ice in the Arctic Basin, since it would alter air–sea
fluxes and especially precipitation (snowfall)
strongly in high altitudes. This would certainly
have a hemispheric impact. Whether the recent
decline of the multiyear sea-ice area in the Arctic
Basin by 18% from 1978 to 1998 reported by
Johannessen et al. (1999) is related to a natural
recent intensification of the North Atlantic Oscillation or is an effect of anthropogenic climate
change is not yet clear. In order to establish reliable data sets on sea-ice thickness (the multiyear
sea-ice decline is related to shrinking sea-ice thickness), an extension of the small networks of
upward-looking sonars established within WCRP’s
Arctic Climate System Study (ACSYS), both in the
Arctic and in the sea-ice area around Antarctica, is
required. The free access to earlier classified data
from submarines in combination with these networks would be a validation data set for the
upcoming altimeter data sets, that could give estimates of sea-ice thickness.
It was one of the roles of WOCE to develop
models that would allow the assessment of the
sensitivity of the climate system to these types of
changes.
1.1.5 Cryosphere and the oceans
Ice–ocean interactions are an important part of the
climate system. Mountain glaciers, small ice caps
and ice sheets influence global sea level. Deep convection in the Arctic and Southern Oceans is largely
related to sea-ice formation on continental shelves
or in coastal polynyas (slope convection) and open
ocean deep convection is often caused by atmospheric forcing at the sea-ice edge. Sea ice is treated
interactively in most coupled ocean–atmosphere
models and improved parameterizations for dynamical sea-ice behaviour have been selected by the
Sea Ice Modelling Intercomparison Project within
ACSYS (Lemke et al., 1997). However, although
meltwater runoff from land-ice and the melting of
tabular icebergs in the Antarctic sea-ice zone influence the thermohaline circulation, as yet no adequate parameterizations exist for these processes.
The positive freshwater balance of the Arctic
Ocean creates a thin and cold, less saline layer on
top of a much warmer and more saline intermediate layer of Atlantic origin. The heat content of
this deeper layer is sufficient to melt the entire
multiyear sea ice. Therefore, changes in the salinity
of the top layer through changes in Arctic river
runoff, snow-depth on sea ice and evaporation
could have a strong impact on Arctic sea ice, and
in turn on Arctic Ocean circulation and at least
regional climate. However, the observation of
solid precipitation is at present not adequate to
detect changes. This is because earlier observations
(for example, snow depth on sea ice measured regularly in spring at many stations where ocean profiling took place) have largely ceased following the
collapse of the Soviet Union. As yet, remote sensing methods of sufficient accuracy to detect these
precipitation changes have not been developed.
1.1.6 Anthropogenic climate change and
the oceans
The ocean as a key component of the climate
system also plays a major role in anthropogenic
climate change. First, ocean heat absorption delays
the full global warming. Second, the oceans, particularly their regional pattern of heat transport
and absorption, lead to significant changes in
regional climate and thus rainfall and temperature
change. Third, the oceans are a major sink for
anthropogenic CO 2 . Fourth, ocean heat absorption leads to thermal expansion of the oceans and
sea-level rise and as a result to coastal erosion and
flooding. Fifth, as already mentioned, changes in
the formation of deep water masses at high latitudes in the North Atlantic and the Southern
Ocean could lead to abrupt changes in the global
ocean thermohaline circulation and a major
rearrangement of global climate.
