current meter arrays (often with temperature sensors), have revealed both long-term trends (e.g.
Joyce and Robbins, 1996) and abrupt changes in
properties (e.g. Hogg and Zenk, 1997). However, a
major challenge remains to link this long-period
subsurface variability to changes in air–sea fluxes
and to produce coupled atmosphere–ocean models
that are capable of reproducing it.
1.2.6.5 Anthropogenic climate change
The oceans are crucial to simulating correctly the
timing and regional impact of anthropogenic climate change. Because of their large heat capacity,
the oceans are able to accumulate heat at rates
that are comparable with the radiation imbalance
resulting from the increasing concentrations of
greenhouse gases. This accumulation of heat in the
oceans will delay the impact of the warming in the
atmosphere. Recent observations (and model simulations) have confirmed that large quantities of
heat are being sequestered in the ocean (Sections
1.2.6.4 and 1.2.7.1; Dickson et al., Chapter 7.3;
Levitus et al., 2000). Indeed, the observations of
Levitus et al. (2000); (Fig. 1.2.8, see Plate 1.2.8,
p. 44) indicate that the ocean warming represents a
significant fraction of the enhanced greenhouse forcing over recent decades but is substantially larger
than variations in solar forcing over this period.
Detection of changes in ocean properties are
now beginning to be used to assess climate change
simulations and to detect (and attribute) anthropogenic climate change in the ocean. For example,
Wong et al. (1999, 2001) observe a freshening of
the Intermediate Waters of the Pacific and Indian
Oceans and an increased salinity in the shallower
thermocline waters (Fig. 1.2.9, see Plate 1.2.9,
p. 44). They attribute these changes to increased
(decreased) precipitation minus evaporation at
high (low) latitudes and suggest there may be an
increase in the hydrological cycle as in simulations
of climate change. Using coupled climate models,
Banks et al. (2000) find similar water mass
changes first emerge above the background variability in the south Indian and Pacific Oceans. On
time scales of hundreds of years, the oceans will
not be in balance with the changed radiative forcing and this will lead to regional changes in precipitation and possibly changes in climate variability
such as ENSO (Timmermann et al., 1999a).
Coupled models do not yet agree in the detail of
these changes but do reveal their sensitivity to the
representation of ocean processes (Hirst, 1998;
Cubasch et al., 2001).
1.2.6.6 Sudden climate change
Palaeoclimate records have demonstrated that the
present equable climate state under which civilization has developed alternates with prolonged
glacial periods during which much of the landmasses poleward of 40–50° have been covered by
continental ice sheets several kilometres thick.
Modern methods have allowed finer and finer resolution of these palaeorecords; these higher resolutions have shown that the changes from glacial
to interglacial conditions take place rapidly over
decades to centuries.
The passage of ice ages and interglacial periods
can be linked to the small changes in the solar
energy received by the earth due to orbital changes
(the Milankovich cycles). However, the solar
energy only changes by a few per cent over millennia while the climate system changes from one
state to the other over a few decades to a century.
In addition, the palaeorecords have revealed rapid
transitions during both glacial and interglacial
periods. Surprisingly these transitions with temperature changes in Greenland of about 10°C occurred
in the period of only a few decades (Fig. 1.2.10).
The events have been categorized into two types.
Dansgaard/Oescher events are sudden warmings of
about 10°C over Greenland during glacial conditions. Heinrich events are linked to sudden surges
SECTION 1 THE OCEAN AND CLIMATE
26
0
5
10
15
Age (thousand years before present)
0.05
0.15
0.25
0
5
1 0
1 5
–60
–50
–40
–30
Temperature (˚C)
Accumulation (m ice yr –1 )
Fig. 1.2.10 Greenland temperatures (solid line) and
accumulated precipitation (dashed line) showing the
rapid (decade long) transition 11.6 thousand years ago at
the end of the Younger Dryas event. Precipitation data
from Alley et al. (1993).
Joyce and Robbins, 1996) and abrupt changes in
properties (e.g. Hogg and Zenk, 1997). However, a
major challenge remains to link this long-period
subsurface variability to changes in air–sea fluxes
and to produce coupled atmosphere–ocean models
that are capable of reproducing it.
1.2.6.5 Anthropogenic climate change
The oceans are crucial to simulating correctly the
timing and regional impact of anthropogenic climate change. Because of their large heat capacity,
the oceans are able to accumulate heat at rates
that are comparable with the radiation imbalance
resulting from the increasing concentrations of
greenhouse gases. This accumulation of heat in the
oceans will delay the impact of the warming in the
atmosphere. Recent observations (and model simulations) have confirmed that large quantities of
heat are being sequestered in the ocean (Sections
1.2.6.4 and 1.2.7.1; Dickson et al., Chapter 7.3;
Levitus et al., 2000). Indeed, the observations of
Levitus et al. (2000); (Fig. 1.2.8, see Plate 1.2.8,
p. 44) indicate that the ocean warming represents a
significant fraction of the enhanced greenhouse forcing over recent decades but is substantially larger
than variations in solar forcing over this period.
Detection of changes in ocean properties are
now beginning to be used to assess climate change
simulations and to detect (and attribute) anthropogenic climate change in the ocean. For example,
Wong et al. (1999, 2001) observe a freshening of
the Intermediate Waters of the Pacific and Indian
Oceans and an increased salinity in the shallower
thermocline waters (Fig. 1.2.9, see Plate 1.2.9,
p. 44). They attribute these changes to increased
(decreased) precipitation minus evaporation at
high (low) latitudes and suggest there may be an
increase in the hydrological cycle as in simulations
of climate change. Using coupled climate models,
Banks et al. (2000) find similar water mass
changes first emerge above the background variability in the south Indian and Pacific Oceans. On
time scales of hundreds of years, the oceans will
not be in balance with the changed radiative forcing and this will lead to regional changes in precipitation and possibly changes in climate variability
such as ENSO (Timmermann et al., 1999a).
Coupled models do not yet agree in the detail of
these changes but do reveal their sensitivity to the
representation of ocean processes (Hirst, 1998;
Cubasch et al., 2001).
1.2.6.6 Sudden climate change
Palaeoclimate records have demonstrated that the
present equable climate state under which civilization has developed alternates with prolonged
glacial periods during which much of the landmasses poleward of 40–50° have been covered by
continental ice sheets several kilometres thick.
Modern methods have allowed finer and finer resolution of these palaeorecords; these higher resolutions have shown that the changes from glacial
to interglacial conditions take place rapidly over
decades to centuries.
The passage of ice ages and interglacial periods
can be linked to the small changes in the solar
energy received by the earth due to orbital changes
(the Milankovich cycles). However, the solar
energy only changes by a few per cent over millennia while the climate system changes from one
state to the other over a few decades to a century.
In addition, the palaeorecords have revealed rapid
transitions during both glacial and interglacial
periods. Surprisingly these transitions with temperature changes in Greenland of about 10°C occurred
in the period of only a few decades (Fig. 1.2.10).
The events have been categorized into two types.
Dansgaard/Oescher events are sudden warmings of
about 10°C over Greenland during glacial conditions. Heinrich events are linked to sudden surges
SECTION 1 THE OCEAN AND CLIMATE
26
0
5
10
15
Age (thousand years before present)
0.05
0.15
0.25
0
5
1 0
1 5
–60
–50
–40
–30
Temperature (˚C)
Accumulation (m ice yr –1 )
Fig. 1.2.10 Greenland temperatures (solid line) and
accumulated precipitation (dashed line) showing the
rapid (decade long) transition 11.6 thousand years ago at
the end of the Younger Dryas event. Precipitation data
from Alley et al. (1993).