Transcript Surface Exchange Processes

Surface–Climate Feedback
Processes
SOEE3410 : Lecture 6
Ian Brooks
The CO2 Budget
CO2 storage terms (PgC)
–
–
–
–
Ocean
Atmosphere
Soil
Plants
: 38,000
: 730
: 1500
: 500
Anthropogenic output (PgC/year)
– 1980’s
– 1990’s
: 5.4 0.3
: 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.
Units: 1 PgC
1 Pg
= 1 Petagram of Carbon
= 1 × 1015 grams
= 1 × 1012 kilograms
= 1 × 109 tonnes
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Measured rate of increase in atmospheric CO2 (emission–
uptake):
– 1980-1989
– 1990-1999
: 3.3 0.1 PgC/year
: 3.2 0.1 PgC/year
These rates are variable on a year-to-year basis
– 1992
– 1998
: 1.9 PgC/year
: 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
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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):
Land-atmosphere flux:
Ocean-atmosphere flux:
1980-1989
-0.2 0.7
1990-1999
-1.4 0.7
-1.9 0.6
-1.7 0.5
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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.
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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.
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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.
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• 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.
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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)
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• 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.
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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 microphysical 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.
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Marine Cloud – Aerosol Feedbacks
Marine stratocumulus cover very extensive regions – large,
persistent cloud decks off the western continental margins.
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• 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.
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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.
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• 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
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.
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Aerosol
Indirect Effect
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Cloud albedo
Radiation budget
+
-
Cloud condensation
nuclei
Global temperature
+
Sulphate aerosol
+
Climate feedbacks
+
SO2
+
Sea-salt
DMS
Mean wind
+ or - ?
+
+
DMS
+ or - ?
+ or - ?
+ or –
Phytoplankton
Marine ecology
Abundance & speciation
?
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Ocean Circulation & Heat Transport
Ocean circulation carries about 50% of total equator-to-pole heat flux
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Ocean Temperature & Hurricane
Activity
• Tropical cyclones form over
distinct regions:
Regions of Tropical Cyclone Generation
– SST > 26C
– 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
-2C
Sea Surface Temperature 35C
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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.

3
PDI   Vmax
dt
0
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)
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.
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Cold trail in N-Atlantic
left by Hurricane Fran.
SST is ~3-5C cooler in
wake of hurricane
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• 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
+
+
T
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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
(1.40.7)×1015W
 hurricanes may drive a
significant fraction of the
meridional heat flux in the
oceans
Emanuel, K. 2001, Contribution of tropical cyclones to
meridional heat transport by the oceans. JGR 106, D14,
14771-14781
• 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 2C 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)
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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
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