The Contemporary Carbon Cycle Overview Marine Carbon cycle Marine sink for atmospheric CO2 The atmospheric imprint Atmospheric inversions The Terrestrial carbon sink Precision oxygen measurements

1
The Contemporary Carbon CycleOverviewMarine
Carbon cycleMarine sink for atmospheric CO2The
atmospheric imprintAtmospheric inversionsThe
Terrestrial carbon sinkPrecision oxygen
measurements
2
Reading
Overviews and reviews The IPCC Third assessment
report. http//www.grida.no/climate/ipcc_tar/ Ch
apter 3, wg1 report The scientific basis
Chapter 2-4, wg3 report mitigation
  Semi-popular overview of sinks for
anthropogenic carbon Sarmiento, J.L. and N.
Gruber. Sinks for anthropogenic carbon, Physics
Today, 55(8), 30-36, 2002. http//lgmacweb.env.ue
a.ac.uk/ajw/Geochemical_cycling/sarmiento_pt_02.pd
f Ocean - atmosphere fluxes Watson, A. J. and
Orr, J. C. (in press). Carbon dioxide fluxes in
the global ocean . Chapter 5 in Ocean
Biogeochemistry a JGOFS synthesis eds Fasham,
M. Field, J. Platt, T. B. Zeitzschel. Available
at http//lgmacweb.env.uea.ac.uk/ajw/Geochemical
_cycling/Watson_and_orr_in_fasham(ed)_2004.pdf  Te
rrestrial net fluxes Schimel DS, House JI,
Hibbard KA, et al. Recent patterns and mechanisms
of carbon exchange by terrestrial ecosystems
NATURE 414 169-172 2001 Available at
http//lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycl
ing/schimel_2001.pdf Atmospheric CO2 and O2
measurements Battle, M. et al. Global carbon
sinks and their variability inferred from
atmospheric O-2 and delta C-13. Science 287,
2467-2470 (2000).  http//lgmacweb.env.uea.ac.uk/a
jw/Geochemical_cycling/battle_2000.pdf
3
More Reading
CO2 measurements in the atmosphere, and what you
can do with them Keeling, C.D., T.P. Whorf, M.
Wahlen, and J. Vanderplicht, Interannual Extremes
In the Rate Of Rise Of Atmospheric Carbon-
Dioxide Since 1980, Nature, 375, 666-670,
1995. Keeling, C.D., J.F.S. Chin, and T.P. Whorf,
Increased Activity Of Northern Vegetation
Inferred From Atmospheric CO2 Measurements,
Nature, 382, 146-149, 1996. Available at
http//lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycl
ing/keeling_cd_1995.pdf http//lgmacweb.env.uea.ac
.uk/ajw/Geochemical_cycling/keeling_cd_1996.pdf
Classic paper on Inverse atmospheric calculation,
and the missing sink. Tans, P.P., I.Y. Fung,
and T. Takahashi, Observational Constraints On
the Global Atmospheric Co2 Budget, Science, 247
(4949), 1431-1438, 1990. http//lgmacweb.env.uea.a
c.uk/ajw/Geochemical_cycling/tans_et_al_1990.pdf
Ocean uptake of CO2. Sarmiento, J.L., and E.T.
Sundquist, Revised Budget For the Oceanic Uptake
Of Anthropogenic Carbon-Dioxide, Nature, 356,
589-593, 1992. Watson, A.J., P.D. Nightingale,
and D.J. Cooper, Modeling Atmosphere Ocean CO2
Transfer, Philosophical Transactions Of the Royal
Society Of London Series B- Biological Sciences,
348, 125-132, 1995.
4
Atmospheric CO2 Past, present and near future
5
The global carbon cycle
(Source, Sarmiento and Gruber, 2002)
6
The global carbon cycle

• Most of the labile carbon on Earth is in the
  deep sea.

• The gross atmosphere-ocean and atmosphere-vegetati
  on fluxes are of the same order.

• The net atmosphere-ocean and atmosphere-vegetation
  fluxes are much smaller than the gross fluxes.

• The flux through the marine biota (net
  productivity) is of the same order as that
  through the land vegetation.

