Dear Umbra,
I'm a bit confused about the possible rise in sea level that may be caused by global warming. I know that in general water expands when warmed, and that is one cause of sea level elevation with respect to global warming. The larger cause for alarm seems to be the melting or collapse of the polar ice caps. I recently read an article that warned that Antarctica, which stores 70 percent of the world's fresh water, could lose the Ross Ice Shelf (a block of ice the size of France) suddenly and without warning and "that its collapse would make sea levels rise by at least 5 meters, with other estimates predicting a rise of up to 17 meters." My question is how quickly would such a rise register?
Saor
Mill Valley, Calif.
Dearest Saor,
I got all swept up and joined your completely logical confusion. If ice shelves collapse, when can we expect the swamping wave? Happily, this is incorrect thinking. Many thanks to the good people at Environmental Defense for straightening out my confusion. My mind now runs clear as a glacial river to the sea of climate comprehension.
Should have worn my high-waters ...
Sea level rise is and will be due to two factors, which Saor mentions. We are warming the atmosphere by increasing greenhouse gas concentrations, the oceans are absorbing large parts of this warmth, warmer water takes up more space, hence the global sea level is rising. The other factor in sea level rise is the collapse of ice shelves and tongues, and the melting of land-based ice caps. These icy items are related but not the same, and here is where the potential confusion lies.
Ice shelves and tongues are part of a giant ice sheet, or cap, covering a land mass (e.g., Greenland and Antarctica). They stick out from the ice sheet into the ocean, like ... like tongues! Or shelves! Atop the ice sheet are various glaciers that flow toward the sea. The glaciers add their icy mass to the shelf, but the importance of the shelf is that it presses back against these glacial flows and stops them from entering the sea.
For you see, the shelf is already in the sea, and the glacial ice is not. Returning to Soar's Ross Ice Shelf example: "Collapse" in this case means detach, disintegrate, perhaps vanish. Collapse conjures an image of a sudden splashy fall into the water. But ice shelves are already in the water, so their detachment and disintegration does not add any mass to the global sea volume in and of itself. Sea rise ensues, however, because no shelf is now there to bar the glacial flow and other melting ice sheet bits from entering the oceans.
The collapse of an ice shelf, then, does result in sea rise -- but in a delayed kind of way. The 5 to 17 meters mentioned in the article actually refers to the rise that could be caused by the ensuing melt of the West Antarctic Ice Sheet, which scientists don't expect to happen in our lifetimes.
In fact, scientists are still actively modeling exactly how much ice is likely to melt into the oceans, how far the global sea level will rise, and how fast all this will happen. Pretty much the only good news is that we don't expect sudden swamping waves following ice shelf collapse. To answer your basic question, usually estimates of dramatic sea rise are "by 2100" or the next few centuries kind of range. The IPCC sea level rise estimate, of 0.22 to 0.44 meters above 1990 levels by 2090, is quite conservative, and there's been a lot of talk that it is low; one recent paper estimated a rise by 2100 more like 1.4 meters.
Many millions of folks live close enough to current sea level that any significant rise will be a calamity, and it looks certain that this is already happening. Sea levels have risen, on global average, 3.1 millimeters a year since 1993. So no swamping waves -- but as the helpful Tony Kreindler at Environmental Defense said to me, "That shouldn't diminish the general disasterishness of it all."
Glacially,
Umbra
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Comments
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SolarBozo Posted 1:41 am
16 Jul 2008
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Mary Gilbert Posted 3:56 am
16 Jul 2008
The problem for small island states and countries with large lowland areas like Bangladesh is not one of plain old rise in sea levels, but of the magnitude of storm events due to major climatic disruption. These events are predicted to increase not necessarily in number but in severity, and this change has already begun. Low areas will be flooded and increasingly washed away. This also leads to saltwater incursion into rock that had previously been holding freshwater, reducing the amount of arable land even more.
We can expect to see mass migration ... in the many millions ... as people leave homelands where they can no longer live.
These millions will be in addition to those from areas where changes in patterns of precipitation are a big factor in the spread of desertification.
Truly, friends, we need to see things on a global level and take compassion more seriously tomorrow than we do today.
Your bluebird of happiness,
Mary
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bailsout Posted 5:42 am
16 Jul 2008
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PermieWriter Posted 11:09 am
16 Jul 2008
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Des Emery Posted 11:18 am
16 Jul 2008
When ice melts it increases the volume of the water into which it is dissolving. When Water freezes, the ice forming on land from snow reduces the volume of the oceans, which extends the shorelines and beaches outward (which is where the landbridge from Asia to Alaska comes from, but slowly is submerged as the last ice age melts away and lets the ocean volume increase).
But ice itself which forms at Zero Degrees Centigrade (32 degrees F) can drop many degrees in cold intensity, which is why Arctic and Antarctic ice persist for so many centuries.
If global cooling occurred, there would be a lot of ice forming into glaciers, on land, thus storing water out of the oceans and lowering sea-levels. Global Warming allows more water, stored away in glaciers to reach sea-level and increase the amount of liquid water, causing coastal flooding.
I hope this short explanation contains enough information to supplement your high school biology.
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sindark Posted 6:12 am
17 Jul 2008
Frozen water floats on liquid water because hydrogen bonds line up molecules in a less tightly packed way that exists in the liquid state.
That being said, warmer water is less dense than cooler water.
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timdennell Posted 10:46 pm
18 Jul 2008
Like all materials H2O also expands when heated (thermal expansion). With H2O this occurs between 4°C and 100°C. A simple school experiment is to take a Pyrex flask of water at room temperature and heat it gently, measuring the rise in water level as it heats. Another is to fill a screw-top (to prevent evaporation) glass container with warm water and mark its surface level on the side. Then place in a fridge for a few hours then see if there's been a change. Invariably the level will now be a little lower. (Perhaps by only a mm or so if a small container, so exact measuring is important.)
NB: All household hot water heaters require thermal expansion tanks to provide a safe place for expanded heated water to go. (Most water meters are installed with devices that prevent backflow, this creates a closed system.) For example, water heated from 32ºC to a thermostat setting of 60ºC in a 40 gallon hot-water heater will expand by almost half a [US] gallon (1.8 litres). This is because when water is heated, its density decreases and its volume expands. Since water is not compressible, the extra volume created by expansion must go somewhere. Pipes in a home may burst unless there is somewhere for the expanded water to go. There are temperature/volume tables to help engineers work out how much a volume of water will expand by at any given temperature.
With seawater salinity differences within the oceans will affect the local water density and thus the expansion rates of local sea levels, but have little effect on the overall global average sea level change.
Mary, above, was absolutely right in pointing out that areas most at risk are those affected by storms. Even a small rise in sea levels can affect the size of a storm surge. A storm surge is primarily caused by the extremely high winds that accompany a hurricane. This wind pushes the water towards land, building it up into huge waves. At the same time, the low pressure caused by a hurricane also causes the water level to rise up in the lowest-pressure spots and to sink in areas of higher pressure, exacerbating the wave build-up caused by the winds. So even a small increase in the volume of seawater can lead to bigger storm surges than before reaching further inland than previously. As well as the devastation this causes, longer term effects include salt contamination of the ground which affects crop productivity. Given how many subsistence farmers live in delta and low lying areas of India, SE Asia and China this could have major impacts.
Even small rises in sea levels also will cause greater penetration of salt into coastal water tables. I understand that some previously healthy Palm trees in Florida are now being killed off because salt has contaminated more of the water table. A small rise in sea levels can have a bigger impact than you'd first think.
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