Dynamics or thermodynamics?

The first step in disentangling the co-evolution of water and climate through geologic time often lies in a simple question: dynamics or thermodynamics? This distinction remains at the core of ongoing debates in paleoclimatology, particularly in the context of late Quaternary monsoon systems, and has implications for understanding global patterns of climate change. Here, I use modern, Quaternary, and Cenozoic data with mechanistic and statistical modeling frameworks to demonstrate novel techniques for distinguishing between atmospheric circulation (dynamic) and humidity (thermodynamic) changes in past climate states. In one example, I use spatial oxygen isotope gradients of South American Monsoon rainfall to identify a precession-paced hysteresis pattern in the migration of climatological moisture convergence since the last deglacial. This migration leads to local changes in mean annual precipitation of up to two meters per year. Using a statistical model, I also discuss evidence for a fundamental change in North American precipitation seasonality at the Paleogene-Neogene boundary. Together, these analyses help to quantitatively identify first-order drivers of hydroclimate evolution, presenting a foundation for investigating richer links between the water cycle, carbon, and life through Earth history.