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