Reining in the upper bound of Earths climate sensitivity (highECS)
Title: Reining in the upper bound of Earths climate sensitivity (highECS)
DNr: NAISS 2023/2-4
Project Type: NAISS Large Storage
Principal Investigator: Thorsten Mauritsen <>
Affiliation: Stockholms universitet
Duration: 2023-07-01 – 2024-01-01
Classification: 10501 10508


One of the greatest recent advances in climate science is that it is now beyond reasonable doubt that human activity is warming the Earth. The next natural question is by how much the Earth will warm for a given emission – a quantity that will be essential to regulating global warming. Yet, the likely range of 1.5-4.5 K for equilibrium climate sensitivity (ECS) for a doubling of the atmospheric CO2 concentration has not been reduced for decades. In particular the risk of ECS being high is concerning, but also represents a scientifically intriguing challenge. In this project I will conduct unconventional and innovative research designed to limit the upper bound of ECS: I will confront leading hypotheses of extreme cloud feedbacks, the primary potential source of a high ECS, with observations from the full instrumental- and satellite records, and proxies from warm- and cold past climates. This will involve both very long model simulations at low resolution and shorter global cloud resolving simulations. If successful in my objective of reining in the upper bound on climate sensitivity this will be a major breakthrough upon a nearly 40-year scientific deadlock and reduce the risk of catastrophic climate change – if not, it will indicate that extreme policy measures may be needed to curb future global warming. Either way, the economic value of knowing is tremendous.