Download Sea Level Rise Due to Melting Ice Sheets and Glaciers

Rising Sea Level – UCAR, CU Boulder
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.
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
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 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)
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 water, does not affect sea level. Like ice
cubes in a glass of water, the sea ice does not effect the
volume in the glass as it melts. Ice is less dense that 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
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.
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.