Saturday, February 8, 2020

What is in a Carbon Sink? Why Can't Mother Gaia use it to Save us from Disaster?

Can the ecosystem remove the CO2 we dumped into the atmosphere? 
One of the reasons for the popularity of Gaia, the Earth Goddess, is the hope that She'll save us from our own reckless behavior. Maybe she'll absorb the CO2 we emitted into the atmosphere? Doesn't the planet have "natural" sinks? Why worry, then? Well, I am afraid that Gaia will react just as shown in the figure above

Roy Spencer is one of the few critics of the current interpretation of climate change who can produce reasonably good scientific credentials. And, indeed, his criticism is often valuable, although -- unfortunately -- sometimes marred by political elements. But that's normal, we are all political animals.

So, Spencer made an interesting observation in a post of a few days ago. He says (boldface mine):

For many years I have seen reference to the average equivalent fraction of excess CO2 that is removed by nature, and I have often (incorrectly) said something similar to this: “about 50% of yearly anthropogenic CO2 emissions do not show up in the atmosphere, because they are absorbed.” I believe this was discussed in the very first IPCC report, FAR. I’ve used that 50% removal fraction myself, many times, to describe how nature removes excess CO2 from the atmosphere.
Recently I realized this is not a very useful metric, and as phrased above is factually incorrect and misleading. In fact, it’s not 50% of the yearly anthropogenic emissions that is absorbed; it’s an amount that is equivalent to 50% of emissions. You see, Mother Nature does not know how much CO2 humanity produces every year; all she knows is the total amount in the atmosphere, and that’s what the biosphere and various geochemical processes respond to.

And that's perfectly correct. I have to admit that I myself made the same mistake more than once. Of course, Mother Gaia doesn't know where the CO2 in the atmosphere comes from. The ecosystem just absorbs part of it.

Then, Spencer went on to claim that the IPCC scenarios were wrong of a factor of 4 with respect to the fraction of CO2 absorbed by the sinks. That was a mistake that he himself corrected later on. That's normal: a good scientist is never afraid of admitting a mistake. Apart from that, Spencer's observation changes nothing of our interpretation of climate change, but it has the merit of focusing on a fundamental point: the interplay of sources and sinks.

Eventually, Spencer presented the following (corrected) model:

Note how the models presented by the IPCC assume that the fraction of CO2 that will be absorbed by the sinks will go down as more CO2 is present in the atmosphere. Spencer's model ("CO2 Budget Model") assumes instead that the ratio will remain constant. If the latter is the case, when we stop burning fossil fuels (and we will, sooner or later), then the CO2 concentration will go down faster and -- hopefully -- we may go back to temperatures comparable to those of the Earth we used to know up to not long ago.

I wish that Spencer were right but, unfortunately, that can't be the case. What he has been doing, in practice, is returning to a criticism that had been already used against Swante Arrhenius when he developed the first version of the greenhouse gas warming theory -- more than a century ago. The criticism was, "but the oceans will absorb the excess CO2!" and it took decades to overcome it. Eventually, a picture emerged in which the sinks of the ecosystem absorb most of the human-generated CO2, but not all of it. In addition, the fraction absorbed will go down as the sinks are gradually filled.

As you may imagine, it is a complex matter. There are all sorts of sinks: the biosphere, the oceans, the weathering reaction, carbon burial, and probably more. You may take a look at the latest IPCC report, or at this recent paper by Walsh. The sinks behave in a complex way and they may still surprise us. But, so far, the experimental data show a small but measurable decline in the absorption rate. I don't have to tell you that this is bad, very bad, extremely bad.

Instead, Spencer assumes that the sinks will continue absorbing at a rate proportional to the difference between the current CO2 concentration and the "reference" value of 295 ppm. In practice, he assumes that the sink is infinite. That's a little too much: nothing on this planet is infinite. Even though we see Gaia as a Goddess, She is not all-powerful. And She is not even benevolent and merciful. She won't save us from ourselves.

A comment from my personal troll, Mr. Kunning Druger

"Now, now, Mr. Bardi. I see that you finally came out for what you are: one of those silly Greenies who are Gaia worshippers. Actually, I think it is worse than that: you just pretend to be a worshipper of Gaia, but under that green skin you are all red: really, you are one of those communists who would use the excuse of a non-existing climate change to impose on us such ugly ideas as free health care and social security for everybody and that would destroy the American way of life. Anyway, about this silly rant of yours, let me first note how it shows your deep elitism: why should those so-called "scientific credentials" be needed to express an opinion? A citizen's opinion is worth another citizen's opinion: that's good, old American democracy. Then, you'll tell us that we need a Ph.D. to vote in our elections, wouldn't you? It is all because you foreigners are envious of our freedom and our democracy that works so well. And all this criticizing Dr. Spencer is only because you are envious of him: he works at a proper American University, in Alabama, whereas you work at a silly, little, provincial university in Europe where people can't even speak proper English. So, keep going like this, the more you disinform us, the more we know who you are."



Ugo Bardi is a member of the Club of Rome, faculty member of the University of Florence, and the author of "Extracted" (Chelsea Green 2014), "The Seneca Effect" (Springer 2017), and Before the Collapse (Springer 2019)