Global Sea Level Rise
Submitted by: Sandra Ashhab and Ram Fishman
December 10th 2006
The issue of Global Sea Level Rise (GSLR) is one of the major concerns associated with
global warming because of the disastrous implication it may have on human society, the
majority of which, both in terms of population and in terms of economical productivity, is
located near the coastline. Since the last ice age, about 18,000 years ago, the global sea
level has risen more than 120m. Since about 3,000 ago and until the 19th century, the sea
level has risen at a rate of 0.1 to 0.2 mm/yr [2], however, there is evidence that this rate
has way increased since the 19th century, and is still accelerating.
Direct measurements of global sea level are conducted through the global system of tide
gauges that continuously record the height of water level with respect to a heightreference surface close to the geoid. Currently there are more than 1750 tidal gauges
around the world, some of which record data from as early as the 1810s. Measurements
from tide gauges indicate that the sea level has risen at an average rate of 1.5 -2 mm/yr
during the 20th Century [1, 2]. However, measurements from tidal gauges should be
interpreted carefully since they might be biased by local conditions such as abnormal
warming or by subsidence.
Since the launch of the joint US/France altimeter satellite TOPEX/Poseidon (T/P) in
1992, more exact sea level data are provided through measurements of water thickness,
using the time taken for a radio signal to reflect from the surface back to the altimeter.
These measurements are suggesting that since the 1990s the global sea level is rising at
rate of 2.5 – 3 mm/yr [1]. Also since the GRACE twin satellites were launched in March
2002 additional detailed measurements of global sea level are provided through
measurements of the Earth's gravity field.
Two principal mechanisms related to global warming are believed be responsible for the
current rise in sea level. The first is an increase in volume; a thermal expansion of the
oceans’ water as their temperature increase, a basic physical property of most materials.
It is important to note that most of the warming will take place above the thermocline,
and it is this layer of near ocean’s surface which will be expanding most. Measurements
of temperature and salinity induce estimates of around 0.5 mm/yr during the 20th century
[3].
The second mechanism, which seems to be more delicately related to global warming, is
an increase in the global seawater mass, as water locked as ice and packed show mainly
in continental glaciers is expected to melt and flow to the oceans. The mass balance
between the oceans and the land locked ice is maintained by a constant flux in both
directions. Evaporation from the oceans and snow precipitation over continents transport
mass from the oceans into continental ice, at a rate that is equivalent to a decrease in the
global ocean level of about 6.5mm/yr [2]. On the other hand, ice melt and rivers flow
transport mass back to the oceans. Global warming and the ensuing climate change is
likely to affect both temperatures and precipitation patters, so it is not in principle
necessary that sea level would rise as a result, since it is generally expected that
precipitation over continents would increase as a result of global warming. Indeed, in the
case of Antarctica, a general rise in ice mass is expected, since precipitation is predicted
to increase, and the extreme cold at these high latitudes means that the ice is less sensitive
to a modest change in temperature of the scale expected. And yet, the global balance of
the various effects, inferred from both measurements and modeling is expected and
already observed to be a reduction in continental ice mass [4]
Glacier mass balance is measured by comparing the amount of snow accumulated during
winter with the amount of ice and snow removed by melting during the following
summer. The balance is said to be negative (positive) if the amount of accumulative snow
during the winter is smaller (larger) than the amount of melting snow and ice during the
summer, which leads to decrease (increase) in glacier mass. To determine the mass
balance, measurements from snow-pack depth probes, snow-pits and crevasse
stratigraphy are used [5].
The IPCC original estimates were of less than 0.5mm/yr GSLR as a result of glacier melt.
These were based on direct observation of mass loss in glaciers. If one adds the estimates
of the volume increase due to thermal expansion, one is still short by about 1mm/year of
the directly observed 20th century GSLR. Some attempts to rectify this discrepancy [6]
were based on suggestions that the tide gauge measurements were an overestimation
resulting from using gauges located in areas that experienced higher than average
warming. Further investigation; however, seems to verify the original measurements as
do the more recent satellite techniques [1]. Moreover, a freshening of the oceans at a rate
equivalent to the addition of 1.4 mm/year of fresh water has also been observed [3]. This
number would pretty much fill the gap to the observed GSLR. While this freshening
could have occurred as a result of melting sea ice as well, which would not increase GSL
(Archimedes’ principle), the assumption that it comes from continental ice is consistent
with the data set, and it generally felt that glacier mass loss has been underestimated.
The projection of future GLSR is also separated into the volume and mass components.
While any projection of this sort will suffer from uncertainties in predicting future
climate change, the mass component is significantly harder to predict due to the large
uncertainty in the effect of global warming on precipitation patterns, and the difficulty of
modeling physical processes in glaciers and ice sheets which are still poorly understood.
