Geoscience Reference
In-Depth Information
differences between air and water are increased;
(iii) widely described positive correlations between
air and river temperature are most likely to
reflect radiative heating in both these media;
and (iv) protecting rivers from insolation, for
example by shading, offers a potential strategy
for minimizing the greatest warming effects.
However, understanding long-term warming
trends, especially in winter when radiation
inputs are small, will require a more thorough
appraisal not only of increased heat gain, but
also possible reduced heat-losses. These might
include river waters entering or flowing through
warmer atmospheres, or less heat lost through
long-wave radiation during increased cloud
cover at low atmospheric pressure. Radiative
advection - i.e. heat transported by water among
oceanic, atmospheric, catchment, groundwater
and surface water components - might also be
important. So far, none of the long-term warming
trends described above has been assessed using
a heat-budget perspective and this represents an
important gap in understanding, with consequent
ramifications
makes the detection of additional climate change
effects very difficult. In temperate locations such
as the UK, both summer and winter discharge
have typically fluctuated by at least 300-350%
between the driest and wettest years over the
last 30-40 years (S.J. Ormerod and I. Durance;
unpublished data). Clear hydrological trends are
therefore difficult to differentiate from background
variation (Wilby, 2006). Some evidence is now
emerging of a spatially coherent tendency towards
increasing precipitation and discharge at higher
latitudes and in north-west Europe, with the
reverse trend in south-east Europe; these patterns
are linked with evidence also of greater seasonality,
with increasing winter discharge (Stahl
et al.
, 2010).
Nevertheless, forecasting climate change effects
on discharge is also characterized by considerable
uncertainty (Fowler and Wilby, 2010). In part,
this reflects variations between projections from
different global circulation models, down-scaling
tools or hydrological models (Prudhomme and
Davies, 2009). Similar problems are likely to arise
wherever climate change effects on discharge are
small relative to existing variation.
At broader spatial extents, altered spatio-
temporal patterns in precipitation, soil moisture,
runoff and discharge into the world's northern
oceans have been apparent from at least the
1970s (Peterson
et al.
, 2002). However, difficulties
arise in distinguishing between the drivers linked
with greenhouse gas emissions and the quasi-
natural effects of the North Atlantic Oscillation.
Given the ecological importance of hydrological
effects in rivers, improved prediction of climate
change effects on precipitation at all scales is an
important priority in modelling consequences for
river biodiversity and ecosystem services.
for
adaptive
management
(see
below).
Discharge
Like temperature, variations in precipitation will be
a major source of climate change effects on stream
and river ecosystems. Factors affecting discharge
are fundamental to most, if not all, ecological
processes in running waters. This stems from the
pre-eminence of the shape, magnitude and timing
of the flood hydrograph as a selective force on
river organisms both directly and through its many
influences on hydraulics, connectivity, habitat
physiography, floodplain inundation, interactions
with the riparian zone and also the transport
or dilution of sediments, natural solutes and
pollutants. Effects might arise not only through
changes in average daily discharge, but also
through the frequency and magnitude of extreme
high- or low-flow events.
In comparison with temperature, however,
precipitation and discharge patterns in many
locations are affected by proportionately large
stochastic variations through time and space. This
Invertebrates, fish and
other organisms
As with the assessment of changing discharge,
establishing long-term climatic effects on river
organisms and ecological processes is characterized
by several difficulties. The incremental nature
of climate change implies that the detection of
significant ecological consequences is likely to
require decades of observation; existing examples
