Geoscience Reference
In-Depth Information
Empirical evidence of effects
and trends
processes: discharge variations, flood and drought
effects, hydraulics, flow-mediated connectivity,
and fluxes of solutes and energy between land and
water. Finally, climate is likely to interact with
other major influences on freshwater ecosystems
such as pollution, abstraction, land-use change and
the effects of invasive non-native species (Wilby
et al.
, 2006). Existing climatic phenomena, such
as the North Atlantic Oscillation and El Ni no/La
Ni na, illustrate already how river organisms are
influenced by variations in precipitation and
temperature between years (Elliott
et al.
, 2000;
Bradley and Ormerod, 2001). Climatic variations
between regions also reveal large effects on river
character (Bonada
et al.
, 2007).
Although climate changes are unfolding
differently across regions and biomes of the
world, all indications are that the magnitude and
rate of future effects will be large and rapid. In
the temperate UK, for example, mean annual
temperatures by the 2050s are likely to be at least
2-3
◦
C greater than now, with extreme summer
temperatures rising well above 30
◦
C. Whereas
rainfall amounts will not change substantially,
greater rainfall seasonality is predicted with
potentially larger floods and also longer rain-free
periods (UKCP09; Jenkins
et al.
, 2009). If realized,
all these changes will have profound consequences
for river organisms, processes and ecosystem
services. An important current need, therefore, is
to develop practical management responses that
might reduce the worst effects. However, well-
informed management responses will depend on
greater knowledge about the effects and processes
involved.
This chapter reviews some of the evidence
available already on changes in river ecosystems
that can be linked to climate as reflected by
observed trends in discharge and temperature over
recent decades. It also reviews some of the evidence
of biological effects in different regions around the
world. Although this review cannot be exhaustive,
we attempt to identify critical knowledge gaps with
respect to: (i) the mechanisms involved; (ii) the
difficulties of understanding interactions with other
stressors; and (iii) the inadequate evidence about
the effectiveness of different management options.
Thermal regimes
Trends in river temperature are liable to be
among the clearest indicators of climatic change
and variation. Throughout the world, several
datasets now reveal long-term warming. River
temperatures in parts of Europe have increased
over at least the last 20-30 years by up to 1
◦
Cper
decade - examples include upland Wales, Scotland,
southern English chalk streams, the upper Rh one,
the Swiss Alps and Austria (Langan
et al.
, 2001;
Daufresne
et al.
, 2004; Hari
et al.
, 2006; Durance
and Ormerod, 2007, 2009; Webb and Nobilis, 2007;
Clews
et al.
, 2010; Figure 9.1). Local variations are
also apparent over similar timescales; in the UK,
average temperature gains range from
ca
0.2
◦
Cper
decade in eastern regions to 0.3-0.4
◦
C in Wales, the
north-west and the south-west (Wilby
et al.
, 2010).
River temperatures have also increased by similar
magnitudes on other continents, including Asia,
Australasia and North America. Most recently,
Kaushal
et al.
(2010) examined 41 historical
datasets from North American rivers, detecting 21
significant, long-term warming trends of 0.1 to
1.0
◦
C per decade that correlated with increasing
air temperature. Warming occurred across a wide
array of locations, but the effects were fastest
in urban areas. Although earlier temperature
data are likely to have been made by mercury
thermometers, from the 1980s onwards electronic
recording has usually been involved, with high
precision and regular data capture, typically every
15 minutes.
Despite the apparent consistency, clarity and
global coherence of these trends, river thermal
regimes and heat budgets are complicated by
several processes of which some understanding
is important in establishing exactly how climate
change effects on rivers are occurring. Local
river heating or cooling depends on the relative
balance between heat radiation (in or out), friction
against the banks and bed, heat exchange with
the surroundings (air, river bed, banks, etc.),
condensation (a warming effect) and evaporation
(a
cooling
effect)
(Webb
et al.
,
2008).
Rivers
