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Considerable advances have been made in the understanding of processes and
feedbacks in the climate system. This has led to a better representation of
processes and feedbacks in numerical climate models, which have become much
more comprehensive. Because of the presence of non-linear processes in the climate
system, deterministic projections of changes are potentially subject to uncertainties
arising from sensitivity to initial conditions or to parameter settings. Such
uncertainties can be partially quantified from ensembles of climate change integrations,
made using different models starting from different initial conditions. They
necessarily give rise to probabilistic estimates of climate change. This results
in more quantitative estimates of uncertainties and more reliable projections
of anthropogenic climate change. While improved parametrizations have built
confidence in some areas, recognition of the complexity in other areas has not
indicated an overall reduction or shift in the current range of uncertainty
of model response to changes in atmospheric composition.
Atmospheric feedbacks largely control climate sensitivity. Important progress
has been made in the understanding of those processes, partly by utilising new
data against which models can be compared. Since the Second Assessment Report
(IPCC, 1996) (hereafter SAR), there has been a better appreciation of the complexity
of the mechanisms controlling water vapour distribution. Within the boundary
layer, water vapour increases with increasing temperatures. In the free troposphere
above the boundary layer, where the greenhouse effect of water vapour is most
important, the situation is less amenable to straightforward thermodynamic arguments.
In models, increases in water vapour in this region are the most important reason
for large responses to increased greenhouse gases.
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. It is likely
that some of the apparent discrepancy is due to observational error and shortcomings
in intercomparison methodology. 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, indicating that the models have successfully
incorporated the basic processes governing water vapour distribution. While
reassuring, this does not provide a definitive check of the feedbacks, though
the balance of evidence favours a positive clear-sky water vapour feedback of
a magnitude comparable to that found in simulations.
Probably the greatest uncertainty in future projections of climate arises
from clouds and their interactions with radiation. Cloud feedbacks depend upon
changes in cloud height, amount, and radiative properties, including short-wave
absorption. The radiative properties depend upon cloud thickness, particle size,
shape, and distribution and on aerosol effects. The evolution of clouds depends
upon a host of processes, mainly those governing the distribution of water vapour.
The physical basis of the cloud parametrizations included into the models has
also been greatly improved. However, this increased physical veracity has not
reduced the uncertainty attached to cloud feedbacks: even the sign of this feedback
remains unknown. A key issue, which also has large implications for changes
in precipitation, is the sensitivity of sub-grid scale dynamical processes,
turbulent and convective, to climate change. It depends on sub-grid features
of surface conditions such as orography. Equally important are microphysical
processes, which have only recently been introduced explicitly in the models,
and carry major uncertainties. The possibility that models underestimate solar
absorption in clouds remains controversial, as does the effect of such an underestimate
on climate sensitivity. The importance of the structure of the stratosphere
and both radiative and dynamical processes have been recognised, and limitations
in representing stratospheric processes add some uncertainty to model results.
Considerable improvements have taken place in modelling ocean processes. In conjunction with an increase in resolution, these improvements have, in some models, allowed a more realistic simulation of the transports and air-sea fluxes of heat and fresh water, thereby reducing the need for flux adjustments in coupled models. These improvements have also contributed to better simulations of natural large-scale circulation patterns such as El Niño-Southern Oscillation (ENSO) and the oceanic response to atmospheric variability associated with the North Atlantic Oscillation (NAO). However, significant deficiencies in ocean models remain. Boundary currents in climate simulations are much weaker and wider than in nature, though the consequences of this fact for the global climate sensitivity are not clear. Improved parametrizations of important sub-grid scale processes, such as mesoscale eddies, have increased the realism of simulations but important details are still under debate. Major uncertainties still exist with the representation of small-scale processes, such as overflows and flow through narrow channels (e.g., between Greenland and Iceland), western boundary currents (i.e., large-scale narrow currents along coastlines), convection, and mixing.
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