A framework for assessing paleoclimate analogy

Our climate is changing due to anthropogenic influences, and we are heading into climate conditions that are largely unknown to modern humans. Considering the great threat that anthropogenic climate change is, there is a need to accurately project how our climate will respond to warmer conditions in the future. Reconstructions of the Earth’s past show that many climatic features that we observe today have – to some extent – been present in the geological archive. So, we can study paleoclimate to advance our understanding of dynamics and processes in warm climates, as well as to explore responses and sensitivities of the Earth’s climate to forcing changes. Given the similarities between the past and the projected future, researchers have been trying to establish analogy between paleoclimate and future climate. This could include analogy in terms of elevated or rising CO₂ concentrations, elevated surface temperatures, and specific processes such as ice sheet melt or an AMOC weakening. However, often paleoclimate – future climate analogies are difficult to interpret, since conditions for analogy are not properly defined, or implications of the analogy are unclear or overstated.

In this work, we propose a practical methodological framework to assess paleoclimate analogy, for general use in the climate research community. The framework consists of five main steps: (1) stating the purpose (e.g. which processes are considered) and relevance of the analogy, (2) assessing feasibility of finding an analogy, (3) a detailed description of the followed methodology, (4) assessment of confidence in the analogue, and (5) clear communication regarding the potential as well as limitations of the analogy. As part of the framework, we identify three main types of analogy: (a) analogy in terms of forcing (e.g. CO₂ concentration), (b) in terms of response (e.g. surface temperatures) and (c) in terms of processes (e.g. tipping behavior). We will briefly treat example applications of the framework to highlight its potential, for different types of analogues on different time scales.