Sea level change
Sea level refers to the elevation of the world's oceans. Sea level change refers to the observed differences in sea level through time. Today, sea level is rising at about 23 centimeters per century. In the recent geologic past sea level has risen as fast as 1300 centimeters per century and has dropped at a comparable rate.
The sedimentary record
For generations, geologists have been trying to explain the obvious cyclicity of sedimentary deposits observed everywhere we look. The prevailing theories hold that this cyclicity primarily represents the response of depositional processes to the rise and fall of sea level. In the rock record, geologists see times when sea level was astoundingly low alternating with times when sea level was much higher than today, and these anomalies often appear worldwide. For instance, during the depths of the last ice age 20,000 years ago when hundreds of thousands of cubic miles of ice was stacked up on the continents as glaciers, sea level was 390 feet lower (~120 meters), locations that today support coral reefs were left high and dry, and coastlines were miles farther basinward from the present-day coastline.
However, for the past 6000 years (long before mankind started keeping written records) the world's sea level has been gradually approaching the level we see today. During the Pleistocene about 120,000 years ago, sea level was for a short time (a few centuries) about 19.5 feet higher than today, as evidenced by wave-cut notches along cliffs in the Bahamas. There are also Pleistocene coral reefs left stranded about 3 meters (~10 feet) above today's sea level along the southwestern coastline of West Caicos Island in the British West Indies . These once-submerged reefs and nearby paleo-beach deposits are silent testament that sea level spent enough time at that higher level to allow the reefs to grow (this level is probably very close to present-day Earth's highest sustainable interglacial sea level).
What causes sea level changes
During times when the Earth is colder than present-day, water that evaporates from the sea gets trapped up on the continents as year-round glacier ice, and the sea level falls in equal measure. When the Earth's temperature rises, that continental ice melts and runs back to the sea and fills the ocean basins once again. Earth scientists think that the total amount of water on Earth is relatively constant, so adding ice to continental glaciers always removes it from the ocean basins, and vice-versa.
Is the Earth's present-day sea level at the maximum possible level?
No, sea level could go higher if more polar glaciers melt. However, compared to the last Ice Age, there are very few continental ice sheets remaining to be melted, and as a result the rate of sea level rise has been gradually decreasing through time. During the last Ice Age, up until about 20,000 years ago when global temperatures started to rise, all of Canada and most of the northern United States and northern Europe were buried kilometers deep with glacial ice. That ice, of course, is long gone, having run back into the sea thousands of years ago. These days, virtually every ounce of water that falls as snow in the winter gets melted by the end of summer, except on a few rare mountaintops and shadowed valleys.
Rates of Change
When world temperatures started to rise at the end of the last Ice Age, sea level rose extremely rapidly at first (1300 cm/century), but slowed considerably about 7,000 years ago to 50 cm/century. Between 5,500 years ago and 3,200 years ago, sea level was rising at only 23 centimeters per century. Between 3,200 years ago and today, sea level rise slowed again to an average of about 4 cm/century. Since the 1930's Dust Bowl era, sea level has been in a rising spurt going at about 23 cm/century again, but there is evidence that indicates spurts like this are not unusual. Data from south Florida shows that during the past 3,200 years there have been at least 2 episodes of sea level drops and subsequent rises of more than one meter, so it is probable that the recent 4cm/century rate is a long-term average of higher frequency rises and falls. Considering that this rising spurt started during a global temperature high, it is likely that the current rise is just one of these high frequency oscillations.
Remaining Glaciers
Today's major continental ice accumulations are on Greenland and Antarctica. There is still a lot of water tied up in those glaciers (enough to raise sea level by 67 meters if it all melted, see ) but due to their extremely high latitudes, only Greenland's portion (at most 7 meters' worth) is ever likely to melt to any appreciable extent (again see ). The world's remaining water contained in glaciers and grounded ice caps (outside of Antarctica and Greenland) is equivalent to about 1/2 meter (call it a foot and a half) of global sea level rise. Today, sea level is rising about 2.3 mm per year (to keep this in perspective, human fingernails grow approximately 50 mm per year), and about 20% of that rise is due to water coming from melting glaciers. The remainder of the rise is due to thermal expansion of the existing seawater (remember, global temperatures are rising). It is important to keep in mind that the supply of glacial ice is not infinite, so this rate of meltwater contribution cannot continue indefinitely.
The rest of Earth's permanent ice cover is already floating in the sea (for example, most of the Earth's northern "Ice Cap" is floating). Unlike continental ice, floating sea ice can not change sea level when it melts. If you doubt this, you can test it for yourself in the kitchen. Fill a glass with water about 3/4 full. Now add ice cubes until the water level is just below the rim of the glass. You'll notice that the tops of the ice cubes are actually above the rim of the glass at this point. Now wait a couple of hours for the ice to completely melt. When the ice is gone, you will find that the water level is still exactly just below the rim of the glass and not one drop has spilled out. This illustrates that floating ice displaces it's own exact volume of water, and that is why the melting of floating ice caps has no net effect on sea level. However, the disappearance of floating ice can still affect the world's climate, because white ice reflects more sunlight energy back into space than open water does. This phenomenon is an example of "positive feedback", where a trend is self-reinforcing.
The effect of plate tectonics on sea level
At times during Earth's long history, continental drift has arranged the land masses into very different configurations from those of today. When there were large amounts of continental crust near the poles, the rock record shows unusually low sea levels during ice ages, because there was lots of polar land mass upon which snow and ice could accumulate. During times when the land masses clustered around the equator, ice ages had much less effect on sea level. Geologists refer to our current world climate as "IceHouse" conditions because of the presence of Antarctica and Greenland near the poles. During the Mesozoic, the world's climate was classified as "GreenHouse" conditions because there was very little land near the poles on which glaciers could grow.
What are the causes of prehistoric global warming and cooling?
The cyclic behaviors of climate change correlate strongly to changes in the Earth's orbital mechanics and the physics of rotating bodies (see
Milankovitch cycles and
ice age). In addition, volcanic eruptions release thousands of tons of dust into the atmosphere, which during historic times has caused noticable but temporary global temperature shifts (see the
Year Without A Summer, caused by the Mount
Tambora volcanic eruption that had occurred in 1815). Similarly, asteroid impacts are also believed to cause global cooling by throwing dust into the atmosphere (see
Cretaceous-Tertiary extinction event).
