The Keeling Curve and Global CO2

This post is taken from a presentation which I gave at a local geologic conference in 2104, with minor modifications.

Major findings of the CO₂ study:

• Atmospheric CO₂ is rising globally as a result of human activities, principally the burning of fossil fuels. 
• Atmospheric CO₂ concentration is now about 40% higher than pre-industrial CO2 concentration.
• CO₂ emissions from fossil fuels mostly originate in the Northern Hemisphere.  A global system of monitoring stations shows the dispersion of these emissions from the Northern Hemisphere to the Southern Hemisphere.
• Atmospheric CO₂ shows a seasonal cycle that is dominated by plant growth in the Northern Hemisphere.
• The amplitude of the seasonal cycle is increasing due to human agriculture.  Agriculture accounts for about 1/3 of the seasonal cycle.
• The Southern Hemisphere has a seasonal cycle that is the opposite polarity of the Northern Hemisphere, but is very weak due to a smaller land mass and less agriculture.
• Atmospheric CO₂ concentrations and carbon isotope ratios are changing much slower than expected if all human carbon emissions remained in the atmosphere.  About half of human carbon emissions are being absorbed by carbon reservoirs (oceans, soils and biomass).  Carbon isotopes show that large volumes of atmospheric carbon are freely exchanged with carbon in carbon reservoirs.  
• The size of the carbon reservoirs exchanging carbon with the atmosphere can be estimated through the dilution of human-derived carbon isotopes in the atmosphere.  The calculation indicates that these carbon reservoirs contain about 7 times the quantity of carbon in the atmosphere.  This solution is about 40% larger than estimates using other methods.
• Human carbon emissions will continue.  Forecasts indicate that atmospheric CO₂ will reach 450 ppm by the year 2036.
• After filtering the seasonal cycle from the carbon isotope data, a multi-year cycle remains.  The multi-year cycle can be correlated with the El Nino climate cycle.  The El Nino cycle changes the rate at which atmospheric carbon is absorbed by the Pacific Ocean, and changes isotopic composition of the atmosphere.

The Keeling Curve and Global CO2

S. D. Robbins                       May 15, 2014

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.

References:

Andres, R.J., T.A. Boden, and G. Marland. 2012.  Monthly

Fossil-Fuel CO2 Emissions: Mass of Emissions Gridded by One Degree

Latitude by One Degree Longitude.  Carbon Dioxide Information Analysis

Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak

Ridge, Tenn., U.S.A.  doi: 10.3334/CDIAC/ffe.MonthlyIsomass.2012

ftp://cdiac.ornl.gov/pub/fossil_fuel_CO2_emissions_gridded_monthly_del13C_v2012/

Andres, R.J., T.A. Boden, and G. Marland. 2009.  Monthly

Fossil-Fuel CO2 Emissions: Mass of Emissions Gridded by One Degree

Latitude by One Degree Longitude.  Carbon Dioxide Information Analysis

Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak

Ridge, Tenn., U.S.A.  doi: 10.3334/CDIAC/ffe.MonthlyIsomass.2009

ftp://cdiac.ornl.gov/pub/fossil_fuel_CO2_emissions_gridded_monthly_del13C_v2009

Andres, R.J., T.A. Boden, and G. Marland. 2012.  Annual Fossil-Fuel CO2 Emissions: Global Stable Carbon Isotopic Signature.  Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.

doi: 10.3334/CDIAC/ffe.db1013.2012

http://cdiac.ornl.gov/ndps/db1013_v2012.html

Boden, T.A., Andres, R.J., and G. Marland 2013. Global CO2 Emissions from Fossil-Fuel Burning, Cement Manufacture, and Gas Flaring: 1751-2010. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.   

