Future challenges in shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 365

11.5 Past perspectives on the shelf seas

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Arguably some of the most interesting and important questions for further research
in the shelf seas arise in relation to the past and future changes in the physics and the
biology of these areas. The dramatic changes in sea level which have occurred since
the last glaciation have greatly modified the area and topography of the shelf seas
with consequences for the strength and distribution of energy dissipation in the tides.
The associated changes in the biology of the shelf seas involves drastic changes in the
levels of carbon fixation and hence the shelf seas' role in the Earth system. We shall
now consider some of the key issues in the oceanography of the shelf seas looking
back to the last ice age.

11.5.1 Changes in tidal dissipation

In previous chapters, we have seen that the tides play a major role in providing much
of the energy input to the shelf seas. The magnitude of this power input to shelf seas
around the deep ocean can now be determined by numerical models driven only by
the lunar and solar tide generating forces. Plots like that of Fig. 2.13 involve the
computation of the tidal response of the ocean on a spatial scale which is small
enough to resolve the detailed topography of the ocean, a calculation first envisaged
by Laplace more than two hundred years ago and only recently made possible by the
advent of massive computing power.
As well as determining dissipation in the shelf seas, these model calculations, as we
saw in Section 2.5.2 , also provide estimates of the dissipation of tidal energy in the
deep ocean. In the current situation, they indicate that, of the global total tidal
dissipation, approximately 2.5-2.7 TW is dissipated in the shelf with 1TW being

consumed in the deep ocean. These values are consistent with independent estimates
of the total tidal dissipation which come from measurements of the acceleration of
the moon in its orbit by laser ranging and from satellite altimetry (Egbert and Ray,
2000 ; Egbert and Ray, 2001 ). We therefore have a rather good picture of the present-
day global tidal dissipation, but questions then arise as to, whether or not, the present
pattern of dissipation has always prevailed as sea level has varied, for example
through glacial- inter-glacial cycles, and whether total dissipation and its distribution
will change as sea level rises in an era of global warming.
There has been a tendency to consider the dissipation rate as fixed; for example,
G. H. Darwin, a son of Charles, assumed a dissipation rate constant over long
periods of geological time to infer the history of Earth-Moon separation (Darwin,
1899 ). There seems, however, no fundamental reason why dissipation should be
invariant; on the contrary, changes in sea level, in seabed topography and even
in the thermal structure of the ocean might all be expected to change the magni-
tude and distribution of dissipation. Such changes are important, not just
in relation to the environment of the shelf seas, but also because it is now
recognised that tidal energy makes a major contribution to mixing in the deep

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