Surface–Climate Feedback Processes SOEE3410 : Lecture 6 Ian Brooks.

Surface–Climate Feedback Processes

SOEE3410 : Lecture 6Ian Brooks

SOEE3410 : Atmosphere and Ocean Climate Change Processes 2

The CO2 Budget

CO2 storage terms (PgC)

– Ocean : 38,000– Atmosphere : 730– Soil : 1500– Plants : 500

Units: 1 PgC = 1 Petagram of Carbon

1 Pg = 1 × 1015 grams

= 1 × 1012 kilograms

= 1 × 109 tonnes

Anthropogenic output (PgC/year)

– 1980’s : 5.4 0.3– 1990’s : 6.3 0.4

Gross exchange of CO2 (PgC/year)

– Land-atmosphere : 120– Ocean-atmosphere : 90

N.B. Gross annual exchange between ocean and atmosphere is:

– A large fraction of total atmospheric storage (16%)

– Much larger than human output

A small imbalance in exchange terms could result in CO2 as large as anthropogenic emission.It is thus important to understand how natural exchanges may respond to changes in climate.


Measured rate of increase in atmospheric CO2 (emission–uptake):

– 1980-1989 : 3.3 0.1 PgC/year– 1990-1999 : 3.2 0.1 PgC/year

These rates are variable on a year-to-year basis

– 1992 : 1.9 PgC/year– 1998 : 6.0 PgC/year

Variability is due to variations in uptake resulting from large-scale annual variability in climate processes: e.g. the El-Niño Southern Oscillation and North-Atlantic Oscillation


The land and ocean uptakes in CO2 can be distinguished from measurements of CO2, O2 and isotopes18O, 13C. The uptake is the difference between gross quantities absorbed and emitted. The estimated uptakes are (PgC/year):

1980-1989 1990-1999

Land-atmosphere flux: -0.2 0.7 -1.4 0.7

Ocean-atmosphere flux: -1.9 0.6 -1.7 0.5


CO2 Cycle Feedbacks (Land)

Higher atmospheric CO2 concentration:• Fertilisation effect increases rate of Net Primary

Production• Leaf stomata partially close, reducing water losses;

increased water use efficiency also increases NPP

Effectiveness of biomass as a sink of CO2 depends on:• Conversion of CO2 to carbon compounds with long

residence times: wood, modified soil organic matter.

NPP may not continue to increase with ever higher CO2 concentrations. CO2 must return to atmosphere eventually – possibly a new equilibrium higher total biomass.


Warming of environment:• On short time-scales: increase in heterotrophic

respiration (especially decay processes) increases, increasing return of CO2 to atmosphere. It is not known what the long-term effects on net land-atmosphere exchange are.

• Changes to regional cloud cover and precipitation are likely to affect regional ecology. This may act locally to promote or reduce CO2 uptake.


CO2 Cycle Feedbacks (Ocean)

Higher atmospheric CO2 concentration:• Flux of CO2 between ocean and atmosphere is driven by

pCO2 while atmospheric concentration increases it will drive increased ocean uptake…but…

• Fraction of anthropogenic CO2 that is taken up by ocean decreases as the concentration increases due to reduced buffering capacity of the carbonate system

• Fractional uptake also decreases because it is ultimately limited by the slow exchange of surface and deep-ocean waters– Surface waters reach equilibrium with atmosphere in ~1 year –

much faster than exchange of surface and deep water.


• There is NO fertilisation effect in the ocean– With limited exceptions photosynthesis takes place

via different reaction pathways than for land-based plants, CO2 availability is not a limiting factor.

– Higher CO2 concentrations decrease pH of ocean waters – may change ecology and rate of calcification.


Warming of ocean surface waters:

• Reduces solubility of CO2 in water, reducing uptake

• Increases vertical stratification of the water column:– Reduces outgassing from upwelling regions (reduced upwelling)– Reduces mixing of surface waters with deep-ocean water, thus

reducing rate of uptake

• Changes to bioproductivity / regional ecology– Changes to rate of calcification– Increased phytoplankton production of DMS has links to aerosol

& cloud properties (see marine cloud – aerosol feedbacks)


• ALL models suggest that the net effect of climate feedbacks on the carbon cycle is to reduce the rate of uptake and increase atmospheric CO2 concentrations.


Water Vapour Feedback

• Within the boundary layer relative humidity tends to remain ~constant, and water vapour content increases with temperature. Note non-linear increase of saturation vapour pressure with T.

• Within the free troposphere water vapour content cannot be inferred from simple thermodynamic arguments. It is controlled by complex dynamic and micro-physical processes that are not all well represented by current models.

