CO2 variations


El Nino and changes in Carbon dioxide concentration.

There will likely be variations to the global CO2 concentrations that will lag variations in sea surface temperatures that have nothing to do with the trend in rising CO2 concentrations that lead the SST’s.

Confusing the causes of the trend and the variations have lead contrarians to false conclusions. 

A coherent and consistent explanation is generally accepted for the rise of CO2 since the start of the industrial revolution. This explanation includes the causes for the trend in CO2 and different causes for the variations in CO2. There is little doubt that the trend is caused by human activity and in particular the burning of fossil fuels just by considering the volumes involved. The trend appears to be approximately half the rate of emissions. The variations in the concentrations of carbon dioxide are more complex and interesting and can throw light on a number of different causes and effects(1). There is the obvious seasonal cycle that is apparent in the keeling curve. There are other short term variations that are concerned with changes in atmospheric and oceanic circulation such as the ENSO cycle.

Focussing on the variations:-
Once the trend in the CO2 concentration rise is identified (although caution is needed here) it can be subtracted from the data to reveal more clearly the variations.

Seasonal cycles become apparent that are concerned with the seasonal decay or growth of plants particularly in the northern hemisphere. Identifying a consistent cycle or by using appropriate filters allows this to be readily removed from the detrended data leading to variations that are closely linked to the ENSO cycle(2).

During the El Nino years with increased sea surface temperature, SST, there is increased CO2 and conversely during la Nina years there is decreased CO2 levels. One may speculate that this increased CO2 associated with the El Nino is a direct result of warming waters (net) absorbing less but evidence suggests that this is not so and more to do with photosynthesis which will be discussed. Furthermore these variations in CO2 lag(1,2,3) the variations in SST by several months also to be discussed.

Carbon isotope ratios.
Plants preferably take up the lighter C12 isotope over C13 and hence the C13/C12 ratio will increase during seasonal plant growth or decrease when plants decay or when plant derived fossil fuels are burned. When we look at the trend in CO2 rising we see that the C13/C12 ratio falls as is consistent with burning of fossil fuels.

(There are two other ways that the C13/C12 ratio could fall:-
1. The biosphere is decaying simultaneously as we emit fossil fuels but this would require the oceans take up all or most of what we emit. There is no evidence that the biosphere is decreasing and I don’t think anyone would consider this as a plausible explanation. If this started to happen in the future then it could indicate a disastrous situation involving a positive feedback to our emissions.
2. Lighter Carbon from the mantle is being released in phase with our emissions! Again not a realistic observation that also fails to describe what is happening to our emissions.)

The seasonal cycles.
 The variations in the carbon isotope ratio follow the growth and decay of plants as expected. Keeling4. In the northern hemisphere with more land mass and more plant growth in the summer months CO2 levels drop due to the extra photosynthesis, and also the C13/C12 ratio increases due to the sinking of more lighter carbon.

The ENSO cycles.
During the La Nina the C13/C12 ratio increases (after factoring out the seasonal changes and changes due to the trend) and this is not consistent with the explanation of SST directly affecting the solubility of CO2 in sea water but more indicative of increased photosynthesis during plant growth. Now we have an explanation of the detrended CO2 concentrations (with seasonal variations removed) lagging the changes in SST. The SST is associated with ENSO cycles. The La Nina component of this causes nutrient rich water to upwell in the eastern pacific encouraging plant growth3 and the Southern oscillation associated with this increases the net precipitation over land globally, and encourages plant growth1,2 months later. A temporary decrease in the rate of sea level rise is consistent with this.

Sea surface temperatures have been rising in the tropical Eastern Pacific through July and August 2015. The atmospheric and oceanic circulations associated with this is of course the ENSO cycle. At the present time we are experiencing the onset of what may likely be a large El Nino. This will have an impact that will differ regionally and globally affecting plant growth on land and in the oceans and have a resulting affect on the sourcing and sinking of carbon dioxide. It is to be expected we will see less upwelling of nutrient rich currents in the eastern ocean, less rain fall on land globally, more drought globally (if not California) and more wild fires globally. All these impacts will almost certainly result in an extra increase in CO2 concentrations, over what would otherwise be expected from our emissions, that are likely to become apparent and that will lag the increase in SST observed in 2015.

1. Interannual extremes in the rate of rise of atmospheric CO2 since 1980   Keeling et al

2. The Carbon Cycle Response to ENSO: A Coupled Climate–Carbon Cycle Model Study.  Jones

3.Climate-driven trends in contemporary ocean productivity. Behrenfeld

4.  Monthly Atmospheric 13C/12C Isotopic Ratios for 11 SIO Stations   Keeling

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