• The mass of the marine biota is 1000 times less
  than that of the land vegetation.

7
The (almost) unperturbed marine carbon
cycle Global mean air-sea flux, calculated from
pCO2 measurements
8
Revision Seawater Carbonate chemistry

• Inorganic carbon exists as several forms in sea
  water
• Hydrated dissolved CO2 gas.
• This rapidly reacts with H2O to form
  undissociated carbonic acid
• CO2(g) H2O ? H2CO3
• Which can dissociate by loss of H to form
  bicarbonate ion
• H2CO3 ? H HCO3-
• which can dissociate by further loss of H to
  form carbonate ion
• HCO3- ? H CO32-

Typically, 90 of the carbon exists as
bicarbonate, 9 as carbonate, 1 as
dissolved CO2 and undissociated H2CO3 (usually
lumped together).
9
Seawater Partial pressure of CO2

• The partial pressure of CO2 of the sea water
  (pCO2sw) determines whether there is flux from
  air to sea or sea to air
• Air-to-sea Flux is proportional to (pCO2air -
  pCO2sw)
• pCO2sw is proportional to dissolved CO2(g)
• CO2(g) ? x pCO2sw where
• is the solubility of CO2. The solubility
  decreases with increasing temperature.
• pCO2air is determined by the atmospheric mixing
  ratio, i.e. if the mixing ratio is 370ppm and
  atmospheric pressure is 1 atm, pCO2air is 370
  ?atm.

10
What sets the net air-sea flux?

• The flux is set by patterns of sea-surface
  pCO2sw, forced by
• Ocean circulation
• Is surface water is cooling or heating?
• Is water being mixed up from depth?
• Ocean biology
• Is biological activity strong or weak?
• Is calcium carbonate being precipitated?
• The rising concentration of atmospheric CO2
• pCO2 of air is rising and this tends to favour a
  flux from atmosphere into the ocean.

11
Circulation influence on air-sea flux

• Warm currents, where water is cooling, are
  normally sink regions (NW Atlantic, Pacific).
• Source regions for subsurface water, where water
  is cooled sufficiently to sink are strong sinks
  (N. N. Atlantic, temperate Southern ocean)..
• Tropical upwelling zones, where subsurface water
  comes to the surface and is strongly heated, are
  strong sources (equatorial Pacific).

12
The overturning thermohaline circulation

• The Northern North Atlantic is a region of strong
  cooling, associated with the North Atlantic
  drift.
• Cooling water takes up CO2 and may subsequently
  sink.
• The water upwells in other parts of the world
  ocean, particularly the equatorial Pacific.
• Upwelling regions are usually sources of CO2 to
  the atmosphere deep water has high CO2 and the
  water is being warmed.
• This circulation controls how rapidly old ocean
  water is brought to the surface, and therefore
  how quickly the ocean equilibrates to changes in
  atmospheric CO2 concentration.

13
Biological influence on air-sea flux.

• Blooms of plankton fix carbon dioxide from the
  water and lower ?CO2, hence pCO2.
• Particularly marked in the North Atlantic which
  has the most intense bloom of any major ocean
  region.
• In the equatorial Pacific, plankton blooms are
  suppressed by lack of iron part of the
  explanation for high pCO2there.
• In the equatorial Atlantic, upwelling is less
  intense and there is more iron from atmospheric
  dust.

14
Ocean carbon pumps

• Deep water has higher (10-20) total carbon
  content and nutrient concentrations than surface
  water. There are several processes contributing
  to this
• The "Solubility pump" tends to keep the deep sea
  higher in total inorganic carbon (?CO2) compared
  to the warm surface ocean.
• The Biological pump(s)" the flux of biological
  detritus from the surface to deep, enriches deep
  water concentrations. There are two distinct
  phases of the carbon in this material
• The "soft tissue" pump enriches the deep sea in
  inorganic carbon and nutrients by transport of
  organic carbon compounds.
• The calcium carbonate pump enriches the deep sea
  in inorganic carbon and calcium.

