ICE CORE EVIDENCE
Abrupt climate change encompasses two extremes of abrupt cooling (thermohaline circulation change) and abrupt warming (positive Arctic feedbacks). The Arctic is implicated in both and we know this from ice cores.
We know abrupt warming and cooling can happen because they show up in the ice cores that now go back 800,000 years,
The historical records shows us that abrupt climate change is not only possible--it is the normal state of affairs. Warming is more abrupt than cooling. The present warm, stable climate is a rare anomaly.
In 2002 National Academies Press published book (available on line) Abrupt Climate Change: Inevitable Surprises
There was a PNAS review on this issue in 2000 Ice-core evidence of abrupt climate changes.
As seen in the graph opposite the ice core record showed frequent sudden warmings and coolings of 8°C (15F) or more. Many of these changes happened in less than 10 years. In one case 11,600 years ago, when Earth emerged from the final phase of the most recent ice age (an event called the Younger Dryas), the Greenland ice core data showed that a 15°F 8°C (15F) warming occurred in less than a decade, accompanied by a doubling of snow accumulation in 3 years. Most of this doubling occurred in a single year.
Younger Dryas abrupt warming after abrupt cooling
The so-called Younger Dryas period of rapid climate climate change involved an abrupt cold reversal of a warming trend followed later by an extremely abrupt warming. The abrupt warming was caused the planet emitting methane. Methane hydrate for years was the top candidate of methane source. Currently wetlands (North and South hemisphere) are regarded as the source with methane hydrate not excluded as contributing.
For the best-characterized warming, the end of the Younger Dryas cold interval ≈11,500 years ago, the transition in many ice-core variables was achieved in three steps, each spanning ≈5 years and in total covering ≈40 years . However, most of the change occurred in the middle of these steps. The warming as recorded in gas isotopes occurred in decades or less. The most direct interpretation of the accumulation-rate record is that snowfall doubled over 3 years and nearly doubled in 1 year. Several records show enhanced variability near this and other transitions, including “flickering” behavior in which climate variables bounced between their “cold” level and their “warm” level before settling in one of them.
The methane increase at the end of the Younger Dryas began 0–30 years after the warming in Greenland, suggesting atmospheric transmission of the signal from the north Atlantic region to methane source regions. The relative changes in methane concentrations in Greenland and Antarctica indicate that the increase at the end of the Younger Dryas involved both tropical and high-latitude sources, and that the previous large increase about 14,700 years ago was dominated by the tropics
Temperatures before and after this event in Greenland and many other regions were slightly higher than recently, showing that warmth is not a guarantee of climate stability. Abrupt changes have been especially large when atmospheric carbon-dioxide concentration, insolation, and other important climatic variables were changing rapidly, with possible implications for general behavior of the climate system..