Perhaps something worth talking about

Mary-Elena Carr has a new post up at the Columbia Climate Blog about the US Climate Change Science Program synthesis report on abrupt climate change. By way of introduction she starts with the etymology

The term abrupt climate change arose in the study of past climate. Some definitions of the term refer to the climate system passing a threshold between different states (for example, with ice to ice-free). As opposed to a gradual change, it can be difficult to anticipate or to understand the passage from one state to another. Some climate tipping points include melting of the Greenland ice sheet or shutdown of major circulation systems.

and points out that the USCCP has evolved (or according to Stoat, devolved) the term to

A large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems.

Carr enlarges upon this

Climate changes that do not involve rapid changes from one state to another can also be abrupt when considered from this human perspective.

Why do we care? A crucial consideration for climate change is the rate at which change occurs. One relevant rate is by comparison to the speed at which human societies or biological systems can react. It is easier to respond to sea level rise over a period of centuries than over a decade.

Abrupt climate change has occurred in the absence of human perturbation to the planet, but in today's world, societies would be extremely vulnerable to rapid changes such as those seen in the past. There is an added concern that anthropogenic forcing might trigger an abrupt change.

This raises, and provides at least one answer, to the perhaps important question of this essay, what is the relevant time for climate change and how long must they persist to be considered a climate change. One must also grapple with the issue of whether local changes, desertification for example, climate changes? Indeed, in their introduction the USCCSP discuss why they have adopted their definition

What is meant by abrupt climate change? Several definitions exist, with subtle but important differences. Clark et al. (2002) defined abrupt climate change as “a persistent transition of climate (over subcontinental scale) that occurs on the timescale of decades.” The NRC report “Abrupt Climate Change” (NRC, 2002) offered two definitions of abrupt climate change. A mechanistic definition defines abrupt climate change as occurring when “the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause.” This definition implies that abrupt climate changes involve a threshold or nonlinear feedback within the climate system from one steady state to another, but is not restrictive to the short time scale (1-100 years) that has clear societal and ecological implications. Accordingly, the NRC report also provided an impacts-based definition of abrupt climate change as “one that takes place so rapidly and unexpectedly that human or natural systems have difficulty adapting to it.” Finally, Overpeck and Cole (2006) defined abrupt climate change as “a transition in the climate system whose duration is fast relative to the duration of the preceding or subsequent state.” Similar to the NRC’s mechanistic definition, this definition transcends many possible time scales, and thus includes many different behaviors of the climate system that would have little or no detrimental impact on human (economic, social) systems and ecosystems.

For this report, we have modified and combined these definitions into one that emphasizes both the short time scale and the impact on ecosystems. In what follows we define abrupt climate change as:

"A large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems."

The USCCSP discusses four major possibilities for abrupt climate change in the next century

(1) rapid change in glaciers, ice sheets, and hence sea level;
(2) widespread and sustained changes to the hydrologic cycle;
(3) abrupt change in the northward flow of warm, salty water in the upper layers of the Atlantic Ocean associated with the Atlantic Meridional Overturning Circulation (AMOC);
(4) rapid release to the atmosphere of methane trapped in permafrost and on continental margins.

to which we shall return and return and return. The bar is open.