The central goal of the Paris Agreement is to strengthen the global response to the threat of climate change by keeping the global temperature rise this century well below 2 degrees Celsius above pre-industrial levels, and to pursue efforts to limit the temperature increase even further to 1.5 degrees Celsius.
Researchers at the University of Bern have developed a new method for calculating the incremental emission reductions required to meet temperature targets such as the 2°C target. Rather than models and scenarios, the calculation method is based solely on observation. The study concludes that international climate policy must become more ambitious.
The Paris climate agreement has a clear goal: Keeping global warming caused by humans to well below 2°C. This limit necessitates a net-zero reduction in greenhouse gas emissions. But, what about the intermediate stages? How much of an emission reduction should there be in the next five, ten, or fifteen years? And what path is being taken in terms of emissions?
There is no consensus on these issues between countries, which complicates the active implementation of the Paris Agreement.
Researchers at the University of Bern have now developed a new method to determine the necessary reduction in emissions on a continuous basis. The main idea: Instead of complex climate models and scenarios, the observed relationship between warming and emissions is applied, and the reduction path is adapted repeatedly according to the latest observations. This new approach has just been published in the journal Nature Climate Change.
Our adaptive approach circumvents the uncertainties, so to speak. In the same way that a thermostat continuously adjusts the heating to the required room temperature, our algorithm adjusts the emission reductions according to the latest temperature and emissions data. This will allow us to approach a temperature goal, such as the 2°C goal, step-by-step and with specific interim goals.
Fortunat Joos
A new calculation method for the emission reduction path
To date, climate models have been used to calculate possible emissions pathways to the net zero goal. These pathways are based on scenarios including economic and social developments. “These calculations for the emission paths are subject to large uncertainties. This makes the decision-making more difficult and might be one reason why the promised reductions made by the 194 signatory countries to the Paris Agreement remain insufficient,” says lead author Jens Terhaar, explaining the background to the study. Like most of the other authors, Terhaar is a member of the Oeschger Center for Climate Change Research at the University of Bern.
“Since the climate agreement actually aims at regulating temperature, we thought to specify an optimal emissions reduction path for this purpose which is independent of model-based projections,” continues Terhaar. According to this initial idea, a calculation method has emerged which is based exclusively on observation data: on the one hand, global surface temperatures in the past, and on the other hand, CO2 emissions statistics.
The Paris Agreement calls for a stocktake of the necessary reductions in global emissions every five years. “The new Bern calculation method is ideally suited to support the stocktake mechanism of the Paris Agreement, as it enables the emission reductions to be recalculated regularly on an adaptive basis,” explains co-author Fortunat Joos of the Oeschger Center. For this purpose, a new algorithm has been developed which is known as the AERA (adaptive emissions reduction approach). In simple terms, the algorithm correlates CO2 emissions with rising temperatures and is adjusted using a control mechanism. In this way, the current uncertainties in the interaction between these variables can be put aside.
“Our adaptive approach circumvents the uncertainties, so to speak,” explains Fortunat Joos. “In the same way that a thermostat continuously adjusts the heating to the required room temperature, our algorithm adjusts the emission reductions according to the latest temperature and emissions data. This will allow us to approach a temperature goal, such as the 2°C goal, step-by-step and with specific interim goals.”
Stronger emissions goals and effective implementation
“The AERA method already confirms that international climate policy must be far more ambitious,” demands Terhaar. According to the Bern study, to achieve the 2°C goal, global CO2 emissions would have to fall by 7 percent between 2020 and 2025. They actually increased by approximately 1 percent in 2021 in comparison with 2020, though. According to the algorithm, limiting global warming to 1.5°C would require as much as a 27 percent reduction by 2025. “We need far stricter emissions goals than those to which nations have committed,” explains Thomas Frölicher, co-author of the study from the Oeschger Center, “and above all else, effective implementation of the goals.”
The Researchers in Bern hope that the new calculation method will succeed in finding its way into international climate policy. “The AERA algorithm is already attracting a lot of interest in the climate research community, as it can also be applied to climate modelling,” explains Jens Terhaar. Until now, climate models with prescribed greenhouse gas concentrations have been used. This meant that at the end of the 21st century, the warming for a specific greenhouse gas concentration was very uncertain. When using the climate models with the AERA, however, emissions are continuously adjusted according to the calculated temperature and the intended temperature goal.
On this basis, the model temperature is eventually stabilized at the intended level and all the models simulate the same warming, but with different emission pathways. “The AERA enables us to study impacts such as heat waves or ocean acidification for different temperature goals – such as 1.5°C versus 2°C versus 3°C – on a consistent basis and with state-of-the-art models,” explains Terhaar. Worldwide, 11 research groups have already started to apply the algorithm under the leadership of the University of Bern in order to study such impacts.
