D.1 Climate Processes and Feedbacks
Processes in the climate system determine the natural variability of the climate
system and its response to perturbations, such as the increase in the atmospheric
concentrations of greenhouse gases. Many basic climate processes of importance
are well-known and modelled exceedingly well. Feedback processes amplify (a positive
feedback) or reduce (a negative feedback) changes in response to an initial perturbation
and hence are very important for accurate simulation of the evolution of climate.
Water vapour
A major feedback accounting for the large warming predicted by climate models
in response to an increase in CO2 is the increase in atmospheric water
vapour. An increase in the temperature of the atmosphere increases its water-holding
capacity; however, since most of the atmosphere is undersaturated, this does not
automatically mean that water vapour, itself, must increase. Within the boundary
layer (roughly the lowest 1 to 2 km of the atmosphere), water vapour increases
with increasing temperature. In the free troposphere above the boundary layer,
where the water vapour greenhouse effect is most important, the situation is harder
to quantify. Water vapour feedback, as derived from current models, approximately
doubles the warming from what it would be for fixed water vapour. Since the SAR,
major improvements have occurred in the treatment of water vapour in models, although
detrainment of moisture from clouds remains quite uncertain and discrepancies
exist between model water vapour distributions and those observed. Models are
capable of simulating the moist and very dry regions observed in the tropics and
sub-tropics and how they evolve with the seasons and from year to year. While
reassuring, this does not provide a check of the feedbacks, although the balance
of evidence favours a positive clear-sky water vapour feedback of the magnitude
comparable to that found in simulations.
Clouds
As has been the case since the first IPCC Assessment Report in 1990, probably
the greatest uncertainty in future projections of climate arises from clouds and
their interactions with radiation. Clouds can both absorb and reflect solar radiation
(thereby cooling the surface) and absorb and emit long wave radiation (thereby
warming the surface). The competition between these effects depends on cloud height,
thickness and radiative properties. The radiative properties and evolution of
clouds depend on the distribution of atmospheric water vapour, water drops, ice
particles, atmospheric aerosols and cloud thickness. The physical basis of cloud
parametrizations is greatly improved in models through inclusion of bulk representation
of cloud microphysical properties in a cloud water budget equation, although considerable
uncertainty remains. Clouds represent a significant source of potential error
in climate simulations. The possibility that models underestimate systematically
solar absorption in clouds remains a controversial matter. The sign of the net
cloud feedback is still a matter of uncertainty, and the various models exhibit
a large spread. Further uncertainties arise from precipitation processes and the
difficulty in correctly simulating the diurnal cycle and precipitation amounts
and frequencies.
Stratosphere
There has been a growing appreciation of the importance of the stratosphere
in the climate system because of changes in its structure and recognition of the
vital role of both radiative and dynamical processes. The vertical profile
of temperature change in the atmosphere, including the stratosphere, is an important
indicator in detection and attribution studies. Most of the observed decreases
in lower-stratospheric temperatures have been due to ozone decreases, of which
the Antarctic "ozone hole" is a part, rather than increased CO2
concentrations. Waves generated in the troposphere can propagate into the stratosphere
where they are absorbed. As a result, stratospheric changes alter where and how
these waves are absorbed, and the effects can extend downward into the troposphere.
Changes in solar irradiance, mainly in the ultraviolet (UV), lead to photochemically-induced
ozone changes and, hence, alter the stratospheric heating rates, which can alter
the tropospheric circulation. Limitations in resolution and relatively poor representation
of some stratospheric processes adds uncertainty to model results.
Ocean
Major improvements have taken place in modelling ocean processes, in particular
heat transport. These improvements, in conjunction with an increase in resolution,
have been important in reducing the need for flux adjustment in models and in
producing realistic simulations of natural large-scale circulation patterns and
improvements in simulating El Niño (see Box 4).
Ocean currents carry heat from the tropics to higher latitudes. The ocean exchanges
heat, water (through evaporation and precipitation) and CO2 with the
atmosphere. Because of its huge mass and high heat capacity, the ocean slows climate
change and influences the time-scales of variability in the ocean-atmosphere system.
