Environmental Engineering Reference
In-Depth Information
Ocean cores - confirmed many predictions of the
Milankovich theory.
Source: After Imbrie and Imbrie (1979).
Figure 9.12
Sedimentary evidence for the sudden termination
of glaciations. Data show variations in δ
18
O ratio from
Ocean Core 806, near Papua New Guinea in the Pacific
Ocean.
Source: After Muller and MacDonald (2000).
frequent evidence from sediments that changes of climate, especially Ice Age
termination, can take place rapidly (Figure 9.12). For example, in one deposit near
Birmingham (UK) a typical northern assemblage of beetles was found dated to 10,025 ±
100 years BP. Ten centimetres higher no Arctic fauna survived at an age of 9970 ± 110
years BP. Conversely the rapid cooling at about 10,900 BP brought a catastrophic
readvance of the ice, which destroyed fully grown forests, and caused desiccation in
Colombia and a marked cooling in Antarctica within a time span of only 200-300 years.
Ice cores from Greenland have also confirmed rapid changes within the last glacial
period. The ending of the Younger Dryas period could have taken only ten to twenty
years with a temperature increase of 7° C and an increase of precipitation of 50 per cent.
Such rapid temperature oscillations - and there are many others recorded in the ice cores
- are now known as Dansgaard-Oeschenger events. It seems highly unlikely that the
orbital variations could have been responsible for such sharp climatic fluctuations as
described here. In some cases this sudden termination preceded the orbit-induced
warming that was supposed to cause it. For such changes we must look to other
mechanisms.
INTERNAL FORCING
Figure 9.8 shows the internal mechanisms which can also generate climate change. Some
are likely to operate only very slowly. For example, changes in Earth's orography take
millions of years to become significant in terms of their effects on the atmosphere,
though they can be extremely important. It is interesting to speculate what the climate of
the northern hemisphere would be like without the Western Cordillera of the United
