Records of sea level

There are records of sea level, but few of them extend very far into the past. Sea levels are monitored by instruments called tide gauges. There are several types, but most record the movement of a float in a well fed from the sea, or by the pressure of sea water acting on a bubble of air. They record the rise and fall of the tides as a line on a chart drawn by a pen on paper fixed to a rotating drum, or as a digital record fed to a computer. The average height the water reaches—the mean tide level—can be calculated from the tide record, and so changes in it can be identified.

There are a few other, older records, including a benchmark carved in a rock at the Isle of the Dead, off Port Arthur, Tasmania, to mark the mean tide level as it was in 1841 (see the sidebar). This seems to show a very small rise in sea level since the benchmark was carved. With these few exceptions, earlier changes in sea level must be calculated indirectly, mainly from the location of layers of sediment.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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