Past climates: Ice ages
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Ice ages can be caused by a chain reaction of positive feedbacks triggered by periodic changes in the Earth’s orbit around the Sun. These feedbacks, involving the spread of ice and the release of greenhouse gases work in reverse to warm the Earth up again when the orbital cycle shifts back. The last ice age ended about 12,000 years ago and in the absence of other factors the next cooling cycle would be expected to start about 30,000 years or more into the future.
Ice age climate
For the past million years or so, shifts in the Earth’s orbit have led to periodic ice ages interspersed with warm periods. We are now in a warm period – the last ice age ended about 12,000 years ago. Before then, large areas of Europe and North America were covered in massive ice sheets, some of which were several kilometres thick. Scientists estimate that global average surface temperature was about 5 °C lower than at present, with larger changes at the poles. Sea levels were also over 100 metres lower, because so much water was locked up in ice on land. In addition, the ice age climate was drier than today’s, because colder air cannot hold as much water vapour.
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The ice age was colder, drier and dustier an today's climate
Image: Tyler Olson / Shutterstock
Differences between hemisphere ice cover during ice ages
During the ice ages, massive ice sheets spread over large areas of the northern hemisphere, while the spread of ice in the southern hemisphere was more limited. This difference was because the Arctic Ocean in the north is surrounded by continents, enabling ice sheets to spread out from the pole across land. In contrast, the Antarctic is a continent surrounded by ocean. This inhibits the spread of ice from the Antarctic shores, so the extent of ice cover in the southern hemisphere wasn’t as large as in the north. However, parts of the Antarctic ice sheet grew significantly thicker during the ice ages.
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The last ice age saw more ice cover in the northern hemisphere than the southern hemisphere
Image: Ittiz / http:/creativecommons.org/licenses/by/3.0/deed.en
Ice age triggers and causes
Orbital cycles change the distribution of sunlight across the Earth’s surface. Scientists calculate that when the combined effect of these cycles reduces summer sunlight in the northern high latitudes, the resulting spread of ice and snow can lead to positive feedbacks, eventually causing ice ages. These feedbacks wouldn’t occur without the initial changes caused by the orbital shifts, so the Earth’s orbit can be said to ‘trigger’ the ice age cycles. But orbital sunlight variations are too small to cause large changes in climate on their own. So without positive feedbacks, the massive ice sheets wouldn’t build up. Positive feedbacks also help to melt the ice when the orbit shifts back.
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Orbital precession (P) and changes in eccentricity (E) and tilt (T) trigger ice age cycles
Image: IPCC
Ice age feedbacks caused by seasonal ice and snow
Because of the tilt of the Earth’s axis, the amount of sunlight hitting the poles varies dramatically through the year – from 24-hour daylight in summer to perpetual darkness in winter. This means large areas of seasonal ice build up each winter and then melt in summer. A slight decrease in summer sunlight can result in less ice melting than usual. A larger-than-usual covering of white ice and snow on darker land or water increases the amount of sunlight reflected from the surface. This cools the surrounding area, enabling yet more ice to survive the summer melt. Scientists calculate this positive feedback contributed significantly to the spread of ice sheets during the ice ages.
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The build up of reflective ice near the poles is an important ice-age feedback
Image: NASA
Ice age feedbacks caused by changing greenhouse gas levels
Orbital shifts triggered a sequence of positive feedbacks, eventually leading to the large ice age climate changes detected in ice core records. Scientists examining the ice core data also observed changes in the amounts of greenhouse gases in the atmosphere during the ice age cycles. Gradually, decreasing global temperatures cause water vapour to fall as rain or snow. Over hundreds of years, the global cooling leads to changes in the carbon cycle, causing carbon dioxide and methane to be absorbed by the oceans or get locked up in frozen ground. This acts as positive feedback, weakening the greenhouse effect and causing further cooling.
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A slice through an ice core, showing bubbles of trapped air
Image: British Antarctic Survey
The rate of ice age temperature change
During the onset of an ice age, it typically takes about 100,000 years for the ice to reach its maximum extent and thickness. Using data from climate proxies such as ice cores and sediment cores, scientists have calculated that global temperature at the peak of the last ice age was about 5 °C colder than today, with temperatures at the poles as much as 10 °C colder. Taking into account the substantial uncertainties, this estimate indicates an average cooling rate of less than one hundredth of a degree per century. The warming trend at the end of the last ice age started around 22,000 years ago and finished around 12,000 years ago – an average warming rate of less than a tenth of a degree per century.
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It took thousands of years for the massive ice-age ice sheet to build up
Image: NOAA
Profile: Dr. Eric Wolff
Eric uses ancient ice to learn about climates past. He and his team spend ten weeks in chilly Antarctica, drilling deep into the polar ice. Once they’ve extracted the cores, they transport them to the laboratory in Cambridge for analysis. The content of trapped air bubbles, the amount of salt in the ice and other measurements reveal clues about the climate when the ice was formed. Eric works with experts around the world to unravel these clues and build up a picture of past climate changes. ‘Working out how our planet worked in the past gives us more confidence in predicting how it will behave in future,’ says Eric.
