Agriculture in an even more sunburnt country - Four Degrees of Global Warming - page 92

development, technology and environmental governance that lead to different
greenhouse gas emissions and changes in the radiative balance of the atmos-
phere (IPCC, 2007a). There is substantial variation between these GCMs in
their capacity to represent past climate and in their response to future emissions
scenarios (e.g. Deser et al., 2012), leading to uncertainty as to when specific
changes such as 4°C increase may occur and consequently the rate of climate
change (IPCC, 2007a). There are also differences between climate variables
and scales, with more reliable projections of temperature than rainfall and more
reliability at global scales than regional or local scales (Giorgi, 2005). Changes
in average climate will be superimposed on large daily, seasonal and yearly
variability, leading to possible significant changes in extreme events, particularly
with 4°C or more warming, but these changes vary significantly between GCMs
and downscaling methods (Solomon et al., 2007). Therefore, there is substantial
and irreducible uncertainty as to the actual changes in climatic factors relevant
to Australian agriculture. This is added to by uncertainty as to the impacts of
such changes on agricultural systems and the effectiveness of adaptations to those
impacts.
It is anticipated that global warming of 4°C or more will increase climate
variability (Braganza et al., 2013, this volume). Australian agriculture is impacted
heavily by climate variability. For example, national wheat production has varied
by as much as 60 per cent from year to year as a result of low rainfall conditions,
halving exports in some years (Howden et al., 2010), the El Niño-Southern
Oscillation, Indian Ocean temperature gradients, atmospheric circulation over
the Southern Ocean and the latitude and strength of the sub-tropical ridge
being particularly important drivers of variability. Over the past decades,
improved knowledge of these drivers of variability has been integrated with
decision-making across the agricultural value chain, reducing risk and enhancing
sustainability of the industries. Unfortunately, recent and future changes in
these key climate drivers are not well simulated in GCMs (Christensen et al.,
2007; Hegerl et al., 2007; Kent et al., 2011; Durack et al., 2012; Guilyardi et
al., 2012). This contributes significantly to the uncertainty of future projections
(Hallegatte, 2009; Wilby et al., 2009) with little likelihood of resolution in the
near term (Hallegate, 2008; 2009). This uncertainty and potential increased
variability will, by themselves as a function of sensible risk management, tend
to lead to more conservative farming practices such as reductions in inputs (e.g.
fertilizer) that will reduce yields.
A Four Degree World will likely be associated with production declines and
associated negative impacts, in most of Southern Australia (e.g. Howden, 2002;
Easterling et al., 2007). Even with atmospheric CO 2 concentrations of 600ppm
or higher, the associated temperature increase and rainfall declines in excess of
20 per cent across much of this region will exceed any benefits arising through
enhanced efficiency with which plants use water, radiation and nutrients (i.e.
CO 2 fertilization effect).