The wheat-growing areas of Western Australia are predicted to experience adverse climate change during the 21st century. Of the three components of change (rainfall, temperature and CO₂) the first two are somewhat uncertain, but current modelling evidence suggests that great pessimism about future yields is probably not warranted. 

Although there was quite substantial climate change in the Western Australia wheatbelt during the 20th century, it had little or no adverse consequences for wheat yields (PD229). Of course, it doesn’t follow that the same will hold during the 21st century. That will depend on the details of how much change occurs, and at what time of year it occurs. These details are highly uncertain.

Even the general pattern of future climate change is inherently uncertain, particularly in the long term. It depends in part on global emissions of greenhouse gases, which in turn depend on economic activity, technology change and climate policy measures over the relevant time frame. Uncertainty about each of these factors is high and increases with the length of time frame. In addition, the world’s climatic system is complex, chaotic and imperfectly understood, so that there is additional uncertainty inherent in the results of global circulation models, the tools used to predict climate.

Uncertainty is greater at the regional scale than at the global scale. And it is greater still when it comes to the within-year timing of changes in rainfall and temperature. We really have little idea about those timings, although they can be crucial in determining the consequences for crop yields (PD229).

Rainfall

Adverse changes in rainfall probably have the largest potential to reduce crop yields. Large reductions in September rainfall would be especially damaging. Unfortunately, rainfall is the factor about which we have the greatest uncertainty. Loosely speaking, regions that are already relatively wet are predicted to get wetter, and regions that are relatively dry (such as the Western Australian wheatbelt) are predicted to get drier, but in truth the models aren’t good enough to give us confidence about what will really happen in any particular place. CSIRO (2007) (somewhat heroically) predicted changes in annual average rainfall for the south-west of -20% to +5% by 2030 and -60% to +10% by 2070 relative to the period 1980-1999.

If the real results do fall within these ranges, 2030 would probably not be catastrophic, unless it includes a large reduction in early-spring rainfall. Clearly, -60% in 2070 would be catastrophic, but it’s the extreme case. Something nearer the midpoint of that range (-20%) would probably be damaging but not catastrophic. The real message about 2070 is that we have huge uncertainty about what rainfall will do – the range of the CSIRO predictions is 70%!

Temperature

By 2050, increases in temperature could perhaps be somewhere in the range of 1 to 3 °C. A positive impact on yield of temperature increases up to 1–3 °C has been reported from relevant crop modelling results when assuming that temperatures increase by the same amount every day across the growing season. Positive yield impacts come from accelerated plant development, leading to avoidance of high maximum temperatures and water stress during grain filling.

On the other hand, if the higher temperatures happen to include an increased frequency of extreme temperatures during grain filling, the result could be very negative. There is a potential for yield reductions of 5% for each day of extreme heat during grain filling.

Again, there is high uncertainty. Future temperature changes could be positive, negative or neutral for crop yields in Western Autralia.

CO₂

CO₂ fertilization of crops is a significant part of the climate/CO₂ story (Attavanich and McCarl, 2012). In future, CO₂ concentrations are likely to increase at about about 4 ppm per year. This is predicted to increase yields of current wheat cultivars in Western Australia by 15–30% over the next 50 years (Asseng and Pannell, 2012). Percentage increases in yield are likely to be greater in dryer agricultural regions, mainly due to increased water-use efficiency. Compared to the changes in rainfall  and temperature, the increases in CO₂ are fairly certain. They are also constant all year, avoiding the tricky problem of predicting the within-year distribution of changes.

Highly adverse changes in rainfall and temperature would be really catastrophic for farmers in the Western Australian wheatbelt, but more likely the overall impacts will be moderate, particularly when the positive yield effects of CO₂ fertilization are factored in.

Reinforcing that judgement, another new paper (Potgieter et al., 2012) concludes that, under a high-emissions scenario, different shires would see changes in crop yields by 2050 of −5% to +6% across most of Western Australia (and Victoria and southern New South Wales), even without allowing for CO₂ fertilization.

I was surprised at how favourable those results are, especially considering that they are for a high-emission scenario. If they are accurate, then any negative impacts would be outweighed by the positive effects of CO₂ fertilization.

I remember reading the original report of the Garnaut review, and being struck by how much it emphasised the risks to agriculture when justifying Australia’s need for a strong policy response to climate change. There certainly are risks to agriculture, but this evidence suggests that they are not as compelling a policy driver as Garnaut indicated. If Potgieter et al. are right, then I would expect that the ongoing decline in public investment in agricultural research will have far worse consequences for agriculture.

This Pannell Discussion draws on part of a paper I’ve recently published with Senthold Asseng, who’s now at the University of Florida (Asseng and Pannell, 2012).

