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Shanghai, China: White, desulfurized steam is leaking from the chimney of a thermal power plant.

Photo: Eric Yang/Getty Images

Numerous countries have pledged to emit as little carbon dioxide and other greenhouse gases as possible into the atmosphere as quickly as possible in order to stop man-made climate change. The world is striving for "net zero emissions," as experts say. The idea behind this is that if humanity stops emitting carbon dioxide, the planet will stop heating up.

In fact, several studies in recent years have independently shown that the expected increase in global temperature this century depends on the total amount of carbon dioxide (CO₂) emitted. Any climate target should therefore include a cap on CO₂ emissions, known as the carbon budget. Such a budget allows for a balance of debits and credits. In other words, man-made emissions can be offset on paper if carbon dioxide is removed from the atmosphere. In a natural way or with human help.

But the calculation is difficult. This is because natural and anthropogenic emissions are defined differently by the Intergovernmental Panel on Climate Change (IPCC) and in the National Greenhouse Gas Inventories (NGHGI). As a result, it is difficult to monitor the progress of individual countries. The discrepancies even mean that the countries' targets do not match scientific standards – which increases the risk of missing net zero, writes a team of researchers in the journal Nature. And immediately provides a suggestion to solve the problem.

IPCC focuses on processes, the greenhouse gas inventory on the situation

It has been known for some time that there are two different definitions. The consequences of this, however, have been unclear until now. The Intergovernmental Panel on Climate Change (IPCC) defines anthropogenic on the basis of actions and processes: which processes are caused by humans and are therefore anthropogenic, and which are natural? According to the study, the reaction of forests to a changing climate is considered natural, but the felling of trees is not, as the group led by climate scientist Matthew Gidden of the International Institute for Applied Systems Analysis explains in the current study.

In contrast, the United Nations Framework Convention on Climate Change (UNFCCC) requires its members to submit a National Greenhouse Gas Inventory (NGHGI) every year – which is determined by location: is a particular carbon sink or source located on "managed" land, such as farmland or nature reserves, or on "natural" land? Only the first case is considered an anthropogenic factor.

"Therefore, if a natural process takes place on cultivated land, it can be identified differently according to these two definitions," write Gidden, who contributed to the IPCC's 6th Assessment Report, and his colleagues. This, in turn, can lead to confusion and, in the worst case, even to the fact that targets are unintentionally missed due to misleading calculations.

To gain clarity, the group has developed a model to re-analyze the IPCC emission pathways and compare them with inventories. One result: "We find that it becomes more difficult to achieve key global emission reduction targets when the NGHGI conventions are applied, as both an earlier net CO2 limit and lower emissions are required," the researchers write. In the case of scenarios with a warming of 1.5 degrees Celsius, for example, net-zero emissions would have to be achieved up to five years earlier than previously assumed.

"Our results are important for the global stocktaking, as they indicate that countries need to jointly make their climate targets more ambitious in order to meet global temperature targets," the team concludes.

The consequences have been recognized – and now?

Is the approach of Gidden and colleagues perfect? "Unfortunately, no – the quantification still contains numerical uncertainties," write climate scientists Chris Jones and Alexander Askew of the Met Office in Exeter, UK, in an article accompanying the study. Because it is still unclear how some natural processes work and how they will change in the twenty-first century, this is only logical.

However, it is also clear that things cannot go on as they are now. "At the end of the day, no amount of creative accounting should obscure the fact that fossil fuel burning must be stopped," Jones and Askew stress. Humanity cannot compensate for its way out of the climate crisis, "but at least there is now a way to better track progress."

Gidden's team also makes it clear: "Emissions must peak as soon as possible and fall significantly this decade."

The next UN Climate Change Conference starts at the end of November

At the 2015 UN Climate Change Conference in Paris, countries around the world agreed to limit global warming to less than 2 degrees Celsius, and if possible even to 1.5 degrees. However, it is already becoming apparent that this goal will be missed: With the current commitments of the countries, the world could even warm by up to 2.9 degrees by the year 2100, according to a report by the United Nations at the beginning of this week.

The findings of the report are to be incorporated into the debates at the COP28 climate conference, which begins next week in Dubai. From 30 November to 12 December, delegates from almost 200 countries will meet there to discuss once again how to finally implement the promises made at previous summits. Gidden and his co-authors hope that their findings will also be taken into account.