15
Ocean biological pumps

• Falling dead organisms, faecal pellets and
  detritus are "remineralised" at depth.
  Remineralization occurs
• By bacterial activity.
• By inorganic dissolution of carbonate below the
  lysocline.
• The different phases have different depth
  profiles for remineralisation.

16
Ocean Carbon The Biological (soft tissue) Pump

• This mechanism acts continually to reduce the
  partial pressure of CO2 (pCO2) in the surface
  ocean, and increase it at depth.
• Over most of the ocean, upwelling water is
  depleted of inorganic carbon and nutrients
  (nitrate and phosphate) by plankton.
• In the process they remove about 10 of the
  inorganic CO2 in the water. Most of this goes to
  form organic matter via the reaction
• CO2 H2O ? CH2O O2.
• Because the buffer factor ? 10, this has a large
  effect on surface pCO2, decreasing it by 2-3
  times.
• The reverse reaction occurs by (mostly bacterial)
  respiration at depth, and increases CO2
  concentration there.

Depth
17
Surface pCO2, nutrient and surface temperature in
the North Atlantic
360
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
340
18
SST
SST (C)
320
8
16
)
atm
m
(
300
6
2
pCO
2
pCO
14
Nitrate (
4
m
280
M
)
12
2
260
Nitrate
18
The biological (calcium carbonate) pump.

• This mechanism also transfers carbon from the
  surface ocean to the deep sea.
• Some of the carbon taken up by the biota in
  surface waters goes to form calcium carbonate.
• The CaCO3 sinks to the deep sea, where some of it
  may re-dissolve and some become sedimented. The
  redissolution can only occur below the lysocline,
  which is shallower in the Pacific than the
  Atlantic.
• In contrast to the soft tissue pump, this
  mechanism tends to increase surface ocean pCO2
  and therefore atmospheric CO2 . The net reaction
  is
• Ca 2HCO3- ? H2O CO2? CaCO3?

19
Coccolithophores -- calcite precipitating plankton
Photo courtesy D. Purdie See the Ehux home
page www.soc.soton.ac.uk/SUDO/tt/eh/index.html
20
The ocean sink for anthropogenic CO2

• The oceans are close to steady-state with respect
  to atmospheric CO2.
• Prior to the industrial revolution, the oceans
  were a net source of order 0.5 Pg C yr-1 CO2 to
  the atmosphere. Today they are net sink of order
  2 Pg C yr-1.
• The main factor controlling ocean uptake is the
  slow overturning circulation, which limits the
  rate at which the ocean mixes vertically.
• Three methods are being used to calculate the
  size of the ocean sink.
• Integration of the pCO2 map (difficult and
  inaccurate).
• Measurements of atmospheric oxygen and CO2 (see
  later).
• Models of ocean circulation. These are of two
  types
• Relatively simple box-diffusion models
  calibrated so that they reproduce the uptake of
  tracers such as bomb-produced 14carbon.
• Ocean GCMs which attempt to diagnose the uptake
  from the circulation. (However, the overturning
  circulation is difficult to model correctly. In
  practice these models are also tested against
  ocean tracers.)

21
Riverine flux
The pre-industrial steady state cycle to balance
the flux of carbon coming down rivers, there must
have been a CO2 flux of the order of 0.5 Pg C
yr-1 from ocean to atmosphere and from atmosphere
to land. Volcanic activity and sedimentation
fluxes provide smaller net inputs and outputs to
the system.
22
Tropospheric bomb radiocarbon
The atmospheric bomb tests of the 50s and 60s
injected a spike of radiocarbon into the
atmosphere which was subsequently tracked into
the ocean. This signal provides a good proxy for
anthropogenic CO2 over decadal time scales.
23
3-D model outputs for surface pCO2

• Capture the basic elements of the sources and
  sinks distribution.
• Considerable discrepancies with one another and
  with the data (Southern Ocean, North Atlantic).

24
How well is the global ocean sink known?
Estimates of the global ocean sink
1990-1999 Reference Sink (Pg C yr-1) IPCC
(2001) 1.7/- 0.5 Estimate (Keeling oxygen
technique) OCMIP-2 Model 2.5/-
0.4 Intercomparison (ten ocean carbon models).
Not very well!
25
Will ocean uptake change in the future?