Water vapour is a strong greenhouse gas. It’s effect is most important in the Free Troposphere, above the boundary layer. An increase in water vapour here would lead to further warming – a strong positive feedback. This is the most important reason for large responses to increased greenhouse gases.


Marine Cloud – Aerosol Feedbacks

Marine stratocumulus cover very extensive regions – large, persistent cloud decks off the western continental margins.


• Marine stratocumulus are an important influence on the radiation budget, reflecting solar radiation and limiting longwave losses at night.

• Cloud reflectivity is sensitive to the droplet size and number distribution – this is determined by the available water vapour and cloud condensation nuclei (CCN) concentrations.


Shiptracks:

This enhanced satellite image shows tracks in low-level marine stratocumulus clouds. Aerosols in ship exhaust act as CCN, increasing the number of droplets in the cloud (since the water available is the same, this reduces the mean droplet size). The change in the droplet spectrum makes the cloud more reflective.


Twomey, S. A., M. Piepgrass, and L. T. Wolfe. 1984: An assessment of the impact of pollution on global cloud albedo. Tellus. 36B, 356-366.

• It has been calculated that a 2% increase in cloud albedo would offset the warming resulting from a doubling of CO2 (Twomey et al. 1984).

• Marine clouds exist in a relatively clean environment (low aerosol concentrations). Large changes in reflectivity can be produced by modest changes in CCN concentration.– Anthropogenic aerosols:

• Significant over land, and where flow is from land to sea.

• Extensive marine stratocumulus regions are primarily in clean oceanic air masses

– Changes in natural aerosol production in response to changes in climate


Aerosol Indirect Effect


Cloud albedo

Cloud condensationnuclei

Sulphate aerosol

SO2

DMS

DMS

Radiation budget

Global temperature

Climate feedbacks

Marine ecologyPhytoplankton

Abundance & speciation

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Sea-salt Mean wind

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Ocean Circulation & Heat Transport

Ocean circulation carries about 50% of total equator-to-pole heat flux


Ocean Temperature & Hurricane Activity

• Tropical cyclones form over distinct regions:– SST > 26C– latitude > 8 north/south of

equator

• There is no well defined relationship between increasing SST and tropical cyclone frequency (theoretical or observed)

• A strong correlation between SST and storm intensity is implied by theory

-2C 35CSea Surface Temperature

Regions of Tropical Cyclone Generation


Integrating the energy dissipated by tropical cyclones over the lifetime of the storms reveals a substantial increase in the total energy dissipated annually over the last 55 years.

Vmax is the maximum wind speed, is the duration of the storm. PDI = Power Dissipation index

A strong correlation with SST is evident in these results.

A measure of total power dissipated annually by tropical cyclones in N-Atlantic and NW-Pacific, and SST (+ offset for comparison).Kerry Emanuel, 2005, Nature, 436/4 August.(doi:10.1038/nature03906)

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3maxdtVPDI


Cold trail in N-Atlantic left by Hurricane Fran.

SST is ~3-5C cooler in wake of hurricane


• Passage of a tropical cyclone causes a significant drop in sea-surface temperature– About 25% due to air-sea

sensible and latent heat fluxes

– 75% due to turbulence generation in upper ocean which deepens the ocean mixed layer, mixing colder water up from below the thermocline.

• Over the following 1-2 months, the water temperature recovers towards its ‘normal’ value

• This results in a net heating of the water column which must be balanced by the meridional heat flux in the ocean

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• The peak meridional heat flux due to ocean circulation has been estimated at 2×1015W

• The net column heating required to restore surface wakes of tropical cyclones over a year is estimated at(1.40.7)×1015W

hurricanes may drive a significant fraction of the meridional heat flux in the oceans

• If hurricane activity – intensity or frequency – increases as a result of climate change, then this may result in an increase in the tropically forced thermohaline circulation– Estimated that 2C rise in

tropical SST could increase the meridional heat flux by up to 30%

– Increases climate sensitivity in mid-high latitudes (warming effect) and decreases climate sensitivity in tropics (cooling effect)Emanuel, K. 2001, Contribution of tropical cyclones to

meridional heat transport by the oceans. JGR 106, D14, 14771-14781


Summary

• Feedback processes between different processes within the climate system are complex and often poorly understood

• Surface-atmosphere interaction involves links between physical, chemical, and biological processes

• Apparently localised processes can have climate impacts on much larger scales

Surface–Climate Feedback Processes SOEE3410 : Lecture 6 Ian Brooks.

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