Transcript The Global Carbon Cycle Overview The atmospheric

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
Reading
Overviews and reviews
The IPCC Third assessment report.
http://www.grida.no/climate/ipcc_tar/
Chapter 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.uea.ac.uk/ajw/Geochemical_cycling/sarmiento_pt_02.pdf
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
Terrestrial 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_cycling/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/ajw/Geochemical_cycling/battle_2000.pdf
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_cycling/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.ac.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.
7 0 0
Atmospheric CO2:
Past, present and near future
Proje cted
(2 100)
6 5 0
6 0 0
5 5 0
5 0 0
Vo stok Re cord
Contem porary Re cord
I PCC I S92a Scen ario
4 5 0
4 0 0
Current
(2 001)
3 5 0
3 0 0
2 5 0
2 0 0
1 5 0
4 5 0
3 0 0
1 5 0
Age of Entrapped Air (kyr BP)
0
The global carbon cycle
(Source, Sarmiento and Gruber, 2002)
The global carbon cycle
•Most of the “labile” carbon on Earth is in the deep sea.
•The gross atmosphere-ocean and atmosphere-vegetation
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.
The (almost) unperturbed marine carbon cycle:
Global mean air-sea flux, calculated from pCO2 measurements
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+ + CO32Typically,
90% of the carbon exists as bicarbonate,
9% as carbonate,
1% as dissolved CO2 and undissociated H2CO3
(usually lumped together).
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.
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.
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).
The overturning thermohaline circulation
Water cools and
sinks
Water warms and
upwells?
•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.
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.
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.
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.
Carbonate
Soft
tissue
•
•
•
•
•
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
Ocean Carbon: The Biological (soft tissue) Pump
Nitrate
1 (or phosphate)
2
3
Concentration
Total
CO2
Surface pCO2, nutrient and surface
temperature in the North Atlantic
360
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
16
300
8
6
14
Nitrate (M)
pCO2 ( atm)
320
18
SST (°C)
340
4
280
12
260
2
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 redissolve 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
Coccolithophores -- calcite precipitating plankton
Photo: courtesy D. Purdie: See the Ehux home
page: www.soc.soton.ac.uk/SUDO/tt/eh/index.html
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.)
“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.
Bomb radiocarbon x 1020 atoms
Tropospheric bomb radiocarbon
300
200
100
1950
1960
1970
1980
1990
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.
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).
How well is the global ocean sink known?
Estimates of the global ocean sink 1990-1999
Reference
Sink (Pg C yr-1)
IPCC (2001)
Estimate
(Keeling oxygen
technique)
1.7+/- 0.5
OCMIP-2 Model
Intercomparison
(ten ocean carbon
models).
2.5+/- 0.4
Not very well!
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….
Source: Manabe and Stouffer, Nature 364, 1993
North Atlantic pCO2
1994-1995
2002-2005
• Data 1994-1995
• Near-continuous
data 2002-present
• Sharp decrease in
ΔpCO2 relative to
mid 90s
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
?
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.
The atmospheric imprint of anthropogenic
carbon
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.
Atmospheric CO2 variations since 1000 AD
Fossil Fuel Emissions
• Well quantified from econometric data (Marland, Andres)
The budget for anthropogenic CO2
(1980s: numbers in Pg C yr-1.)
•Well-known numbers (<10% 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
1980s budget of anthropogenic carbon dioxide.
Land uptake?
(1.9 by difference)
Accumulation in atmosphere
3.3 Pg C yr-1
Fossil fuel release
5.4 Pg C yr-1
Ocean uptake
1.9 Pg C yr-1
Deforestation
1.7 Pg C yr-1?
The Mauna Loa atmospheric record.
Accurate measurements of CO 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.
2
C. Dave Keeling
Late 1990s measurement
network
The Mauna Loa atmospheric record.
•Overall increase in
atmospheric CO
of~4% per year.
2
•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.
360
355
350
345
Mauna Loa, Hawaii
Monthly mean CO2
Concentrations
340
335
330
325
320
315
310
1955
1960
1965
1970
1975
y ear
1980
1985
1990
1995
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
8
CO2 GROWTH RATE
Global
(NOAA)
6
Fossil Fuel
Cape Grim
(CSIRO)
4
2
Mauna Loa
(Scripps/NOAA)
Pinatubo
30
La Nina
0
-30
El Nino
1955
1960
1965
1970
1975
1980
1985
1990
1995
2000
(R J Francey, pers. Com)
The Mauna Loa
atmospheric record…contd.
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 CO of~4% per year.
2
•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.
Keeling, C.D., et al., Nature, 382, 146-149, 1996
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.
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.
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 <= 0.6 Pg yr-1.
They deduced:
– Global net ocean sink <= 1 Pg C yr
– Large N. Hemisphere mid-latitidue terrestrial sink (2-3
-1
• Subsequently it has been found that their ocean
Pg C yr-1)
sink was too small, land sink too large, but the
existence of a substantial NH land sink is now
established.
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.
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.
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 τ = 1 yr
The Mass of the atmosphere M = 1.6 x 1020
mol
The interhemispheric flux is then
= 1.9 PgC
IF  gM
2
Box model calculations for the period 1980-1989
.
