Figure 21. Dryas flowers in the short arctic summer, by Rusty Lotti Bond.

The study of rapid or abrupt change still is in its infancy and while new data are published every month, the paleo record is still frustratingly incomplete. As definitions of abrupt change are being refined, scientists continue to pose hypotheses regarding mechanisms, but only a few of these mechanisms have been tested using climate models. Even for one of the best known abrupt change events, the Younger Dryas about 11,000 years ago, neither the global extent of temperature or precipitation change nor the accompanying changes in ocean circulation and atmospheric trace gases are well known. The next decade is sure to bring many new developments on this topic.

Even from the proxy data that exist, one thing is relatively certain: the climate system has changed in the past in ways that are much larger and faster than anything we have observed in the last few centuries or estimated over the past 10,000 years. Most of these changes involve major rearrangements of the ocean-atmosphere-land-ice system.

During a brief period called the Younger Dryas, after temperatures in most of the Northern
Hemisphere had begun to warm from the last ice age, they rapidly returned to near-glacial
conditions. After about 1,000 years, they abruptly warmed again, with temperatures in Greenland warming by eight degrees Celsius (+14°F) in a decade. By comparison, the change in the global mean annual temperature over the last 140 years has been less than 1°C, with a similar temperature 1-2°C range estimated for the entire Holocene Epoch, which is roughly the past 10,000 years. (It should be noted that larger abrupt events have occurred, particularly on regional scales, during the Holocene period, but that on the global scale the climate has experienced little variability overall.)

Figure 22. Norwegian Sea image from NASA.

Scientists make progress by posing hypotheses or statements about how the world works that can be rigorously tested by data or models. One well-supported hypothesis is that the climate system appears to have different stable modes, and switches between them rapidly. The paleo record of the last few hundred thousand years reveals warm, interglacial times like today that persist for ten thousand years or more, and glacial times, when large ice sheets covered the northern hemisphere continents.

During each glacial cycle, the Earth returns to conditions that are relatively similar: every ice age is just as cold, with the same reductions in atmospheric carbon dioxide and other conditions. Glacial times appear to last longer, many tens of thousands of years, leaving the Earth in a glacial state most of the time. While most scientists believe that changes in the Earth's orbit slowly drive these changes, we need to gain a better understanding of the switches that trigger abrupt changes from one state to another.

For some abrupt changes, theories have been put forward to describe the triggers of change. Rapid ice sheet melting, for example, has been implicated in the rapid transition from cold, glacial to warm, interglacial conditions. This particular mechanism is well supported by observations, although the modeling of rapid ice sheet melting is still evolving. For other rapid changes, the causes and mechanisms are intensely debated by scientists. For example, the cause of the large rapid changes in atmospheric carbon dioxide that accompany the glacial-interglacial cycles, and the mechanisms that trigger the abrupt Dansgaard-Oeschger oscillations that occurred in the North Atlantic during the last glacial period are not well understood. If scientifically possible, society needs to be able to predict such abrupt changes before they happen in the future in order to anticipate and mitigate the impacts of such changes.