A study performed by Robbin Bastiaansen, Anna von der Heydt, the University of Utrecht, the Netherlands, and Peter Ashwin, the University of Exeter, UK, indicates that it could still be rather difficult to precisely determine the Equilibrium Climate Sensitivity in complex climate models.
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The equilibrium climate sensitivity is derived using a limited amount of data from a relatively brief simulation and is used to compare and assess models.
However, the authors of the study conclude that such outcomes could be severely underestimating long-term warming as late climate tipping cannot be excluded by the commonly used methods to estimate equilibrium climate sensitivity. The work performed is considered to be part of the European TiPES project on tipping points present in the Earth System.
The equilibrium climate sensitivity is considered to be an essential number in climate science since it is ideally suited for the comparison and assessment of climate models. The number has been specified as the complete increase in global mean temperature following a doubling of CO2 present in the air. Since the Earth’s system has been large and complicated, attaining a final equilibrium temperature can take thousands of years.
Yet, advanced climate models only require a few months of calculations on supercomputers to perform simulations following 150 years of climate change. Therefore, it is not feasible to have the models run for years on end to simulate thousands of years of climate change in order to find the model's equilibrium climate sensitivity.
Rather, an easier technique has been put to use: Once a model has been subjected to the simulation of a couple of hundred years of climate evolution, the data is gathered and then utilized to evaluate how much the mean global temperature goes up if the model was allowed to run until the equilibrium temperature was reached.
However, this method could be underestimating temperature rise. The study published in the journal Proceedings of the Royal Society A reveals that such techniques have the potential to fail in simple climate models and thus also may be insufficient for bigger and advanced climate models.
One issue that has been determined by the team is that climate models, as well as the real climate system, may exhibit an abrupt temperature increase even after years of an apparently stable climate.
In other words, an abrupt transition in a part of the climate system later than 150 years, such as the partial collapse of an ice sheet or sudden desertification of a large part of a continent, can greatly influence the global mean temperature and current methods are unable to truly estimate the resulting warming.
So, we show that to be sure of the long-term behavior of a modern global climate model, there are no shortcuts to doing extensive simulations. If you want to know what ultimately is the response/temperature for a given amount of added CO2, there will be no easy, straightforward and sound way to determine that for sure – even in models.
Robbin Bastiaansen, Anna von der Heydt, University of Utrecht
The TiPES project is an EU Horizon 2020 interdisciplinary climate science project based on tipping points in the Earth system. Around 18 partner institutions in over ten countries are working collaboratively on this project.
TiPES is coordinated and headed by The Niels Bohr Institute at the University of Copenhagen, Denmark, and the Potsdam Institute for Climate Impact Research, Germany. The TiPES project has received financial support from the European Horizon 2020 research and innovation program, grant agreement number 820970.
Bastiaansen, R., et al. (2022) Climate response and sensitivity: time scales and late tipping points. Proceedings of the Royal Society A. doi.org/10.1098/rspa.2022.0483.