Considerable progress has been made in the understanding of ocean processes relevant
for climate change. Increases in resolution, as well as improved representation
(parametrization) of important sub-grid scale processes (e.g., mesoscale eddies),
have increased the realism of simulations. Major uncertainties still exist with
the representation of small-scale processes, such as overflows (flow through narrow
channels, e.g., between Greenland and Iceland), western boundary currents (i.e.,
large-scale narrow currents along coastlines), convection and mixing. Boundary
currents in climate simulations are weaker and wider than in nature, although
the consequences of this for climate are not clear.
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Box 4: Climate Models: How are they built and how are they applied?
The strongest natural fluctuation of climate on interannual time-scales
is the El Niño-Southern Oscillation (ENSO) phenomenon. The term
"El Niño" originally applied to an annual weak warm ocean
current that ran southwards along the coast of Peru about Christmas-time
and only subsequently became associated with the unusually large warmings.
The coastal warming, however, is often associated with a much more extensive
anomalous ocean warming to the International Dateline, and it is this
Pacific basinwide phenomenon that forms the link with the anomalous global
climate patterns. The atmospheric component tied to "El Niño"
is termed the "Southern Oscillation". Scientists often call
this phenomenon, where the atmosphere and ocean collaborate together,
ENSO (El Niño-Southern Oscillation).
ENSO is a natural phenomenon, and there is good evidence from cores of
coral and glacial ice in the Andes that it has been going on for millennia.
The ocean and atmospheric conditions in the tropical Pacific are seldom
average, but instead fluctuate somewhat irregularly between El Niño
events and the opposite "La Niña" phase, consisting of
a basinwide cooling of the tropical Pacific, with a preferred period of
about three to six years. The most intense phase of each event usually
lasts about a year.
A distinctive pattern of sea surface temperatures in the Pacific Ocean
sets the stage for ENSO events. Key features are the "warm pool"
in the tropical western Pacific, where the warmest ocean waters in the
world reside, much colder waters in the eastern Pacific, and a cold tongue
along the equator that is most pronounced about October and weakest in
March. The atmospheric easterly trade winds in the tropics pile up the
warm waters in the west, producing an upward slope of sea level along
the equator of 0.60 m from east to west. The winds drive the surface ocean
currents, which determine where the surface waters flow and diverge. Thus,
cooler nutrient-rich waters upwell from below along the equator and western
coasts of the Americas, favouring development of phytoplankton, zooplankton,
and hence fish. Because convection and thunderstorms preferentially occur
over warmer waters, the pattern of sea surface temperatures determines
the distribution of rainfall in the tropics, and this in turn determines
the atmospheric heating patterns through the release of latent heat. The
heating drives the large-scale monsoonal-type circulations in the tropics,
and consequently determines the winds. This strong coupling between the
atmosphere and ocean in the tropics gives rise to the El Niño phenomenon.
During El Niño, the warm waters from the western tropical Pacific
migrate eastward as the trade winds weaken, shifting the pattern of tropical
rainstorms, further weakening the trade winds, and thus reinforcing the
changes in sea temperatures. Sea level drops in the west, but rises in
the east by as much as 0.25 m, as warm waters surge eastward along the
equator. However, the changes in atmospheric circulation are not confined
to the tropics, but extend globally and influence the jet streams and
storm tracks in mid-latitudes. Approximately reverse patterns occur during
the opposite La Niña phase of the phenomenon.
Changes associated with ENSO produce large variations in weather and climate
around the world from year to year. These often have a profound impact
on humanity and society because of associated droughts, floods, heat waves
and other changes that can severely disrupt agriculture, fisheries, the
environment, health, energy demand, air quality and also change the risks
of fire. ENSO also plays a prominent role in modulating exchanges of CO2
with the atmosphere. The normal upwelling of cold nutrient-rich and CO2-rich
waters in the tropical Pacific is suppressed during El Niño.
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Cryosphere
The representation of sea-ice processes continues to improve, with several
climate models now incorporating physically based treatments of ice dynamics.
The representation of land-ice processes in global climate models remains rudimentary.
