Reliable emissions pathway planning tool to meet Paris temperature target — ScienceDaily

The central goal of the Paris climate agreement is clear: to limit man-made global warming to well below 2°C. This limit requires a reduction in greenhouse gas emissions to net zero. But what do the intermediate stages look like? How big should the emission reduction be in the next five, ten or fifteen years? And which emission path is followed? There is no consensus between countries on these issues, making it difficult to actively implement the Paris Agreement.

Researchers at the University of Bern have now developed a new method to continuously determine the necessary reduction in emissions. The core idea: Instead of complex climate models and scenarios, the observed connection between warming and emissions is used and the reduction path is constantly adapted to the latest observations. This new approach has just been published in the journal Nature Climate Change.

A new calculation method for the emission reduction path

So far, climate models have been used to calculate possible emissions pathways to the net-zero target. These paths are based on scenarios that include economic and social developments. “These calculations of the emission paths are subject to large uncertainties. This makes decision-making more difficult and could be one reason why the reduction commitments of the 194 signatory countries to the Paris Climate Agreement are still insufficient,” says lead author Jens Terhaar, explaining the background to the study. Like most other authors, Terhaar is a member of the Oeschger Center for Climate Research at the University of Bern.

“Since the climate agreement is actually aimed at temperature regulation, we considered specifying an optimal emission reduction path that is independent of model-based projections,” Terhaar continues. From this initial idea, a calculation method was developed that is based exclusively on observation data: on the one hand global surface temperatures in the past, on the other hand CO2 emission statistics.

The Paris Climate Agreement calls for an inventory of the necessary reductions in global emissions every five years. “The new Bern calculation method is ideally suited to support the inventory mechanism of the Paris Agreement, as it enables regular adaptive recalculation of emission reductions,” explains co-author Fortunat Joos from the Oeschger Center. For this purpose, a new algorithm known as AERA (Adaptive Emissions Reduction Approach) was developed. Put simply, the algorithm correlates CO2 emissions with rising temperatures and is adjusted using a control mechanism. In this way, the current uncertainties in the interaction of these variables can be eliminated.

“Our adaptive approach bypasses the uncertainties, so to speak,” explains Fortunat Joos. “Just like a thermostat continuously regulates the heating to the desired room temperature, our algorithm adjusts the emission reductions to the current temperature and emissions data. We are thus approaching a temperature target, such as the 2°C target step by step and with specific intermediate targets.”

Stricter emission targets and effective implementation

“The AERA method already confirms that international climate policy needs to be much more ambitious,” Terhaar demands. According to the Bern study, global CO2 emissions would have to fall by 7 percent between 2020 and 2025 in order to achieve the 2°C target. Compared to 2020, however, they actually increased by around 1 percent in 2021. According to the algorithm, in order to limit global warming to 1.5 °C, a reduction of 27 percent would be required by 2025. “We need significantly stricter emission targets than those to which the states have committed themselves,” explains Thomas Frölicher, co-author of the study from the Oeschger Center, “and, above all, effective implementation of the targets.”

The Bern researchers hope that the new calculation method will succeed in finding its way into international climate policy. “The AERA algorithm is already arousing great interest in climate research because it can also be used for climate modelling,” explains Jens Terhaar. So far, climate models with prescribed greenhouse gas concentrations have been used. This meant that at the end of the 21st century, warming for a given concentration of greenhouse gases was very uncertain. However, when using the climate models with the AERA, the emissions are continuously adjusted according to the calculated temperature and the intended temperature target. Based on this, the model temperature is eventually stabilized at the intended level and all models simulate the same heating but with different emission pathways. “The AERA enables us to consistently investigate effects such as heat waves or ocean acidification for different temperature targets – such as 1.5 °C versus 2 °C versus 3 °C – using state-of-the-art models,” explains Terhaar.

Worldwide, 11 research groups led by the University of Bern have already started using the algorithm to study such effects.

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Materials provided by University of Bern. Note: Content can be edited for style and length.

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