http://cdiac.ornl.gov/ftp/ndp030/global.1751_2010.ems

Boden, T.A., G. Marland, and R.J. Andres. 2013. Global, Regional, and National Fossil-Fuel CO2 Emissions. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. doi 10.3334/CDIAC/00001_V2013

http://cdiac.ornl.gov/trends/emis/overview_2010.html

Ciattaglia, L., C. Rafanelli, H. Rodriguez, and J. Araujo. 2010. Atmospheric CO2 record from continuous measurements at Jubany Station, Antarctica, in Trends Online: A Compendium of Data on Global Change, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.

http://cdiac.esd.ornl.gov/trends/co2/jubany.html

Climate Prediction Center Internet Team, 2013, ONI Index, NOAA/National Weather Service, NOAA Center for Weather and Climate Prediction , Climate Prediction Center, 5830 University Research Court, College Park, Maryland 20740.

http://www.cpc.ncep.noaa.gov/data/indices/oni.ascii.txt

http://www.nwfsc.noaa.gov/research/divisions/fe/estuarine/oeip/cb-mei.cfm

Colombo, T., and R. Santaguida. 1998. Atmospheric CO2 record from in situ measurements at Mt. Cimone. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.

http://cdiac.esd.ornl.gov/trends/co2/mtcim.html

Conti, J., et al, 2013.  International Energy Outlook, U. S. Energy Information Administration, Office of Energy Analysis, U.S. Department of Energy, Washington, D.C.    DOE/EIA-0484(2013)

http://www.eia.gov/forecasts/ieo/

http://www.eia.gov/forecasts/ieo/pdf/0484(2013).pdf

Gaudry, A., V. Kazan, and P. Monfray. 1996. Atmospheric CO2 record from in situ measurements at Amsterdam Island. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.

http://cdiac.esd.ornl.gov/trends/co2/amster.html

Houghton, R.A. 2008. Carbon Flux to the Atmosphere from Land-Use Changes: 1850-2005. In TRENDS: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.

http://cdiac.ornl.gov/trends/landuse/houghton/houghton.html

Keeling, C.D., S.C. Piper, R.B. Bacastow, M.  Wahlen, T.P. Whorf, M. Heimann, and H. A. Meijer, Exchanges of atmospheric CO2 and 13CO2 with the terrestrial biosphere and oceans from 1978 to 2000.

I. Global aspects, SIO Reference Series, No. 01-06, Scripps Institution of Oceanography, San Diego, 88 pages, 2001.

http://scrippsco2.ucsd.edu/data/atmospheric_co2.html

Keeling, R.F., S.C. Piper, A.F. Bollenbacher and J.S. Walker. 2008. Atmospheric CO2 records from sites in the SIO air sampling network. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.

http://cdiac.esd.ornl.gov/trends/co2/sio-keel.html

Sieminski, A., 2013, International Energy Outlook 2013, Center for Strategic and International Studies, Washington, DC.

http://www.eia.gov/pressroom/presentations/sieminski_07252013.pdf

U.S. Energy Information Agency, Data Tables, U.S. Energy Information Agency, Office of Energy Analysis, U.S. Department of Energy, Washington D.C. data tables, 2014

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Carbon Isotopes in the Atmosphere -- Part II

Finding Niño -- Correlating CO2 Carbon Isotopes in the Atmosphere with the El Niño Cycle

Abstract:

Carbon dioxide released by fossil fuels has a lighter isotopic composition than CO2 in the atmosphere.   The distinctive signature of light carbon released from fossil fuels provides a tool for tracking the movement of carbon through the atmosphere.  That same distinctive signature can also be used to measure the exchange of carbon between the atmosphere and carbon reservoirs on the earth’s surface.

Carbon istotope ratios in the air have been measured at monitoring stations around the globe since 1977.  Despite superficial similarity to the bulk CO2 record, isotope records tells a different story, and give deeper insight into the workings of the earth’s carbon systems. 

Part I of this post discussed how we can measure the size of carbon reservoirs exchanging carbon with the atmosphere.  We defined the term "Carbonsphere" representing the sum of all reservoirs freely exchanging carbon with the atmosphere.   We estimated the size of the carbonsphere as 5200 gigatonnes, about seven times the carbon volume of the atmosphere, based on the dilution of light isotopes from fossil fuel emissions.   

