This post is taken from a presentation which I gave at a local geologic conference in 2104, with minor modifications.
Abstract
The Keeling Curve is a remarkable series of atmospheric CO2 measurements
taken at Mauna Loa, Hawaii, from 1960 to the present. The curve shows seasonal cycles and a steady
rise in the concentration of CO2,
beginning about 315 ppm and currently approaching 400 ppm. Long-term CO2 records are also available from a number of
other observatories, located from the Arctic Ocean to the South Pole. Integration of the global dataset with
carbon emissions data provides additional insights about the world’s carbon
cycle.
Atmospheric CO2
concentrations and CO2
carbon isotopes show seasonal and long term trends which vary by latitude. The seasonal cycle is strongest in the
Northern Hemisphere, and the Northern Hemisphere leads the Southern Hemisphere
in terms of rising CO2. People and plants in the Northern Hemisphere
cause changes in atmospheric CO2
which propagate from the Northern Hemisphere to the Southern Hemisphere. The dispersion of CO2 from human sources can be seen as a progression through the global data. This progressive change is seen in bulk concentration
of CO2, in carbon
isotopes of CO2, and the amplitude of the seasonal cycle. Long-term changes in global CO2 are consistent
with known volumes of fossil fuel emissions.
A simple model can be constructed based solely on human carbon emissions and agricultural biomass, that matches the observed seasonal cycle and long-term trends in bulk CO2,. This model shows that it is reasonable to
conclude that human activities are influencing carbon dioxide in the atmosphere. The Energy Information Agency (EIA) predicts
rising carbon emissions for the foreseeable future, from about 35 gigatonnes CO2 annually to
nearly 50 gigatonnes CO2
by the year 2040, in the base-case forecast.
Carbon dioxide from fossil fuels and deforestation carries a
distinctive isotopic signature, which marks the movement of man-made CO2 through the
atmosphere and carbon reservoirs (soils, biomass, and oceans). This movement of carbon, as seen in both
carbon isotope data and bulk CO2
data, reveals complexity in the carbon cycle.
Discrepancies between the datasets imply the active exchange of carbon
between the atmosphere and carbon reservoirs.
More than 85% of anthropogenic CO2 emissions, as tagged by carbon isotopes, do not
remain in the atmosphere, but are absorbed by carbon reservoirs. However, some of the anthropogenic carbon in
the atmosphere is exchanged for natural carbon from carbon reservoirs, so that
atmospheric CO2
concentration is maintained at a level equivalent to about 44% of cumulative
annual CO2
emissions over the long term. The size
of carbon reservoirs is estimated at more than 7 times the volume of carbon
present in the atmosphere, based on a dilution calculation of anthropogenic
carbon isotopes in the atmosphere. The
role of the ocean in exchanging carbon with the atmosphere is illustrated by
the correlation of atmospheric carbon isotope data with the Oceanic Niño Index
(ONI), which is a measure of the El Niño/La Niña climate cycle based on
sea-surface temperatures.
Understanding the patterns of atmospheric CO2 may provide a
tool for recognizing and measuring changes in global climate. Additional monitoring of carbon reservoirs,
particularly of the world’s oceans, will be necessary to develop a
comprehensive model of the earth’s carbon cycle.
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A Few
Words about CO2 Carbon Isotopes
There are two stable isotopes of carbon, C13 and
C12. C12 is the more abundant isotope;
the natural ratio of C12 to C13 is about 99 to 1. The standard measure of carbon isotopes
compares the C12/C13 isotope ratio of the sample in question to the C13/C12
ratio of a standard limestone, according to the expression:
d C13/C12 = ((C13/C12 sample / C13/C12 standard) – 1)*1000.
This expression, commonly termed “del 13”, amplifies
small but meaningful differences in the isotopes, which are diagnostic of
certain processes and occurrences of carbon.
The standard is a uniform Cretaceous limestone with a d 13 value defined as zero. Positive values indicate a heavier
composition, i.e., a greater concentration of C13 than the standard. Negative values indicate a lighter
composition, i.e., a smaller concentration of C13 than the standard.
Plants fractionate carbon, favoring the lighter
isotope C12. Anything derived from
plants, including oil, gas, and coal (and algae, animals and people) carries a
light (negative) d C13/C12 signature. Limestone carries a d C13/C12 ratio
near zero. The atmosphere, in 1977, had
a d C13/C12 ratio of about -7.5; it is
currently about -8.3, reflecting the influence of fossil fuels. Oceans have a slightly positive d C13/C12 ratio of dissolved inorganic carbon, although
Northern Hemisphere waters show a negative ratio due to the greater use of
fossil fuels in the Northern Hemisphere.
