How does the Earth recover after an Ice Age?
Around 20,000 years ago vast tracts of the Earth’s surface, including much of northern Europe and North America, were covered in ice. In fact, due to the volume of water that was tied up in the ice sheets, the sea level at the time was, on average, around 125m lower than it is today ; the UK was joined to Europe by dry land, and you could walk from Alaska to Russia across the Bering Straits.
But it wasn’t just sea level that was different; the enormous mass of ice that was sitting on the continents, over 3 km thick in places , squashed the land downwards so that the whole shape of the Earth was altered. The land beneath the thickest ice was pushed down by up to half a kilometre, while the land outside the ice sheets bulged upwards by several hundred metres (Figure1a). The reason for this is that the Earth’s mantle – the 2900 km-thick layer within the Earth which lies beneath the rocky layer that we stand on – behaves like a viscous fluid. Imagine what would happen if you managed to fill a lilo with really thick honey and then stood on it: For a while nothing would happen, then, slowly, you’d start sinking as the honey beneath your feet flowed outwards because it couldn’t support your weight. However, since you’ve closed the valve on the lilo, that honey can’t escape completely, and instead it forms a bulge around your feet. This is exactly what happens when a large ice sheet grows on the surface of the Earth.
Ice sheet deglaciation
Around 19,000 years ago, the ice sheets of North America and northern Europe began to melt, and the processes described above were reversed (Figure 1b). The meltwater from the ice sheets flowed into the oceans, raising the sea level once again, and the land which had been beneath the ice began to rebound upwards, a process known as ‘postglacial rebound’. And, just as the lilo wouldn’t immediately return to its original shape once you stepped off it, the Earth also returns to its original shape very slowly. In fact, postglacial rebound continues today, albeit at an exponentially-decaying rate. The land beneath the former ice sheets, e.g. around Hudson Bay and central Scandinavia, is still rising by over a centimetre a year [3, 4], while those regions which had bulged upwards around the ice sheet are subsiding – regions such as the Baltic states and much of the eastern seaboard of North America (Figure 2).
Sea level change
Due to this ongoing ‘relaxation’ of the Earth, when we try to measure how quickly sea-level is changing, the answer we get depends not only on how much meltwater is being added to the ocean, but also whether the land we are standing on to measure sea-level change is rising or falling (Figure 3). The rate of rebound or subsidence has to be added to or subtracted from the raw observations made by, for example, tide gauges (see the Permanent Service for Mean Sea Level website for more information).
But the rebound of the Earth is not the only complicating factor when we try to measure how much sea level is rising. When meltwater is added to the oceans, sea level doesn’t rise by the same amount everywhere; in some places it rises by more than the average amount, and in others it rises by less. The reason for this variation is gravity.
Gravity is the force which pulls two masses towards each other; the larger the mass, the greater the attraction. This is why the ocean stays stuck to the Earth – because the Earth is a very large mass. Since the ocean is a liquid, its surface must track a surface of equal gravity (this is also why water in a glass placed on a table forms a flat surface), and we call this surface the ‘geoid’. Now, the distribution of mass throughout the Earth is not uniform, so the pull of gravity is not the same everywhere. Therefore, the geoid, or sea surface, doesn’t follow the outline of a perfect sphere; it bulges downwards where the interior of the Earth is very dense, and it bulges upwards where there is a large mass on the surface of the Earth, such as an ice sheet (Figure 4).
Solid Earth Shape
The reason that this complicates our measurements of sea-level change is because when an ice sheet melts it alters the distribution of mass on the surface of the Earth, and this alters the shape of the geoid. In addition, the rebound of the solid Earth beneath the former ice sheet alters the distribution of mass within the Earth, also altering the shape of the geoid. As the meltwater enters the ocean, it follows the new geoid shape, and since this shape differs from the shape before the ice sheet melted, the sea-level rise is not uniform . In some locations, far from the melting ice sheet, sea-level rise will be larger than the average value. While close to the melting ice sheet, sea level will actually fall because there is no longer a strong gravitational attraction towards the former ice sheet, and this cancels out the increase in sea level due to the addition of meltwater (Figure 5).
In an ideal world we would have a record of sea-level change over the whole of the Earth’s surface for the last 20,000 years, and this would enable us to work out how much ice has melted during that time, and from where. In reality, the sea-level records that we have are sparse in both space and time, and therefore we need to use mathematical models to work out the possible melting scenarios which fit the sea-level records that we do have . Using this method we are gradually building up a picture of how the ice sheets have changed over the last 20,000 years.
Modern ice sheet melt
We have to be similarly devious to work out how quickly our remaining ice sheets are melting. Satellites measure changes in the height of the sea surface, pinpointing areas where it is rising the fastest (Figure 6). This information relating to the spatial pattern of sea-level rise could potentially enable us to work out the source of the meltwater , but the signal is complicated because some of that sea-level rise is due to the water in the ocean becoming hotter and expanding, rather than due to the addition of meltwater . Therefore, we must turn to the ice sheets themselves to try to measure how quickly they are melting. As they melt, the land beneath them rebounds rapidly upwards, just as it did when the ancient ice sheets began to melt. By placing GPS receivers on mountains poking through the ice sheets (Figure 7) we are slowly building up a picture of this modern-day rebound, and hence a picture of the mass of ice which is being lost to the oceans today.
- Sea level rise
- Antarctica’s contribution to global sea level rise
- Climate change
- Glaciers and climate change
- Numerical ice sheet modelling
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