Chapter 6 – Case Study 4: Abrupt Climate Change

Publisher Summary This chapter is a scoping analysis that leads to a quantitative risk assessment of the human fatality risk of an abrupt climate change in the north Atlantic coastal states. It considers the risk indicators associated with a disruption in the thermohaline circulation (THC). In particular, it takes a look at the changes in the rate of Atlantic Ocean overturning circulation expressed as a volume of water per time, which is a major driver of the THC. The phenomenon considered for the abrupt climate change is the possible shutdown of the THC in the Atlantic Ocean triggered by global warming. The coupling between the atmosphere and the oceans is a critical aspect of global climate dynamics. Relative to the atmosphere, the ocean responds to perturbations slowly, and stores and transports large amounts of energy. The transport of large amounts of heat by the oceans feeds back to the atmosphere and is responsible for many aspects of climate. The large-scale ocean circulation that is responsible for the transport of heat from the equator toward the poles in the surface of the Atlantic and the return flow of cold, deep water in the reverse direction is part of the global THC. The circulation is driven by density changes between freshwater and saline water and between warm and cold water; hence the term “thermohaline” circulation is used. The circulation is called “overturning” because colder and more saline water at the surface sinks as it is heavier and displaces the less dense water at depth.