Environment: understanding atmospheric and ocean flows
Environment: understanding atmospheric and ocean flows

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Environment: understanding atmospheric and ocean flows

5.1 Melting ice caps and sea ice extent

Figure 27 compares the surface melting on the Greenland ice cap in 1992 and 2005 as measured by satellite. For ice to form, the snow has to survive the following summer. But an increasing area of the Greenland ice cap is melting in summer, so annual snow layers are not being converted to ice in these regions.

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Figure 27 A comparison of the surface melt of the Greenland ice cap in 1992 and 2005

It is an extremely complex process to estimate the melt of the whole ice cap, and the current best value is that Greenland was losing around 160–270 billion tonnes of ice per year in the first decade of the 21st century. All of this melt is contributing to the predicted sea-level rise of around half a metre to a metre by 2100; a rise of a metre could affect around 150 million people worldwide (Anthoff et al., 2006).

These impacts are primarily through increased flooding, rather than widespread loss of land: note the sea level changes predicted for this century are 10–20 times smaller than those of the much longer ice age cycles. The fresh water from the Greenland ice sheet could also slow Broecker’s conveyor (Section 4.1), causing other climate impacts.

For the Arctic sea ice, the signal of climate change is clear: it is getting thinner and the amount of it that survives the summer is reducing. Figure 28 shows the trend in extent of sea ice in September each year from 1979–2016 (the summer minimum, of which the median for 1981–2010 is shown as a pink line in Figure 29). The sea ice minimum is decreasing at a rate of approximately 90 000 square kilometres each year. Near the start of the observations, in 1980, the September ice area was around 7.9 million square kilometres (white area in Figure 29(a)). In 2012, the September ice area was less than half this (3.6 million square kilometres, Figure 29(b)), though in subsequent years it recovered to some extent.

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Figure 28 The minimum extent of Arctic sea ice in September of each year from 1979–2016
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Figure 29 Arctic sea ice extent in September: (a) 1980; (b) 2012. These years have been selected to show the largest observed changes. The median September sea ice extent from 1981–2010 is shown as a pink line.

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