Airborne Fraction of CO2: Concentration vs. Load

By November 26, 2009 February 24th, 2013 CO2

The Airborne Faction Discussion:  This topic is controversial only in the minds of radical climate skeptics. Actually there is controversy, but it is totally different from what the radical climate skeptics (RCS) is saying. What RCS propaganda says is that because the airborne fraction is not increasing, there is no basis for climate change. What the scientists are talking about is completely different. The scientists are talking about the "fraction" of CO2 in the sky relative to everything else is staying the same. The RCS are saying that CO2 is not increasing and there for there is no basis for global warming – a totally irrelevant statement compared to the topic and investigation of this study.  The RCS does not understand what "airborne fraction" means.
Let me splain Lucy:

Pre- industrial atmosphere had 10 bazillion units of  atmosphere and 3 bazillion units of CO2 = 3% airborne fraction.

Today’s atmosphere has 103.3 bazillion units of atmosphere and 4 bazillion units of CO2 = 3% airborne fraction. EXCEPT – there’s more stuff in the atmosphere, there is more CO2 there, approximately 38% more than preindustrial times – greater density. The atmosphere is thicker. There is more CO2 here to warm the planet. The greenhouse affect is not controlled by the airborne fraction, it is controlled by the amount of greenhouse gases present.

The RCS is confusing CO2 ratio with CO2 load.  It is a very common misunderstanding that I have to deal with all of the time in water quality discussions. So because the public doesn’t know jack, they get fooled. It certainly "looks like" the scientists are saysing that there is no basis for climate change.  I think that the RCS is intentionally misleading in order to further their agenda – or they are just plain old stupid, irresponsible and unethical for not understanding the propaganda that they are distributing.

Folks tend to try and relate concentration with load all the time, but there is a huge difference. The airborne fraction doesn’t really mean anything by itself. It is used to define other things.
The study suggests that something is absorbing more CO2 than expected, the researchers know not  what. It could be little green men from space. There is a significant unknown factor in natural sequestration that is +/- 20% of the total load (natural and anthropogenic combined – nature is not picky). These guys see 40% remaining in the atmosphere. That is the bottom end of the discussion, the high end, that I have seen is 64% stays in the atmosphere.  And BTW – This is a really, really good thing for us because it keeps the atmospheric load of CO2 from increasing even more rapidly.

This group of studies is one that looks at the atmosphere. There are other studies that also look at the atmosphere that say the airborne fraction IS increasing, which means that the sinks ARE slowing.  But the most telling part of the new discoveries is that the studies that look specifically at the sink are seeing that they ARE slowing. This is a disconnect between scientists – one the the RCS likes to mention so disdainly. Actually it’s a disconnect  about how the public understands science. These papers are very specific and don’t explain inter-relatedness very well. Other studies are just as certain about the facts, very robust and up-to-date facts about CO2 sinks decreasing in their capacity to absorb, they are just as correct as this paper (see below). What this paper does not do, and never intended to do was to say why the airborne fraction is staying the same even though emissions are different. It very specifically analyzed the question "Is the fraction constant or changing?" 


Those other papers that say it isn’t? Well now you have to understand how the scientists measure how much of anything is in the sky. It’s a big guess. We aren’t smart enough yet to weigh the atmosphere. We do have records of how much fossil fuel has been burned, how much of our forests have been turned into CO2, how much CO2 has been emitted to the atmosphere from agricultural practices and loss of C from soils, etc. but we can’t weigh the atmosphere (very well).What is happening is that the atmospheric scientists are tying down the portion of the fraction that goes to different places better with the help of the scientists who are studying the specific sinks. Exactly how much will still be an estimate until we can weigh the sky, or weigh theCO2 lost be farming practices, or weigh the oceans to see how much carbon they have absorbed, etc.

Knorr, Is airborne fraction of anthropogenic CO2 emissions increasing, Geophysical Research Letters, November 2009.pdf