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Warming over the next hundred years is almost certainly expected to differ from that of the next few decades, as evolving spatial patterns of sea surface temperatures modify the global radiative feedback and thus climate sensitivity. On much longer timescales, very slow feedbacks associated with the reduction of large ice sheets and changes in vegetation and biomes are expected to enhance the ultimate "Earth system" warming in response to greenhouse gas forcing. Present-day radiative feedbacks govern the modern geophysical climate commitment (i.e., the near-future warming that will occur from only past emissions), while past climate states, such as the mid-Pliocene, permit an examination of climate feedbacks operating on much longer timescales.
In Chapter 1, I describe the framework of global mean radiation balance in the climate system and the separation of forcings and feedbacks central to this thesis. I then introduce the mechanisms by which spatial patterns of sea surface temperatures alter the global mean radiative feedback. Lastly, I introduce the mid-Pliocene Warm Period (mPWP) and its utility as an analogue of future climate.
In Chapter 2, I use the framework of global energy balance to estimate the magnitude of global mean warming following an abrupt cessation of anthropogenic emissions -- a geophysical climate commitment -- both in the present-day and in the future, following realistic emissions pathways. I find that the probability of our past emissions committing us to surpassing 1.5°C of global warming, at least temporarily, is already greater than 40% (as of 2020), and increases to 66% by 2029.
In Chapter 3, I estimate the forcings and feedbacks operating in the mid-Pliocene in a global climate model hierarchy. I find that the non-CO2 boundary conditions of the mPWP induce a spatial pattern of warming that results in a more sensitive climate relative to the modern-day, potentially reducing estimates of modern-day climate sensitivity that are constrained by mPWP warming. On long timescales, however, these feedbacks imply that Earth System Sensitivity may be higher than previously recognized.
In Chapter 4, I separate the climate response to the loss of the West Antarctic Ice Sheet from northern hemisphere boundary condition changes on the sea surface temperature patterns of the mPWP in a coupled climate model. I find that increasing high latitude ocean heat transport results in gradual Southern Ocean warming in response to a loss of the West Antarctic Ice Sheet, driving the higher sensitivity of this paleoclimate state. The results suggest that a loss of the West Antarctic Ice Sheet could produce more future global warming than previously appreciated.