The cryosphere consists of those regions of Earth that are seasonally or perennially
covered by snow and ice. Sea ice is important because it reflects more incoming
solar radiation than the sea surface (i.e., it has a higher albedo), and it insulates
the sea from heat loss during the winter. Therefore, reduction of sea ice gives
a positive feedback on climate warming at high latitudes. Furthermore, because
sea ice contains less salt than sea water, when sea ice is formed the salt content
(salinity) and density of the surface layer of the ocean is increased. This promotes
an exchange of water with deeper layers of the ocean, affecting ocean circulation.
The formation of icebergs and the melting of ice shelves returns fresh water from
the land to the ocean, so that changes in the rates of these processes could affect
ocean circulation by changing the surface salinity. Snow has a higher albedo than
the land surface; hence, reductions in snow cover lead to a similar positive albedo
feedback, although weaker than for sea ice. Increasingly complex snow schemes
and sub-grid scale variability in ice cover and thickness, which can significantly
influence albedo and atmosphere-ocean exchanges, are being introduced in some
climate models.
Land surface
Research with models containing the latest representations of the land surface
indicates that the direct effects of increased CO2 on the physiology
of plants could lead to a relative reduction in evapotranspiration over the tropical
continents, with associated regional warming and drying over that predicted for
conventional greenhouse warming effects. Land surface changes provide important
feedbacks as anthropogenic climate changes (e.g., increased temperature, changes
in precipitation, changes in net radiative heating, and the direct effects of
CO2) will influence the state of the land surface (e.g., soil moisture,
albedo, roughness and vegetation). Exchanges of energy, momentum, water, heat
and carbon between the land surface and the atmosphere can be defined in models
as functions of the type and density of the local vegetation and the depth and
physical properties of the soil, all based on land-surface data bases that have
been improved using satellite observations. Recent advances in the understanding
of vegetation photosynthesis and water use have been used to couple the terrestrial
energy, water and carbon cycles within a new generation of land surface parametrizations,
which have been tested against field observations and implemented in a few GCMs,
with demonstrable improvements in the simulation of land-atmosphere fluxes. However,
significant problems remain to be solved in the areas of soil moisture processes,
runoff prediction, land-use change and the treatment of snow and sub-grid scale
heterogeneity.
Changes in land-surface cover can affect global climate in several ways. Large-scale
deforestation in the humid tropics (e.g., South America, Africa, and Southeast
Asia) has been identified as the most important ongoing land-surface process,
because it reduces evaporation and increases surface temperature. These effects
are qualitatively reproduced by most models. However, large uncertainties still
persist on the quantitative impact of large-scale deforestation on the hydrological
cycle, particularly over Amazonia.
Carbon cycle
Recent improvements in process-based terrestrial and ocean carbon cycle models
and their evaluation against observations have given more confidence in their
use for future scenario studies. CO2 naturally cycles rapidly among
the atmosphere, oceans and land. However, the removal of the CO2 perturbation
added by human activities from the atmosphere takes far longer. This is because
of processes that limit the rate at which ocean and terrestrial carbon stocks
can increase. Anthropogenic CO2 is taken up by the ocean because of
its high solubility (caused by the nature of carbonate chemistry), but the rate
of uptake is limited by the finite speed of vertical mixing. Anthropogenic CO2
is taken up by terrestrial ecosystems through several possible mechanisms, for
example, land management, CO2 fertilisation (the enhancement of plant
growth as a result of increased atmospheric CO2 concentration) and
increasing anthropogenic inputs of nitrogen. This uptake is limited by the relatively
small fraction of plant carbon that can enter long-term storage (wood and humus).
The fraction of emitted CO2 that can be taken up by the oceans and
land is expected to decline with increasing CO2 concentrations. Process-based
models of the ocean and land carbon cycles (including representations of physical,
chemical and biological processes) have been developed and evaluated against measurements
pertinent to the natural carbon cycle. Such models have also been set up to mimic
the human perturbation of the carbon cycle and have been able to generate time-series
of ocean and land carbon uptake that are broadly consistent with observed global
trends. There are still substantial differences among models, especially in how
they treat the physical ocean circulation and in regional responses of terrestrial
ecosystem processes to climate. Nevertheless, current models consistently indicate
that when the effects of climate change are considered, CO2 uptake
by oceans and land becomes smaller.
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