As indicated by Kattenberg et al. (1996), the
warming response is dependent on the rate of
increase of greenhouse gases because of uptake of
heat by the oceans. In coupled models, a slow
increase in greenhouse gas concentrations of
0.25% yr
91 to doubled preindustrial levels results
in 70% of the full equilibrium warming at the time
of doubling. For a more rapid increase of 4% yr
91
,
only about 40% of the equilibrium warming is
achieved at the time of doubling. The present rate
of increase of greenhouse gas concentrations is
below 1% yr
91
, accounting for the combined effect
of all greenhouse gases. Thus only about 60% of the
warming we are committed to because of increases
in past greenhouse gas concentrations should have
been realized to date. In addition to slowing the
rate of warming, changes in precipitation patterns
1.1 Climate and Oceans
7
Grassl
fluxes and especially precipitation (snowfall)
strongly in high altitudes. This would certainly
have a hemispheric impact. Whether the recent
decline of the multiyear sea-ice area in the Arctic
Basin by 18% from 1978 to 1998 reported by
Johannessen et al. (1999) is related to a natural
recent intensification of the North Atlantic Oscillation or is an effect of anthropogenic climate
change is not yet clear. In order to establish reliable data sets on sea-ice thickness (the multiyear
sea-ice decline is related to shrinking sea-ice thickness), an extension of the small networks of
upward-looking sonars established within WCRP’s
Arctic Climate System Study (ACSYS), both in the
Arctic and in the sea-ice area around Antarctica, is
required. The free access to earlier classified data
from submarines in combination with these networks would be a validation data set for the
upcoming altimeter data sets, that could give estimates of sea-ice thickness.
It was one of the roles of WOCE to develop
models that would allow the assessment of the
sensitivity of the climate system to these types of
changes.
1.1.5 Cryosphere and the oceans
Ice–ocean interactions are an important part of the
climate system. Mountain glaciers, small ice caps
and ice sheets influence global sea level. Deep convection in the Arctic and Southern Oceans is largely
related to sea-ice formation on continental shelves
or in coastal polynyas (slope convection) and open
ocean deep convection is often caused by atmospheric forcing at the sea-ice edge. Sea ice is treated
interactively in most coupled ocean–atmosphere
models and improved parameterizations for dynamical sea-ice behaviour have been selected by the
Sea Ice Modelling Intercomparison Project within
ACSYS (Lemke et al., 1997). However, although
meltwater runoff from land-ice and the melting of
tabular icebergs in the Antarctic sea-ice zone influence the thermohaline circulation, as yet no adequate parameterizations exist for these processes.
The positive freshwater balance of the Arctic
Ocean creates a thin and cold, less saline layer on
top of a much warmer and more saline intermediate layer of Atlantic origin. The heat content of
this deeper layer is sufficient to melt the entire
multiyear sea ice. Therefore, changes in the salinity
of the top layer through changes in Arctic river
runoff, snow-depth on sea ice and evaporation
could have a strong impact on Arctic sea ice, and
in turn on Arctic Ocean circulation and at least
regional climate. However, the observation of
solid precipitation is at present not adequate to
detect changes. This is because earlier observations
(for example, snow depth on sea ice measured regularly in spring at many stations where ocean profiling took place) have largely ceased following the
collapse of the Soviet Union. As yet, remote sensing methods of sufficient accuracy to detect these
precipitation changes have not been developed.
1.1.6 Anthropogenic climate change and
the oceans
The ocean as a key component of the climate
system also plays a major role in anthropogenic
climate change. First, ocean heat absorption delays
the full global warming. Second, the oceans, particularly their regional pattern of heat transport
and absorption, lead to significant changes in
regional climate and thus rainfall and temperature
change. Third, the oceans are a major sink for
anthropogenic CO 2 . Fourth, ocean heat absorption leads to thermal expansion of the oceans and
sea-level rise and as a result to coastal erosion and
flooding. Fifth, as already mentioned, changes in
the formation of deep water masses at high latitudes in the North Atlantic and the Southern
Ocean could lead to abrupt changes in the global
ocean thermohaline circulation and a major
rearrangement of global climate.
As indicated by Kattenberg et al. (1996), the
warming response is dependent on the rate of
increase of greenhouse gases because of uptake of
heat by the oceans. In coupled models, a slow
increase in greenhouse gas concentrations of
0.25% yr
91 to doubled preindustrial levels results
in 70% of the full equilibrium warming at the time
of doubling. For a more rapid increase of 4% yr
91
,
only about 40% of the equilibrium warming is
achieved at the time of doubling. The present rate
of increase of greenhouse gas concentrations is
below 1% yr
91
, accounting for the combined effect
of all greenhouse gases. Thus only about 60% of the
warming we are committed to because of increases
in past greenhouse gas concentrations should have
been realized to date. In addition to slowing the
rate of warming, changes in precipitation patterns
1.1 Climate and Oceans
7
Grassl