In this post, we will examine fluctuations in atmospheric carbon isotopes, and show how these can be correlated to the El Niño/La Niña climate cycle.  A number of mathematical operations on the base carbon isotope data reveal a clear correlation to the El Niño cycle.

 d C13/C12 CO2 isotope fluctuations correlate with the El Niño/La Niña climate cycle.

The El Niño/La Niña cycle controls how the Pacific Ocean exchanges carbon with the atmosphere.  The mechanism is not clear.   Two hypotheses are considered.  First, ocean currents may move carbon from shallow water into the deep ocean during La Niña events.  Or second, ocean temperatures may cause selective absorption and release of carbon isotopes, favoring absorption of light isotopes in cool water, and heavy isotopes in warm water.  The isotope cycles represent the ocean "breathing" -- taking in light isotopes during the cool phase, and exhaling during the warm phase.  Isotope data from dissolved carbon dioxide in the Pacific Ocean would answer the question.

 Carbon isotope data should be monitored throughout the earth’s carbon reservoirs to recognize and quantify the movement of carbon, and to understand the destiny of carbon emitted by burning fossil fuels.

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Carbon Isotopes in the Atmosphere

As described in previous posts, the isotope ratio d C13/C12 is the standard expression of stable carbon isotopes.   The d C13/C12 formula allows recognition of small but meaningful changes in the ratios of carbon isotopes.    In this post, d C13/C12 will be referred to by the expression “del 13”.

Let’s begin by comparing the bulk CO2 record (the “Keeling Curve”) to the del 13 record. 

Figure 1 shows atmospheric CO2, as measured at monitoring stations located from the Arctic Ocean to the South Poel.   The chart shows increasing CO2 concentration in the atmosphere due to fossil fuel emissions.  The record shows a strong seasonal cyclicity resulting from plant growth in the northern hemisphere, as discussed in previous posts.  The chart is color-coded according to the latitude of the monitoring stations.  

Figure 2, location of CO2 and Carbon Isotope Monitoring Stations.

Figure 3 shows the isotopic ratio d C13/C12, otherwise known as “del 13”, in atmospheric CO2.

The del 13 record resembles the Keeling Curve.  There is a strong cyclity in the isotope record resulting from seasonal plant growth and decay in the Northern Hemisphere, as discussed in a previous post. 

In general , the isotope record is a mirror image of the bulk CO2 record.   The long-term bulk CO2 is increasing due to fossil fuel emissions, and the del 13 record is decreasing, reflecting the light isotopic composition of fossil fuels.  The del 13 ratio of fossil fuel emissions is about – 26, compared to the del 13 ratio of the atmosphere, at about – 7.5.   The seasonal cyclicity is likewise a mirror image.  As plants take up carbon in the summer, the concentration of atmospheric CO2 decreases, while the del 13 ratio increases, because the plants preferentially remove light isotopes from the atmosphere. 

Let’s begin the investigation of atmospheric carbon isotopes by removing the seasonal cycle.   For reference, we will first look at the bulk CO2 data, after filtering the seasonal cycles with a one-year rolling average, seen in Figure 4 below.

 Let’s compare the del 13 carbon isotope data, after removing the seasonal cycle with the same technique, shown in Figure 5.

 This is an amazing chart!    There are two surprises immediately apparent in the del 13 chart, in comparison to the bulk CO2 chart.   First, there is a wide separation between the curves on the del 13 chart, whereas the bulk CO2 curves are in a narrow band.   Second, the del 13 chart shows large waves moving through the data, whereas the bulk CO2 curves are smooth and nearly linear.   Let’s explore these two differences.

The del 13 chart shows a wide separation of curves by latitude.  The time required for equilibration between northern and southern hemisphere is much longer in the isotope data than in the bulk CO2 data.   The falling del 13 ratio at the South Pole lags the readings in Alaska by about eight years, while the rising bulk CO2 concentration at the South Pole lags the northernmost readings by only about two years.