• Yes the models forecast that the sink will
  increase in the short term as increasing
  atmospheric CO2 forces more into the oceans.
• But, the buffering capacity of the ocean becomes
  less as CO2 increases, tending to decrease
  uptake.
• Also, if ocean overturning slows down, this would
  tend to decrease the uptake.
• Changes in ocean biology may also have an impact.

26
Source Manabe and Stouffer, Nature 364, 1993
27
North Atlantic pCO2

• Data 1994-1995
• Near-continuous data 2002-present
• Sharp decrease in ?pCO2 relative to mid 90s

28
Possible Marine biological effects on Carbon
uptake, next 100 years.
Process Effect on CO2 uptake

• Iron fertilisation -- deliberate or
• inadvertent
• NO3 fertilisation
• pH change mediates against calcite-
• precipitating organisms
• Reduction in THC offset by increased
• efficiency of nutrient utilisation
• Other unforeseen ecosystem changes

?
29
Marine carbon cycle summary

• The ocean CO2 sink is affected both by
  circulation and biology. Changes in either would
  affect how much CO2 is taken up by the ocean.
  Climate change may cause both.
• Because different methods agree roughly on the
  size of the global ocean sink, it has generally
  been assumed that we know it reasonably well.
• However, there is an increasing discrepancy
  between the most accurate methods. Our present
  understanding allows us to specify the sink only
  to 35.
• We cannot at present specify how it changes from
  year to year or decade-to-decade.
• Acccurate knowledge of the ocean sink would
  enable us (via atmospheric inverse modelling) to
  be much more specific about the terrestrial sinks
  useful for verification of Kyoto-type
  agreements.

30
The atmospheric imprint of anthropogenic carbon
31
Pre-industrial steady state.

• Fluxes into and out of the atmosphere were
  approximately at steady state before 1750.

• Small variations correlate with climate change
  (?) i.e little ice age 1600.

32
Atmospheric CO2 variations since 1000 AD
33
Fossil Fuel Emissions

• Well quantified from econometric data (Marland,
  Andres)

34
The budget for anthropogenic CO2
(1980s numbers in Pg C yr-1.)

• Well-known numbers (lt10 uncertainty)
• 1) Rate of fossil fuel release 5.4
• 2) Rate of build-up in the atmosphere 3.3
• Poorly known number (? 0.8 Pg uncertainty?)
• 3) uptake by ocean 1.9
• Very poorly known number (? 1.3 Pg C yr-1).
• 4) Rate of (mostly tropical) deforestation 1.7
• Extremely poorly known number calculated
• to balance budget (ie 1 4 2 - 3).
• 5) Uptake by extra-tropical vegetation 1.9

35
1980s budget of anthropogenic carbon dioxide.
Land uptake? (1.9 by difference)
Deforestation 1.7 Pg C yr-1?
Fossil fuel release 5.4 Pg C yr-1
Ocean uptake 1.9 Pg C yr-1
36
The Mauna Loa atmospheric record.
Accurate measurements of CO2 mixing ratio in
dried air have been made by C. Dave Keeling since
1958 at Mauna Loa observatory, Hawaii. From the
70s on, there have been an regular measurements
at an increasing number of stations around the
globe.  
C. Dave Keeling
Late 1990s measurement network  
37
The Mauna Loa atmospheric record.

• Overall increase in atmospheric CO2 of4 per
  year.

• Inter-annual and inter-decadal changes in the
  rate of rise not due to changes in fossil fuel
  emissions -- indicate changes in the natural
  sinks.

• An increasing amplitude of the northern
  hemisphere seasonal cycle correlating with
  increased global temperatures.

• Increasing length of the growing season.

38
Variation in the growth rate of atmospheric CO2,
1957-1999

• Rate of growth is highly variable not due to
  change in fossil fuel source.
• Variation correlates with Southern oscillation
  El Ninos.
• Indicates the Natural sinks for atmospheric CO2
  are highly variable.
• Though the land sink dominates variability, ocean
  is also important

39
The Mauna Loa atmospheric recordcontd.
Accurate measurements of CO2 mixing ratio in
dried air have been made by C. D. Keeling since
1958 at the Mauna Loa Laboratory in Hawaii, and
more recently at many other stations around the
world. The Mauna Loa record shows

• Overall increase in atmospheric CO2 of4 per
  year.

