of the Laurentide ice sheet into the North Atlantic,
the extent of which are indicated by drop-stones in
the ocean sediments. At the end of the last ice age
when conditions had warmed to near-interglacial
levels, the climate of the north Atlantic suddenly switched back to near-glacial conditions for
more than a thousand years (the Younger Dryas
event).
The challenge to climate scientists is to find a
mechanism by which the climate system can move
so quickly between two climate states. Studies
of deep ocean sediments and ice cores as well as
coupled climate model simulations have identified
changes in the thermohaline circulation in the
North Atlantic as the probable mechanism. The
warm events are thought to have occurred when
the thermohaline circulation penetrated further
into the Nordic Seas, whereas the cold events
coincided with times at which the thermohaline
circulation has slackened, reducing the transport
of warm water to the North Atlantic (Dansgaard
et al., 1993; Bond et al., 1997; Alverson and
Oldfield, 2000; Rahmstorf, 2001). The strength of
the thermohaline circulation is related to the
strength of the winter convection in the northern
North Atlantic. This, in turn, is related to the
degree of stratification of these northern waters
through their accumulation of fresh water.
Model simulations by Manabe and Stouffer
(1988) first demonstrated the existence of two
stable states for the Atlantic thermohaline circulation. Similar results have since been obtained from
a number of different climate models, most
recently by Rahmstorf and Ganopolski (1999) and
Ganopolski et al. (1998). The models used for
such climate runs have coarse resolution and thus
do not adequately (for an oceanographer at least)
represent many key ocean processes. It is therefore
difficult to determine whether our present climate
system is close to the critical conditions for the
switch to glacial conditions or distant. What is
indisputable, however, is that on palaeo time
scales the earth’s climate has made such switches.
There have been major changes in the patterns
of wintertime convection, in water mass properties
and in sea ice thickness and extent in the northern
hemisphere during recent decades (Dickson et al.,
Chapter 7.3). Concerns have been raised that such
changes may be a precursor of a rapid change in
the thermohaline overturning of the North Atlantic
Ocean.
1.2.7 Impacts of ocean climate
The distribution of the oceans’ physical (and other)
properties – the oceans’ climate – is important in its
own right as well as being central to the broader
climate system. For example, ocean climate determines the productivity of the oceans (and hence
impacts on fisheries), and ocean currents are
important for both commercial and recreational
use of the oceans. Here we introduce two important components of ocean climate – sea-level change
and the impact of the ocean on inshore and shelf
conditions.
1.2.7.1 Sea-level change
Climate variations over periods of decades to centuries produce changes in both globally averaged
and regional sea level. Global-average sea-level
change results from changes in the volume of the
world’s ocean as a result of changes in both the
density and the total mass of the ocean.
On time scales of millennia, sea level has fluctuated by over a hundred metres as the volume of
large ice sheets has varied. For example, sea level at
the time of the last glacial maximum (about 20 000
years ago) was about 120 m lower than today
(Fleming et al., 1998). Most of the storage was in
ice sheets over North America and Europe with
lesser amounts in Greenland, Antarctica and elsewhere. While most of the melting occurred between
18 000 and 6000 years ago, there are indications
that sea level has continued to rise slowly over the
last several thousand years (Fleming et al., 1998).
The massive transfer of water from the ice sheets to
the oceans at the end of the last ice age has resulted
in ongoing isostatic land movements (post-glacial
rebound) at rates of up to several mm yr
91 (e.g.
Peltier, 1998). Changing density of the ocean as a
result of warming of the ocean also changes global
average sea level.
Sea-level variations over the last 200 years
Estimates of global-average sea-level change during the twentieth century are based principally
upon the tide gauge data set of the Permanent
Service for Mean Sea Level (PSMSL); (Spencer and
Woodworth, 1993). A recent assessment is that
globally averaged sea level has risen at the rate of
between 1 and 2 mm yr
91 during the twentieth
century (Church et al., 2001). Douglas (1992)
and Woodworth (1990) find no evidence for an
1.2 Ocean Processes and Climate Phenomena
27
Clarke, Church and Gould
the extent of which are indicated by drop-stones in
the ocean sediments. At the end of the last ice age
when conditions had warmed to near-interglacial
levels, the climate of the north Atlantic suddenly switched back to near-glacial conditions for
more than a thousand years (the Younger Dryas
event).
