No one doubts that abrupt climate change has happened - the evidence for it is overwhelming - but there are considerable controversies as to where, how, why or even when it happened. The uncertainties bear on the following points:

• Spatial footprints: What is the geographical extent of a given climate event? Did it occur in both hemispheres? Did it happen at the same time everywhere, or can we track down its propagation on the Earth's surface? For example, there is an argument as to whether the Younger Dryas (YD) cooling occurred in the Southern Hemisphere as well as the Northern Hemisphere.  The main evidence in the Southern Hemisphere is a glacial advance in New Zealand, the date of which is unsure. It is a matter of pressing urgency to determine if the YD occurred in other parts of the Southern Hemisphere and in the Tropics.

•  Interpretation of climate records: accurate, global coverage of relevant climate variables is very recent, and in many respects still insufficient. Thus to gain knowledge of the past, paleoclimatologists must rely on "proxy variables" measured in various geological objects, such as ice cores, sediment cores (deep-sea and lakes), tree-rings, corals, etc. Since those are, by nature, indirect measurements, the exact amplitude of some isolated events is very difficult to determine. In the colorful prose of the NAS report: imagine that the climate system is "a device [...] hidden in a box in a dark room.You have no knowledge of the hand that occasionally sets things in motion, and you are trying to figure out the system's behavior on the basis of some old 78-rpm recordings of the muffled sounds made by the device. Plus, the recordings are badly scratched, so some of what was recorded is lost or garbled beyond recognition. If you can imagine this, you have some appreciation of the difficulties of paleoclimate research and of predicting the results of abrupt changes in the climate system."

• Causes of abrupt climate change : A viable scenario for abrupt climate change must fulfill 4 requirements (Broecker, 2003).
  1. the scenario must characterize the states among which the climate system has jumped.
  2. it must identify a mechanism by which the system can be triggered to jump from one of these states to another (see question 7).
  3. it must invoke a teleconnection mechanism by which the message can be rapidly transmitted across the planet.
  4. it must have a flywheel capable of holding the system in a given state for many centuries.

   

  There is generally no consensus about any of these 4 points, although in general terms, the flywheel is often the ocean, and the transmitter the atmosphere.

Reference:

Broecker, WS., Does the trigger for abrupt climate change reside in the oceans or in the atmosphere?,  Science 300 (5625): 1519-1522 JUN 6 2003.