Impacts of Climate Change on Agriculture - Climate Resilient Agriculture for Ensuring Food Security - page 75

Under clear-sky conditions, the ambient UV-B
fl ux in tropical rice-growing areas is already
among the highest on the earth's surface because
the stratospheric ozone layer is naturally thinner
than at high latitudes and because solar angles are
higher. Thus, with stratospheric ozone depletion,
the UV-B fl ux in tropical areas is likely to exceed
that experienced anywhere in the world.
Clouds can reduce UV-B transmission through
the atmosphere. Thus, in the tropics where there
is a strong monsoon-driven seasonality in cloud
cover, actual UV-B radiation during certain times
of the year may be lower than predicted for clear
skies. However, the quantitative effects of clouds
on UV-B have not been clearly determined.
The IPCC assumes a further increase of sur-
face ozone (O 3 ) until the end of the century
(Vinzargan 2004 ) which may lead to consider-
able crop losses at least until 2030, especially in
China (Van Deningen et al. 2009 ).
The release of chlorofl uorocarbons has
severely depleted the atmosphere's protective
ozone layer. In general, each 1 % reduction in the
ozone layer causes a 2 % increase in the amount
of ultraviolet radiation that reaches the earth. In a
recent study, two-thirds of the 300 species and
cultivars examined appeared susceptible to ultra-
violet radiation damage. This study suggests that
25 % depletion in the ozone layer could reduce
soybean yields by 20 % (Pitovranov et al. 1988 ).
Unlike soybeans, some crops may be more
tolerant of ultraviolet radiation. However, such
crops also may be more susceptible to disease.
For example, although wheat seems to tolerate
ultraviolet radiation, “Red Hard” infection rates
increased from 9 to 20 % when experimental
ultraviolet radiation was increased from 8 to
16 % above ambient levels (Kettunen et al. 1988 ).
Disease rates in rice also have increased when
rice is exposed to higher ultraviolet radiation than
normal (Bergthorsson et al. 1988 ).

level of biological organization, from plant
molecules to entire ecosystems. Among the
observed effects are:
• A number of plant molecules, such as DNA,
lipids, and proteins strongly absorb UV-B and
can, in turn, induce specifi c changes in tissue
and whole-plant structure and function
(Caldwell et al. 1989 ).
• UV-B can reduce plant growth and yield
through reductions in biomass production,
seed yield, and yield quality (Barnes et al.
1988 ).
• UV-B can alter plant morphology through
reductions in plant height and leaf area,
increased tillering, and changes in plant geom-
etry (Barnes et al. 1988 ).
• Plant physiological processes are impacted by
UV-B. Photosynthesis is often reduced, and
the production of plant secondary metabolites
increased (Caldwell et al. 1989 ).
• Plant competitive interactions can shift due to
the differential sensitivity of competing plant
species (Fox and Caldwell 1978 ; Barnes et al.
1988 ).
• Pest-pathogen relationships may be altered
due to changes in plant secondary metabolites
(Caldwell et al. 1989 ) .

4.10.2 Ozone

Ozone is a major secondary air pollutant, which
at current concentrations has been shown to have
signifi cant negative impacts on crop yields
(Fig. 4.10 ) (Van Dingenen et al. 2009 ). Whereas
in North America and Europe, emissions of
ozone precursors are decreasing, in other regions
of the world, especially Asia, they are increasing
rapidly (Van Dingenen et al. 2009 ).
Ozone reduces agricultural yield through sev-
eral mechanisms. Firstly, acute and visible injury
to products such as horticultural crops reduces
market value. Secondly, ozone reduces photosyn-
thetic rates and accelerates leaf senescence which
in turn impacts on fi nal yield. In Europe and
North America, many studies have investigated
such yield reductions.

4.10.1 UV-B Effects

Effects of UV-B on terrestrial biota, both direct
and indirect, have been demonstrated at every