Uncommon Trends: Progress and Challenges in Extending Climate Knowledge Over the Common Era

Prof. Julien Emile-Geay

University of Southern California

Monday April 27, 2026, 2:30 PM ET

Abstract:

The shortness of the instrumental record imposes fundamental limits on our ability to train and independently test climate prediction systems, particularly at decadal and longer scales. A related problem is the difficulty to precisely quantify how unusual recent trends are in a broader historical context. In the nearly three decades since the publication of the “Hockey Stick” curve by Mann et al (1998), climate field reconstruction has progressed by leaps and bounds, allowing to tackle previously unanswerable questions. In this talk I will summarize recent progress in reconstructing the climate of the Common Era (1 CE – present), leaning on new data standards, crowdsourced compilations, and advances in data assimilation. I will show how new, underutilized sources of information may be leveraged to independently validate these estimates, as well as climate model simulations. I will present an analysis of surface temperature trends over intervals of 10, 25, 50 and 100 years at various spatial scales using several variants of the Last Millennium Reanalysis, which confirms that recent trends are unprecedented over the Common Era. The talk will conclude with an assessment of promising applications and remaining challenges.

Biosketch:

Dr Julien Emile-Geay is a mathematical paleoclimatologist working as a professor in the Earth Science department at the University of Southern California. Using deterministic and probabilistic models, he creates mathematical representations of the climate system to shed light on its long-term behavior. He is particularly interested in the role of the tropics in climate, and in constraining how much of climate variations arise from within, or are being triggered by external factors, natural or human. Prior to joining USC in 2008, he studied at the Ecole Normale Supérieure in Paris, France (BS/MS), obtained a PhD from Columbia University in the City of New York and conducted postdoctoral research at the Georgia Institute of Technology. Dr Emile-Geay has co-authored over 65 peer-reviewed publications and one book on data analysis. To learn more, visit https://commonclimate.github.io.