Fossil fuel CO2 emissions and CO2 emissions from
deforestation carry a very light d C13/C12,
in the range of -25 to -28. The
distinctive isotopic signature of CO2 from fossil fuels and
deforestation is useful in tracking the movement of carbon through the
atmosphere and oceans. Boden, Marland
and Andres (2013) published estimates of the annual CO2 released by
fossil fuels and the d C13/C12 ratio of those
emissions. Those estimates were used in
this work.
Part
1: The Global Record
Global CO2 is rising, and the isotopic composition of atmospheric
CO2 is becoming
lighter.
A network of observatories, mostly operated by Scripps
Oceanographic Institute, monitors global atmospheric CO2 concentrations
and CO2 carbon isotopes.
Bulk CO2 has been monitored since 1957, and CO2
carbon isotopes since 1977. In all
figures, data from the Northern Hemisphere is indicated in cool colors, and
data from the Southern Hemisphere is indicated with warm colors.
Global CO2 observations show a seasonal
cycle and a steady rise in the concentration of CO2. Today’s concentration of atmospheric CO2 is
about 25% higher than in 1957, and about 40% higher than in 1800.
The carbon isotope ratio (d
C13/C12) of atmospheric CO2 is becoming lighter, which is consistent
with the isotopic signature of fossil fuels mixing with the atmosphere.
The amplitude of the CO2 seasonal
cycle is largest in the high latitudes of the Northern Hemisphere, and
diminishes southward. The polarity of
the Northern Hemisphere cycle persists to about 30 degrees South Latitude. From that point southward, the polarity of
the cycle is reversed, but with low amplitude.
Part
2: Long Term Trends
The Northern Hemisphere leads the Southern Hemisphere
in rising CO2 values and falling CO2 carbon isotope values.
The Northern Hemisphere holds 2/3
of the world’s landmass, and nearly 90% of the world’s population, fossil-fuel
consumption, and agriculture. People and
plants in the Northern Hemisphere cause changes in atmospheric CO2 which
propagate from the Northern Hemisphere to the Southern Hemisphere.
The seasonal cycle in bulk CO2 was
removed by taking a one-year rolling average at each observatory. The Northern Hemisphere leads the Southern
Hemisphere in rising CO2. The concentration of CO2 is
highest in the Arctic, and is progressively lower by latitude to the South
Pole. The progression marks the
dispersion of fossil fuel emissions from the Northern Hemisphere to the
Southern Hemisphere.
The seasonal cycle in the CO2
carbon isotope ratio was removed by taking a one-year rolling average at each
observatory. The Northern Hemisphere
leads the Southern Hemisphere in terms of falling d
C13/C12 of CO2 (becoming isotopically lighter). The isotopic composition of CO2 is
lightest near the North Pole, and becomes progressively heavier to
Antarctica. The progression marks the
dispersion of fossil fuel emissions from the Northern Hemisphere to the
Southern Hemisphere.
A simple model was constructed to
investigate the plausibility of the idea that human activity causes changes in
atmospheric CO2. The model
begins at the global baseline CO2 concentration in 1970. Model inputs included 60% of global fossil
fuel emissions, allocated to Northern and Southern Hemispheres by GDP. Global agricultural biomass was scaled by
year to human population, and allocated to the Northern and Southern
Hemispheres by population. Volumes of
carbon were converted to CO2 concentration by hemisphere, and
defined the concentration at the poles.
Concentrations at intermediate latitudes were created by mixing
concentrations from each hemisphere, with weighting by latitude. The ease with which the model was created suggests
that human activity is plausibly responsible for much of the change in atmospheric
CO2.
Part 3: Seasonal Cycle
The Northern Hemisphere dominates the global CO2
seasonal cycle.
The Keeling Curve is characterized
by a strong seasonal cycle, dominated by the Northern Hemisphere. The concentration of atmospheric CO2
falls during the Northern Hemisphere growing season, when land plants remove
carbon from the air through photosynthesis.
The concentration of CO2 rises in the fall, winter and spring
as decay returns carbon to the atmosphere as CO2.
