Are the natural sinks at an End?

Articles are currently being circulated in the media claiming that natural CO₂ sinks have “suddenly and unexpectedly” ceased to function, such as the article in the British magazine Guardian “Trees and land absorbed almost no CO2 last year. Is nature’s carbon sink failing?“. The natural CO₂ reservoirs are the biological world, consisting of all living organisms, plants, animals and humans. In addition, the oceans, which store around 50 times the amount of CO₂ in the atmosphere. It is known and has been proven for many decades that both the biological world and the oceans are strong CO2 sinks. Currently, more than half of all anthropogenic emissions are absorbed by the two major sink systems, as shown in Figure 1.

What has happened that suddenly the sink effect is supposedly diminishing? Even at first glance, the diagram reveals that the sink effect shown, which is attributed to land plants, is subject to extraordinarily strong fluctuations, much more so than in the case of the oceans, for example. This should immediately make us suspicious when we talk about a “one-off” event within the past year.

A closer look at all the articles published on this topic quickly reveals that they all refer to a single publication. The “scientific” basis and trigger for the current discussion is apparently this article: “Low latency carbon budget analysis reveals a large decline of the land carbon sink in 2023“. 

In order to find an appropriate answer to this, it is necessary to take a closer look and use original data to examine how the changes in concentration develop. In the publications “Emissions and CO ₂ Concentration – An Evidence  Based Approach” and “Improvements and Extension of  the Linear Carbon Sink Model” I carefully analyzed the relationship between emissions, concentration increase and sink effect and developed a robust, simple model of the sink effect that not only reproduces the measurement data of the last 70 years very accurately, but also allows reliable forecasts. For example, the concentration data for the years 2000-2020 could be predicted with extremely high accuracy from the emissions and the model parameters determined before the year 2000. However, the most recent series of measurements used in the publications ended in December 2023 and annual averages were used, so the phenomena that are currently causing so much excitement are not yet taken into account.

Detailed analysis of the increase in concentration until August 2024

Since details of the last two years are now important here, the calculations of the publication mentioned are continued here with monthly data up to August 2024 in order to get a clear picture of the details and to include the latest data. The starting point is the original Maona Loa measurement datawhich are shown in Figure 2.

The monthly data is subject to seasonal fluctuations caused by the uneven distribution of land mass between the northern and southern hemispheres. Therefore, the first processing step is to remove the seasonal influences, i.e. all periodic changes with a period of 1 year. The result can also be seen in Figure 2 (orange color).

The global sea surface temperature is also subject to seasonal fluctuations, but to a much lesser extent, as shown in Figure 3.

Formation and analysis of the monthly increase in concentration

The “raw” increase in concentration is calculated by subtracting successive measuring points:    

It is easy to see that the monthly fluctuations in the increase in concentration are considerable and that the particularly high peak at the end of 2023 is by no means a singular event; in particular, the positive peak is preceded by a much larger negative one, which has not been mentioned in the press.  The much higher increase in 2015 was also not taken into account. This would have made it easy to refute the bold hypothesis of a declining sink effect, as the smoothed data (orange) shows that there is a clear trend of a declining increase in concentration after 2015.

After smoothing (orange), it is easier to recognize the actual trend than with the raw, noisy differences. As these are monthly values, the values must be multiplied by 12 in order to draw conclusions about the annual increase in concentration. There is no doubt that the right-hand side of the diagram shows that there has actually been an increase in concentration since 2023, which is interpreted in the current discussion as a decrease in sink performance.

Interpretation of the increase in concentration growth as a result of natural emissions

To illustrate this, the figure shows the sink capacity (green) from the difference between anthropogenic emissions (blue) and concentration growth (orange).
This calculation is a consequence of the ontinuity equation based on the conservation of mass, according to which the concentration growth G_i in month i results from the difference between the total emissions and all absorptions A_i , whereby the total emissions are the sum of the anthropogenic emissions E_i and the natural emissions N_i , i.e.
G_i = E_i + N_i – A_i
The effective monthly sink capacity S_i is calculated as the difference between the monthly anthropogenic emissions E_i and the monthly concentration growth G_i , i.e.
S_i = E_i – G_i
Following the continuity equation above, the effective sink capacity S_i therefore is the difference of ocean and plant absorptions A_i and natural emissions N_i:
S_i = A_i – N_i

It is easy to see that the effective sink capacity (smoothed over several months) does not fall below 0 at the right-hand edge of Figure 5 either. However, it is actually decreasing in 2023-2024. We must now remember that, according to the continuity equation, the “effective sink capacity” consists of absorptions (sinks in the narrower sense) and natural emissions. It could therefore also be that the peaks in concentration growth are caused by natural emissions.  These are not mentioned at all in the publications that are currently sounding the alarm.

It is generally known and a consequence of Henry’s Law that the gas exchange between the sea and the atmosphere depends on the sea surface temperature. We therefore expect increased CO₂ emissions from the oceans as the temperature rises, somewhat exaggeratedly comparable to a bottle of bubbles in an oven.
This consideration motivates the introduction of temperature as a model parameter in the description of the effective sink capacity. The details of the elaboration of this model extension can be found in the above-mentioned publication and in its simplified description.

Figure 6 shows what the addition of temperature means for modeling the increase in concentration.

While the green curve represents the widely known linear sink model (see here (2024), here (2023), here (2019) and here (1997)), as described in the publication or in the simplified representation, the orange curve also takes into account the dependence on temperature according to the new publication. It turns out that the current “excessive” increase in concentration is a natural consequence of the underlying rise in temperature. The sink performance, the linear dependence of absorption on the CO₂ concentration, has not changed at all.

It is therefore high time to establish temperature as a “normal” cause of CO₂ concentration changes in the public debate as an influencing factor instead of making wild speculations about the absence of sinks without evidence.