Further reading

Asseng, S. and Pannell, D.J. (2012). Adaptating dryland agriculture to climate change: farming implications and research and development needs in Western Australia, Climatic Change (forthcoming). http://link.springer.com/article/10.1007/s10584-012-0623-1

Attavanich, W. and McCarl, B. (2012). The effect of climate change, CO₂ fertilization, and crop production technology on crop yields and its economic implications on market outcomes and welfare distribution, Agricultural and Applied Economics Association, 2011 Annual Meeting, July 24-26, 2011, Pittsburgh, Pennsylvania, IDEAS page for this paper.

Potgieter, A., Meinke, H., Doherty, A., Sadras, V.O., Hammer, G., Crimp, S. and Rodriguez, D. (2012). Spatial impact of projected changes in rainfall and temperature on wheat yields in Australia, Climatic Change (forthcoming). http://link.springer.com/article/10.1007/s10584-012-0543-0

The wheat-growing areas of Western Australia experienced substantial climate change (particularly rainfall decline) during the 20th century. However, the resulting impacts on wheat yields were negligible, even after factoring out changes in technology and prices.

Western Australia is by far the largest grain-crop-producing state of Australia, and wheat is by far its main crop.

The wheatbelt underwent significant climate change during the 20th century, commencing even before climate change was a high-profile issue. The region has had a 20% rainfall decline over the past 110 years, more than any other wheat-growing region in Australia.

There has been a specific pattern to the rainfall decline, with most of it occurring in winter. Figure 1 shows a typical example, for the Mullewa region.

Figure 1. Average monthly rainfall for Mullewa for 1945–1974 (filled bars) and 1975–2008 (open bars)

The other important climatic variable is temperature. Average temperature in the region has increased slightly (0.8 °C) over the past 50 years, but there has been a disproportionate increase in the frequency of hot days during grain filling (Asseng et al. 2011), when wheat yields are adversely affected by high temperatures. Some of that impact may have been off-set by increases in temperature during winter, which helps to increase yields.

Ludwig et al. (2009) used crop modelling to investigate the effects of past climate change on crop yields over the past 60 years, across various locations of the Western Australian wheatbelt. Remarkably, they concluded that there has been no change in wheat yield potential. The reasons they proposed for the lack of impact of reduced rainfall are:

1. Crops have low demand for water during the cool winter months in which the decline has occurred
2. Given the unpredictability of weather, farmers do not apply sufficient inputs (particularly fertilizer) to achieve the higher yields that are theoretically possible in wet years, and
3. Most Western Australian soils have low water-holding capacity, so a large proportion of unused water is lost below the root zone of crops.

A third change, not considered by Ludwig et al. (2009), has been the increase in atmospheric CO₂. Higher CO₂ has presumably contributed to the climatic changes (especially tempterature) but has another effect on crop yields, through so-called “CO₂ fertilization” (Attavanich and McCarl, 2012). Increases in atmospheric CO₂ concentration over the past 50 years have increased wheat yields in Western Australia by approximately 2–8 %.

Overall, I think it’s quite interesting and surprising that, despite really significant changes in climate in the region, these changes have had no significant negative impact on yields, especially when CO₂ fertilization is factored in.

It highlights that the specific details of the climate changes (such as their within-year timing) really matter. Changes that would be damaging at one time of the year can be benign at another. This makes it even harder to accurately predict the impacts of future climate changes, even if we get their average magnitudes right (which is already tough).

At the same time as climate change was having no impact on wheat yields in Western Australia, other things were having big positive impacts, including changes in crop varieties, increased fertiliser use, herbicides, reduced tillage, improved machinery allowing earlier sowing, retention of crop residues, and the use of ‘break’ crops that reduce root diseases. These have combined to increase average wheat yields in the region by around 100% over the past 30 years.

Some scientist have argued that farmers in this region should already be making changes to adapt to climate change. In the light of these results, that advice seems misguided.

This Pannell Discussion is based on part of a paper I’ve recently published with Senthold Asseng, who’s now at the University of Florida (Asseng and Pannell, 2012).

Further reading

Asseng, S. and Pannell, D.J. (2012). Adaptating dryland agriculture to climate change: farming implications and research and development needs in Western Australia, Climatic Change (forthcoming). http://link.springer.com/article/10.1007/s10584-012-0623-1

Attavanich, W. and McCarl, B. (2012). The effect of climate change, CO₂ fertilization, and crop production technology on crop yields and its economic implications on market outcomes and welfare distribution, Agricultural and Applied Economics Association, 2011 Annual Meeting, July 24-26, 2011, Pittsburgh, Pennsylvania, IDEAS page for this paper.

Ludwig, F., Milroy, S.P., Asseng, S. (2009). Impacts of recent climate change on wheat production systems in Western Australia. Climatic Change 92: 495–517.