The CO2 carbon isotope
ratio d
C13/C12 shows a strong seasonal cycle as a mirror image of the cycle in bulk CO2
concentration. Land plants in the
Northern Hemisphere strongly fractionate carbon isotopes. Plants preferentially absorb C12 during the
growing season, raising the d C13/C12 ratio of the atmosphere. During decay, plants return C12 to the
atmosphere, and atmospheric d C13/C12 falls.
Amplitude of the seasonal cycle is
relatively small in the Southern Hemisphere, reflecting a smaller land mass and
sparse population. Seasonal cycles in
low latitudes of the Southern Hemisphere follow the polarity of the Northern
Hemisphere, but with a phase shift.
Seasonal cycles in high latitudes of the Southern Hemisphere carry the
opposite polarity to the Northern Hemisphere.
Amplitude of the seasonal cycle is
increasing over the past 40 years, particularly at high northern
latitudes. The increase in amplitude correlates
well to the increase in human population over the past 40 years. By inference, the increase in seasonal
amplitude also correlates to a proportional increase in human agriculture. The correlation implies that agriculture
accounts for about one-third of the amplitude of the seasonal CO2
cycle.
The polarity reversal of the seasonal cycle occurs at about
30 degrees South Latitude, near the southern boundary of the Hadley convection
cell. The atmosphere north of -30
degrees latitude contains air which is mixed with air from the Northern Hemisphere;
CO2 concentrations and carbon isotopes follow the seasonal cycle of
the Northern Hemisphere. The atmosphere
south of -30 degrees latitude carries the seasonal cycle of the Southern
Hemisphere. This finding suggests that
additional CO2 observatories between -30 degrees and -40 degrees
south latitude could monitor climate-change induced expansion of Hadley
circulation, by detecting air from the Northern Hemisphere, according to the
Northern Hemisphere seasonal CO2 cycle.
Part 4: CO2
Emissions
Long-term
changes in atmospheric CO2 are consistent with known volumes of human CO2 emissions.
The rate of human CO2
emissions is increasing. Anthropogenic CO2 emissions,
including deforestation, have grown from about 5 gigatonnes annually in 1900 to
about 38 gigatonnes in 2009. The
greatest part of that increase occurred in the last 50 years.
The average CO2 concentration of the Northern Hemisphere leads the
Southern Hemisphere by 2.5 to 4 ppm CO2. Net annual fossil
fuel emissions in the Northern Hemisphere, converted to CO2 concentration, neatly match the difference in CO2 concentration between the hemispheres. Deforestation, (which is more prevalent in
the Southern Hemisphere) was not included in the emissions numbers, which might
account for the growing discrepancy in recent years.
Despite international efforts to reduce carbon emissions,
global industrial CO2
emissions are rising sharply. According
to the EIA base economic forecast, fossil fuel CO2 emissions are
expected to rise 45% by the year 2040, from 33 gigatonnes to 48 gigatonnes per
year. Emissions including deforestation
bring the total in 2040 to 53 gigatonnes, assuming a constant rate of
deforestation from 2005 to 2040.
Global average CO2 is rising at a rate equal to about 44 percent of annual CO2 emissions, including deforestation. The forecast of future CO2 concentrations, based on expected emissions, calls for world average CO2 to exceed 450 ppm around the year 2036.
Part 5: The Carbonsphere
The
Carbonsphere consists of atmospheric carbon and all reservoirs (ocean, biomass,
and soils) freely exchanging carbon with the atmosphere.
The global mix
of fossil fuels has a d C13/C12 value of about -28. Deforestation is assumed to have a d C13/C12
value of about -25. These contrast
sharply with the atmospheric d C13/C12 value of about -8, and slightly positive
oceanic d C13/C12 values. The
distinctive isotopic signature of human carbon emissions allows us to track the
movement of carbon through the atmosphere, and to detect the exchange of carbon
with carbon reservoirs on the earth’s surface.
A 2-year lag is required for the concentration of bulk CO2 to equilibrate
from sources in the Northern Hemisphere to the Antarctic.
Differences in the behavior of bulk CO2 and CO2
carbon isotopes indicate the active role of carbon reservoirs in the ocean,
plants, and soil in exchanging carbon with the atmosphere.
Bulk carbon requires about a 2-year lag for CO2 concentration to
equilibrate from the Arctic to the Antarctic.
In contrast, CO2
carbon isotopes require an 8-year lag for equilibration.
The difference indicates that the specific molecules
released by fossil fuels are cycled through carbon reservoirs and replaced in
the atmosphere by other molecules from those reservoirs. The difference in equilibration lag of bulk
carbon and carbon isotopes indicates residency time in those reservoirs.