Figure 6 shows the 2- year time lag required for the concentration of CO2 at the South Pole to equilibrate with the far northern hemisphere.  

Figure 7 shows the 8-year time lag required for the CO2 del 13 ratio at the South Pole to equilibrate with the far northern hemisphere.

What can account for the difference in the time required for equilibration between bulk carbon and carbon isotopes?   I suggest that light isotopes released in the northern hemisphere by fossil fuels have a long residency time in carbon reservoirs.    The difference in equilibration times shows exchange of carbon between the atmosphere and carbon reservoirs.  These reservoirs are not simply carbon sinks, but are actively exchanging carbon with the atmosphere.   Light carbon from fossil fuels is absorbed by carbon reservoirs near the point of emission; the bulk CO2 concentration of the atmosphere is maintained by the release of heavier carbon from the reservoir back to the atmosphere.

There is a second surprise in the del 13 chart, compared to what we see in the bulk CO2 data.  When we remove the seasonal cycle from the bulk CO2 data, the curves are very smooth, almost linear.   However, when we remove the seasonal cycle from the del 13 data, we see a series of large waves, observed at every monitoring station across the globe.  These are events which were not removed by the seasonal filter.  There are a few events which occurred only in the northern hemisphere, and a few which occurred only in the southern hemisphere. 

There is a remarkable paradox in the del 13 chart.   The paradox lies in the different responses of the atmosphere to perturbations of the carbon isotope ratio.   Following a perturbation in del 13 as a result of fossil fuel emissions in the northern hemisphere, nearly a decade is required for the air at the South Pole to reach to the same level of isotopic composition.   But the waves moving through the del 13 chart occur nearly simultaneously at every monitoring station on earth!   Although the del 13 values do not equilibrate to the same value, this signal is felt around the world with a lag of less than six months.   I would speculate that this indicates two carbon reservoirs; one on land, and the other in the ocean.   The land system locks up carbon in forests and soils, accounting for the long residency time, while the ocean system more readily propagates changes around the globe. 

On that hunch, I plotted measurements of the El Niño – La Niña cycle on the del 13 plot.   Figure 8 shows an apparent correlation of strong El Nino events to periods of rapidly falling del 13. 

Figure 8.  Atmospheric carbon isotopes and El Niño events.

To clarify the wave-like signal in the data, I took the average of all curves, and a linear regression through the average curve.  

Figure 9.   Atmospheric CO2 del 13 ratios, with average curve and linear regression.

I then subtracted the linear fit from the data, to produce a chart of the residual values after removing the linear trend.

Figure 10.  Chart of Residual del 13, after subtraction of linear trend.

We can compare the residual chart with the El Nino events.  El Nino events tend to correspond to negative slopes on the residual chart. 

Figure 11.  Chart of Residual del 13, with El Niño events.

If we recall Part I of this post on carbon isotopes, a relative increase in del 13 corresponds to a larger carbonsphere; relative decreases in del 13 correspond to a smaller carbonsphere.   It is the slope of the residual function that is significant, rather than the peaks and valleys.   Changes in slope indicate a change in conditions.  A positive slope indicates an expanding carbonsphere – fossil fuel emissions are being diluted into a larger volume of carbon reservoirs.  A negative slope indicates a shrinking carbonsphere – fossil fuel emissions are being diluted into a smaller volume of carbon reservoirs. 

So, to complete the transformation of the del 13 data, we now take the derivative, or instantaneous slope of the residual curve.   On this chart, positive values will indicate an expanding carbonsphere, and negative values will indicate a shrinking carbonsphere. 

Figure 12.   Derivative of Residual del 13 data; all curves.

