Geoscience Reference
In-Depth Information
hectares per year due partly to high rainfall variability
and partly to unsuitable human agricultural activities
such as overgrazing and over-intensive cultivation.
Desertification was estimated to affect nearly 70 per cent
of the total dryland area in the 1990s.
response to forcing mechanisms are also poorly
understood.
•
More information regarding the
processes involved
in the coupling between the atmosphere and the
oceans
. A particular problem concerns the coupling
of sea-surface temperatures (a vital part of the
atmospheric model) with ocean surface energy flux,
fresh water supplies and momentum or wind stress
conditions (important parts of the ocean model).
Attempts to link large parts of complex systems
generally leads to 'drift' of the model's climate
system towards a new, unrealistic, mean state.
H POSTSCRIPT
Our ability to understand and anticipate climate change
has increased considerably since the first IPCC Report
appeared in 1990, but many problems and uncertainties
remain. Key research needs include (not in order of
importance):
•
The
links between land-surface and atmospheric
processes
, including the hydrological cycle and
interactions between ice sheets and glaciers, on
the one hand, and climate, on the other, need to be
examined.
•
The development of more refined
forcing
scenarios
through a better understanding of forcing mecha-
nisms (past, present and future) such as those relating
to economic growth, forest clearances, land use
changes, sulphate aerosols, carbonaceous aerosols
generated by biomass burning, and radiative trace
gases other than CO
2
(e.g. methane and ozone). The
incorporation of past anthropogenic forcing will
generate more realistic warming in model simu-
lations - so avoiding the present 'cold start' problem.
•
The imperative to make a clear distinction between
climate change related to anthropogenic causes and
the
natural variability
of the space-time climate
structure. Information on natural climatic variability
can be gained from instrumental data, palaeoclimatic
reconstructions and numerical models. An important
feature of natural variability is rapid changes, which
are at present little understood. These changes are
characteristic of complex non-linear systems that
are rapidly forced. Examples of such changes include
rapid circulation changes in the North Atlantic and
feedbacks associated with terrestrial ecosystem
changes.
•
Better understanding of
feedback
processes, notably
those involving clouds and the surface radiation
budget, interactions between the land biosphere and
the carbon cycle, between climate and atmospheric
chemistry, and those involving sea ice.
•
The need for more information regarding the dis-
tribution of
clouds and their radiative effects
. It is
clear that high and low clouds may have very
different radiative effects and that under certain
conditions cloud albedo may have the capacity to
counterbalance much of the potential CO
2
warming
effect.
•
The need for
the systematic collection of long-term
instrumental and proxy observations
to do with solar
output, atmospheric energy balance components,
hydrological cycles, ocean characteristics, atmos-
phere and ocean coupling, and ecosystem changes.
•
Finally, because numerical models of the world
climate system are becoming ever more complex,
there is a growing requirement for
massive com-
puting facilities
; sometimes, weeks of expensive
computer time are needed to run a single climatic
simulation to achieve an equilibrium state.
•
The need to increase the resolution of global climate
models so that
small-scale physical processes
can
be represented (e.g. those relating to clouds). This is
part of the move to improve scale-coupling between
global climate models and regional and smaller scale
models.
•
Greater understanding of
oceanic processes
,
including the heat flux at the ocean surface, the
upwelling diffusion-energy balance, and the role of
the oceans in absorbing CO
2
, especially by biological
processes. The role of the oceans in heat transport
and the delays it introduces into the climate system's
