November 10, 2009

Airborne Fraction of Human CO2 Emissions Constant over Time

A couple of months back, there was a discussion taking place over at Joe Romm’s ClimateProgress blog concerning a report that the earth’s ability to take-up atmospheric carbon dioxide was declining. A declining CO2 sink, of course, meant that things climatological were going to be even worse than expected, because a growing proportion of anthropogenic CO2 emissions were going to remain in the atmosphere, thus pushing the rise of CO2 concentrations and the degree of climate change higher.

At the time, an alert reader pointed out to Joe Romm that there was in fact, no indication from data and observations that a larger percentage of human CO2 emissions were ending up in the atmosphere. In fact, the data showed that the fraction of CO2 emitted into the atmospheric by human activities has remained constant for the past 40 years.

This fact runs directly counter to the idea that the earth’s natural CO2 sinks are weakening—instead it indicates that natural sinks have been expanding as anthropogenic CO2 emissions have increased. After all, in order to keep the airborne fraction of CO2 emissions constant over time, increasing anthropogenic CO2 emissions must be countered by an increasing CO2 sink.

Joe Romm was a bit dismissive (to say the least) of this line of argument.

Here was one such exchange (Comment 13 of this thread):

Comment 13. Chip Knappenberger says:
March 30, 2009 at 5:15pm

Mr. Romm,

I am not sure how you justify this statement:

“At the same time that CO2 emissions are soaring, CO2 sinks are saturating.”

Take your numbers for the rate of CO2 increase each year and divide them by the numbers for the annual global CO2 emissions each year (available from the Carbon Dioxide Information Analysis Center, ftp/ ndp030/ global.1751_2005.ems) and see what you get.

Hint: the ppm/emissions ratio shows no trend at all which means that there is no decline in the CO2 sink—otherwise, this ratio would be increasing.

-Chip Knappenberger

[JR: It is Dr. Romm, Chip, and, hint, it is what the scientific literature says. Try reading it, some time. Start with the Global Carbon Project.]

Just in case JR (Dr. Romm) really is interested in what the latest scientific literature on the topic says, there is a new paper in the journal Geophysical Research Letters that directly examines this issue. The paper is by Wolfgang Knorr a senior researcher in the Department of Earth Sciences at the U. K’s University of Bristol, and is aptly titled with the question “Is the airborne fraction of anthropogenic CO2 emissions increasing?”

Dr. Knorr carefully analyzed the record of anthropogenic CO2 emissions, atmospheric CO2 concentrations, and anthropogenic land-use changes for the past 150 years. Keeping in mind the various sources of potential errors inherent in these data, he developed several different possible solutions to fitting a trend to the airborne fraction of anthropogenic carbon dioxide emissions. In all cases, he found no significant trend (at the 95% significance level) in airborne fraction since 1850. (Note: It is not that the total atmospheric burden of CO2 has not been increasing over time, but that of the total CO2 released into the atmosphere each year by human activities, about 45% remains in the atmosphere while the other 55% is taken up by various natural processes—and these percentages have not changed during the past 150 years)

Figure 1. The annual increase in atmospheric CO2 (as determined from ice cores, thin dotted lines, and direct measurements, thin black line) has remained constantly proportional to the annual amount of CO2 released by human activities (thick black line). The proportion is about 46% (thick dotted line). (Figure source: Knorr, 2009)

The Abstract of the paper directly addresses Romm’s concerns, but, unfortunately, Knorr finds no support for Romm’s take on the issue:

Several recent studies have highlighted the possibility that the oceans and terrestrial ecosystems have started loosing part of their ability to sequester a large proportion of the anthropogenic CO2 emissions. This is an important claim, because so far only about 40% of those emissions have stayed in the atmosphere, which has prevented additional climate change. This study re-examines the available atmospheric CO2 and emissions data including their uncertainties. It is shown that with those uncertainties, the trend in the airborne fraction since 1850 has been 0.7 ± 1.4% per decade, i.e. close to and not significantly different from zero. The analysis further shows that the statistical model of a constant airborne fraction agrees best with the available data if emissions from land use change are scaled down to 82% or less of their original estimates. Despite the predictions of coupled climate-carbon cycle models, no trend in the airborne fraction can be found.

The University of Bristol press release is even more explicit:

New data show that the balance between the airborne and the absorbed fraction of carbon dioxide has stayed approximately constant since 1850, despite emissions of carbon dioxide having risen from about 2 billion tons a year in 1850 to 35 billion tons a year now.

This suggests that terrestrial ecosystems and the oceans have a much greater capacity to absorb CO2 than had been previously expected.

The results run contrary to a significant body of recent research which expects that the capacity of terrestrial ecosystems and the oceans to absorb CO2 should start to diminish as CO2 emissions increase, letting greenhouse gas levels skyrocket. Dr Wolfgang Knorr at the University of Bristol found that in fact the trend in the airborne fraction since 1850 has only been 0.7 ± 1.4% per decade, which is essentially zero.

Further, Knorr noted that we still have a good bit of work to do to completely understand why this is the case, especially if we want to be able to predict the future course of atmospheric CO2 concentrations (a necessity if we want to predict future climate change). Knorr concludes:

Given the importance of the [the anthropogenic CO2 airborne fraction] for the degree of future climate change, the question is how to best predict its future course. One pre-requisite is that we gain a thorough understand of why it has stayed approximately constant in the past, another that we improve our ability to detect if and when it changes. The most urgent need seems to exist for more accurate estimates of land use emissions. Another possible approach is to add more data through the combination of many detailed regional studies such as the ones by Schuster and Watson (2007) and Le Quéré et al. (2007), or using process based models combined with data assimilation approaches (Rayner et al., 2005). If process models are used, however, they need to be carefully constructed in order to answer the question of why the AF has remained constant and not shown more pronounced decadal-scale fluctuations or a stronger secular trend.

In other words, like we have repeated over and over, if the models can’t replicate the past (for the right reasons), they can’t be relied on for producing accurate future projections. And as things now stand, the earth is responding to anthropogenic CO2 emissions in a different (and perhaps better) manner than we thought that it would. Despite what Joe Romm would have you believe.


Knorr, W., 2009. Is the airborne fraction of anthropogenic CO2 emissions increasing? Geophysical Research Letters, 36, L21710, doi:10.1029/2009GL040613.

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