• Inter-annual and inter-decadal changes in the
  rate of rise not due to changes in fossil fuel
  emissions -- indicate changes in the natural
  sinks.

• An increasing amplitude of the northern
  hemisphere seasonal cycle correlating with
  increased global temperatures.

• Increasing length of the growing season.

40
Keeling, C.D., et al., Nature, 382, 146-149, 1996
41
Distribution of CO2 in the atmosphere

• Seasonality is most pronounced at high latitudes
  Northern Hemisphere. Southern Hemisphere
  seasonality is small.
• The seasonality is mostly due to the land biota
  almost all in the N. Hemisphere.
• The marine biological signal is buffered by
  carbonate chemistry and its seasonality is
  smoothed out not apparent in the atmospheric
  signal.

42
Calculation of sinks by inversion

• Principle Models of global atmospheric transport
  are used to deduce where the net source/sinks
  must be, in order to give rise to the observed
  (small) variations in atmospheric CO2
  concentrations.
• If the locations of the (anthropogenic) sources
  are known, the (natural) sinks can be specified.
• Good for inter-hemispheric distributions.
• Less good for latitudinal distributions.
• Poor for longitudinal distributions.

43
Tans, Fung and Takahashi
Observational constraints on the global
atmospheric CO2 budget, Sciecne 247, 1431 (1990).

• Combined constraints from observed
  interhemispheric gradient with ocean surface pCO2
  data.
• N. Hemishere ocean data suggested N.H ocean
  uptake lt 0.6 Pg yr-1.
• They deduced
• Global net ocean sink lt 1 Pg C yr-1
• Large N. Hemisphere mid-latitidue terrestrial
  sink (2-3 Pg C yr-1)
• Subsequently it has been found that their ocean
  sink was too small, land sink too large, but the
  existence of a substantial NH land sink is now
  established.

44
Tans et al Fig 5 Observed mean annual CO2
concentrations (circles and solid curve) as a
function of sine of latitude (-1 is S. Pole).
These are compared with calculations from a model
(squares and dashed curve), and expressed as
deviations from a mean CO2 concentration.
45
Present distribution of Land sources and sinks
N. hemisphere
Tropics
S. hemisphere

• Firm conclusions
• A substantial sink in the Northern Hemisphere
  mid-latitudes.
• Unknown distribution among the continents
• The tropical land areas are thought to be nearly
  neutral.
• All sinks are variable from year to year and
  decade to decade.

46
Box inverse model
Quantity Symbol Value PgC a-1
Fossil fuel flux FF 5.4
Accumulation in the atmosphere AA 3.4
Interhemispheric flux IF 1.9
Northern hemisphere ocean sink NO
Southern hemisphere equatorial ocean sink SO
Northern hemisphere land sink NL
Equatorial Southern land sink EL

• We take the interhemispheric gradient to be
  g 2 x 10-6 v/v
• The residence time wrt interhemispheric exchange
  is t 1 yr
• The Mass of the atmosphere M 1.6 x 1020 mol
• The interhemispheric flux is then
• 1.9 PgC

47
Box model calculations for the period 1980-1989
. (1) Total Mass balance
NOSONL EL FF-AA 2.0 (2)
N. hemisphere mass balance
0.9FF-NO-NL-IF 0.45AA 1.5 (3) N.
hemisphere ocean sink by observation NO
0.6?0.15 (4) Total ocean sink by model and
observation NOSO 1.4?0.5 This is a
system four equations in four unknowns
Calculation From (3) and (4) SO0.8 Sub
values for FF, NO, IF in (2) NL0.9FF-NO-IF-0.45
AA 0.9 Sub (4) into (1) NL EL 0.6
Hence EL -0.3 These calculations imply a
modest NH land sink in mid latitudes, and a small
net source in the land tropics (could be lots of
deforestation lots of re-growth). Note the
sensitivity of the calculations to errors. This
arises because the sinks are calculated as
comparatively small differences between large
numbers. In the 1980s, the total natural sink
(fossil fuel input - accumulation in atmosphere)
was on average 2.0. In the early 1990s, (period
1991-1994) natural sinks were nearly double this.
Today they are in the range 3 4 Pg C yr-1
48
Possible causes of the NH mid-latitiude sink

