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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).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.This relative abundance is expressed as the Sr ratio, where strontium-86 is chosen to represent the stable isotopes strontium-88, strontium-86, and strontium-84, which occur in constant proportions in natural materials.
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’.
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’.
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. Gravity is the force which pulls two masses towards each other; the larger the mass, the greater the attraction.
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Therefore, we must turn to the ice sheets themselves to try to measure how quickly they are melting.