4 The end of the last ice age: the Holocene

As noted earlier, the great ice sheets took about 100 000 years to grow and only about 10 000 years to decay. So what happened at the end of the last ice age? Figure 19 shows the EPICA ice core CO₂ concentration and Antarctic air temperature for the most recent 20 000 years, which is within the last ice age. The temperature scale shows the difference from the average temperature of the last 1000 years, so 0 °C represents no change from (fairly) recent climate.

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Figure 19 The global atmospheric CO₂ concentration (relatively smooth line) and Antarctic air temperature change (spiky line) from the EPICA ice core over the last 20 000 years up to 1813 (CO₂) and 1911 (temperature)

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A graph of temperature and atmospheric CO₂ concentration changes from the EPICA ice core over the last 22 000 years BP. The horizontal axis is time, measured in years before present (BP), and the minimum is 22 000 years BP with ticks every 2 000 years. The maximum on this axis is 0 which is the present day. The left-hand vertical axis is temperature change relative to the present day (mean of the past 1000 years) and the minimum is −12 °C and the maximum 4 °C, with ticks every 2 °C. There is a dashed line parallel to the horizontal axis at a temperature change of 0 °C (which refers to temperatures similar to today). The right-hand vertical axis is atmospheric CO₂ concentration with a minimum of 165 ppm, a maximum of 305 ppm and ticks every 20 ppm. The temperature data shows a lot of variability compared with the atmospheric CO₂ data. At 22 000 years BP the temperature change is around −9 °C whilst atmospheric CO₂ is about 190 ppm. From about 18 000 years BP both the temperature and atmospheric CO₂ rise until about 14 000 years BP when for a very short period of time the temperature change is −1 °C and atmospheric CO₂ is as high as ~240 ppm. By 12 800 years BP the temperature rapidly falls to −5 °C below present whilst atmospheric CO₂ remains approximately constant at around 240 ppm. This time period is labelled ‘Younger Dryas’. From about 11 600 years BP to 11 000 years BP the temperature and atmospheric CO₂ again rise with the temperature change at around 0 °C and atmospheric CO₂ about 265 ppm. There is another feature where the temperature and atmospheric CO2 fall at 8 200 years BP to −2 °C and 260 ppm, respectively. This is labelled as the ‘8.2 ka event’. From this time period onwards both temperature and atmospheric CO₂ concentration increase. The CO₂ data finish slightly before 0 years BP with a value of around 280 ppm and after this the temperature data show a rapid increase to around 3 °C above the mean of the past 1000 years.

Figure 19 The global atmospheric CO₂ concentration (relatively smooth line) and Antarctic air temperature change (spiky line) from the ...

Figure 19 shows again the high correlation between the two variables: 20 000 years ago it was up to 10 °C colder in Antarctica, and global CO₂ concentration was more than 200 ppm lower than today. Over the most recent 10 000 years the temperature was within about 2 °C of current temperatures, and this climatically stable time period is called the Holocene. Figure 18 shows that such a warm, stable period has been very unusual in the last 800 000 years, yet it is only during the Holocene that agriculture and the civilisations that rely on it have developed. Homo sapiens has flourished in the stable climate era.

Figure 19 shows that up to approximately 14 000 years ago the planet appeared to be leaving the ice age, and Antarctic temperatures rose to within 1 °C of the 0 °C line. But then there was a very rapid cooling of 4–5 °C (and most of this in just a couple of decades), and lower temperatures resumed from 12 900 to 11 600 years before the present. This cold period affected most of the planet and is called the Younger Dryas, after a pretty Arctic alpine flowering plant called the white dryas (Figure 20). This species spread its geographical range as temperatures fell and the tundra biome expanded in area.

Figure 20 The white dryas. The Latin name of this flower is Dryas octopetala, meaning ‘dryas flower with eight petals’ – although it can have up to 16 petals.

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A colour photograph of two white dryas flowers. The flowers are white with many petals and have a yellow centre.

Figure 20 The white dryas. The Latin name of this flower is Dryas octopetala, meaning ‘dryas flower with eight petals’ – although it can ...

Another interesting event shown in Figure 19 happened just before 8000 years ago (called the ‘8.2 ka event’ where ‘ka’ is an abbreviation meaning 1000 years), when there was a definite but relatively small temperature and CO₂ decrease which was associated with drier conditions in some parts of the world. This represents the largest climatic variation that civilisation has currently had to cope with.

So what happened in the Younger Dryas and 8000 years ago to make the planet suddenly colder? The changes occurred too fast for the Milankovitch cycle to be responsible. It is now believed that the only way to cause that much cooling is by a sudden change in part of the global ocean circulation. Just as there are global patterns of air circulation, so there are also much slower, but enormous, movements of water around the oceans, driven by changes in water temperature and salinity, which you will look at next.