Is Temperature Causing the Increase in CO2?

Every once in a while a paper gets published in a low-to-no impact journal suggesting that the increase in CO2 concentrations was caused by the increase in global temperatures and not by human emissions. There are several blogs and unpublished manuscripts that make similar claims. Now for sure, since CO2 is less soluble in warmer water, increasing sea surface temperatures, so increasing temperature does cause an ocean-to-atmosphere CO2 flux. But can this explain why CO2 levels are increasing? Some say yes. The most recent paper[1] was published in a no-name MDPI journal called Sci, and Judith Curry promoted it on her blog. 

Papers and arguments like this are obviously nonsense, but this paper is a little unique in that it includes the very evidence that proves its central thesis wrong. The following chart comes from the paper. It takes data from the IPCC AR6 accounting of the carbon budget, showing the ocean, land and human sources and sinks. From the numbers the authors included in this graph, the land is a net carbon sink, and the oceans are a net carbon sink. Meanwhile human activity is responsible for a source of +11 GtC and a sink of -0 GtC. So if the increase in CO2 was caused by an increase in temperature and not from human emissions, where did the CO2 come from? Temperature can't create CO2; it has to come from somewhere, and land and oceans are both net carbon sinks so they can't be sources for the increase in CO2. This is pretty clear from the chart below from the very paper trying to argue the opposite.

The information from the above chart comes from figure 5.12 in AR6 below. It breaks down the various natural fluxes on land and ocean as well as anthropogenic fluxes from land use and fossil fuel use. The natural carbon cycle is a net sink. Natural sinks total -221.8 GtC/yr and natural sources total 216.8 GtC/yr, so without human sources, the natural carbon cycle would be a net sink of 5.9 GtC/yr. But since human emissions total 11 GtC/yr the carbon cycle is a net source of 5.1 GtC/yr. The data show that the oceans and land can't be the sources of atmospheric CO2.

So why do papers like this get written? When Curry posted about this paper on her blog, Demetris Koutsoyiannis, one of the authors, was responding to comments, and I had the opportunity to interact with him in the comment section. When I made the above point, he said something to me that I think explains why people get this so wrong:

I conjecture that the logic behind your arithmetic is this. A tree, before taking a molecule in its photosynthesis, first examines its origin, whether it was emitted by land or ocean or humans, and uses it only in the first case (and likewise for the ocean).

The odd thing about this, though, is that it seems to me this is exactly the kind of thinking that makes the paper seem plausible to him. That is, he seems to think that nature can blame natural CO2 and absolve anthropogenic CO2 of that same responsibility. My response to him was to give him an example: if you have an income of $1000/week and your expenses are $990/week, you can save $10/week. But if someone has figured out a way to steal $50/week from your account, you’re going in debt (or your savings are being depleted) by $40/week. Why? Because someone is stealing your money. The theft is flipped your budget from being in the black to being in the red, and the weekly theft is 100% of the problem.

But I think there may be a better way to illustrate this without resorting to a financial analogy. We can think of carbon within the climate system as existing in three pools - the land, oceans and atmosphere - and carbon can be exchanged between these pools. Photosynthesis and respiration cause carbon to be exchanged between land and the atmosphere. Carbon exchanges between the oceans and atmosphere because of Henry's Law. The total amount of carbon in the climate system is roughly fixed. Volcanism adds about 0.1 GtC/yr to the system, and the chemical weathering of rocks can remove as much as 0.3  GtC/yr, though that number may be a little too high. These natural additions and subtractions to these climate system are small, and they nearly offset, so I'm going to include them within the climate system for simplicity. On decadal and century time scales, the amount of natural carbon in the climate system is relatively fixed. 

However, of the 11 GtC/yr carbon emissions from human activity, 9.4 GtC/yr comes from fossil fuel emissions. This carbon doesn't come from within the carbon cycle - it comes from carbon that was sequestered from the climate system for hundreds of millions of years. Carbon from fossil fuel emissions is added to the climate system from the outside. I tried to illustrate this with the diagram below. As you can see, the amount of carbon emitted from fossil fuels is small compared total amount of carbon in these pools, especially given the size of the land and ocean pools. But fossil fuel emissions goes directly to the atmosphere, and about half of it flows from the atmosphere into the ocean and land pools. The increase of carbon in the atmosphere goes from 600 GtC (about 280 ppm) to 900 GtC (about 420 ppm) and the rest of it flows to the oceans and land carbon pools..

Here's how this looks in the annual fluxes between the atmosphere, oceans and land. While 1.6 GtC/yr comes from land use changes (a flow from land to atmosphere), about 9.4 GtC/yr comes from outside the climate system and flows into it. The increase in carbon into the carbon cycle matches exactly the amount of carbon added to it from human activity. This requires that 100% of the increase in CO2 comes from human activity.

So what this paper is asking us to believe is that the 700 GtC added to the atmosphere from human activity (about 600 GtC from fossil fuels) just magically disappears, and temperature just magically creates carbon in the exact same quantities that would have come from human activity. The carbon isn't coming from the land and oceans, they are net sinks. And the additional CO2 is not coming from volcanism or the chemical weathering of rocks; their contributions are small and nearly offset. But human carbon emissions total about 2x the annual growth in atmospheric CO2; the only way the carbon budget balances is if human activity is responsible for virtually all the increase in atmospheric CO2.

References:

[1] Koutsoyiannis, D.; Onof, C.; Kundzewicz, Z.W.; Christofides, A. On Hens, Eggs, Temperatures and CO2: Causal Links in Earth’s Atmosphere. Sci 2023, 5, 35. https://doi.org/10.3390/sci5030035