• Land use Change
• Anthropogenic fertilization, chiefly nitrogen
  deposition
• CO2 fertilization

49
Land-Use change

• REVERSE PIONEER REGROWTH OF FOREST
• In the last century, large areas of forest near
  population centres in N. America were cleared for
  crops.
• With the coming of the railways, the centres of
  crop production moved to the mid-western
  prairies. Farmland was abandoned and new-growth
  forest re-established.
• The process is continuing today.
• Similar, less dramatic trend in Europe and
  Russia.
• FOREST CONSERVATION
• Suppression of fire
• Suppression of insect infestation
• INCREASED ORGANIC SEDIMENTATION IN RESERVOIRS?

50
Land use change and the US carbon
budgetestimates from carbon accounting

• Houghton RA, Hackler JL, Lawrence KT
• The US carbon budget Contributions from land-use
  change
• SCIENCE 285 (5427) 574-578 JUL 23 1999

51
Sources of anthropogenic nitrogen

• Agricultural fertilizer
• Animal husbandry
• Runoff from farms
• Ammonia emissions
• NOy emissions from transport, other fossil fuels

52
Current deposition of atmospheric NOy (mmol N m-2
yr-1)
53
Effect of fertilization on tree growth
Cross-section of trunk of Picea abies from the
fertilised and irrigated (IL) treatment at the
Flakaliden study site -- Boreal forest, Northern
Sweden.
54
CO2 Fertilization effect.

• CO2 is a limiting factor on growth of plants.
  Higher CO2 may therefore stimulate net growth.
  CO2 fertilization is usually quantified by the
  "beta factor"

?
0.3 0.2 0.1 0
where ? is usually in the range 0-0.3 P,P0 are
the carbon assimilation rates at CO2
concentrations C,C0
55
Free-air CO2 Enrichment (FACE) experiments

• Designed to enrich the CO2 in air over a circle
  of vegetation, with minimal other disturbance.
• A ring of towers able to release CO2, sensors to
  detect wind speed and direction and measure CO2
  concentration.
• Continuous rapid monitoring of the CO2
  concentrations. Control system to decide which
  towers to release from and adjust release rates
  to keep concentration constant.

56
Uncertainties about CO2 Fertilization

• Easily measurable in many plants in greenhouse
  situations, but it is difficult to extrapolate
  this to the natural world. Questions include
• How big is the effect in natural ecosystems?
• How is it modified by other limiting nutrient
  availabilities?
• Does it result in continuous storage of carbon in
  plants and soils, or is a new equilibrium state
  rapidly reached?

57
Sink saturation?

• Assume that the sink is mostly due to CO2
  fertilization.
• Rising CO2 has an immediate effect on
  photosynthesis
• Leading to net ecosystem uptake of CO2.
• Rising CO2 has a delayed effect on global
  temperatures.
• Rising temperatures will enhance respiration in
  the future
• Leading to net ecosystem release of CO2
• Therefore presently observed uptake of CO2 may be
  a transitory phenomenon only, and the sink will
  saturate.
• The sink may be even more transitory if it is due
  in whole or in part to land use change, or
  nitrogen fertilization.

58
Courtesy John Grace, U. Edinburgh
59
Sink saturation?

• FACE experiments suggest uptake of CO2 due to CO2
  fertilization is itself transitory.
• But soil warming experiments suggest that the
  temperature effect on soil respiration may also
  be transient.

60
Carbon cyclechange of carbon in vegetation and
soils according to the Hadley Centre coupled
carbon-climate model.
61
Precision atmospheric oxygen measurements
Locations of precision O2 measurements

• Since 1990, direct measurements of oxygen
  concentrations, at ppm accuracy, have been made
  at certain sites throughout the world, by R. F.
  Keeling and others.

