As discussed in Chapter 2, the Younger Dryas is the most studied example of an abrupt change and provides insights about possible mechanisms. In many ways the Younger Dryas serves as a defining event that embodies the notion of abrupt climate change. Heinrich and Dansgaard/Oeschger events are generally thought to be governed by physical laws that are closely related to those involved in the Younger Dryas; indeed, many consider the Younger Dryas to be just an unusually big Heinrich or Dansgaard/Oeschger event. However, there are potentially many types of abrupt change, as described in Chapter 2, so we need to consider the full array of possible processes, rather than only those currently in favor for explaining the Younger Dryas and Dansgaard/Oeschger events. Other major events of interest in the paleoclimatic records include decadal Holocene droughts, the dry-moist cycles of North Africa, the Little Ice Age, and the cooling event that occurred about 8,200 years ago (the “8.2K event”). Abrupt shifts in the dominant modes of the modern climate are also being documented, and the active mechanisms in these shifts might be relevant to larger abrupt changes in the distant past or in the future.

AN OVERVIEW OF MECHANISMS

Understanding and simulating abrupt climate change poses special challenges in climate science. Many climate changes are well described as relatively small deviations from a reference state, often assumed to be in equilibrium with external forcing. Powerful simplified conceptual approaches (“linearizations”) are therefore appropriate and can explain many features in a faithful way. In a linear model, doubling the forcing doubles the response. The linear approach does not hold for abrupt climate change, in which a small forcing can cause a small change or a huge one, so fully nonlinear and transient considerations and simulations are required. In particular, transitions between qualitatively different climate states, as are seen in the paleoclimatic records, require abandoning the “time-slice” perspective in which equilibrium runs of climate models under different forcings are used to try to infer the path of climate change.

Three past classical model types have been important in this context because of their ability to simulate paleo abrupt climate change either spontaneously or in response to changes in controlling parameters or external forcing. These are the two-box Stommel model (1961), simple energy-balance models such as that of Sellers (1969), and the Lorenz models (1963, 1990).



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