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.
----------
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.
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.
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.
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).
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.
------
References:
Atmospheric CO2 Carbon Isotope Data:
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.
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
El Nino/La Nina Climate Cycle
Oceanic Nino Index
Carbon Flux Models -- NOAA
Interactive mapper: http://cwcgom.aoml.noaa.gov/erddap/griddap/aomlcarbonfluxes.graph
Previous posts om this site regarding atmospheric CO2:
http://dougrobbins.blogspot.com/2013/05/the-keeling-curve-seasonal-co2-cycles.html
1) The Keeling Curve
http://dougrobbins.blogspot.com/2011/05/keeling-curve.html
http://dougrobbins.blogspot.com/2011/05/keeling-curve.html
2) The Keeling Curve and Seasonal Carbon Cycles
3) Seasonal Carbon Isotope Cycles
4) Long-Term Trends in Atmospheric CO2
5) Modeling Global CO2 Cycles
6) The Keeling Curve Summary: Seasonal CO2 cycles and Global CO2 Distributionhttp://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
http://dougrobbins.blogspot.com/2013/11/how-big-is-carbonsphere.html
