| Summary: | Abrupt shifts in the climate system have been observed in paleoclimate records and are the subject of much concern for the future climate as anthropogenic greenhouse gas emissions continue to drive global climate change. Some of these abrupt changes are thought to be associated with climate “tipping points”, where some component of the climate system changes rapidly and irreversibly at a threshold value of CO2. One way to understand such shifts comes from conceptual, mathematical, models of the climate that suggest it is possible to have multiple climate equilibria for a given external forcing value (such as CO2 concentration). A tipping point in such models occurs when the CO2 concentration crosses a threshold where one climate equilibrium disappears or loses stability, and the system must rapidly adjust to a different equilibrium state. While this mathematical insight provides an elegant explanation for abrupt climate shifts, the gulf between the simple hypothetical systems that display this behavior and Earth’s climate system is vast. Earth’s climate is different from such a simple system in that it has many more components, diffusive properties that smooth out abrupt transitions, and is subject to noisy perturbations and time-changing forcing that can cause it to be far from its equilibrium state. These differences shed doubt on the applicability of the dynamical systems perspective to the Earth’s climate, and on whether we should ever expect to actually observe large-scale climate tipping points. In my thesis, I try to bridge the gap in our understanding between the tipping points displayed by simple conceptual systems and the complicated abrupt shifts seen in full-complexity models by studying two elements of the climate system whose status as a “tipping point” remain debated: winter Arctic sea ice, and the Atlantic Meridional Overturning Circulation (AMOC). In Chapter 2, I analyze six global climate models (GCMs) that show a range of Arctic winter sea ice loss rates under global warming scenarios, from ...
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