Editor’s comments

Oscillations in the last 130 000 years

The most recent large climate oscillation spanning the last 130 000 years (130 ka) has been the subject of the most intensive study, because it offers a relatively detailed climate record from the land, from the oceans and from the ice cores. In the last few years, a considerable amount of new data on the warm period known as the Eemian (the last major interglacial) has become available. This interval has seen the global climate system switch from warm interglacial (similar to present day) to cold glacial conditions, and back.

For the most recent Eemian-to-Holocene phase there remains significant ambiguity in terms of the errors in geological dating techniques, causing problems in correlation of events between different regions, so that it is difficult to know whether climate changes were truly synchronous globally. In many other respects, however, the record that has been assembled for this period is remarkably detailed.

As an example, the combined sources of evidence suggest that there was a cold and dry event near the middle of the Eemian, at about 122 ka, which was characterized by a change in circulation of the North Atlantic, a several-degree decline in the Nordic seas and Atlantic sea-surface temperatures, and an opening up of the west European forests to a mixture of steppe and trees. This intra-Eemian cold phase was less dramatic than had been suggested by the ice-core records, but still a major climatic change. Evidence from a high-resolution marine core record at Site ODP 658 suggests that this event, which might possibly have come on in a few decades or less, lasted no more than 400 years.

According to the marine records, the Eemian interglacial ended with a rapid cooling event about 110 000 years ago, which also shows up in ice cores and pollen records from across Eurasia. It has been suggested that the final cooling event took less than 400 years, and it might have been much more rapid. Following the end of the Eemian, a large number of other sudden changes and shortterm warm and cold alternations have been recognized.

• Interstadials are sudden and short-lived warm events which occurred many times during the generally colder conditions that prevailed between 110 000 and 10 000 years ago. Between 115 000 and 14 000 years ago, there are 24 of these warm events. Each warm interstadial is linked to a cold interstadial and each of these ‘Dansgaard–Oeschger’ cycles lasted approximately 1500 years.
• Heinrich events are extreme and short-lived cold events, which were first recognized as periods with very intense ice-rafting in the North Atlantic.
• The Younger Dryas: The Younger Dryas cold event at about 12 900–11 500 years ago seems to have had the general features of a Heinrich event. The sudden onset and ending of the Younger Dryas have been studied in detail in the ice-core and sediment records on land and in the sea suggests that the main Younger Dryas-to-Holocene warming took several decades in the Arctic, but was marked by a series of warming steps, each taking less than 5 years. About half of the warming was concentrated into a single period of less than 15 years. The occurrence of a rapid rise in atmospheric methane concentration at the same time suggests that the warming and moistening of climate (causing more methane output from swamps and other biotic sources) were globally synchronized.
• Sudden climate transitions since the start of the Holocene: Following the sudden start of the Holocene, there have been a number of rapid, widespread climate changes recorded from the palaeoclimatic record around the world. The Greenland ice-core data again show a clear record of these events. At least in the North Atlantic region, these changes seem to have been paced according to approximately the same 1500-year rhythm as that found for the last glacial and earlier glacial periods.
• Sudden climate jumps in the more recent past: Different sources seem to suggest differing speeds and intensities for Holocene climate events. The Little Ice Age began in late medieval times and played a role in extinguishing Norse colonies on Greenland. It ended at about AD 1650.

It is still unclear how the climate on a regional or even global scale can change as rapidly as present evidence suggests. It appears that the climate system is more delicately balanced than had previously been thought, linked by a cascade of powerful mechanisms that can amplify a small initial change into a much larger shift in temperature and aridity. At present, the thinking of climatologists tends to emphasize several key components,

1. North Atlantic circulation as trigger or amplifier of rapid climate changes
2. Carbon dioxide and methane concentration as a feedback in sudden changes
3. Surface reflectivity (albedo) of ice, snow and vegetation
4. Water vapour as a feedback in sudden changes
5. Dust and particulates as a feedback in sudden changes
6. Seasonal sunlight intensity as a background to sudden changes

The authors point out that most of the very rapid climate transitions during the last 100 000 years do not show any clear association in timing with the background Milankovitch rhythms, especially the fluctuations at periodicities below 10 ka. In these cases their ultimate trigger must lie in other factors – probably a combination of many processes that sometimes line up to set the climate system on a runaway course in either the direction of cooling or warming.

This review has shown that, over the last 150 ka, there have been numerous large climate changes that have occurred on the timescale of individual human lifetimes – for example, the end of the Younger Dryas. Other substantial climate shifts documented in the Quaternary took, at most, a few centuries.
Sudden climate transitions during the Quaternary, Progress in Physical Geography 23,1 (1999) pp. 1–36