The initial chart is rather noisy.   A better signal to noise ratio can be obtained by taking the average of all curves, to produce the following curve.  Positive values indicate an expanding carbonsphere (the light isotope is being diluted into a larger volume), and negative values indicate a shrinking carbonsphere (the light isotope is being diluted into a smaller volume). 

Figure 13.  Derivative of residual del 13 data, from average of all curves.

La Nina/El Nino

El Niño is an oceanic phenomenon, involving anomalously warm surface waters in the Pacific Ocean.  The warm waters develop off the western coast of South America, and extend westward across the equatorial Pacific Ocean.  El Niño events have profound meteorological impact, and influence weather around the globe.   The opposite of the El Niño event is termed La Niña, and involves anomalously cool Pacific waters.

Figure 14.  Pacific Ocean Temperature Anomalies, showing El Niño and La Niña events;(from  NASA).

                  http://www.elnino.noaa.gov/lanina.html

The National Oceanographic and Atmospheric Administration keeps a record of the strength of the El Nino – La Nina cycle, and expresses that record as the Oceanic Nino Index (ONI).    The data are a time series of three-month average sea surface temperature anomalies.    For the purposes of this blog post, I have reversed the sign of the ONI values, making La Nina events positive, and El Nino events negative.

Figure 15.  Here is the chart of the Oceanic Nino Index (polarity reversed).  

We can superimpose the chart of the Oceanic Niño Index, and the slope of the residual del 13 measurements.   Despite some noise, there is a clear and perceptible correlation between the curves.  

Figure 16.   Averaged derivative of residual del 13 data, and Oceanic Niño Index (from NOAA).

The ONI curve does not match the isotope data, in terms of the timing of events.  There is a brief lag between the ONI curve (representing surface temperature anomalies, and the del 13 data indicating isotopic changes in the atmosphere.  I added a six month lag to the ONI curve, in order to make a better match to the observed isotope data. 

Figure 17.  Average derivative of the residual del 13 data, and ONI curve with a 6 month lag.

We’ve performed a number of transformations of the atmospheric CO2 carbon isotope data, in order to reach the curve that corresponds to the Oceanic Niño Index. 

Figure 18.   Here is a summary slide indicating the transformations. 

The meaning of the correlation is not clear at this time, but it is clearly a significant phenomenon for global climate study.  I can advance two hypotheses. 

Deep Current Hypothesis

My first thought was that La Niña conditions indicated currents which displaced waters of the shallow Pacific Ocean into deeper water.  When La Niña conditions prevail, carbon which is enriched in light isotopes due to fossil fuels is transported and sequestered in the deep ocean.  The shallow water would be replaced by deeper waters, which still carry pre-industrial del 13 ratios (of about -6.5, based on ice core data).   Such a current would expand the Carbonsphere (as discussed in the previous post) and dilute light isotopes from fossil fuels into a larger volume of carbon reservoirs.   El Niño conditions would be stagnant, allowing heat to build up in shallow waters, and light isotopes from fossil fuels to accumulate.  El Niño would shrink the Carbonsphere, relative to La Niña.

CO2 Solubility and Isotope Differentiation Hypothesis

My daughter suggested a different hypothesis to me; one that is more probably correct.  She suggested that temperature changes in the shallow ocean should change the solubility of CO2, and the rate of exchange with the atmosphere.  An extension of that thought is that changes in the temperature of the water may differentiate the carbon isotopes being exchanged with the atmosphere.   Thus, during La Niña events, with cold Pacific water, light isotopes may be better absorbed by the water, raising the del 13 of the atmosphere.   During La Niño events with warm Pacific water, heavy isotopes may be better absorbed by the water, lowering the del 13 of the atmosphere.

NOAA is now conducting research and modeling on sea-air carbon exchange, with a focus on the Pacific Ocean, and the El Nino-La Nina cycle.  However, I have not seen data regarding isotope differentiation through that process.  Figure 19 shows one map from that study.

