Oceanic Hazards - Natural Hazards - page 166

into a harbor and prevent its being used for most of the
short (two to three months), ice-free navigation season.

SEA LEVEL RISE AS A
HAZARD

Intro duction

Sea level rise also poses a significant long-term hazard.
Over the past century, there has been a measurable rise
in global (eustatic) sea level of 15.1

Pack-ice 1 m thick pushing up gravel and muds along the
south shore of Cornwall Island, Canadian Arctic. Scars in
upper part of the photograph represent ice push at slightly
higher sea levels.

Fig. 8.8

1.5 cm. In the
early 1980s, this rate was viewed as accelerating to
3mmyr -1 , fuelling speculation that the Earth was about
to enter a period whereby sea levels could rise
50-100 cm eustatically in the next 100 years. A sea level
rise of this magnitude has destructive implications for
the world's coastline. Beach erosion would accelerate,
low-lying areas would be permanently flooded or
subject to more frequent inundation during storms, and
the baseline for watertables raised. Much of the rise in
sea level in the twentieth century has been attributed
to the melting of icecaps, concomitant with increasing
temperatures because of global warming. Whether
global warming is occurring and whether it is natural or
human-induced are moot points. Global warming and
thermal expansion of surface ocean waters are forecast
to cause sea level rises of 20-40 cm by 2100. This
section will examine the evidence for sea level rise, its
causes, and the reasons for sea level fluctuations.

have frequently occurred in as little as 15 minutes. All
that is required is for loosely packed floating ice to be
present near shore, and for a slight breeze to drive it
ashore.
Ice pushing can also occur because of thermal
expansion, especially in lakes. With a rapid fall in tem-
perature, lake ice can contract and crack dramatically.
If lake water enters the cracks, or subsequent thawing
infills the crack, warming of the whole ice surface can
begin to expand the ice and push it slowly shoreward.
If intense freezing alternates with thawing, then over-
riding of the shore can be significant for lakes as small
as 5 km 2 . Larger lakes tend not to suffer from ice
expansion because ice will simply override itself,
closing cracks that have opened up in the lake.
The main effect of grounded sea-ice or ice push is
the destruction of shore structures. In lake resorts in
the north-eastern United States and southern Canada,
docks, breakwalls, and shoreline buildings are almost
impossible to maintain on small lakes because of
thermal ice expansion. On larger lakes, and along the
Arctic Ocean coastline in Canada, northern Europe,
and Russia, wind-driven ice push has destroyed coastal
structures. In the Beaufort Sea area of North America,
oil exploration and drilling programs have been
severely curtailed by drifting pack-ice. Sites have had
to be surrounded by steep breakwalls to prevent ice
overriding. In the Canadian High Arctic, oil removal
was delayed for years while the government tried to
determine the cheapest and safest means of exporting
oil. Pipelines laid across the seabed between islands
are exposed to drifting icebergs, or to ice-push damage
near the shoreline. Alternatively, harbor facilities for
oil tankers are threatened by ice push. Even where ice
push is not a major problem, drifting sea-ice can push

Current rates of change in sea level
wor ldwide

(Cazenave et al., 2002; Nerem et al., 2002; Colorado
Center for Astrodynamics Research, 2003)

Figure 8.9 presents sea level curves measured on tide
gauges, 1930-1980, for five major cities generally per-
ceived as being threatened by a rise in ocean levels:
London, Venice, New York, Tokyo and Sydney. The
cities are not representative necessarily of eustatic
change in sea level because local ( isostatic ) effects
operate in some areas. London is in an area where the
North Sea Basin is tectonically sinking, Venice suffers
from subsidence due to groundwater extraction, and
Tokyo has had tectonic subsidence due to earthquake
activity. However, the diagrams illustrate that some of
the world's major cities would require extensive engi-
neering works to prevent flooding given predicted
rates of sea level rise. In addition, Figure 8.9 presents
the changes in sea level for three cities: Stockholm;
Wajima, Japan; and Crescent City on the west coast of