Geoscience Reference
In-Depth Information
Temperature
Australian agricultural systems are strongly influenced by average temperatures,
which affect the choice of production system, and by high and low temperature
extremes, which can cause severe impacts through, for example, heat stress
damage and frost damage. Most of the major crops have strong physiological
responses to temperature, with this driving their growth and reproductive stages
(phenology). For instance, temperature increases across the season will shorten
the duration of cereal crops (i.e. the time from germination to harvest), reducing
the time to accumulate solar radiation and hence biomass and grain yield will
decline. McKeon et
al
.
(1988) suggest that a 2°C increase in temperature in
Queensland would reduce wheat yields by 6 per cent while Wang et al. (1992)
suggest that a 3°C increase in temperature could reduce mean yields in Wagga
Wagga and Mildura by up to 50 per cent and in Horsham by 25 to 60 per cent,
depending on cultivar.
Increased temperature will tend to increase evapotranspiration and vapour
pressure deficit (the difference between how much moisture the air is holding
and how much it can hold; e.g. Monteith, 1965), resulting in more rapid
depletion of soil moisture in spring when the grain is filling and ripening. This
will tend to reduce grain number, reduce harvest index, and in conditions of
high soil nitrogen may result in 'haying off' and subsequent major reductions in
effective yield (van Herwaarden, et al., 1998). A key adaptation to temperature
increase involves selecting varieties with greater thermal time requirements and
integrating this with changed planting dates and methods such as dry sowing,
which allows earlier sowing than traditional methods (Chapman et
al., 2012).
In horticultural crops, higher temperatures would affect attributes such as sugar
content and flavour as well as market synchronization (Webb and Whetton,
2010).
Extreme temperatures are also important. The climate projections derived
from the GCMs and emissions scenarios suggest that well before 4°C is reached
almost every region in Australia each year will be experiencing what were
historically exceptionally hot conditions (Hennessy et
al
.
, 2008) with much
higher frequencies of extreme temperatures than were historically recorded. In
particular, high temperatures during flowering (anthesis) of crops can result in
difficulties with pollination and other problems, dramatically lowering potential
yield and also decreasing grain quality (e.g. Howden, 2002). High temperatures
are increasingly being recognized as having a major impact on global food
production (Lobell et
al
.
,
2010).
Rainfall
Rainfall is a critical factor in Australian agriculture. It affects what crop varieties
are used, where and how crops are grown, as well as having an impact on annual
production, product quality and soil degradation, amongst many other elements.
Climate projections for 4°C or more indicate substantial rainfall reductions in
