Future challenges in shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 367

differences are mainly due to the change in sea level, but the changes in topography
due to loading of the solid earth by the ice mass have also been taken into account.

11.5.2 Changes in the role of the shelf seas in the carbon cycle

At the time of the last glacial maximum, the total area of the shelf seas was reduced
to 10 6 km 2 . On this

basis alone we would expect the potential of the shelf seas to fix carbon and draw
down atmospheric CO 2 (the continental shelf pump, see Section 10.9.3 )tohave
increased greatly as sea level rose between 18 000 and 8000 years BP (Before
Present). But it is not just a question of the shelf area. As we saw in Chapter 11 ,
the water column structure of a temperate shelf sea exerts an important control on
CO 2 recruitment from the atmosphere with stratified areas acting as sinks and
permanently mixed regions acting as sources releasing CO 2 back into the atmos-
phere. Using a numerical model to hindcast the tidal changes since the last glacial
maximum, it has been possible to determine the extent of stratified and mixed
regimes by applying the SH criterion ( Chapter 6 ) with assumptions about the
annual cycle of heating and cooling (Rippeth et al., 2008 ). These hindcasts of tidal
conditions are supported by some evidence from a recent study of changes in
frontal positions over the last 20 000 years based on bottom temperature estimates
derived from proxies (Marret et al., 2004 ). Making the bold assumption that
present-day heat exchange for stratified and mixed regions (Thomas et al., 2004 )
has remained invariant over the glacial-inter-glacial transition, it is possible to
estimate the evolution of the net carbon sink in the temperate shelf seas. The results
indicate an increase of the shelf sea sink by a factor of

10 6 km 2 which is only 6 25% of its present value of 26

∼ ∼

3 since 18 000 BP, the effect
of which would have been to oppose the increase in atmospheric CO 2 which has
taken place since the glacial period.

∼

11.6 Response of the shelf seas to future climate change

...................................................................................
Finally, we should look forward in time and consider the important practical ques-
tions concerning the changes in the shelf seas on a time scale of decades to centuries
which will occur in response to climate change.
A question of immediate interest is, how will the ocean's tidal system react to future
increases in sea level due to global warming? Model predictions indicate that tidal
dissipation will decrease with rising global sea level (Green, 2010 ), continuing the
trend which occurred as the Earth moved into the present inter-glacial. For realistic
sea-level changes (1-2 metres over the next 100-200 years), dissipation on a global
scale will decrease rather little (

5%) but there will be significant local effects on the
tides in some shelf sea areas (e.g. Hudson Bay, Sea of Japan, the Severn Estuary and
Barents Sea). Given the biogeochemical and ecological significance of the internal tide, a
challenging question involves determining how changes in the seasonal cycles of heat
flux across the sea surface may lead to shifts in the timing and extent of stratification at

<