New study unveils the human and scientific dimensions of IPCC sea level rise projections

The Intergovernmental Panel on Climate Change (IPCC), a body of the United Nations, has long been at the forefront of climate science, regularly producing state-of-the-science assessments and possible solution pathways for policymakers.  In a first-of-its-kind ethnographic study, researchers from Indiana University Bloomington and Princeton University provide insights into the human and scientific dynamics that shape global climate assessments and sea level rise projections in the IPCC’s Sixth Assessment Report (AR6).  

Based on unprecedented access to all four of the IPCC Lead Author Meetings during the multi-year creation of AR6, the study focuses on “the curve” - a visual representation of sea level rise projections under various emissions scenarios.  The curve uniquely incorporates both high-confidence projections and low-likelihood, “worst-case” scenarios related to complex events such as Marine Ice Cliff Instability (see Figure 1*).  

The researchers conducted 71 interviews with all 18 coordinating lead authors (CLAs) and lead authors (LAs) of Chapter 9, where the curve is featured.  They also observed plenary sessions during the IPCC approval process and Conferences of the Parties (COP) of the United Nations Framework Convention on Climate Change.  

“Our project followed a set of authors throughout the AR6 process, focusing on how experts made decisions about how to characterize the state of climate science over time,” explains lead author Jessica O’Reilly, an Associate Professor of International Studies at Indiana University, Bloomington. “We paid attention not only to the knowledge they produced, but the sociocultural dynamics of the writing team, such as conflict resolution and trust building.”

The Collaborative Process Behind “The Curve”

The creation of the curve was a result of a highly collaborative and interdisciplinary process among IPCC authors.  Scientists from various fields contributed to its development, including scientists specializing in ice sheet dynamics, climate system studies, and statistical modeling.  During this process, the authors had to negotiate how to balance well-established science with emerging, uncertain phenomena.  The researchers found that IPCC authors faced challenges in reconciling conflicting methodologies while maintaining the scientific integrity of projections.  

“Research on the West Antarctic Ice Sheet has had some exciting new developments in the past decade, and not all experts agree with these findings,” says O’Reilly.  “Trying to incorporate more established science alongside these new findings in a way that is comprehensible and useful to decision makers was a key challenge for these authors. They had strong, diverging views within the chapter, and external inputs from the review process, to contend with. In more traditional science, everyone would wait for the dust to settle, but IPCC deadlines and the urgent need for climate action meant that the authors had to find a way forward.”

Visual Communication for Policy and Public Engagement

The study also looks at the deliberate design choices behind the curve.  The IPCC used an iterative process to develop two versions of the curve: a detailed version with annotations for scientific audiences and a simplified version for policymakers and the public.  Both versions use “storylines,” a narrative framing device that present plausible future scenarios in a way that can make uncertain science more relatable and actionable.  

“In contrast with a standard scientific report, taking a storyline approach helps the IPCC convey the policy relevance of the scientific assessment,” explains O’Reilly. “It also provides an opportunity to bring together other disciplinary approaches to understanding and solving climate change. With the curve figure, using a storyline approach gave the authors the opportunity to explore sea level rise possibilities that may have low probability of occurrence but high risk due to their high potential impacts on individuals and society as a whole.”

A Reflection on the IPCC’s Dual Role

The study concluded that the development of the curve reflects the IPCC’s dual role as a scientific and social institution.  The collaborative dynamics, debates, and conflict resolution among authors were pivotal to fostering a rigorous process and legitimate assessment.  Additionally, the creation of the curve represents a departure from previous approaches, bringing high-impact scenarios to the forefront to ensure policymakers consider all potential risks.  

“Today, many institutions made up of experts face skepticism from the public due to the obscure nature of both the material they evaluate and their decision processes.  IPCC is not immune to such skepticism.” says co-author Michael Oppenheimer, a Professor at the School of Public and International Affairs and the High Meadows Environmental Institute at Princeton University. “It’s better for IPCC, for science in general, and for the public if the process of evaluating and making judgments about science is clear and transparent. At the same time, such an approach provides IPCC with insights about its internal processes which are key to making improvements in its assessments over time.”

*NOTE: Go to original press release link to see Figure 1

Expert Contact: Jessica O’Reilly, Indiana University, Bloomington (jloreill@indiana.edu)

Media Contact: Cara Clase, Princeton University (cara.clase@princeton.edu)

The paper, “The curve: An ethnography of projecting sea level rise under uncertainty” was co-authored by Jessica O’Reilly (International Studies, Hamilton Lugar School of Global and International Studies, Indiana University, Bloomington) and Michael Oppenheimer (School of Public and International Affairs and the High Meadows Environmental Institute, Princeton University).  The paper appeared in Global Environmental Change in December of 2024.  This study was supported by Indiana University Bloomington, Princeton University, and the National Science Foundation.