From the Past to the Future: 40 Years of Shared Effort to Understand Climate

As a PhD student encountering the “Bretherton Diagram” for the first time, I was struck by the complexity of the Earth system and simultaneously motivated to bridge a gap in the model hierarchy—between general circulation models and simple box models. In 1990, I was fortunate to meet the late Daniel Wright, and together we set out to develop a compact and affordable model that would allow us to conduct our own climate simulations. This reduced-complexity climate model paved the way for a new class of models that enabled (i) large ensembles for probabilistic climate simulations, (ii) million-year integrations to explore paleoclimate dynamics, and (iii) the coupling of physical and biogeochemical cycles for direct tracer simulations.

Such reduced-complexity models opened the door to new collaborations with the highly organized paleoscience community. Suddenly, a tool became available that helped illuminate emerging and intriguing paleoclimate records, including shifts in ocean tracer distributions and abrupt changes in isotope records from Greenland ice cores, along with their associated fingerprints in Antarctic ice cores. The Dansgaard–Oeschger events continue to challenge our understanding, but, to quote Alfred Wegener, “we are like a judge confronted by a defendant who declines to answer, and we must determine the truth from the circumstantial evidence.”

Over four decades of effort, climate modeling and paleoclimate analysis have drawn closer together and now routinely produce this much-needed circumstantial evidence. Coupled physical–biogeochemical simulations help to exclude or support hypotheses concerning the sequence of events during glacial inceptions and terminations, as well as global shifts in climate dynamics during the Mid-Pleistocene Transition. The combination of paleoclimate modeling and high-resolution analyses of paleoclimate records also provides a foundation for consensus-building on the highly debated issue of high-impact events and tipping points, which will be assessed in the forthcoming IPCC AR7. In this way, paleoclimate science is centrally relevant to inform us about possible surprises or global disruption of the climate system in response to the continuing use of fossil fuels.

In this lecture, I will recall some of these highlights, share anecdotes, and discuss several of the open “big questions” that we still need to tackle. It has been a great privilege to be part of the climate science community and to be guided and inspired by colleagues and friends around the world. I am immensely grateful to the students, postdocs and colleagues with whom I could work in our shared effort to understand the climate system.