Geoscience Reference
In-Depth Information
intensity, and increased sea-levels. Depending
upon the magnitude of these changes, both posi-
tive and negative responses may occur within
different sedimentary environments.
The long-term links between climate and
sediment system response are clearly illustrated
in studies that have examined the response of
large fluvial systems to Quaternary or Holocene
climatic change. The Ganges river system, for
example, represents one of the largest sediment
dispersal systems in the world. It extends for a
distance of over 3000 km, presently discharging
around 300
Among the more complex models in use are the
coupled atmosphere-ocean general circulation
models (AOGCMs). Examples of these include
the recent HadCM3 model developed at the
UK's Hadley Centre (see Hadley Centre 2004),
the projections from which are discussed below.
Such models assume that future greenhouse
gas emissions will follow the widely used IS92a
scenario (Houghton et al. 1992), whereby CO
2
levels will double during the twenty-first cen-
tury, there will be mid-range economic growth,
and no measures made to reduce greenhouse gas
emissions. The dynamics of, and interactions
between, the complex variables involved in such
models means, however, that although climate
projection models are progressively improving,
there remain many unknowns and significant
uncertainties about the feedbacks that occur
between different parameters. Hence, a range of
climate simulation models have been developed
that are based on different input data and account
for different future scenarios (Hadley Centre
2004; McCarthy et al. 2001).
Although future changes in variables such as
temperature, rainfall and sea-level are clearly
significant from a sedimentological perspective
(outlined below), it is important that these
projections are placed in the context of past
environmental change. The Quaternary has,
for example, been characterized by marked
climatic shifts associated with glacial and inter-
glacial phases. At the global scale, these oscilla-
tions resulted in marked changes in climate, but
the actual effects were spatially very variable.
Hence, although previously temperate regions
in the northerly latitudes cooled and were sub-
ject to glacial or periglacial conditions, more arid
regions to the south became more temperate in
character. Consequently the nature of environ-
mental change and, as a result, the processes
influencing sediment transport and accumula-
tion differed between regions. Such spatial vari-
ability is likely to be a key factor in the context
of any future climatic change. It is also relevant
to note that the magnitudes of projected future
change in, for example, temperature and sea-level
are both above and below those experienced in
the recent past. During the early Holocene, for
10
6
t of
sediment annually (Goodbred 2003). Despite its
vast size, strong sediment linkages occur between
the source areas in the Himalaya, through the
catchment basins on the Ganges Plain, to the
depocentres in the Bengal Basin. This is believed
to reflect the strong seasonal control that exists
on river flow, with around 80% of fluvial dis-
charge and 95% of sediment load delivered over
the 4 month summer monsoon period. As a
result, the system is highly susceptible to changes
in atmospheric circulation patterns and, in
particular, any change in the strength of the
summer monsoon. This has altered several times
over the past 150 kyr, resulting in changes in
rates and patterns of fluvial sediment produc-
tion, erosion, transport and accumulation, and
is manifested by shifts from periods of upstream
erosion and entrenchment, to periods of sediment
accumulation on the delta plain and within the
Bengal Basin.
Large-scale studies thus establish clear links
between sediment dispersal system behaviour
and climatic regime. Predicting future changes
in sediment system dynamics is, however, com-
plicated by the uncertainty that exists in rela-
tion to the magnitude and regional variability of
future climate change. Future climate projections
are based, in part, upon constructed scenarios of
future human behaviour (in relation to issues
such as greenhouse gas emissions and land-use
practice). They are also dependent, however,
upon atmospheric circulation patterns and
atmospheric interactions with large-scale ocean
currents and land features (including albedo,
vegetation cover and soil moisture content).
×
10
9
m
3
of water and 520
×
