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Predicted climate change impacts
Predicted climate change impacts on northern farming systems
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By Steven Crimp and Mark Howden, ANU
AT A GLANCE…
An increasing body of scientific evidence regarding the impact of human activity on the earth’s climate has shifted the debate from “Is climate change real?” to “What can we do about it?” Adapting current management activities must include considerations of both climate variability and change. Advisers have a vital role in helping to develop information-rich farming systems that will improve responses to current climate variability and that can enhance adaptation to climate changes.
FIGURE 1: Probability distributions of mean daily maximum temperature (left) and mean daily minimum temperatures (right) for Dubbo for two periods, 1960 to 1985 and 1986 to 2018
FIGURE 2: Mean annual dry spell length (left) and seasonal dry spell length for December to January (DJF), March to May (MAM), June to August (JJA) and September to November (SON)
Historical changes in climate?
Globally averaged air temperature has warmed by over 1°C since records began in 1850, and each of the last four decades has been warmer than the previous one. This warming is driven by increasing concentrations of all the major long-lived greenhouse gases in the atmosphere, with carbon dioxide (CO 2 ) concentrations rising above 400ppm and the CO equivalent (CO 2 -e) of all gases reaching 500ppm for the first time in at least 800,000 years.
In Australia, the pattern of warming (average temperature) has been largely similar to that experienced globally, with warming of just over 1°C since 1910. Examining the shift in the distributions of monthly day and night-time temperature shows that very high monthly maximum temperatures that occurred around 2 per cent of the time in the past (1951–80) now occur around 12 per cent of the time (2003–17). Very warm monthly minimum, or night-time, temperatures have shown a similar change from 2 per cent of the time in the past (1951–80) to 12 per cent more recently.
This shift in the distributions towards hotter temperatures and more extreme high temperature conditions has occurred across all seasons, with the largest change being in spring.
In the Dubbo region over the period 1950 to 2018 (the length of the temperature record), warming has occurred in both minimum and maximum temperatures, with mean temperatures now approximately 1.2°C warmer than in 1950. For the period 1950 to 1985 a maximum daily temperature of 25°C occurred, on average, 18 per cent of the year i.e. 66 times. More recently (1986 to 2018), this temperature now occurs on average 31 per cent of the time i.e. 113 times. Similarly, mean minimum temperatures have warmed with the frequency of a minimum temperature of 19°C increasing from 11 to 29 times each year on average (Figure 1).
The Dubbo rainfall record exhibits a declining trend, with declines during the December to February period most pronounced. Mean dry spell lengths have also increased, with the average time between rainfall events now four days
longer during June to August (i.e. an average dry spell length of 8 days for 2018: Figure 2). Despite declines in annual rainfall totals, the number of heavy rainfall events (i.e. greater than the 90 percentile) across the whole year has increased, most notably during the September to November period. The maximum number of consecutive dry days has increased across the whole year (i.e. from 33 to 38 days) with March to May and September to November periods increasing by two and five days respectively (i.e. now 30 and 23 days respectively).
The current acceleration of global warming is expected to continue based on future greenhouse gas (GHG) emissions trajectories.
What is expected to happen in the future?
In response to the continued growth in atmospheric greenhouse gas (GHG) concentrations, scientists estimate that global average temperatures could increase by up to 4.8°C by the end of the present century, dependent on global population growth, technological advancement and economic growth. To put this in context, the difference between our historical temperatures and those of the last ice age was only about 5°C. So even though 4.8°C does not sound like much,
TABLE 1: Projected changes in temperature and rainfall for Dubbo Variable Season Historical mean (1986 to 2005) 2030 2070 Annual 17.1°C 0.7 (0.5 to 0.9) 2.1 (1.7 to 2.6) Summer 24.4°C 0.9 (0.5 to 1.6) 2.4 (1.8 to 3.1) Mean temperature and temperature change (°C) Autumn 17.6°C 0.6 (0.4 to 0.9) 2.0 (1.4 to 2.6) Winter 9.9°C 0.4 (0.2 to 0.6) 1.7 (1.1 to 2.0) Spring 16.7°C 0.8 (0.6 to 1.2) 2.3 (1.9 to 2.9) Annual 604 mm 0 (-12 to +11) +7 (-10 to +22) Summer 160 mm -1 (-15 to +17) +13 (-10 to +26) Mean rainfall (mm) and rainfall change (% change) Autumn 141 mm +14 (-11 to +42) +13 (-9 to +45) Winter 139 mm -4 (-12 to +3) +5 (-25 to +34) Spring 162 mm -8 (-25 to +11) -6 (-25 to +17) Present average temperatures and rainfall are calculated for the period 1986 to 2005. The data contained in this table represents information compiled from the Queensland Department of Environment and Science, SILO database and New South Wales Department of Environment and Heritage. it signals a huge change in how the climateocean-land systems of the earth function and hence how agriculture will operate.
