Geoscience Reference
In-Depth Information
of Bothnia (north arm of the Baltic Sea) and Norway's
Hudson Bay and northern Labrador coast, will mostly
offset twenty-first-century sea-level rise. Conversely, rising
sea levels impose hydro-isostatic loads on coastal crust,
adding to those from net seaward sediment flux from
continental glacial (and fluvial) erosion.
However, deglaciation driving rising sea level today is
still too weak, and isostatic recovery too slow, to increase
vertical rates of crustal movement in the near future. The
greatest coastal sensitivity occurs instead where sea-level
rise compounds existing crustal
subsidence
, in the flexural
isostasy zone beyond rebound areas, natural subsidence
around large deltas and subsidence through groundwater
abstraction and beneath large conurbations. All four
processes probably contribute to 'background' subsidence
of 2-4 mm yr
-1
in the southern North Sea composite delta
of the Thames, Rhine, Waal, Maas and Schelde (see
Figure
or twice the forecast eustatic rise. This compromises the
original design and marine flood protection value of the
Thames Barrier and Delta Plan coastal defences in
Belgium and the Netherlands (see box,
Chapter 17,
p. 416).
Many other deltas around the world are similarly
threatened, along with trailing-edge coastlines, especially
where active extension continues to stimulate subsidence.
Do almost continuous glacio-isostatic/eustatic adjust-
ments also increase the risk of seismo-volcanic responses
to background tectonically-induced stress ? It is very
difficult to assess specific risks at millennial time scales,
and even more so over merely a century, but there is
evidence of strong causal relationships between rapid
Quaternary sea-level change, volcanic eruptions and
earthquakes. There is also evidence that seasonally
higher sea-level loading, associated with multiple storm
sequences or stages in ENSO events, may disturb sub-
surface magma chambers sufficiently to trigger eruption
on mid-ocean volcanoes. These processes are also focused
in narrow coastal zones (typically 200-400 km wide)
where, subduction, crustal shortening and uplift are
concentrated at leading-edge,
interplate
boundaries -
hence mainly the island arcs and coastal orogens of the
Pacific rim. Repetitive crustal flexure here, exacerbated
by the close proximity and regularity of iso/eustatic
loading and unloading, exceeds that in stable cratons
underpinning much of the former North American,
Scandinavian and eastern Siberian ice sheets. Similarly, in
the Tethyan intercontinental collision orogenic belt,
Mediterranean, Persian Gulf and peninsular south-east
Asian coasts experience combined tectonic and iso/
eustatic stresses. Although
GEOSPHERE
IPCC Fourth Assessment Climate Change scenarios provide
the clearest guide to probable changes in geological
processes and environments during the twenty-first
century but they need to be seen in context. Not only can
we expect the impacts of anthropogenic climate change
to continue for many centuries beyond
AD
2100, even if
and when greenhouse gas emissions are stabilized, but
there are existing, longer-term background changes to
consider first. This section will not explore the further
feedbacks of anthropogenic disturbances to land surfaces,
vegetation systems on climate, nor of gas compositional
aspects of climate change for weathering, ocean processes
etc., although such impacts are recognized. The relentless
movement of tectonic plates, reconfiguring Earth's
continents and ocean basins, takes place over such long
time scales as to appear beyond the scope of this chapter.
Even global impacts of regional and geologically recent
events on Quaternary climates and hence human evolu-
tion, such as closure of the Panama isthmus and elevation
of the Tibetan plateau, lie far beyond more immediate
concerns over climate change. However, geomorphic
processes significantly alter the land surface at decadal to
millennial time scales, much more closely approximating
climate change rates and responding to changing climate
energy and material fluxes. The very need to distinguish
natural climate and environmental variability from
anthropogenic forcing acknowledges that background
geological processes
are
at work and
may
be reinforced or
diminished by anthropogenic impacts.
Tectonic and oceanic change
Tectonic processes of interest are sea-floor spreading,
uplift/subsidence rates and seismo-vulcanicity with
specific Quaternary scenarios. Maximum horizontal plate
motion rates of
120 mm yr
-1
generate such small
millennial-scale changes as to be insignificant even in
ocean straits, where their impact on ocean currents might
otherwise be greatest. Vertical rates of
20 mm yr
-1
,
however, are comparable to 3-4 mm yr
-1
(and increas-
ing) IPCC forecast sea-level rise and therefore will offset
or exacerbate coastline impacts, depending on regional
tectonic trends (see
Chapter 11,
p. 235 and
Chapter 17
p. 407). Uplift will match or exceed sea-level rise at
emergent leading-edge coasts, or those still experiencing
glacio-isostatic rebound after the melting of Late
Quaternary northern ice sheets and drainage of their pro-
glacial lakes. Rebound rates of
10 mm yr
-1
in the Gulf
10 per cent of global shallow
