Commentary By Donald Rapp on the paper: “The phase relation between atmospheric carbon dioxide and global temperature” by Ole Humlum, Kjell Stordahl, Jan-Erik Solheim, accepted for publication in: Global and Planetary Change.
This paper analyzed data on annual variations in carbon dioxide concentration, various measures of earth temperature, and rate of emissions of carbon dioxide for the period 1980 to 2011. They compared the rate of change of CO2 concentration with measures of the rate of change of global temperature. While both CO2 and temperature generally increased during this 31-year period, the rates of change varied significantly during the period. They showed that changes in CO2 correlated somewhat with changes in sea surface temperature (SST) but the CO2 change lagged the SST change by about 11-12 months. They concluded that “A main control on atmospheric CO2 appears to be the ocean surface temperature”. They mentioned possible connection to the giant 1998 El Niño but did not elaborate on the connection of the entire sequence of data to El Niño indices.
In the present posting I desire to make a few comments on this paper by Humlum et al. Of course, as noted by the authors, the common belief is that rising CO2 produces an increase in the rate of warming, not vice versa. Their data suggests quite the opposite.
Consider the figure at the top of this post.
The uppermost curve shows the NINO3.4 index from 1980 to 2011. Peak El Niños are labeled with letters A to F.
The middle curve shows the change in CO2 concentration per year plotted on a monthly basis. The peaks in this curve are also subjectively labeled A to F. The average change in CO2 concentration per year can be interpreted either as a ramp or a step-function. Arbitrarily adopting the step function, the average change in CO2 concentration per year varied from year to year about 1.5 ppm/yr prior to the 1998 El Niño, and varied from year to year about 2.0 ppm/yr after the 1998 El Niño. These are depicted as horizontal dashed lines x and y.
The lowermost curve shows the annual change in anthropogenic CO2 emissions plotted on a per month basis.
A rough rule of thumb is that each Gt of carbon (3.67 Gt of CO2) produces the equivalent of about 0.5 ppm of CO2 in the atmosphere if none of it is absorbed. The figure below shows that annual variations in global emissions of carbon are typically about 2 x 104 metric tons per year which if unabsorbed, would produce annual changes in CO2 that are far too small to account for the observed variations in the average change in CO2 concentration per year.
The point made by Humlum et al. is that the average change in CO2 concentration per year lags the change in ocean temperature by about 11-12 months. As Tisdale showed in his book, El Niños leave behind them a pool of warm surface waters. As a result, the average change in CO2 concentration per year tends to lag the NINO3.4 index by a bit more than a year. This correlation is far from perfect but it seems to have some validity, particularly for the major El Niño that started toward the end of 1997. The data suggest that the ability of the oceans to absorb CO2 emitted by human activity responds to the state of the NINO3.4 index with a delay of a bit over a year.
Human activity is presently emitting roughly 8 Gt/yr of carbon, which if unabsorbed, would be sufficient to increase the atmospheric concentration of CO2 by about 4 ppm per year. Over a period of years, (very) roughly half of that CO2 is absorbed by earth systems (oceans, biosphere, …) and the other half ends in the atmosphere raising the atmospheric concentration by about 2 ppm. However, on a year-by-year basis, the proportion of emitted CO2 that is absorbed by the earth systems varies considerably, mainly due to the presence of warm surface waters in the Pacific produced quasi-periodically by El Niños. According to the graphical data below, the annual increase in CO2 concentration can be as high as 3 ppm (following the 1998 El Niño) or as low as 1 ppm (between peaks B and C). During the most recent period after the 1998 El Niño, variations in annual increase in CO2 concentration seem to have varied roughly as 2 ± 0.5 ppm or ±25%. These results seem to suggest that while roughly half of emissions end up in the atmosphere over an extended period, annual variations in the distribution of emitted CO2 between the atmosphere and the earth system are significant, and strongly dependent on prevalence of El Niños.
Tisdale showed that from 1976 to about 2005, there was a pronounced prevalence of El Niños over La Niñas. He argued that this could account for all of the warming of the earth during that period without invoking the greenhouse effect. However, it seems likely that during this period, a greater proportion of emitted CO2 ended up in the atmosphere due to prevalence of El Niños, and this might have amplified the natural El Niño warming effect via greenhouse gas forcing. McLean et al. (2009) estimated that 70% was due to El Niños while Foster et al. (2010) fell back on climate models that attribute only 15-30% of temperature variation in the 20th century to variability of the El Niño index. As is usual in climate matters, one has only to glance at the authors to know in advance what spin the results are likely to show. The Foster paper included the crème de la crème of climategate characters while the Mclean paper was written by skeptics.
The proportion of global heating from 1976 to 2005 due to prevalence of El Niños over La Niñas vs. greenhouse gas forcing remains uncertain. Nevertheless, the state of the Pacific Ocean is clearly important, not only for its impact on the atmospheric temperature, but also because it regulates the annual rise in CO2 concentration.
McLean, J. D., C. R. de Freitas, and R. M. Carter (2009) “Influence of the Southern Oscillation on tropospheric temperature” Journal of Geophysical Research, 114, D14104.
Foster, G., J. D. Annan, P. D. Jones, M. E. Mann, J. Renwick, J. Salinger, G. A. Schmidt and K. E. Trenberth (2010) “Comment on “Influence of the Southern Oscillation on tropospheric temperature” by J. D. McLean, C. R. de Freitas, and R. M. Carter”, Journal of Geophysical Research, 115, D09110.