AdApting urbAn wAter systems to climAte chAnge - IWA

10 Adapting urban water systems to climate change
A handbook for decision makers at the local level
Section 1
Vulnerability of urban water systems in the face of climate change
11
Regional and temporal variability
Discussions about climate change are often accompanied by generalisations: ‘heat waves
will be more common’ or ‘rainfall will increase.’ However, the difference between global
mean levels and specific local manifestations should be kept in mind. Examples abound:
While global mean surface air temperature is indeed expected to increase, this increase
will be subject to regional differences, with land surfaces and high northern latitudes
experiencing the greatest warming. Globally averaged mean precipitation is also projected
to increase but that in the subtropics to decrease.
1.2 Sensitivity within urban water systems
Cities concentrate population, infrastructure, economic activity and wealth, and will
therefore be disproportionately affected by the local impacts of climate change. In
addition, cities located in coastal areas and/or on the banks of rivers are particularly
vulnerable to sea level rise and flooding. Cities are also characterised by the predominance
of impermeable surfaces – which are less capable of absorbing increased rainfall and
therefore increase the intensity of rainfall runoff – and are prone to the urban heat island
effect which amplifies heat waves.
A regional breakdown of
climate change impacts on
water resources can be found
in the IPCC’s Technical paper
on climate change and water
(Bates, Kundzewicz, Wu
and Palutikof, 2008).
Perth’s experience of nonlinear
climatic change and
its approach to dealing with
variability are presented in
Section 2.1.
Some regions will also experience multiple manifestations: for example, mean precipitation
will decrease in subtropical and mid-latitude regions, but, since the intensity of rainfall will
also increase, longer periods will pass between rainfall events (Bates, Kundzewicz, Wu
& Palutikof, 2008). A city might therefore be confronted with both flooding and drought
within a relatively short time span. These examples of regional variability highlight the
need for cities to make use of projections downscaled to a more precise scale: see Section
2.1 for more information.
Variability will also manifest itself on a temporal scale: long-term trends will interact with
short-term ones, and greater climatic variability will lead to an increase in the frequency
of extreme events (UN-Water, 2010). In addition, climatic variability will not always occur
in a gradual linear way. Instead, the predictability of climate change is reduced by the fact
that step changes can and do happen, with climatic conditions shifting from one state to
another radically different one in a short period of time. As a result, although long-term
climatic predictions are useful, cities should also prepare for the possibility of abrupt
change as part of their planning.
Other pressures on water systems
Although the public and decision makers have become more aware of climate change,
it is vital not to lose sight of the dominant role of the impacts of other human activities
on freshwater and related systems. In many cases, the effects of other anthropogenic
interventions are hard to separate from the effects of variations in climate. For example,
river flows are substantially affected not only by climate-related precipitation and temperature
patterns, but also by human activities such as dam building, land use changes
and pollutant loads. Similarly, observed decreases in groundwater levels should first and
foremost be attributed to overabstraction rather than climate-driven changes in recharge
rates (Bates, Kundzewicz, Wu & Palutikof, 2008).
Human activities can exacerbate the
negative impacts of climate change
by increasing the vulnerability of
systems to a changing climate (Bates,
Kundzewicz, Wu & Palutikof, 2008).
While this handbook only focuses on
adaptation to the impacts of climate
change on the water cycle, it takes
as a central assumption that sound
planning should take all driving
forces into consideration. In some
cases, placing the focus exclusively
on climate change may lead a city to miss more obvious leverage points, which might
be more cost-effectively addressed and lead to better results and greater co-benefits.
Attribution of impacts to driving forces should therefore be a key exercise within a city’s
assessment of its baseline situation.
© dreamstime.com/Samrat35
Finally, as population growth will overwhelmingly take place in cities, urban water
managers will face a growing challenge to maintain safe and adequate water supplies and
wastewater services for urban residents. Urban population growth in the next decades
will take place at a very rapid scale and mostly in developing countries, exacerbating many
of the problems linked to urban poverty, increasing the size of vulnerable populations in
cities and placing additional pressure on dwindling supplies of resources such as water
(Pageler, 2009).
The vulnerability of urban water supply, wastewater and stormwater systems to climate
change is outlined in this section. These systems will be strongly affected by the various
manifestations of climate change, with impacts primarily relating to their physical infrastructure
but also to their functionality.
This section is supported by the table in Annex 2 which presents a selection of examples
of the impacts of some predicted climate change manifestations. The table mainly
focuses on urban water but also touches upon other urban sectors which are strongly
related to water, such as energy, health, food production and green spaces. Annex 2 is
intended as an illustration of the variety of impacts which can be anticipated and of the
links between the water sector and other urban sectors. This section of the handbook is
also accompanied by the illustration on pages 28 and 29, which shows some of the main
challenges of climate change for cities.
© iStockphoto.com/thejack