62
.Precision atmospheric oxygen measurements

• The concentrations are affected by
• fossil fuel burning
• net land vegetation net uptake
• Seasonal uptake/release of oxygen from both the
  land and the ocean biota -- unlike the case of
  CO2 which is little affected by ocean seasonal
  cycle, because of the long air/sea exchange time
  for CO2.

63
Oxygen and CO2 Comparison

• O2 decrease year-on year of the same order as the
  CO2 increase.
• Seasonal cycles of O2 in antiphase with those of
  CO2.
• The Southern Hemisphere O2 seasonal signal is
  much larger than is the case for CO2.

64
Deductions from Oxygen

• 1)    Net land and ocean sinks of carbon
• The molar ratio of oxygen utilisation relative to
  carbon dioxide release during the following three
  processes are all known
• a) fossil fuel burning, Rff (DO2/DCO2)-1.3
• b) photosynthesis/ respiration,
  Rpr(DO2/DCO2)-1.1
• c) ocean uptake of CO2 (DO2/DCO2)0
•  
• They can be plotted on a vector diagram of mean
  annual O2 change versus CO2 change. From a
  knowledge of how much fossil fuel has been
  burned, the size of the net ocean and land sinks
  can be determined.

65
Complications with the O2 signal.. 

• Ocean release The oceans are not in general
  neutral w.r.t atmospheric oxygen.
• As the ocean warms, O2 becomes less soluble and
  some dissolved oxygen outgasses from ocean to
  atmosphere.
• Seasonally mixed ocean regions such as the N.
  Atlantic tend to be oversaturated with O2 in
  summer and undersaturated in winter, when deep
  mixing brings up old water. If mixing patterns
  change this can mean a net source or sink.
• From these causes, there is evidence that the
  oceans have outgassed O2 in recent decades.
• Uncertainty in CO ratios
• Different fossil fuels (e.g. coal, gas) have
  markedly different ratios of carbon released to
  oxygen used.
• Different vegetation types may have different CO
  ratios ratios may vary between soil and
  vegetation, and may change seasonally even for
  the same vegetation type.

66
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67
Conclusions

• The carbon cycle was closely in steady state
  prior to the industrial revolution. It is now
  substantially perturbed.
• Natural CO2 sinks both on land and in the ocean
  are very important for slowing the rate of global
  warming due to greenhouse gases.
• These sinks change substantially from year to
  year and decade to decade. Changes are
  synchronous with climate oscillations such as El
  Niño/ La Niña cycles.
• The cause of the land sink for anthropogenic
  CO2is poorly understood. It may turn into a
  source because of global warming, in the next 100
  years.
• The ocean CO2 sink is affected both by
  circulation and biology. Changes in either would
  affect how much CO2 is taken up by the ocean.
  Climate change may cause both.

68
Conclusions

• We know 3 or 4 possible reasons for the global
  vegetation sink, but presently we cannot be sure
  which of these are most important.
• We cannot be sure how long the sink will
  continue, and whether it will increase or
  decrease. Many lines of evidence point to a
  decrease.

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70
Questions

• Should land sequestration of carbon be considered
  as a serious option for climate change
  mitigation, given
• our poor understanding of current land sinks
• their possibly transitory nature
• their vulnerability to climate change
• The precautionary principle if near-catastrophic
  outcomes of present practices cannot be ruled
  out, should we be putting maximum effort into
  emissions reductions?

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Distribution of CO2 in the atmosphere

• The space and time-averaged Northern hemisphere
  concentration is about 2ppm higher than that in
  the Southern hemisphere. This is because nearly
  all fossil fuel emissions occur in the North.
• Combined with atmospheric transport model, we can
  estimate the latidudinal distribution of natural
  sinks.

Two views of global CO2 versus time and latitude
between 1986 and 1993
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Box inverse model
10ºN
SH equatorial region
NH
Accumulation 055AA
Accumulation 0.45AA
Interhemispheric flux IF
Fossil fuel (90) 0.9FF
Fossil fuel (10) 0.1FF
---Unknown sinks ---- NO NL SO EL
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Pre-industrial steady state.

