Why is the CO₂ level in the atmosphere rising?

This webpage is the online version of the infographic on this subject.

This is what most people believe

It seems so obvious. In the same period that we have massively emitted CO₂, the concentration in the atmosphere has risen sharply. The believe is that humans have disturbed the natural balance and that anthropogenic emissions are the decisive cause of the rise ("if you put more into it than nature can absorb, the concentration increases").

The major assumptions to blame human CO₂

➊ Without human perturbation the natural up and down flows of 210 PgC/yr Petagram Carbon. 1 Petagram = 1 Gigaton = 1 billion tons are supposed to be in perfect balance and can’t be the cause of the CO₂ rise. The Residence Time of CO₂ is around 4.1 years (i.e. the average time CO₂ remains in atmosphere).

➋ Almost half of the human CO₂ accumulates in the atmosphere and is the sole cause of the yearly CO₂ rise (+5 PgC/yr). It remains almost indefinitely in the atmosphere (>100,000 instead of 4.1 years).

➌ The third argument is not from the GCB but from ice core drillings on Antarctica. The CO₂ found in deep layers suggest that the levels in the past 800,000 years were much lower (less than 300 ppmv) than the present level.

This is what we know about the GCB

Show picture text:
1. Only the annual human emission (11 PgC/yr) and the concentration in the atmosphere (417 ppm in 2023) are accurately known (ppm = parts per million). The much larger natural flows have an uncertainty of ±20%.
2. In the well-mixed atmosphere human CO₂ cannot be distinguished from natural CO₂. There is no separate natural down flow that can compensate the natural up flow.
3. The carbon mass in the sink (land/oceans) of 41,000 PgC is around 50 times larger than in the atmosphere.
4. The average time CO₂ remains in the atmosphere (the Residence Time τ_a) is around 4.1 years. The Residence Time in the sink (τ_s) is around 190 years.
5. The up and down flow are not directly related and take place at different times and/or places.
6. The down flow is defined by the mass in the atmosphere and its residence time. In this case: 216 PgC/yr = 885 PgC / 4.1 yrs. A higher atmospheric concentration means more uptake and/or less emission, lower concentration means less uptake and/or more emission.
7. The up flow is defined by the mass in the sink and its residence time: 210 PgC/yr = 41,000 PgC / 190 yrs.

The assumptions to blame human CO₂ are incorrect

➊ Are natural flows without human perturbation in perfect balance?

Although never measured, the IPCC assumes that without human perturbation the natural inflow is always exactly equal to the natural outflow. However:

• In the land and ocean sinks most of the CO₂ is transformed into other carbon compounds, such as carbohydrates, (bi)carbonates, calcium carbonates, etc.
• The physical, chemical and biological processes that define the amount of carbon that is stored or released to/from these sinks, are complex and chaotic This Wikipedia-page gives an impression of the complexity of the global carbon cycle. .
• The carbon sinks are very large compared to the atmosphere (~50 times larger).
• A small imbalance, even for many years, is quite possible and would have no noticeable impact on the sub-surface reservoirs. Due to their great uncertainty, we do not know whether the up and down flows are equal.
• Based on the numbers we know (see above) we cannot tell what causes to atmospheric increase. The airborne fraction (the part of human emissions that remains in the atmosphere) can be anything between 0 and 5 PgC/yr. See waterfall charts below.

➋ Does human CO₂ accumulate in the atmosphere, due to a much longer residence time?

• All the CO₂ exchanged between well-mixed atmosphere and the oceans (and other waters) is the result of differences in the concentration above and below the surface (Henry’s Law).
• The complex chemical and biological processes do not alter the fact that the oceans will absorb CO₂, if the concentration in the atmosphere is high compared to that in the surface water.
• The surface layer of the oceans is not saturated with CO₂, due to the large upwelling and downwelling to and from the deeper ocean (Levy,2013). The small surplus of human CO₂ (5%) will therefore not remain in the atmosphere longer than any other CO₂ (4.1 years according to IPCC).

It is very illogical to assume that the absorption capacity of the oceans is very large at 280 ppm (a quarter of all CO₂ is exchanged in one year), but that at a slightly higher concentration the absorption suddenly becomes factors smaller. This is even more obvious as we realize that since 1750 natural emissions have increased with 40 Petagram Carbon per year, that is 3.5 times more than human emissions, with only a modest change in residence time (IPCC AR5).

