were coincident.

Seasonally averaged values of Fs and ΔTs for 60° to 90°N

Fs ΔTs

Quinn et al., ACPD, submitted, 2007.

Seasonally averaged values of Fs and ΔTs for 60° to 90°N

ΔTs > 0

Fs ΔTs

Seasonally averaged values of Fs and ΔTs for 60° to 90°N

ΔTs > 0

O3 and CH4

ΔTs maximum is in the winter when forcing

is at a minimum

BC / Snow albedoFs and ΔTs are both at a

maximum in the spring ?

Not a seasonal average

Fs ΔTs

Seasonally averaged values of Fs and ΔTs for 60° to 90°N

ΔTs > 0

O3 and CH4

ΔTs maximum in winterwhen forcing is at a

minimum

BC / Snow albedoFs at a maximum in spring

along with is ΔTs?

BC – Added atmospheric

heating will ultimately increase LW radiation and warm the surface

Indirect Effect -Largest Fs is in the

spring butthis is when ΔTs is at a minimum

Not a seasonal average

Fs ΔTs

Comparison of Short-Lived Pollutants and Well-Mixed Greenhouse Gases

Includes cloud LWemissivity so is an overestimate of the seasonal average

Need an apples to apples comparison!

Framework for Discussion of Climate Impactsof Short-lived Pollutants

• What is the best way to calculate forcing and the resulting temperature response in order to assess impacts on Arctic melting and to find “smoking gun(s)”?

• Seasonal averages do not capture the response due to episodic intense events. What is the impact of a short, intense forcing vs. a continuous moderate forcing on a non-linear system?

• Transient simulations do not allow for equilibration with atmospheric temperatures.

• How do we implement an apples-to-apples comparison in order to design an effective mitigation strategy?

• What are the largest uncertainties in current models and measurements? How can we reduce those uncertainties?

State of Understanding• forcing & response calculations• uncertainties• required measurements• local vs. remote forcing• potential feedbacks

Recommendations for Policy Makers• What is the climate impact of each pollutant?• What are the sources of that pollutant?• What are mitigation strategies for each pollutant?

Day 1

Day 3

Day 2

Emerging Issues• boreal forest fires• shipping activity• source of BC deposited to snow pack• BC and O3 mitigation strategies• effects of a warming Arctic