The Royal Society Report - Push-Pull

used for time spans of days or months, or the entire crop
growing season, or per year. At the leaf or plant scale, it
can be applied when considering the flow of CO 2 and
water vapour into and out of leaves. The highest WUEs can
often be achieved when productivities are very low.
Improvements therefore need to be balanced against the
need to maintain yields.
2.2.2 Water use and its impacts
Agriculture currently accounts for around 70% of annual
use of global water resources (FAO 2002; WRI 2005). In
hot, dry regions, much larger amounts of water are needed
to produce the same grain yield than in less stressed
regions (Wallace & Gregory 2002).
Most of the water used in agriculture is for irrigation.
Globally, irrigated areas of land are increasing, although the
rate of increase appears to be slowing (Faurèsa et al. 2003).
Although irrigated areas account for less than 20% of the
world’s cropped land, they produce nearly 50% of the
global food (Döll & Siebert 2002). Reduction in irrigated
areas or the amount of irrigation could therefore have very
serious impacts on global food supply.
Significant abstraction of water for irrigation has resulted in
large reductions in river flows (Ma et al. 2003) leading to
general environmental degradation and in extreme cases to
an acceleration of desertification and more ‘super’ dust
storms. Increased agricultural activity driving increased
desertification can drive climate change at an increased rate.
Water levels in many major regional aquifers and ground
water levels in many regions have fallen to unprecedented
levels (Wu 2007). Exploitation of land and unsustainable
practices, particularly in arid regions, can result in severe
degradation of soils and potential desertification, initiated by
loss of vegetation and soil erosion.
Using predictions of future availability of irrigation water
(eg Scholze et al. 2006), it will be important to identify the
most vulnerable people, places and sectors (climate
change hotspots) but there is currently a shortage of good
quality information of this kind. At a regional scale, the
major problems in water supply are in regions with low
rainfall and high evaporative demand, and those with
expanding populations, such as North Africa, Southern
Africa and the Near East (Wallace & Gregory 2002; FAO
2003). Wealthy countries that are short of water often
import food from elsewhere, meaning that ‘virtual water’ is
traded, which may be to the detriment of the environment
in the source country.
The food supply chain and other crop trades exert many
pressures on global water resources, with a resultant strain
on the human population and ecosystems worldwide
(Chapagain & Orr 2008a). The production of food, biofuel
and other commodities can drive over-abstraction and
pollution of groundwater and freshwater ecosystems in
many water-scarce parts of the world. Decisions on the
use of water for irrigated agriculture are therefore
increasingly moral and ethical choices, as well as
economic ones. Understanding how much water a nation
(or a business) requires—its water footprint (WF)—and
how this water is consumed (different crops grown in
different climatic zones with different cropping, processing
and transport methods) is the first step in forming views on
the appropriateness of different food choices.
The agricultural WF of the UK is 74.8 Gm 3 /yr or 73% of the
total WF. The internal WF of UK agriculture is 28.4 Gm 3 / yr
while the external component is 46.4 Gm 3 /yr. A larger
share of the internal WF is related to livestock production
and cereal products (wheat and barley), whereas the larger
share of the external WF (EWF) is related to products
originating from oil crops, cotton products, livestock
products and stimulants (coffee, tea and cocoa). Most of
the products responsible for the EWF are not grown in the
UK, mainly because of unsuitable agro-climatic conditions
(Chapagain & Orr 2008b).
Reducing the use of agricultural water is an aim that
requires combined agronomic, physiological,
biotechnological/genetic and engineering approaches
which may be collectively described as water saving
agriculture. As Kofi Annan, UN Secretary General,
declared, ‘we need a Blue Revolution in agriculture that
focuses on increasing productivity per unit of water—more
crop per drop’ (UN 2000). This issue has been summarised
recently by Pennisi (2008).
2.2.3 Increasing risks of flooding
Existing weather patterns leading to river and coastal
flooding have a dramatic effect on crop production.
Particularly sensitive areas in this context are the deltas of
southeast Asia which provide much rice for local and
regional consumption. The consequences of increasingly
turbulent and unpredictable weather patterns, driven by
climate change, have been discussed in many studies
(eg Scholze et al. 2006). Rising sea levels leading to
exacerbated coastal flooding are predicted to have
dramatic effects on many countries.
2.3 Temperature
Recent reports suggest that global temperature increases
are occurring more rapidly than previously predicted (Field
2009). In early February 2009, for example, southeastern
Australia experienced temperatures of up to nearly 50°C.
A risk of more frequent catastrophic crop failure is
correlated with an increase in the frequency of extreme
events (Semenov 2009).
Temperature is an important factor in controlling changes
in the development of plants. An increase in temperature
caused by climate change is predicted to speed plant
development (Sadok et al. 2007). When combined with the
lengthening of the cropping season, this change may
increase yield. However, when assessing the effects of
temperature on crop yield, it is necessary to take account
of extremes, particularly if these occur during the sensitive
stages of growth. Different developmental stages vary in
sensitivity to temperature extremes. For instance, very
significant reductions in the yield of wheat can be caused
12 I October 2009 I Reaping the Benefits The Royal Society