In assessing future climate change due to volume increase, it is common to compare the
canonical scenarios known as the Special Report for Emission Scenarios (SRES),
established by the IPCC to compare different stabilization policies undertaken by society
in the next two centuries. Here we will refer to only three of these emission scenarios,
B1, A1B, and A2 (Figure 1).
Figure1: Three different scenarios B1, A1B and A2, for stabilizing CO2 emission
including the 20th century stabilizing scinario [7]
Temperature rise, calculated by two GGCM for the three different SRESs and their
corresponding GSLR by the end of the 21st (Figures 2 and b) century are summarized in
table 1 below [7]
Temperature rise [C]
Scenarios B1
1.1-1.5
A1B
A2
1.9-2.6
2.2-3.5
GSLR by the end of the 21st century
13-18
18-25
19-30
[cm]
Table1: The expected temperature rise and related GSLR for the 21st century according
to the three different emission scenarios [7].
Figure 2
a
b
Figure 2a represents temperature projections for the two coming centuries according to the three different
scenarios and b represents their corresponding sea level rise [7].
In fact, even if all GHG emissions were completely stopped by the year 2000, the climate
system would still be in the process of stabilizing, and will commit to an estimated 0.5C
temperature rise, which can cause a GLSR of about 10cm [7] by the end of the century.
Even while temperature increase is slowed down considerably, GSLR seems to respond
at a much slower rate. We were able to find three possible causes for that: the slower
response time of the oceans, the fact that thermal expansion is proportional to volume and
the fact that the thermal expansion coefficient is rising with temperature.
When it comes to projecting the mass component of GSLR, things are much more
complicated. General estimates are in the range 21cm GSLR due to glacier melting, of
which about 37% are contributed by Greenland ice sheet alone [8].
Based on their study, Gregory et al [8] calculate glacier mass loss based on a local grid of
temperatures and glacial cover and find an increase of 20% compared to the global
average calculation. Most importantly, there has been some evidence that glacial melt is
proceeding at a faster pace than expected [1]. Especially worrisome is the Greenland ice
sheet. Some of the processes observed are highly non-linear, poorly understood, and hard
to model. Recent studies [2] have suggested a loss of mass in the ablation zone and have
brought to light the important role played by bottom melting below floating glaciers.
Neglect of this term led to erroneous results in earlier analyses. Moreover, glacier mills
(or Moulins) are thought to play an additional role in increasing mass loss form
continental ice sheets. Moulins are vertical tubular chutes that carry water from the ice
sheet surface into its interior. Moulins reach the depth of several tens of meters but
sometimes they go all the way to the bottom of the glacier that can be hundreds of meters
deep. If moulins flow from the base of the glacier towards the ocean (under the effect of
gravity) they can cause “basal sliding” off the bedrock beneath them causing an abrupt
considerable mass loss into the ocean [9] Although increased mass loss due to moulins is
related to high latitude warming, such abrupt mass loss doesn’t necessarily translate to
sea level rise.
To summarize, conventional projections of GSLR during the 21st century are in the order
of 50 cm [7, 8] due to mass and volume increase together. However, this estimate is
subject to considerable uncertainty due to, but not only, the different abovementioned
reasons. The Greenland and Antarctic ice sheets together have the potential to raise sea
level by as much as 70 m, so that only a small fractional change in their volume would
have a significant effect on the GSL. Moreover, it is widely accepted the GSL will
continue to rise for centuries even after GHG concentrations stabilize, so that its eventual
effect can be significantly larger than might seem during this century.
References:
[1] Miller, L. and B. C. Douglas, 2004. Mass and volume contributions to twentieth century sea
level rise. Nature, 428, 406-409.
[2] IPCC-TAR: http://www.grida.no/climate/ipcc_tar/wg1/index.htm
[3] Antonov, J. I., Levitus, S. & Boyer, T. P. Steric sea level variations 1957–1994:
importance of salinity.
J. Geophys. Res. 107, doi:10.1029/200/JC000964 (2002).
[4] Sea Level, Ice, and Greenhouses FAQ:
http://www.radix.net/~bobg/faqs/sea.level.faq.html
[5] Wikipedia:
http://en.wikipedia.org/wiki/Glacier_mass_balance#Measurement_methods
[6] Cabanes, C., Cazenave, A. & LeProvost, C. Sea level rise during past 40 years
determined from satellite and in situ observations. Science 294, 840–842 (2001).
[7] Meehl, GA and co-authors, 2005. How much more global warming and sea level rise?
Science, 307 (5716): 1769-1772.
[8] Gregory, J. M. and J. Oerlemans, 1998. Simulated future sea-level rise due to glacier melt
based on regionally and seasonally resolved temperature changes, Nature, 391, 474-476.
[9] Wikipedia: http://en.wikipedia.org/wiki/Glacial_motion