Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 333

melting would continue through to the end of the century due to the increased warmth
already in the system.
In addition to the above terrestrial ice-melt factors, roughly a quarter to a third of
recent sea-level rise has not been due to melt but thermal expansion of the oceans. As
the oceans warm, they expand and the sea-level rises. This means that the sea-level
rise in a warmer world thousands of years from now, with all terrestrial ice melted,
would exceed the 80 m or so anticipated rise from melting alone.
Before dealing with possible surprises, it is worth reminding ourselves that our
current understanding of historic, let alone possible near-future, sea-level rise is
fraught with uncertainties (see the previous chapter). Uncertainties are not the same as
surprises, even if one can lead to, or spring from, the other. As previously mentioned
our estimates for the various components of historic sea-level change include a
considerable margin of error. So, in one sense we could say that we can only explain
about half the change seen over the past couple of centuries. In addition, we not only
do not understand how Antarctica is likely to behave in a warmer future, we do not
know entirely what is going on today.
What we do know is that the marine ice shelves of the Antarctic Peninsula are
vulnerable to collapse, but because the bulk of these is already under water their
future melting will not contribute as much to sea-level rise as the continental ice shelf
(grounded on rock). The Antarctic Peninsula is particularly sensitive to climate change
as it juts away from the main body of the continent and the South Pole. We also know
that much of West Antarctica consists of floating marine shelves and so will certainly
contribute to sea-level rise in a warmer future, long before East Antarctica. It appears
that parts of the West Antarctic Ice Sheet (or WAIS) contributed to the (5-6 m)
higher sea levels seen in the previous interglacial. Conversely, East Antarctica is
largely composed of grounded continental ice shelves and is higher (hence cooler)
than West Antarctica. On one hand these ice shelves are more stable and so will not
melt, except with warming over a considerable period of time (hundreds of thousands
of, if not one or two million, years) or unless global warming is really exceptional,
such as perhaps with an IETM analogue event.
Yet uncertainty abounds as to what is currently happening in much of Antarctica (let
alone what might happen in the future). There were in 2006 18 long-term monitoring
stations in Antarctica. Eleven of these show warming over the past few decades and
seven show cooling. This may suggest that Antarctica is warming up. Yet, it must be
remembered that Antarctica is a continent the size of Europe and the USA combined,
and so 18 stations are not sufficient for detailed monitoring. This insufficiency is
even more evident when one considers that 16 are arranged around the coast and
just two (at the South Pole and at Vostok) are in the continent's interior. Indeed,
even the South Pole station is not particularly useful for getting an idea of what
is happening in East Antarctica, as it is not that far from the West Antarctic Ice
Sheet, in the transition border zone between the part of the continent that is thought
to be losing water and the part that is accumulating it. Consequently, that station
does not tell us as much about the continent's interior as we would like. Remote
sensing via satellite has also been limited as no satellite has passed directly over the
South Pole (see Figure 5.9). However, this will change with results from the second
European Space Agency CryoSat satellite. Following the first CryoSat's crash due to a