Now， the typical pattern for an interglacial period， and we have studied several， is that the concentration of carbon dioxide and methane gas actually reaches it... its peak， that is， there is the most carbon dioxide and methane gas， uh， greenhouse gases in the atmosphere just after the beginning of the interglacial period. And then， for reasons which are not entirely clear， the concentration of greenhouse gases gradually goes down. Now， the climate continues to warm for a while because there is a lag effect. But uh， gradually as the concentration of greenhouse gases goes down， earth starts to cool again， and eventually you slip back into an Ice Age.
In fact， back in the 1970s， a lot of theorists were predicting that， you know， the climate would start to cool and we’d slowly enter into the new Ice Age. And then they were puzzled as to why it didn’t seem to be happening.
Now， what are the implications for the future？ Well， um... it is a little tricky. I mean， you could say， well， here is an example of ... um ... human activity， the agricultural revolution which actually was beneficial； we altered the climate for the better， perhaps， by preventing an Ice Age. But then industrialization， of course， has drastically increased the amount of carbon dioxide that humans are putting into the atmosphere， the burning of fossil fuels tends to put a lot of CO2 into the atmosphere. Um... so we are entering into uncharted territory now， in terms of the amount of carbon dioxide， the concentrations of carbon dioxide that are now being put into the atmosphere as a result of industrialization and the use of fossil fuels.
The Younger Dryas， from c. 12，900 to c. 11，700 years ago (BP)， was a sharp decline in temperature over most of the northern hemisphere， at the end of the Pleistocene epoch， immediately preceding the current warmer Holocene. It was the most recent and longest of several interruptions to the gradual warming of the Earth's climate since the severe Last Glacial Maximum， c. 27，000 to 24，000 BP. The change was relatively sudden， taking place in decades， and resulted in a decline of 2 to 6 degrees Celsius， advances of glaciers and drier conditions， over much the temperate northern hemisphere. It is thought to have been caused by a decline in the strength of the Atlantic meridional overturning circulation， which transports warm water from the equator towards the North Pole， and which in turn is thought to have been caused by an influx of fresh cold water from North America into the Atlantic. The Younger Dryas was a period of climatic change， but the effects were complex and variable. In the southern hemisphere， and some areas of the north such as the south-eastern United States， there was a slight warming.