Geoscience Reference
In-Depth Information
example, global temperatures were rising at rates
of around 1-2°C /1000 years. This is below the
rates of most projections through to 2100 (see
below). By contrast, sea-level has risen by around
120 m over the last 17,000 years, and most of
this rise occurred prior to 6000 ybp. Hence rates
of rise were in the region of 10 mm yr
−1
, con-
siderably above current projections of change
( Jones 1993a).
Although future projections are subject to a
high degree of uncertainty, what is evident from
recent data is that marked increases in atmo-
spheric concentrations of greenhouse gases have
occurred over the past 200 years. The following
increases have, for example, been reported for the
period 1750 -2000: CO
2
(280 ppm to 368 ppm),
CH
4
(700 ppb to 1750 ppb) and N
2
O (270 ppb
to 316 ppb) (McCarthy et al. 2001). These are
all predicted to increase further through to 2100
(e.g. atmospheric CO
2
concentrations are pre-
dicted to increase to between 540 and 970 ppm).
Linked to these greenhouse gas increases are pro-
jected changes in global mean surface temper-
atures, which will have potential consequences
for sea-levels, sea-surface temperatures and clim-
ate circulation systems. Global mean surface
temperatures have, for example, increased by
0.6
variable regionally, but show increases over the
northern mid-latitudes, tropical Africa and south-
east Asia. Decreases are predicted in Australia,
central America and southern Africa.
Such changes in temperature and rainfall
are likely to have an impact upon the function-
ing of many terrestrial sedimentary systems. As
patterns of river channel erosion and sedimenta-
tion are influenced by streamflow over time and,
especially, by flood frequency, any changes in
the hydrological cycle will significantly influence
fluvial sediment transport and depositional
processes (see Chapter 3). Changes in precipita-
tion may result in modified drainage densities and
either higher or lower sediment yields depend-
ing on the regional effect. In either case there
would be significant change in the downstream
depositional environments. Lakes are also
highly susceptible to changes in air temperature
and rainfall because these influence rates of
evaporation, lake-level and hydrochemical and
hydrobiological regimes (see Chapter 4). Under
extreme conditions, lakes may disappear entirely.
Responses are also likely to vary between open
(exorheic) and closed (endorheic) lakes. The
latter are dependent upon rates of fluvial input
and evaporation and are thus highly sensitive to
changes in both. Hence lake sediments are good
sources of information about past climatic and
environmental conditions.
Climate changes are also likely to have an
impact upon rates of sea-level rise and on the
functioning of large-scale ocean-atmosphere
circulation systems. Over the past 100 years,
global mean sea-level has increased at an aver-
age rate of 1-2 mm yr
−1
. Predictions from the
HadCM3 models suggest that mean sea-level
will rise by 0.38 m by the end of the twenty-
first century (range 0.09 - 0.74 m; see Hadley
Centre 2004). This will occur primarily because
of thermal expansion of the oceans and the
melting of glaciers and ice caps. Significant
regional variations in the magnitude of these
rises are likely, however, superimposed upon
which will be spatial variations in rates of
isostatic change. Even relatively small increases
in sea-level will, however, exert a significant
influence on most coastal sedimentary systems
0.2°C over the twentieth century, although
these increases have been more significant over
land areas than the oceans. There has also been
an increase in the number of hot days and a
reduction in the number of days experiencing
frosts. In addition, Arctic sea-ice has thinned
by around 40% in the past few decades and
decreased in extent by 10 -15% since the 1950s
(McCarthy et al. 2001).
Based on outputs from the HadCM3 models,
which show differences between current climate
(defined as the period 1960 -1990) and climate at
the end of the twenty-first century (2070-2100),
mean surface air temperatures are predicted
to increase by 0.3°C (range 0.6 -9.2°C; see
Hadley Centre 2004). Increases are evident
across much of the globe with the exception of
the southern Pacific Ocean. Annual precipitation
rates are also projected to increase by an average
of 0.2 mm day
−1
(range
±
3.7- 8.9 mm day
−1
; see
Hadley Centre 2004). These projections are more
−