The challenge to climate scientists is to find a
mechanism by which the climate system can move
so quickly between two climate states. Studies
of deep ocean sediments and ice cores as well as
coupled climate model simulations have identified
changes in the thermohaline circulation in the
North Atlantic as the probable mechanism. The
warm events are thought to have occurred when
the thermohaline circulation penetrated further
into the Nordic Seas, whereas the cold events
coincided with times at which the thermohaline
circulation has slackened, reducing the transport
of warm water to the North Atlantic (Dansgaard
et al., 1993; Bond et al., 1997; Alverson and
Oldfield, 2000; Rahmstorf, 2001). The strength of
the thermohaline circulation is related to the
strength of the winter convection in the northern
North Atlantic. This, in turn, is related to the
degree of stratification of these northern waters
through their accumulation of fresh water.
Model simulations by Manabe and Stouffer
(1988) first demonstrated the existence of two
stable states for the Atlantic thermohaline circulation. Similar results have since been obtained from
a number of different climate models, most
recently by Rahmstorf and Ganopolski (1999) and
Ganopolski et al. (1998). The models used for
such climate runs have coarse resolution and thus
do not adequately (for an oceanographer at least)
represent many key ocean processes. It is therefore
difficult to determine whether our present climate
system is close to the critical conditions for the
switch to glacial conditions or distant. What is
indisputable, however, is that on palaeo time
scales the earth’s climate has made such switches.
There have been major changes in the patterns
of wintertime convection, in water mass properties
and in sea ice thickness and extent in the northern
hemisphere during recent decades (Dickson et al.,
Chapter 7.3). Concerns have been raised that such
changes may be a precursor of a rapid change in
the thermohaline overturning of the North Atlantic
Ocean.
1.2.7 Impacts of ocean climate
The distribution of the oceans’ physical (and other)
properties – the oceans’ climate – is important in its
own right as well as being central to the broader
climate system. For example, ocean climate determines the productivity of the oceans (and hence
impacts on fisheries), and ocean currents are
important for both commercial and recreational
use of the oceans. Here we introduce two important components of ocean climate – sea-level change
and the impact of the ocean on inshore and shelf
conditions.
1.2.7.1 Sea-level change
Climate variations over periods of decades to centuries produce changes in both globally averaged
and regional sea level. Global-average sea-level
change results from changes in the volume of the
world’s ocean as a result of changes in both the
density and the total mass of the ocean.
On time scales of millennia, sea level has fluctuated by over a hundred metres as the volume of
large ice sheets has varied. For example, sea level at
the time of the last glacial maximum (about 20 000
years ago) was about 120 m lower than today
(Fleming et al., 1998). Most of the storage was in
ice sheets over North America and Europe with
lesser amounts in Greenland, Antarctica and elsewhere. While most of the melting occurred between
18 000 and 6000 years ago, there are indications
that sea level has continued to rise slowly over the
last several thousand years (Fleming et al., 1998).
The massive transfer of water from the ice sheets to
the oceans at the end of the last ice age has resulted
in ongoing isostatic land movements (post-glacial
rebound) at rates of up to several mm yr
91 (e.g.
Peltier, 1998). Changing density of the ocean as a
result of warming of the ocean also changes global
average sea level.
Sea-level variations over the last 200 years
Estimates of global-average sea-level change during the twentieth century are based principally
upon the tide gauge data set of the Permanent
Service for Mean Sea Level (PSMSL); (Spencer and
Woodworth, 1993). A recent assessment is that
globally averaged sea level has risen at the rate of
between 1 and 2 mm yr
91 during the twentieth
century (Church et al., 2001). Douglas (1992)
and Woodworth (1990) find no evidence for an
1.2 Ocean Processes and Climate Phenomena
27
Clarke, Church and Gould