Figure 19.  Carbon Flux map from NOAA study.  The upper map shows carbon flux in absolute terms; the lower map shows relative variability from the normal pattern.  Positive values (reds) indicate less uptake of CO2 by the ocean from the atmosphere.  The year chosen is a strong La Nina year.  However, the published maps do not address the behavior of carbon isotopes as a function of temperature.  

Carbon isotope data in the waters of the tropical Pacific is needed to resolve the question.  If the current hypothesis is correct, the cool waters of La Niña would be have high, pre-industrial del 13 values of about -6.5 (from ice-core data).   If the isotope differentiation model is correct, La Niña waters would be enriched in light isotopes relative to the atmosphere, lower than -8.2.   This question of interpretation would seem to be easily resolved by additional data.   Physical solubility data and modeling would be helpful, but direct measurements of carbon isotopes in water would be definitive.   The isotope data must target chemical species related to aqueous carbon dioxide – carbonate, bicarbonate, and carbonic acid. 

Further, if the current hypothesis is correct, La Niña is transporting a measurable quantity of atmospheric carbon into the deep ocean.   From that information, the volume of water and quantity of heat carried by the current could also be calculated, providing key data in understanding the pace of global warming on earth.

Isotope data through all of earth’s carbon reservoirs would be helpful in understanding the movement of carbon through those systems, and the destiny of carbon emitted by burning fossil fuels.

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References:
Atmospheric CO2 Carbon Isotope Data:

Keeling, R.F. S.C. Piper, A.F. Bollenbacher, and S.J. Walker. 2010. Monthly atmospheric ¹³C/¹²C isotopic ratios for 11 SIO stations. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.

http://cdiac.ornl.gov/trends/co2/iso-sio/iso-sio.html

Global Emissions average isotope data

Boden, T.A., G. Marland, and R.J. Andres. 2013. Global, Regional, and National Fossil-Fuel CO₂ Emissions. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. doi 10.3334/CDIAC/00001_V2013

http://cdiac.ornl.gov/ndps/db1013_v2012.html

ftp://cdiac.ornl.gov/pub/db1013_v2012/db1013.global.dat

El Nino/La Nina Climate Cycle

http://www.elnino.noaa.gov/lanina.html

Oceanic Nino Index

http://www.nwfsc.noaa.gov/research/divisions/fe/estuarine/oeip/cb-mei.cfm

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml

Data:  http://www.cpc.ncep.noaa.gov/data/indices/oni.ascii.txt

Carbon Flux Models -- NOAA

http://www.pmel.noaa.gov/co2/file/CO2+Flux+Map

http://www.oco.noaa.gov/oceanCarbonUptake.html

http://www.pmel.noaa.gov/co2/story/Surface+CO2+Flux+maps

Interactive mapper:  http://cwcgom.aoml.noaa.gov/erddap/griddap/aomlcarbonfluxes.graph

Previous posts om this site regarding atmospheric CO2:

1)  The Keeling Curve
     http://dougrobbins.blogspot.com/2011/05/keeling-curve.html

2)  The Keeling Curve and Seasonal Carbon Cycles

      http://dougrobbins.blogspot.com/2012/03/keeling-curve-and-seasonal-carbon.html

3)   Seasonal Carbon Isotope Cycles

      http://dougrobbins.blogspot.com/2012/03/seasonal-carbon-isotope-cycles.html

4)   Long-Term Trends in Atmospheric CO2

      http://dougrobbins.blogspot.com/2012/03/long-term-trends-in-atmospheric-co2.html

5)   Modeling Global CO2 Cycles

      http://dougrobbins.blogspot.com/2012/04/modeling-global-co2-cycles.html

6)   The Keeling Curve Summary:  Seasonal CO2 cycles and Global CO2 Distribution
       http://dougrobbins.blogspot.com/2013/05/the-keeling-curve-seasonal-co2-cycles.html

7)   Carbon Isotopes in the Atmosphere, Part I -- How Big is the Carbonsphere?
       http://dougrobbins.blogspot.com/2013/11/how-big-is-carbonsphere.html