The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 133

Laurentide
ice sheet

Cordilleran
ice sheet

Fig. 4.3 ApproximatesouthernextentoficeovernorthernNorthAmericaduringthedeepestpartoflate-Quaternary

glacials.Notethatthepresent-daycoastlineisportrayedwhereasduringglacialsthelowersealevelswould
havemeantalargerlandarea.

we shall shortly see, shows a broadly similar pattern of high concentration during
interglacials and low during glacials, with the lowest concentration during the LGM.
Atmospheric methane reflects the global extent of wetlands (including, in modern
times, rice paddies). As previously mentioned, when the Earth is warmer the greater
ocean evaporation leads to greater rainfall which in turn adds to the extent of wetlands.
Conversely, when the planet is cooler and drier there is less methane produced. So
methane concentration can also be loosely said to track temperature and indeed, being
a powerful greenhouse gas (see Chapter 1), amplify temperature changes.
Importantly, the temperature and gaseous changes recorded in the Vostok cores in
Antarctica are reflected in their northern hemisphere counterparts. The 18 O isotope
records of Vostok and Greenland cores broadly, though not nearly exactly, follow each
other. Figure 4.6a shows a low-resolution graph (with 25 000-year filter) of how 18 O
has changed over the past 90 000 years from the present to early in the last glacial.
The LGM transition to the current Holocene interglacial can be clearly seen.
The reason for using a filter on the above data is that a high-resolution analysis
of northern and southern hemisphere ocean temperatures during the last glacial