Global Warming

74 Climates ofthe past

−− 3838 − 36 − 34 − 32 − 30 −28 ‰ −^35 −^30

Gerzensee

Lake sediments

DYE 3

δ^18 O δ^18 O ‰

Deep ice core

9.500 yr BP

8500 yr BP

9000 yr BP

Depth of sediment

Depth of

ice core

Allerod

Younger Dryas

Bolling

10.700 BP

− 10

1

1750

1785

Pre-Boreal

Younger Dryas

1786

7 °C

∼50yr

1787

m

1800

1850

m

1700

2

3

4

m

− 8 −6 ‰

Figure 4.7Records of the variations of the oxygen isotopeδ^18 O from lake

sediments from Lake Gerzen in Switzerland and from the Greenland ice core

‘Dye 3’ showing the ‘Younger Dryas’ event and its rapid end about 10 700 years

ago. Dating of the ice core was by counting the annual layers down from the

surface; dating of the lake sediment was by the^14 C method. A change of five

parts in a thousand inδ^18 O in the ice core corresponds to about a 7◦C change

in temperature.

Broecker for the Younger Dryas event.^11 As the great ice-sheet over

north America began to melt at the end of the last ice age, the melt water

at first drained through the Mississippi into the Gulf of Mexico. Even-

tually, however, the retreat of the ice opened up a channel for the water

in the region of the St Lawrence river. This influx of fresh water into the

North Atlantic reduced its saltiness, thus, Broecker postulates, cutting

off the formation of deep water and that part of the ocean ‘conveyor

belt’.^12 Warm water was therefore prevented from flowing northward,

resulting in a reversal to much colder conditions. The suggestion is

also that a reversal of this process with the starting up of the Atlantic

‘conveyor belt’ could lead to a sudden onset of warmer conditions.

Although debate continues regarding the details of the Younger

Dryas event, there is considerable evidence from paleodata, especially

those from ocean sediments, for the main elements of the Broecker