| Summary: | The Earth system — the interconnected global set of living and non-living systems comprising our planet — has undergone profound changes throughout its history. Some were slow and gradual; others dramatic and seemingly abrupt. The latter include “hyperthermal” global warming events, “Snowball Earth” glaciations, and mass extinctions. Although the details are different, underpinning them all is the nonlinearity of the Earth system: small changes sometimes trigger much larger responses. This thesis addresses four related problems in this subject area.
First, I search for general patterns in extreme climate-carbon cycle disruptions throughout the last 66 million years. I find a persistent tendency towards extreme warming rather than cooling events, and that extremes are strongly non-Gaussian. Through theory and computation, I suggest a simple explanation: the carbon cycle fluctuates more when it is warmer. This could also explain why hyperthermals were often synchronized with changes in Earth’s orbit, and suggests that there will be more extreme hyperthermal-like events in a future warmer world.
Second, I interrogate past global temperature changes for signs of long-term stabilizing feedbacks. One example is the silicate weathering feedback, which is often invoked to explain recovery from large Earth system disruptions, and Earth’s persistent habitability thus far. I show that stabilizing feedbacks indeed controlled temperature on timescales of thousands to hundreds of thousands of years, but not on longer timescales. This supports silicate weathering as an important long-term climate controller, but raises questions about the role of chance in Earth’s continued habitability.
Third, I revisit the problem of how global glaciations (“Snowball Earth” events) are initiated; this happened multiple times in the deep geologic past. Using a simple model of atmospheric radiation and the weathering feedback, I show that the most relevant “tipping point” leading to glaciation may be a critical rate of decrease in effective solar radiation, as could result from volcanic aerosols or the collapse of a methane greenhouse. This also suggests that planets well within the conventional “habitable zone” can still be surprisingly susceptible to glaciation.
Fourth and finally, I consider abrupt catastrophes for life. Earth system disruptions have previously triggered mass extinctions, and empirical work shows that the rate of environmental change may have been the determining factor. Using this observation, the existing theory of “evolutionary rescue”, mathematics surprisingly like the global glaciation problem, and agent-based simulations, I argue that rate-induced collapse in response to environmental change may be a common feature of evolutionary-ecological systems on a vast range of scales.
Beyond pure intellectual interest, I hope that insights from these four contributions will help, however modestly, to enable a flourishing Earth system long into the future.
|
|---|