(1) Total Mass balance:
(2) N. hemisphere mass balance:
(3) N. hemisphere ocean sink by observation:
(4) Total ocean sink by model and observation:
NO+SO+NL +EL = FF-AA = 2.0
0.9FF-NO-NL-IF = 0.45AA= 1.5
NO= 0.60.15
NO+SO = 1.40.5
This is a system four equations in four unknowns:
Calculation:
From (3) and (4):
SO=0.8
Sub values for FF, NO, IF in (2):
NL=0.9FF-NO-IF-0.45AA = 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
Possible causes of the NH mid-latitiude sink
• Land use Change
• Anthropogenic fertilization, chiefly
nitrogen deposition
• CO2 fertilization
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 newgrowth 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?
Land use change and the US carbon budget:
estimates 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
Sources of anthropogenic nitrogen
• Agricultural fertilizer
• Animal husbandry:
– Runoff from farms
– Ammonia emissions
• NOy emissions from transport, other
fossil fuels
Current deposition of atmospheric NOy (mmol
N m-2 yr-1)
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.
CO2 Fertilization effect.
P  P0 1  Ln(C / C0 )
where  is usually in the range 0-0.3
P,P0 are the carbon assimilation rates
at CO2 concentrations C,C0
Effect of beta-factor
1.3

0.3
1.2
0.2
1.1
0.1
P/P0
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
1
0.9
0.8
0.5
1
1.5
C / C0
2
2.5
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.
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?
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.
Courtesy John Grace, U. Edinburgh
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.
Carbon cycle:change of carbon in vegetation and soils
according to the Hadley Centre coupled carbonclimate model.
Precision atmospheric oxygen measurements
Alert
Cold Bay
Niwot Ridge
Mauna Loa
La Jolla
American
Samoa
Cape Grim
South Pole
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.
….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.
Oxyg en
-10.1
Alaska
0
-20
-40
•
•
O2 decrease yearon 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.
-34.3
-3.5
Tasmania
0
-20
-40
-27.0
Carbon dioxide
Alaska
370
CO2(ppm v/v)
•
O2(ppm v/v -- relative to standard)
Oxygen and CO2
Comparison
350
364.2
356.9
352.8
370
Tasmania
350
91
9
1
.0
92
19
.0
93
9
1
.0
0
.0
.0 97.0
.0
5.
8
6
4
9
9
9 19
9
19
19
19
19
362.9
Deductions from Oxygen
1)
O2 mole fraction change (ppm)
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.
S
L
initial
0
l
ue l y
il f on
ss ng
Fo urni
b
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
-2
-4
final
-6
La
nd
up
ta
k
e
Ocean uptake
-8
0
2
4
6
CO2 mole fraction change (ppm)
8
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 C:O ratios:
– Different fossil fuels (e.g. coal, gas) have markedly different
ratios of carbon released to oxygen used.
– Different vegetation types may have different C:O ratios:
ratios may vary between soil and vegetation, and may change
seasonally even for the same vegetation type.
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.
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.
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?
Distribution of CO2
in the atmosphere
•The space and timeaveraged 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
Box inverse model
10ºN
SH + equatorial region
NH
Accumulation
055AA
Accumulation
0.45AA
Interhemispheric flux
IF
Fossil fuel (90%)
0.9FF
---Unknown sinks ---NO
NL
SO
EL
Fossil fuel (10%)
0.1FF
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.
Atmospheric CO2 variations since 1000 AD
1980s budget of anthropogenic carbon dioxide.
Land uptake?
(1.9 by difference)
Accumulation in atmosphere
3.3 Pg yr-1
Fossil fuel release
5.4 Pg yr-1
Ocean uptake
1.9 Pg yr-1?
Deforestation
1.7 Pg yr-1?
The Mauna Loa atmospheric record.
•Overall increase in
atmospheric CO
of~4% per year.
2
•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.
360
355
350
345
Mauna Loa, Hawaii
Monthly mean CO2
Concentrations
340
335
330
325
320
315
310
1955
1960
1965
1970
1975
y ear
1980
1985
1990
1995
“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.
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.
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.
….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.
Deductions from Oxygen
1)
O2 mole fraction change (ppm)
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.
L
S
initial
0
l
ue l y
il f on
ss ing
Fo urn
b
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
-2
-4
final
-6
La
nd
up
ta
ke
Ocean upt ake
-8
0
2
4
6
CO2 mole fraction change (ppm)
8
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.
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.
Alert
Cold Bay
Niwot Ridge
Mauna Loa
La Jolla
American
Samoa
Cape Grim
South Pole
Locations of precision O2 measurements
Oxygen and CO2 Comparison
Oxyg en
0
Alaska
- 20
- 40
- 34.3
- 3.5
0
Tasmania
- 20
- 40
O2(ppm v/v -- relative to standard)
- 10.1
- 27.0
Car bon dioxide
Alaska
CO2(ppm v/v)
370
350
364.2
356.9
352.8
370
362.9
Tasmania
350
91
19
•
•
•
.0
92
19
.0
93
19
.0
94
19
.0
95
19
.0
96
19
.0
97
19
.0
98
19
.0
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.