3.5 Interglacial periods and sea levels

The EPICA ice core is a record of temperature variations in Antarctica, but what was happening in the rest of the world? Temperatures in other areas varied in a similar pattern of cycles. Other proxy data, such as from sediments found at the bottom of the oceans and lake beds, and the dating of rocks and analysis of ice cores from high-altitude mountain glaciers, show that during the cold periods a large proportion of the northern hemisphere was covered by an ice sheet that was, in places, several kilometres thick. Glaciers advanced, eroding valleys and mountains, and northern hemisphere wildlife moved south to more temperate regions. At the lowest temperatures the ice sheets covered about 10% of the entire planet – up to 30% of all the land. This meant sea levels were very much lower than today, so the area of exposed land was larger. Figure 14 shows how different the ice sheets and coastlines were.

Figure 14 The maximum extent of the ice sheets of the northern hemisphere during the 800 000 years of EPICA ice core data. Oceans are coloured dark blue and continents yellow. Ice is shown as lighter shades of blue.

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A schematic map of the northern hemisphere. The oceans are coloured dark blue, and the continents are coloured yellow. On top of the landmasses and coloured in light blue are the maximum extent of the ice sheets over the last 800 000 years BP. All of Canada, Greenland and Iceland are covered in ice sheet. Virtually all of the UK and most of northern Europe is also covered in ice. A small part of Siberia and small part of Alaska remain ice free.

Figure 14 The maximum extent of the ice sheets of the northern hemisphere during the 800 000 years of EPICA ice core data. Oceans are ...

The sea froze as far south as the northern Spanish coast, and almost all of Britain was buried beneath the ice. These periods are called the ice ages. A vast quantity of water was locked in these ice sheets, so sea level was as much as 120 m lower than today, and there was dry land between Britain and the rest of Europe. During times between these cold periods, the ice sheets melted and the water from land ice meant that sea levels rose. These are called interglacials.

Note that it is only the melting of land ice that changes sea levels: melting sea ice does not change the sea level. Recall that ice is less dense than water, so it floats. As sea ice melts, it forms a smaller volume of water than the volume of ice. In fact, the volume of water formed is exactly the same as the volume of ice that was below the water surface when it was floating, so no change in sea level occurs (Figure 15). Of course, when ice on the land melts and flows into the seas, this does raise sea levels.

Figure 15 Floating ice does not increase sea level when it melts, because the volume underwater is the same as the volume of water when the whole piece of floating ice melts. Ice on the land does increase sea level when it melts, because it adds new volume to the ocean.

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A simple colour diagram of a block of land ice resting on land and a block of floating ice in water shown side-on. An arrow points from the land ice into the water and this side of the diagram is labelled ‘Land ice raises sea level when it enters ocean’. The floating ice is partly submerged at the surface of the water. A dotted line box delineates the submerged part and this is labelled ‘Submerged volume of sea ice is equal to volume of water when sea ice melts’.

Figure 15 Floating ice does not increase sea level when it melts, because the volume underwater is the same as the volume of water when ...

Activity 6 Rates of change of temperature

Timing: Allow about 10 minutes

Look carefully at the Antarctic temperature record in Figure 13(a). Are there any general observations you can make about the rates of change of temperature between the relatively warm and relatively cold periods?

Maximise

Figure 13 (repeated) (a) Antarctic temperature changes from the EPICA ice core from 800 000 years before present (BP) up to 1911. The vertical temperature scale has 0 °C for the mean temperature over the past 1000 years, and goes from −10 °C to +5 °C relative to this (Jouzel et al., 2007). (b) Map showing location of Dome C in Antarctica.

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Figure 13(a) is a graph of temperature data inferred from the EPICA ice core going back 800 000 years. The horizontal axis is time measured in years before present (BP) and the minimum is 800 000 BP. The scale is linear and there are ticks every 100 000 years. The maximum on this axis is 0 which is the present day. The vertical axis is temperature change from the present day with a minimum of −10 °C and a maximum of 5 °C. A temperature change of 0 °C refers to the temperature of the present day which is defined as the mean temperature over the past 1000 years. The data on the graph is an irregular orange line which seems to oscillate between two states: one with a temperature close to the present day, and one approximately 8–10 °C colder. At the present day the temperature change is 3 °C, and at around 14 000 years BP the temperature change is about −10 °C. Around 130 000 years BP the temperature change is about 5 °C, but at around 140 000 years BP the temperature change is about −9 °C. At around 250 000 years BP the temperature change is about 3 °C, but at around 275 000 years BP the temperature change is about −9 °C. Around 340 000 years BP the temperature change is about 4 °C, but at around 345 000 years BP the temperature change is about −9 °C. Around 405 000 years BP the temperature change is about 3–4 °C but at around 450 000 years BP the temperature change is about −9 °C . Further back in time the temperature oscillations decrease in magnitude and the warm periods are colder than present by about −1 °C to −2 °C, whilst the cold periods are mostly around −8 °C. The final four cold periods are at times of around 550 000 years BP, 640 000 years BP, 720 000 years BP and 800 000 years BP. Figure 13(b) is an outline map of Antarctica, the landmass is white and a blue dot marks the location of the South Pole. To the South-east of the South Pole is a green dot marking the location of Dome C, the location of the EPICA ice core. The area surrounding Antarctica is shown in blue and represents the oceans: to the top left is the Atlantic Ocean, to the top right the Indian Ocean and the bottom left is the Pacific Ocean. In the bottom right is a scale indicating 1000 kilometres. Dome C is around 1700 km from the South Pole.

Figure 13 (repeated) (a) Antarctic temperature changes from the EPICA ice core from 800 000 years before present (BP) up to 1911. The ...

Answer

The record in Figure 13(a) shows that the temperatures fall relatively slowly but rise relatively quickly – particularly in the most recent 450 000 years.

Assuming (correctly) that the timing of Antarctic temperature changes is a proxy for the timing of changes in the amount of ice on the planet, the ice sheets in Figure 14 took about 100 000 years to grow, and yet they rapidly decreased in size – typically in only approximately 10 000 years. Consequently, sea levels fall slowly as the ice sheets grow, and rise relatively quickly as they decay again. The obvious question from Figure 13 is what causes these regular fluctuations in temperature and ice cover. One of the most influential is the Milankovitch cycles of the Earth’s orbit. You will look at this in more detail next.