Guest Post by Willis Eschenbach
There’s an interesting measure of atmospheric CO2, called the “airborne fraction”. The airborne fraction is the fraction of the CO2 emitted each year which remains in the atmosphere. When humans emit say 9 gigatonnes of carbon, only about half of that remains in the air. The other half of the emitted carbon is absorbed, “sequestered” in some semi-permanent fashion, by various carbon sinks in the land and the ocean.
Dr. James Hansen of NASA, another in the long line of climate alarmists who don’t mind shafting the poor with expensive energy, has come out with a most surprising statement in his latest paper, Climate forcing growth rates: doubling down on our Faustian bargain, hereinafter Hansen 2012. The statement involves Hansen et al.’s explanation for a claimed recent decrease in the airborne fraction. Here’s their graphic showing the changes in the airborne fraction since 1960.
[ORIGINAL CAPTION] Fossil fuel CO2 emissions (left scale) and airborne fraction, i.e., the ratio of observed atmospheric CO2 increase to fossil fuel CO2 emissions. Final three points are 5-, 3- and 1-year means.
I do wish people would show the underlying data and not just averages, but setting that aside, here are the authors’ claims about the drop in the airborne fraction (blue line) post 2000:
We suggest that the surge of fossil fuel use, mainly coal, since 2000 is a basic cause of the large increase of carbon uptake by the combined terrestrial and ocean carbon sinks. One mechanism by which fossil fuel emissions increase carbon uptake is by fertilizing the biosphere via provision of nutrients essential for tissue building, especially nitrogen, which plays a critical role in controlling net primary productivity and is limited in many ecosystems (Gruber and Galloway 2008). Modeling (e.g., Thornton et al 2009) and field studies (Magnani et al 2007) confirm a major role of nitrogen deposition, working in concert with CO2 fertilization, in causing a large increase in net primary productivity of temperate and boreal forests.
This is an interesting argument, but it has a few moving parts. Let me list them.
1) Increased coal use leads to increased net primary productivity (NPP) .
2) Increased NPP is evidence of increased carbon absorption.
3) Increased carbon absorption leads to increased biologically driven carbon sequestration.
4) Increased biologically driven sequestration explains the post-2000 decrease in airborne fraction.
I’m good with claims number 1 and number 2, but from there they get increasingly unlikely for various reasons. I’ll go get the data and show the actual airborne fraction, but first, let me quote a bit more from Hansen 2012, this time regarding Pinatubo.
Remarkably, and we will argue importantly, the airborne fraction has declined since 2000 (figure 3) during a period without any large volcanic eruptions. The 7-year running mean of the airborne fraction had remained close to 60% up to 2000, except for the period affected by Pinatubo.
and also …
Thus we see the decreased CO2 airborne fraction since 2000 as sharing some of the same causes as the decreased airborne fraction after the Pinatubo eruption (figure 3).
I looked at the chart, and I looked at the dates. Pinatubo was in June of 1991. Here’s what I get from the data:
So to start with, from both his graph and mine I’m saying absolutely no way to Hansen’s claim that there was a “decreased airborne fraction after the Pinatubo eruption”. Hansen seems obsessed with Pinatubo. He previously has claimed (falsely) that it represented a successful test of his GISS climate model. See here, here , and here for a discussion of how poorly the models actually did with Pinatubo.
He is now claiming (again falsely) that there is some drop in the airborne fraction after Pinatubo. I’m sorry, but that’s a totally false statement. There’s no sign of any unusual drop post-Pinatubo in this record at all, neither in the annual data nor in the average data. The majority of the drop he seems to be pointing to occurred well before Pinatubo occurred …
In passing, let me comment that any reviewer who let any of that Pinatubo nonsense past them should resign their commission. It was the first thing I noticed when I looked at the paper.
There’s a second problem with what Hansen et al. have done. They say regarding their 7-year average (blue line) that: Final three points are 5-, 3- and 1-year means. Sadly, this means that the final point in the 7-year average is forced to be equal to the last point in the raw data … easily the worst choice of ways to handle the final points of any average, almost guaranteed to have the largest error.
But that method does have one advantage in this case. It greatly exaggerates the amount of the recent drop. Note for example that had the data ended one year earlier, the final point in his average would have had a value 60% … here’s what the 7-year average figured their way would have looked like if the data had ended in 2010.
As you can see, their curious treatment of the 7-year average at the end of the data is the only thing that makes the trend look so bad. When changing the data length by one year makes that kind of change in an average, you can assume that your results are far, far from robust.
But neither of those is the main problem with their claim. The main problem is that the general slight decrease in the airborne fraction is an expected result of the exponential decay of the excess atmospheric CO2. As the green line shows, the actual results are in no way different from the value we’d expect to see. The green line shows the result of the exponential decay of the excess CO2 if we assume a half-life of about 46 years. The expected value decreases slightly from 1970 to 2011.
It’s worth noting that if CO2 emissions leveled off entirely, the airborne fraction would gradually decay to zero. This is because if emissions level off, eventually the excess CO2 level will be such that the annual sequestration will equal the annual emission with nothing to remain airborne.
To close, let me return to their claim:
We suggest that the surge of fossil fuel use, mainly coal, since 2000 is a basic cause of the large increase of carbon uptake by the combined terrestrial and ocean carbon sinks.
I must confess that I hadn’t looked at fuel use by type in a while, so I was unaware of a large spike in coal use.
So yes, coal use has indeed spiked since 2000, with a jump in coal emissions putting it back out in front of oil. I assume, although I’ve not checked, that this is the result of the huge increase in coal for electricity generation in India and China. And good on them, the folks in that part of the planet desperately need cheap energy.
Returning to the claims in Hansen 2012, it is true that the carbon uptake by the various sinks has constantly increased over time. This increase, however, appears to be much more related to the exponential decay of the CO2, and has less to do with the changes in the biosphere. We know this because the change in the amount sequestered is much larger than the change in the NPP.
Here are the figures. In 1960 the natural sinks were sequestering about 1 gigatonne of excess carbon annually. By 2011, this had risen to 4.5 gigatonnes annually. I agree that CO2 fertilization is real, but clearly this 4.5-fold increase in total tonnage of excess carbon sequestered cannot all be the result of increased NPP from CO2 fertilization.
So while I’m glad to hear that Hansen thinks that coal is good for something, I fear his explanation for the increase in the amount sequestered is not correct. The increases in the amount sequestered have been much, much larger (450% since 1960) than the increase in the amount of sequestration due to greater NPP.
Before I leave, let me remind folks what cheap electricity and energy from coal does for us all, rich and poor alike, every day of the year.
Figure 5. Daily output of coal energy. SOURCE
That huge benefit to the poor and the rich is what Hansen is trying to get rid of … but he and others have very little with which to replace it. So all that happens is that the price of energy goes up, and the poor once again are impoverished the most.
Brilliant plan, that fellow Hansen truly cares about the future … he just doesn’t seem to care if he hurts people in the present.
My best to everyone,