Rapid Events

Climatic oscillations on timescales of decades frequently show up not only in the stable isotope record, but also in dust and chemical traces. Changes in insolation or global ice volume cannot account for these oscillations because of the longer response times involved. The question arises as t o what climatic mechanisms could have caused temperature shifts of up 10°C in such short periods. A well studied example of a rapid climate event is the termination of the Younger Dryas at the end of the last glaciation.

The Boiling-Allerod-Younger Dryas sequence has been known from con- tinental climate indicators for a long time. The warming a t the end of the last glaciation was interrupted several times by abrupt returns t o glacial conditions, culminating in the cold spell of the Younger Dryas. The main forcing for this last major shift in global climate was the disintegration of the Laurentian ice sheet. Its abrupt termination is characterized by large shifts in stable isotopes and other environmental parameters (Dansgaard et al., 1989; Johnsen et al., 1989; Alley et al., 1993; Chappelaz et al., 1993). Figure 4 illustrates these changes over time. From the Dye 3 ice core d a t a (top left) it is obvious that the change in atmospheric circulation was much faster than the general warming. The 5 permille increase in oxygen-18 corresponds t o an increase in air temperature of about 7OC that took place within 50 years, but the deuterium excess, accompanied by atmospheric dust concentrations, shifted t o lower levels within only 20 years. The warming caused a quick retreat of sea ice cover, the polar front moved northwards from its Younger Dryas position, and a relatively cold water body opened up as a new source of moisture. T h e intermediate temperatures between the tropical and polar waters lowered the latitudinal temperature gradient and thus the degree of storminess. T h e latter is described by a reduction in dust concentrations

Dye3: Term~inalioo of the Younger Dryas Summit GRIP: Eem event

1

6 10 -35 -30 0 5 dex ^(O/OO) 6Ie0 Dust (mg/l)

Summit GRIP and GISP2: Environmental record during Belling-AIIered-Younger Dryas

Figure 4. Change in atmospheric circulation pattern during rapid climatic events. The similar trend of methane during the B~lling-Aller~d-Younger Dryas sequence underlines the global significance of the data.

by a factor of three. This dramatic shift in atmospheric circulation pat- terns in the northern hemisphere is even more underlined by the change in precipitation. The snow accumulation data from the GISP 2 core (lower right part of Figure 4) have been used t o reconstruct the B~lling-Aller~d- Younger Dryas sequence. By multiparameter analyses it has been possible t o identify annual layers t o high accuracy. This layering suggests that the amount of precipitation not only changed very rapidly a t the transition t o

the Bolling-Allerod warm period, but it almost doubled a t the termination of the Younger Dryas event within only three years. These short timescales place severe constraints on the subtle mechanisms that lead t o such large changes in atmospheric circulation. There must be some thresholds or trig- gers in the subtle interplay between the thermohaline circulation a n d . the advection of heat and salt in the North Atlantic climate system.

On the same timescale, rapid events also took place in the Eemian. The example of event 1 in Figure 4 (top right) is taken from a n undisturbed part of the GRIP record, 59 m above the section where the layering starts t o become inclined. All parameters show behavior similar t o those of the Younger Dryas, except the deuterium excess signal, which parallels the dl'O record throughout the Eemian, but is strongly in antiphase during the late glacial and the Younger Dryas. This implies general differences in the sources of atmospheric moisture and their circulation patterns during the interglacial as compared t o the Younger Dryas example. The rapidity of these shifts again points t o a highly variable ocean with changing heat and moisture transport. Thus the questions arise as t o whether such extreme events are restricted to the North Atlantic climate system, and also whether such shifts in our stable climate conditions could be triggered by human interference.

At least the first question can be answered to some extent. Since methane is a climate parameter of global relevance, the Summit record also provides a link between ice cores and tropical climates.

5 . Ice Cores and Tropical Climates

Although up to now the sample frequency of methane cannot be compared with the high-resolution records of other climatic parameters, it is obvious from the lower part of Figure 4 that this indicator of biotic activity under- went similar changes during the Bolling-Allerod-Younger Dryas period. The record in Figure 5 confirms the earlier Vostok d a t a with a better time reso- lution in the northern hemisphere. It is obvious that atmospheric methane concentrations more or less paralleled the Greenland climate from 40,000 B P t o the Holocene. Changes in concentration also follow the course of the ob- served warm interstadials. The increase in CH4 concentrations t o Holocene levels occurred a t a time when the northern wetlands were still ice-covered, which implies another methane source was the spreading wetlands a t low latitudes, as pointed out earlier by Street-Perrott (1994). Indeed, there is increasing evidence of a link between the North Atlantic SST and rainfall patterns in the tropics. Deepwater formation in the North Atlantic drives a

Summit

GRIP:

8

Figure 5. Evolution of atmospheric temperature and methane from 40,000 B.P. t o Holocene levels. All major warm periods are accompanied by high methane levels. A rapid climatic event a t the beginning of the Holocene is marked by the shaded area a t 8,200 B.P.

large-scale ocean circulation system, which transports heat across the equa- tor. A shutdown of this huge thermohaline conveyor belt can be caused by low salinity surface waters, as happened for instance during the Younger Dryas. This steepens the temperature gradient between North Atlantic, South Atlantic, and Indian Oceans, which in turn drives the equatorial rain belts. T h e reconstruction of past salinities in the North Atlantic (Duplessy et al., 1992) showed that a low salinity event also occurred around 8000 BP, corresponding t o a known interval of low lake levels in Africa (Street-Perrott

et al., 1983). In the top part of the record in Figure 5 a sharp drop in temperature coincides with a lower methane concentration in the ice core.

6 . Outlook

The extreme dynamics of atmospheric circulation in the past, and the r&es of changes in temperature and precipitation as revealed by the new ice core measurements, cast a shadow on impact scenarios for anticipated climate and environmental change. The obvious stability of the present climate compared with that of the past glacial-interglacial cycle is not an isolated conclusion drawn from environmental d a t a from a remote region. The find- ings are of global significance. We are therefore confronted with the question of whether the rapid events of the past could also be triggered by human interference today. In 1756 Voltaire stated: Three things influence human thought: climate, politics and religion. At least for the climate issue, the link between thinking and acting is still missing.

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