A rise in sea levels is one of the most well-known consequences of global warming. There are two ways that higher temperatures cause higher sea level: (1) melting ice sheets and glaciers and (2) thermal expansion of seawater.
|Past, present, and future global mean sea level (shown as the difference from the 1980-1999 mean). Grey shading shows the estimated sea level change in the past. The red line indicates sea level measurements from tide gauges, and the red shading denotes the range of variations in the tide gauge data. The green line shows the global mean sea level observed from satellite altimetry. Blue shading represents the range of model projections for the IPCC's A1B scenario, which has been calculated independently from observations. (Image: IPCC AR4)|
How much is sea level rising?
Since 1900, sea level has risen between 1 and 2 millimeters per year (10-20 cm or 3.9-7.9 inches per century) on average. During the preceding 3,000 years, scientists estimate that sea level was almost constant, rising at a rate of 0.1 to 0.2 mm per year.
Scientists use averages from a large number of tide gauges around the globe to estimate global average sea level. Since 1992, global sea level has been observed using satellite data too, with more accurate results than tide gauges (according to the Intergovermental Panel on Climate Change AR4 report). Satellite measurements show a rate of sea level rise of 3 mm per year, far more than the tide gauges. Some scientists suspect that the satellite is incorrectly calibrated. Others suspect that the difference may be because the satellite measurements cover much of the globe while tide gauges are near coastlines.
Change in global sea level in the future is predicted to occur at a faster rate. The amount of sea level rise depends in large part on the amount of warming. According to the IPCC Forth Assessment Report (2007) by the mid-2090s global sea level may be 22 to 44 centimeters above its 1990 level and rising at about 4 mm per year.
Sea Level Rise Due to Melting Ice Sheets and Glaciers
There is between 24 and 30 million cubic km of ice on land. About 90% of this ice is in Antarctica. Most of the rest is in Greenland and a tiny fraction is locked up in mountain glaciers elsewhere.
As global temperatures have risen, some of this ice has begun to melt. And as ice melts, meltwater flows into the ocean, gradually raising sea level. Of course, each year the amount of ice on land changes with the seasons, but overall global warming has begun to disturb this equilibrium. Melting has outpaced snowfall, and the loss of ice has been greatest on mountain glaciers in the mid-latitudes and tropics and on the Greenland ice sheet. The Antarctic ice sheet seems less disturbed, however, there’s evidence that the rate of ice flowing towards the ocean has increased.
Melting ice can produce a positive feedback loop - darker areas (especially oceans, but also land) beneath the high-albedo snow or ice absorb more light than the bright snow or ice had done, and thus warm still further, melting more snow and ice. This will be further discussed in the next week of the course.
Melting ice can alter the salinity of seawater as freshwater is added to the ocean. Changes in salinity alter seawater density, which can change major ocean currents that transport heat through the ocean driven by the currents, stimulating more climate change.
Complete melting of glaciers and ice sheets would raise sea levels worldwide almost 70 meters (230 feet) above current levels. 7.2 meters of this rise would be from the Greenland ice sheet, while 61.1 meters of this rise would be from the Antarctic ice sheet. Melting glaciers would add another half of a meter.
For some perspective on all this melting ice, consider this: sea level has risen about 120 meters since the last glacial maximum approximately 20,000 years ago when ice covered large parts of the Northern Hemisphere and wooly mammoths roamed the Earth.
Misconception Alert: Not all melting ice causes sea level rise!
As ice on land melts, water flows into the ocean, and sea level rises. But sea ice, the stuff that forms a frozen cap floating on top of the water, does not affect sea level. Like ice cubes in a glass of water, the sea ice does not affect the volume in the glass as it melts. Ice is less dense than liquid water; that's why ice floats. When ice melts, it becomes denser; the same mass fits into a smaller volume. The change in volume exactly offsets the extra bit of the ice that was sticking up above the water's surface.
Melting sea ice is a sign of climate change and, in fact, causes more warming through the ice-albedo feedback (as discussed in the following week of this course). Melting sea ice also changes ocean salinity (the sea ice has little salt in it, so it releases virtually fresh water as it melts) and ocean temperature. These changes can alter ocean currents, which also affect climate.
Sea Level Rise Caused by Thermal Expansion of Water
Just as most materials expand when heated, water expands as it warms due to climate change. Thermal expansion of water plays a role in sea level rise. The ocean gradually takes up much of the additional heat due to climate change. This temperature rise is likely to be quite small at first; however, the volume of water affected is immense.
For example, suppose 1 liter of water, initially at 20° C, was heated to 21° C. It would expand by 0.021% (see the table of volume and temperature). It would increase in volume by 0.21 milliliters. This tiny increase seems trivial, but the ocean contains about 1,400,000,000 cubic kilometers of water. Even a tiny fractional increase adds up to a very large increase in volume, and hence substantial sea level rise.
The different layers of the ocean (surface layers and deep ocean) are not heated equally. Also, the volumes of the different layers are not the same; nor are their initial temperatures, which affects the rate at which they expand (see table).
- The surface layer of the ocean contains roughly 50,000,000 km3 of water, and has temperatures ranging from freezing near the poles to around 30° C in the tropics.
- The mid-ocean, where the thermocline produces the transition from warm surface to cold deep water, holds about 460,000,000 km3 of water and spans a large range of temperatures.
- The deep ocean holds the most water, some 890,000,000 km3, but because of its relatively cool temperatures of 4° C or less is also less prone to expansion as its temperature rises slightly.
How heat is affecting the ocean and how it will affect the ocean in the future is an area of active research. Supercomputer models of climate change predict that thermal expansion of seawater will play as large a role, or possibly greater, than meltwater runoff in raising sea levels in the coming decades and centuries.