What happens in the event of a perturbation?

To get a better understanding of the impact of human emissions, we consider a situation where the natural flows are precisely in balance, and where this balance is perturbed by a single large (human) emission.

• Imagin up and down flow are in balance.
• At one moment in time 100 PgC is added to the atmosphere.
• The mass in the atmosphere increases from 885 to 985 PgC.
• The down flow is proportional to the mass, so will increase to 240 PgC/y (= 985 / 4.1).
• This reduces the mass in the atmosphere and increases the mass in the sink.
• A year-by-year (simple) simulation is given in the Excel-table.
• The mass in the atmosphere decreases to almost the old level (2% remains after 10 years), blue bars in the chart.
• Most of the added CO₂ (98%) ends up in the sink, green line.
• The adjustment time (= time to re-equilibrate) is 4.0 years, slightly smaller than the residence time.

Human CO₂ does not accumulate in the atmosphere

The time to re-equilibrate from a perturbation is shorter than the residence time.

• Stallinga (2023) shows that the adjustment time is always shorter than the residence times.
• The extra CO₂ is distributed to atmosphere and sink in the ratio based on the size of the reservoirs. In this case: land/ocean absorbs around 50 times more than the atmosphere.

Only a small percentage of human CO₂ remains in the atmosphere

• Since 1750 humans have emitted around 700 PgC (incl. land use change). From up to 10 years ago, only 2% of all that is still in the atmosphere. From the last 10 years a larger part is in the atmosphere.
  • If we stabilize human emission at current level, around 7% of the CO₂ in the atmosphere is human caused.
  • If we would stop emitting today (net-zero), the human contribution will quickly go down to less than 2%.
  Source: Stallinga 2023

Temperature is a far more likely cause for the rising CO₂

As only a small percentage of the human emission remains in the atmosphere, it cannot be the main cause of the CO₂ rise. There are many arguments that the increased temperature is far more important for the rise of the CO₂ level.

Temperature change is a likely cause for CO₂ change

Temperature is important factor in Henry’s Law. So, for the oceans and other waters: Higher temperature ➜ less solubility in water ➜ more emission / less absorption.

Soil respiration is exponentially related to temperature (Lee 2011). The temperature induced increase is >25% in the past 50 years (Zhang 2016).

There are direct observations that confirm the impact of temperature on CO₂

• There is a significant correlation between changes in T and CO₂, where CO₂ always lags T.
• Koutsoyiannis (2023) investigated the causal relationship based on accurately measured data since 1979: “Changes in CO₂ concentration cannot be a cause of temperature changes. On the contrary, temperature change is a potential cause of CO₂ change on all time scales.”.
  
  

CO₂ rise can be fully explained by temperature variations

Based on a linear regression the CO₂ rise can be fully explained by the temperature.

Other (historical) data confirm natural cause of CO₂ rise

➌ Ice core data do not refute a natural cause for the CO₂ rise

• Until 1985 most ice core studies indicated that CO₂ concentrations were higher than the current atmospheric level (up to 2450 ppm). After 1985, lower pre-industrial CO, values were reported, and used as evidence for a recent man-made CO, increase (Jaworowski 1992).
• The current ice core results suffer from serious problems, casting doubt on the reliability of the data presented. Most important is the dissolvement of CO₂ in the water and ice in the many years before the air bubbles in the ice are fully closed. So, the absolute value of the measured concentration is a fraction of the original value.(Jaworowski 1992).
• Ice core reconstructions over the past 800,000 years give a very flattened representation. A single observation in an ice layer represents a period of on average 730 years, with peaks up to more than 5000 years. Short fluctuations (<5000 years), even with much higher concentrations, are therefore not visible.

Direct measurements and other proxy data show higher historical levels

Other observations that show (much) higher historical CO₂ values and/or more variation, have largely been disregarded: direct scientific measurements in the period before 1959, CO₂ ice core reconstructions in Greenland, and CO₂ proxies from plant stomata.

Human-caused carbon emissions on climate is non-discernible

In recent years, a decrease in atmospheric δ13C has been observed, reflecting the relative change of stable carbon isotopes 12 and 13. This decrease is often attributed to the combustion of fossil fuels. However, δ13C proofs to be consistent with an input isotopic signature that is stable over the entire period of observations (>40 years), i.e., not affected by increases in human CO2 emissions.

Conclusions

References