Nature as a net sink?
Many people find it difficult to understand that human emissions are not the cause of the increase, while nature (land and sea) ultimately absorbs CO₂. We emit 11 PgC per year, while the amount in the atmosphere increases by 5 PgC/year and so the rest is absorbed by land and sea. The common notion is that nature acts as a net sink, which gives the impression that it cannot contribute to an increase.
However, it is easy to see that it can contribute to an increase, if we realize that natural emissions have increased much more and that a small imbalance between the natural flows is possible and even very realistic. The erroneous reasoning is equivalent to saying that the increase in temperature in a house can only be caused by the small electric heater that supplies 5% of the total energy, without taking into account the much larger changes in the energy use of the central heating system or any temperature changes outside.
The flawed bookkeeping of the Global Carbon Budget
The annual determination of the Global Carbon Budget, or GCB (Friedlingstein, P. et al. (2023)) is based on bookkeeping. The GCB a large project that yearly presents an overview of all the fluxes and reservoirs of the global carbon cycle. The CO₂ change in the atmosphere in one year is the sum of all the human and natural emissions, minus the sum of all the absorptions in that year. The conclusions are however colored by the all-determining assumption that without human perturbation, the natural inflows and outflows are always in perfect balance with each other. This leads to the conclusion that every year roughly half of the human-produced CO₂ accumulates in the atmosphere, while the other half is absorbed by the land and oceans.
The following waterfall charts illustrate the flows to and from the atmosphere. In the left one from the GCB, the gray part is the human contribution on top of the (green) natural flow. Some CO₂ is absorbed (depicted as light green), while some persists in the atmosphere (represented as light blue). For clarity, the Y-axis in the graphs does not start at 0 but at 203 PgC/year. In reality the green bars are much longer.
In the left graph, the natural inflow and outflow are exactly the same, so that all increase is the result of human emissions. It is however important to realize that only the increase in atmospheric concentration and annual human emissions are accurately known. We cannot measure the other values with precision. The margin of error is according to the IPCC approximately ±20%, but it is probably even greater.
The use of bookkeeping (as every accountant knows) is only possible if all the fluxes to and from the atmosphere are accurately known. This is definitely not the case. The natural flows to and from the atmosphere are not directly related, have different causes and occur at different times and/or places. Most of the CO₂ is transformed into other carbon compounds on land and sea, based on complex and chaotic processes. Significant variations exist between different locations, as well as between day and night. Seasonal changes and local temperatures also have a substantial impact. In plant respiration, there isn't a fixed ratio between the uptake from photosynthesis and the release through respiration. Besides the amount of sunlight, numerous other factors play a role.
The only thing we really know is the conservation law as presented in the right chart. The 11 PgC/yr human contribution cannot be distinguished from the much larger natural emissions. We only know that the total up flux is around 5 PgC/yr larger than the down flux. Based on the measurements we cannot tell what part of human emissions is absorbed and we cannot tell what the increase without human emissions would be. So, based on the accurately measured data we cannot conclude what caused the atmospheric increase.
The total carbon mass contained in land and ocean reservoirs is 40 to 50 times larger than in the atmosphere, so a relatively small uptake or release cannot be measured. A minor imbalance, even for many years, is quite possible and would have no noticeable impact on the sub-surface reservoirs. A change in the oceanic carbon mass of 0.8% over a period of 275 years is enough to explain the full rise of 140 ppm since the preindustrial period.
Taking everything into account, the likelihood of natural flows perfectly negating one another is exceedingly slim. A small imbalance (for whatever reason) over a shorter or longer period is not only possible, but also very realistic. All parameters that determine weather and climate exhibit fluctuations on all time scales. It is reasonable to believe that this applies to CO₂ concentrations as well.
Illogical behavior of human CO₂
If we take another good look at the Global Carbon Budget as presented in the left chart of Figure 1, you can see that the behavior of human emissions is very strange. Approximately 45% of the emissions remains almost indefinitely in the atmosphere, so with a extremely long residence time. The other 55% (sink) is absorbed immediately by land and oceans, so this part has a residence time of 0 years. To explain the accumulation of human CO₂ it is apparently needed to assume a very deviant behavior for these molecules, as the residence time of all other CO₂ is around 4 years.
This inconceivable behavior comes from the presumption that the absorption rate of human CO₂ is not dependent from the concentration, but from the emission (55% of the emission is absorbed). This violates however all physical laws. Nature has no knowledge about how and why CO₂ entered the atmosphere. It can only respond to the amount of CO₂ that is present in the atmosphere at a certain moment of time. Thus, the amount of emission can never be a driver for the absorption.
Natural emissions have increased much more
Even if we do assume a perfect balance between the natural flows in the period before the industrial time, the waterfall chart from Figure 1 is still misleading. It shows the changes from the human emissions since 1750, but it forgets that also the natural flows have changed since that time. The following chart shows all the changes since 1750. The total change in emissions is 54 PgC/yr of which 20% human emissions.
It clearly shows that contribution of human emissions is now much smaller. The large majority of the of the CO₂ rise is the result of the increase of the natural emissions.
The increased natural fluxes can fully explain the CO₂ rise even though nature is a net sink, i.e. the annual change of CO₂ in the atmosphere is smaller than the amount of human emissions in the same period. As seen in The green CO₂ level the only assumptions for that are a single residence time for all CO₂ and an increase in the natural flows as has been observed.