• Fluxes into and out of the atmosphere were
  approximately at steady state before 1750.

• Small variations correlate with climate change
  (?) i.e little ice age 1600.

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Atmospheric CO2 variations since 1000 AD
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1980s budget of anthropogenic carbon dioxide.
Land uptake? (1.9 by difference)
Deforestation 1.7 Pg yr-1?
Fossil fuel release 5.4 Pg yr-1
Ocean uptake 1.9 Pg yr-1?
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The Mauna Loa atmospheric record.

• Overall increase in atmospheric CO2 of4 per
  year.

• Inter-annual and inter-decadal changes in the
  rate of rise not due to changes in fossil fuel
  emissions -- indicate changes in the natural
  sinks.

• An increasing amplitude of the northern
  hemisphere seasonal cycle correlating with
  increased global temperatures.

• Increasing length of the growing season.

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Riverine flux
The pre-industrial steady state cycle to balance
the flux of carbon coming down rivers, there must
have been a CO2 flux of the order of 0.5 Pg C
yr-1 from ocean to atmosphere and from atmosphere
to land. Volcanic activity and sedimentation
fluxes provide smaller net inputs and outputs to
the system.
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Calculation of sinks by inversion

• Principle Models of global atmospheric transport
  are used to deduce where the net source/sinks
  must be, in order to give rise to the observed
  (small) variations in atmospheric CO2
  concentrations.
• If the locations of the (anthropogenic) sources
  are known, the (natural) sinks can be specified.
• Good for inter-hemispheric distributions.
• Less good for latitudinal distributions.
• Poor for longitudinal distributions.

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Tans et al Fig 5 Observed mean annual CO2
concentrations (circles and solid curve) as a
function of sine of latitude (-1 is S. Pole).
These are compared with calculations from a model
(squares and dashed curve), and expressed as
deviations from a mean CO2 concentration.
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.Precision atmospheric oxygen measurements

• The concentrations are affected by
• fossil fuel burning
• net land vegetation net uptake
• Seasonal uptake/release of oxygen from both the
  land and the ocean biota -- unlike the case of
  CO2 which is little affected by ocean seasonal
  cycle, because of the long air/sea exchange time
  for CO2.

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Deductions from Oxygen

• 1)    Net land and ocean sinks of carbon
• The molar ratio of oxygen utilisation relative to
  carbon dioxide release during the following three
  processes are all known
• a) fossil fuel burning, Rff (DO2/DCO2)-1.3
• b) photosynthesis/ respiration,
  Rpr(DO2/DCO2)-1.1
• c) ocean uptake of CO2 (DO2/DCO2)0
•  
• They can be plotted on a vector diagram of mean
  annual O2 change versus CO2 change. From a
  knowledge of how much fossil fuel has been
  burned, the size of the net ocean and land sinks
  can be determined.

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Deductions from O2 continued. 

• 2) Gas exchange rate and primary productivity of
  the oceans
• To examine the contribution of the oceans to the
  oxygen signal,
• 1)      The year-to-year decrease is removed
  leaving only a seasonal signal.
• 2)      The CO2 seasonal signal is multiplied by
  the ratio Rpr of O2/CO2 changes due to land
  vegetation, and subtracted from the O2 signal
  this leaves only the signal due to seasonal
  release/uptake by the oceans.
• 3)      The magnitude of this signal at different
  stations is a function of the amount of oxygen
  released by the marine biota, and the rate at
  which it escapes into the atmosphere. Using
  additional information on the sea-surface
  concentrations of oxygen, estimates of both of
  these can be made.

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Precision atmospheric oxygen measurements

• Since 1990, direct measurements of oxygen
  concentrations, at ppm accuracy, have been made
  at certain sites throughout the world, by R. F.
  Keeling and others.

Locations of precision O2 measurements
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Oxygen and CO2 Comparison

• O2 decrease year-on year of the same order as the
  CO2 increase.
• Seasonal cycles of O2 in antiphase with those of
  CO2.
• The Southern Hemisphere O2 seasonal signal is
  much larger than is the case for CO2.