Looking for details on data presented in IPCC AR6 WG1 report

Question

I am working on a table of data from a chart that appears in the IPCC AR6 WG1 report, Global Carbon and Other Biogeochemical Cycles and Feedbacks-Chapter 5, Figure 5.12 | Global carbon (CO2) budget (2010–2019) on page 700.

I must admit from the outset that I have not, nor do I intend to, read the entire report. I am, however, searching to see if someone more familiar with Figure 5.12 who can provide a bit more of an explanation of the details presented in Figure 5.12. Also, I am NOT a climate scientist, so apologies in advance if my questions are readily apparent to those more familiar with the science.

Quick key:

Specifically, I would like to understand more about the CO2 fluxes to and from the atmosphere that are nicely summarized toward the top of the chart:

Question 1: The carbon fluxes shown toward the left side, beneath Net land flux display two values alongside Gross photosynthesis of 113 (orange) and 29 (red). I assume that Gross photosynthesis refers to GPP (as opposed to Net primary Production or NPP) but I'm puzzled as to what the 29 (red) refers to. Both the 113 (orange) and 29 (red) point toward the earth indicating carbon flux away from the atmosphere (e.g. reducing carbon in the atmosphere) which I understand happens with Naturally occurring photosynthesis. The 29 (red), however, indicates an anthropogenic activity, purportedly related to Gross photosynthesis that reduces carbon in the atmosphere. I'm puzzled as to what that 29 (red) may be.

Question 2: Similar to the carbon fluxes shown on the left side, beneath Net land flux, I'd like to know more about what constitutes the carbon fluxes shown on the right side, beneath Net ocean flux. Here I am looking for any information on the carbon flux that makes up the Ocean-atmosphere gas exchange, including the carbon flux toward the atmosphere of 54.6 (orange) and 23 (red) (e.g. increasing carbon in the atmosphere), and the flux away from the atmosphere of 54.0 (orange) and 25.5 (red) (e.g. reducing carbon in the atmosphere).

Excerpts or simply page numbers in the report where I may learn more about the above numbers would be greatly appreciated.

Answer 1

Oh dear, I agree that this is a poorly explained figure. Let's look at the caption:

Yellow arrows represent annual carbon fluxes (in PgC yr–1) associated with the natural carbon cycle, estimated for the time prior to the industrial era, around 1750. Pink arrows represent anthropogenic fluxes averaged over the period 2010–2019.

I don't think that second sentence is correct. My interpretation is that the pink arrows are the anthropogenic perturbation of the natural carbon cycle that give us the observed present-day fluxes. They're not meant to represent separate processes from the more obviously labelled yellow arrows.

So, total respiration and fire emissions were 111.1 PgC/yr in 1750 and are 111.1+25.6=136.7 PgC/yr in the present day. Similarly, gross photosynthesis was 113 PgC/yr in 1750 and is 113+29=142 PgC/yr in the present day, which represents approximately a 25% enhancement (due mainly to CO2 fertilization). That fits with some text in the Fig 5.12 caption from an earlier draft of the report:

The relative change of Gross photosynthesis since pre-industrial times is estimated as the range of observation-based of 31 ± 3 % (Campbell et al., 2017) and land-model of 19±12% (Sitch et al., 2015) estimates. This is used to estimate the pre-industrial Gross photosynthesis, assuming a present-day range of 116–175 PgC yr-1 (Joiner et al., 2018). The corresponding emissions by Total respiration and fire are those required to match the Net land flux...

Note that most of those pink values on the net fluxes row come from Friedlingstein et al (2020), who describe them as "the overall perturbation of the global carbon cycle caused by anthropogenic activities". Where I think this figure differs from Friedlingstein et al slightly is that it imposes closure of the cumulative 1750 to 2019 carbon fluxes (see column 1 of Table 5.1), which means that the budgetary imbalance of 20 PgC going back to 1850 is assigned to the pre-industrial land and ocean sinks.

Re the oceans, in some ways this is a simpler case than the land. All parts of the ocean are continually exchanging CO2 with the atmosphere: some CO2 molecules dissolve into surface waters (entering through diffusion or bubble entrainment and collapse), and some CO2 molecules outgas from surface waters (leaving through diffusion or bubble cavitation). Figure 5.12 shows estimates of those gross fluxes in 1750 in yellow: 54.0 PgC/yr entering the ocean and 54.6 PgC/yr leaving the ocean, giving a net loss from the ocean to atmosphere of 0.6 PgC/yr.

Anthropogenic emissions have perturbed the system so that there are now gross CO2 fluxes of 54.0+25.5=79.5 PgC/yr entering and 54.6+23.0=77.6 PgC/yr leaving the ocean, giving a net gain of CO2 by the ocean of -0.6+2.5=1.9 PgC/yr. Both of those gross fluxes have increased because the CO2 concentrations in the atmosphere and surface ocean have both increased but, on average, the atmospheric concentration has increased more. This produces a gradient in CO2 concentration across the atmosphere-ocean interface that drives the net flux into the ocean. This gradient is maintained partly by continued fossil fuel emissions into the atmosphere and partly by physical and biological ocean processes that transfer dissolved CO2 down away from the surface into deeper ocean waters.