Greenhouse gas balances: The calculation is not yet working out

When the Global Carbon Project (GCP) published its annual carbon budget report in a preprint a few days ago, the focus was initially on fossil emissions - of course, because the figures were presented at the current UN climate summit in Glasgow.

A good 34.8 billion tons of carbon dioxide flowed out of chimneys, factory chimneys and exhaust pipes worldwide in 2020. That is 5.4 percent less than in 2019, thanks to the Corona kink, but the prediction for this year is that fossil emissions will almost climb back to pre-crisis levels. China, economically faster than most other industrialized countries and even faster in expanding coal energy, is now responsible for almost a third of fossil emissions. "We already expected that there would be a certain rebound effect after Corona," says marine ecologist Judith Hauck from the Alfred Wegener Institute in Bremerhaven. "But we were surprised that emissions were rising so sharply."

Hauck coordinated the ocean contribution to the GCP report. The budget calculation, published annually by an international team, includes all relevant sources and sinks of carbon dioxide on earth. On the side of carbon sources are fossil emissions (around 89 percent in the past ten years) and land use change (11 percent). On the storage side - sinks - are the ocean (26 percent), the vegetation on land (29 percent) and the atmosphere (48 percent). In order for the budget to be balanced, which is prescribed by the conservation of mass, sources and sinks should add up to zero.

How important it is to consider all contributions is shown by the risk of double counting of carbon dioxide sinks, which was also discussed again at the current climate summit. This applies above all to the contribution made by land use change - this includes deforestation, peat drainage and afforestation, but also, for example, the conversion of pastures into fields. In the GCP report, there was the most movement in this sector compared to previous years. The key is the collection of the data. Because deforestation is now counted differently, the trend in emissions in this sector is slightly declining, which, with fossil emissions rising at the same time, has led to roughly constant CO₂ emissions over the last ten years - with the exception of the Corona kink. However, this correction is in the error range of the land use sector.Uncertainties can be attributed, for example, to illegal logging, which is difficult to record - “a core problem in the calculation,” says Julia Pongratz from the Ludwig Maximilians University in Munich, coordinator of the land use contribution.

Possible errors in the invoice

But aren't there satellite images everywhere now?

“We've been using them for a long time,” says Pongratz, who has heard this question many times.

“But forest cover can change for various reasons.” In the GCP, man-made land changes such as deforestation or afforestation are part of the land use sector;

natural growth and death due to droughts but to the land emissions sector.

In the national carbon inventories, which are laid down in the requirements for the Framework Convention on Climate Change, the emissions per area are considered.

What happens on the areas cultivated by humans is attributed to humans.

That is also not a problem, says Pongratz, "unless you count the depression twice". Double counting means: You write down the additional plant growth from carbon dioxide fertilization as a contribution from land use, but forget to deduct this contribution from the depression, i.e. from the part that nature absorbs pro bono. “It cannot be ruled out that some of the sinks will be counted twice,” says Pongratz. One is in the process of investigating this further. The comparison and conversion between the national inventories and the GCP report has been added this year. This is "very valuable", says Pongratz, since the inventory data is not up-to-date, especially for developing countries,

But the GCP report still has gaps.

Anyone who paid attention at the beginning of the text should have noticed that the sinks add up to more than 100 percent - so the calculation does not exactly work.

“The deviation is pleasantly small,” says Julia Pongratz.

Finally, the five contributions would be calculated independently using many models and data sets.

Her colleague Hauck thinks that at the moment we are either underestimating emissions or somewhat overestimating the sinks.

"This illustrates the remaining gaps in our understanding of the global carbon cycle."