In Australia, national projections suggest up to 1.3°C of additional warming could be experienced by 2030 and up to 5.1°C of warming by 2090, with the greatest warming being in inland Australia and the lesser warming along the southern coast and Tasmania. Global studies indicate that a rule of thumb is that global potential crop production drops by 6 per cent per degree of warming.
While changes in rainfall are more uncertain, projections suggest drier conditions in the southern half of Australia, particularly in the south-west and during the cool season months of May to October with as much as 20 per cent less by 2030 and up to 50 per cent less rainfall by 2090.
At a regional scale, projected change in climate for Dubbo (representing a central town in this study region) are summarised in Table 1. In addition to warmer temperatures and no change (2030) to slight increases in mean annual rainfall (2070), evaporation rates are likely to increase.
The impacts of climate change on wheat production for the Dubbo region have been simulated using the Agricultural Production Simulator (APSIM).
If the 1990 to 2018 climate were to change, with a mean increase in temperature of 0.7°C with no change in annual rainfall (i.e. the mean 2030 projection) small improvements (approximately 55 kg per hectare) might be possible for 5 , 25 and 50 percentile yields. The 75 percentile yields could also improve by as much as 185 kg per hectare, but the 95 percentile yields could decline by as much as 470 kg per hectare.
Looking at the 2070 scenarios, if temperatures were to increase by 2.1°C and annual rainfall were to increase by 7 per cent, significant changes in yields are likely. Increases in 75 and 95 percentile yields are possible (i.e. 420 kg per hectare and 435 kg per hectare respectively). This is coupled with possible large declines in both 25 and 5 percentile yields (i.e. declines of 380 kg per hectare and 1020kg per hectare respectively). With little change in 50 percentile yields from those simulated for the baseline period, production variability could increase by more than 35 per cent by 2070 (i.e. the difference between high yields and low yields is likely to increase significantly).
This simple example does not take into consideration the compounding effects such as changes in runoff (Figure 3). This simulation
exercise does begin to make a case for adaptation at a range of spatial scales including farm-level and regional scales as well as changes to strategic planning and polices at the state and national level.
Runoff integrates the effects of changes in temperature, rainfall and evaporation. For example, where the map shows a 25 per cent reduction in runoff per degree and global temperatures rose by 2°C then runoff is likely to halve (i.e. 2 times 25 per cent) with major implications for water resource management including for irrigated agriculture.
Adapting to projected climate changes
Climate change is likely to pose a significant challenge for Australian agriculture. Of greatest concern are likely to be changes in water availability, and the change in frequency of climatic extremes (e.g. heatwaves, drought and floods).
Many of the actions required for adapting to climate change are extensions of those currently used for managing climate variability. For this reason, efforts to improve current levels of adaptation to climate variability will have positive benefits in addressing likely climate change impacts.
Examples of likely farm level adaptation options include longer-term decisions at a family farm level – to sell up, to buy more land or
FIGURE 3: Mid-range assessment of changes in average runoff per degree global temperature increase (IPCC 2014)
Runoff integrates the effects of changes in temperature, rainfall and evaporation. For example, where the map shows a 25 per cent reduction in runoff per degree and global temperatures rose by 2°C then runoff is likely to halve (i.e. 2 times 25 per cent) with major implications for water resource management including for irrigated agriculture. where to invest. These are especially pertinent for farmers in low rainfall regions and it will increasingly be more difficult to find no-regret decisions if climate change progresses as anticipated. These decisions, along with industry infrastructure (silos etc.) and industry support (drought policy) are hard decisions requiring full understanding of the likely future risks.
The value of adaptation
There is a growing international body of research examining the benefits of adaptation to climate variability and change, showing a number of adaptation options are available to reduce the possible impacts of climate change.
In Australia a number of studies have examined the economic benefits of adaptation in the wheat industry at both national and regional scales under a range of likely future climate conditions. Hochman et al. (2017) highlighted that the adoption of new technology and management systems has held actual yields fairly steady – without these advances, water-limited yield would have dropped by 27 per cent. It was estimated that rainfall declines should have accounted for about three-quarters of the fall in simulated yield potential, while observed warming should have accounted for about a quarter of the fall in yield potential.
Continued adaptation to climate change has been estimated to add an additional AU$500M per annum to Australia’s annual income from wheat exports via the introduction of improved water-use efficiency options and may mitigate potential yield losses by up to 18 per cent through broader scale adaptation.
The results suggest a number of adaptation options exist to manage increased future downside risk but the effectiveness of adaptation is driven by the extent of future change. Under conditions of large climate change, tactical adaptation will only have limited effectiveness and more extensive adaptation options, often defined as transformation adaptation, may be required.
Advisers have a key role to play in changing the nature of the climate change dialogue. In the space of about five years many grain growers and their advisers have moved from asking “What is climate change?” or “Is it real?” to “How do we manage for climate change?” and “What will the impact be on agriculture?”
