What is "abrupt climate change?

There is an abundance of scientific evidence that shows major and widespread climate changes have occurred with startling speed. For example, roughly half of the warming of the North Atlantic Ocean since the last ice age was achieved in only a decade, and this warming was accompanied by significant changes in climate across most of the globe. Research over the past decade has shown that these abrupt - or nonlinear - climate changes have been especially common when the climate system was being forced to change most rapidly. Thus, the rate of buildup of carbon dioxide in the atmosphere may increase the possibility of large, abrupt and unwelcome regional or global climate events.

The mechanisms of past abrupt climate changes are not yet fully understood, and climate models typically underestimate the size, speed and extent of those changes. Hence, future abrupt changes cannot be predicted with confidence. Yet, because of greenhouse warming and other human alterations of the earth system, and the long lifetime of carbon dioxide in the atmosphere, certain thresholds are likely to be crossed and we will not know we have crossed them until it is too late to alter the outcome.