Is Sea Level Rise
a Reliable Indicator of Global Climate Change?
Let's first concentrate on the scale:
Recent data suggest a rise on the order of 3 mm a year.
But this is minute compared to the large scale modulation of
sea level by ice ages.
Therefore, the cause of sea level rise could be dominated either by:
the thermal expansion of the oceans as they warm in
response to human induced greenhouse gas additions to the atmosphere
or
The continued addition of fresh water due disequlibrium between
galacial ice and global temperatures.
The components of Sea Level Changes:
Projections and Data:
IPPC Projections:
What is somewhat curious/important about the instrumental record is that it shows only
one inflection point (around 1930) and after that, a more or less steady rate of 3 mm per
year has been established. (note that the zero level is defined to be for today)
Dominant source of uncertainty is this expectation: Higher temperatures are also likely
to increase the amount of snowfall over central Greenland and Antarctica.
The higher snowfall is likely to offset part of the sea level rise from other
factors because the additional
snow is composed of water that would otherwise be in the ocean.
How well does the IPPC do?
If we zoom in deatil on sea level rise we see noise and features in the data - but these
seem to average out over sufficiently long timescales.
Overall, however, the data are highly consistent with a noisy linear rise in data and do not
show any signs (yet) of an exponential increase. This would indicate that notipping point
in this system has yet been reached.
However, a closer statistical analysis done with the IPPC data does suggest that
we have moved into a period of higher rate changes which otherwise appear lost
in the noise. This is why you average together noisy data:
The Feature at 1997/98 was the
last strong El Nino
Now Some Points about melting ice:
The standard classroom demo of filling glass with ice water to the brim and
then letting the ice melt to show that the "water level" has gone down is not a good
analog to the oceans due to scale problem. (ratio of total floating ice to oceans liquid
volume is very small).
In general, floating water ice in water does not produce any change in water level
because the ice has already displaced the amount of water it will take up when it melts.
However, floating water ice in salt water is different, because salt water is more
dense (less volume occuppied by same mass). In this case, melting ice increases
the volume of water by 2.5 -3% (depends on mean ocean salinity and temperature). In principle,
this could mean a total sea level rise of 30-40 mm. Maybe this is what is causing the
observed increase in slope post 1993?