Atmospheric CO2 concentration rises at a rate equal to about 44% of
human CO2 emissions, including
deforestation. If all human carbon
emissions remained in the atmosphere, the concentration of atmospheric CO2
would be much higher.
Atmospheric CO2
d C13/C12 falls at a rate incorporating about 12% of human CO2 emissions.
If all human carbon emissions remained in the atmosphere,
the concentration of atmospheric CO2
would be much higher, and the d C13/C12 isotope ratio would be much lower. Calculations indicate that atmospheric CO2 rises at a rate equal to
about 44% of human CO2
emissions. In contrast, CO2 carbon isotopes
indicate that only 12% of human carbon emissions remain in the atmosphere. More than 85% of human carbon emissions, as
tagged by carbon isotopes do not remain in the atmosphere, but are cycled into
other carbon reservoirs. At the same time,
natural carbon, carrying a heavier isotopic signature, is exchanged from the
carbon reservoirs, maintaining a bulk CO2 concentration accumulating at a rate of 44% of
human CO2
emissions.
In the inverse
of the calculation above, the d C13/C12 isotope ratio of the atmosphere shows
the total volume of carbon reservoirs interacting with the atmosphere. The calculation determines the total
reservoir volume necessary to produce the observed dilution of d C13/C12 from
human emissions in the air. Carbonsphere
reservoirs are assumed to be in equilibrium with the atmosphere, which is
demonstrated by the relatively good fit to the solution for 30 years. The model assumes a balance of fractionation
between the carbonsphere reservoirs and the atmosphere. The solution calls for a 6000 gigatonne
carbonsphere in 1979 (including the atmosphere). This solution is about 40% larger than
estimates based on carbon inventory methods
.
Part 6: Finding Niño
The CO2
carbon isotope record can be correlated with the El Niño/La Niña
climate cycle, indicating large volumes of carbon exchange between the
atmosphere and the tropical Pacific Ocean.
Multi-annual cycles, or “waves” are present in the CO2 carbon isotope
data after removing the seasonal cycle.
Lower amplitude but correlative waves also exist in the bulk CO2 chart. Periods of rapidly falling d C13/C12
correspond to El Niño
climatic events, suggesting variability in the rate at which the tropical
Pacific Ocean takes up or releases C12 to the atmosphere. The amplitude of the waves interrupts and
sometimes reverses the secular trend, indicating that the volumes of carbon
exchanged sometimes exceed the volume of human carbon emissions.
A series of mathematical operations can reduce the global CO2 isotope record to
a single trace which indicates the rate at which atmospheric carbon is
exchanged with the Carbonsphere.
Assuming no fractionation in the exchange process, positive values
indicate a faster rate of absorption of C12 by the Carbonsphere. Negative values indicate a slower rate of C12
absorption by the Carbonsphere. With a
slight time lag, the trace can be correlated with the Oceanic Niño Index (ONI). The ONI is a measure of sea surface
temperatures published by NOAA, indicating the prevalence of El Niño or La Niño conditions.
The trace derived from the d C13/C12 isotope data indicates
variability in the rate of exchange of C12 between the atmosphere and carbon
reservoirs. That curve correlates well with the Oceanic Niño Index, a measure of the El Niño/El Niña climate cycle. La Niña conditions, which are
characterized by abnormally cool sea surface temperatures, correspond with
accelerated absorption of atmospheric C12 by the ocean. El Niño conditions, characterized by warm sea surface temperatures,
correspond to decreased absorption of atmospheric C12 by the ocean.
The quality and quantity of oceanic carbon data is weak in
comparison to atmospheric CO2
data. Oceanic carbon data is limited by
a lack of continuous readings or fixed observation sites, and is strongly
influenced by local biological activity.
Nevertheless, at the broadest scale, oceanic dissolved inorganic carbon
is isotopically lighter in the Northern Hemisphere, reflecting the influence of
fossil fuels in the Northern Hemisphere.
Improved, systematic data collection in the oceans and other carbon
reservoirs will be necessary to develop a comprehensive model of the earth’s
carbon cycle.
Note: Since the original poster was presented in May, 2014,
I have noticed that the low d C13/C12 values are located in the Atlantic Ocean,
and may be associated with the Greenland Current. It is possible that these values reflect a
natural process, such as a significant volume of glacial meltwater lowering the d
C13/C12 value of the seawater.
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