Greenhouse Gases and Climate Sensitivity

Wed, 10 Sep 2025, 17:00 CEST

Conveners: Christo Buizert & Eric Wolff

Earth's climate and environmental conditions have changed dramatically through geologic time, and the concentration of atmospheric greenhouse gases is an important driver of such change. Ice-core and proxy-based reconstructions of past carbon dioxide levels allow a better understanding of its role in past natural climate change and the dynamics of the Earth system. Climate sensitivity, commonly defined as the global average warming resulting from a doubling of atmospheric CO2, depends on a series of complex feedback processes that include changes to cloud formation, vegetation, and ice cover. Coupled changes in past climate and radiative forcing can be used to provide direct empirical constraints on Earth’s climate sensitivity – the most important parameter in projecting future climate change from anthropogenic carbon emission.

Guest speakers:

• James Rae (University of St. Andrews, UK)
• Jessica Tierney (The University of Arizona, USA)

Refining reconstructions of atmospheric CO₂ and climate sensitivity from climates of the past

James Rae

Climates of the past provide invaluable insights into global change under elevated CO₂, including modes of Earth system functioning and climate sensitivity. CO₂ reconstructions are central to these efforts and have seen concerted development in recent years. Here I present an overview of the current state of CO₂ reconstruction from the mid-Pleistocene to the PETM and beyond. These studies demonstrate CO₂'s pervasive control on global climate, including the coupling of declining CO₂ levels and glacial intensification in both the Pleistocene and the Ordovician, while highlighting the need for further research to better resolve and refine our fossil record of the atmosphere.

Estimating climate sensitivity from the paleoclimate record

Jessica Tierney

Paleoclimates are crucial for constraining Earth's climate sensitivity (ECS), especially the upper tail of the distribution which is not well constrained by historical observations. However, changing boundary conditions (ice sheets, vegetation, and continental configuration) complicate how we use past climates to infer present-day ECS. In this talk I'll discuss recent approaches we've taken to tackle this problem including addressing the paleo-"pattern effect". So far, we've discovered ancient climates have more amplifying feedbacks than the present day, which raises a question as to whether the present climate sits represents an ECS minimum.