We found a secret history of megadroughts written in tree rings. The wheatbelt’s future may be drier than we thought


An almost-dry dam, surrounded by wheat fields, in WA’s wheatbelt region.
Shutterstock

Alison O’Donnell, The University of Western Australia; Edward Cook, Columbia University, and Pauline Grierson, The University of Western AustraliaDrought over the last two decades has dealt a heavy blow to the wheatbelt of Western Australia, the country’s most productive grain-growing region. Since 2000, winter rainfall has plummeted by almost 20% and shifted grain-growing areas towards the coast.

Our recent research, however, found these dry conditions are nothing out of the ordinary for the region.

In fact, after analysing rings in centuries-old tree trunks, we found the region has seen far worse “megadroughts” over the last 700 years. Australia’s instrumental climate records only cover the last 120 or so years (at best), which means these historic droughts may not have previously been known to science.

Our research also found the 20th century was the wettest of the last seven centuries in the wheatbelt. This is important, because it means scientists have likely been underestimating the actual risk of drought – and this will be exacerbated by climate change.

What we can learn from ancient trees

We estimate the risk of extreme climate events, such as droughts, cyclones and floods, based on what we know from instrumental climate records from weather stations. Extending climate records by hundreds or even thousands of years means scientists would be able to get a much better understanding of climate variability and the risk of extreme events.

_Callitris_ trees overlooking a salt lake
Callitris trees overlooking a salt lake. We pulled a column of wood from these tree trunks to investigate past climate changes in the region.
Alison O’Donnell, Author provided

Thankfully we can do just that in many parts of the world using proxy records — things like tree rings, corals, stalagmites and ice cores in Antarctica. These record evidence of past climate conditions as they grow.

For example, trees typically create a new layer of growth (“growth ring”) around their trunks, just beneath the bark, each year. The amount of growth generally depends on how much rain falls in the year. The more it rains, the more growth and the wider the ring.

Tree rings of Callitris columellaris.
Alison O’Donnell, Author provided

We used growth rings of native cypress trees (Callitris columellaris) near a large salt lake at the eastern edge the wheatbelt region. These trees can live for up to 1,000 years, perhaps even longer.

We can examine the growth rings of living trees without cutting them down by carefully drilling a small hole into the trunk and extracting a column (“core”) of wood about the size of a drinking straw. By measuring the ring widths, we developed a timeline of tree growth and used this to work out how much rain fell in each year of a tree’s life.

This method allowed us to reconstruct the last 668 years of autumn-winter rainfall in the wheatbelt.

A tree trunk with a blue scientific instrument attached
A tree borer – a hollow drill used to extract ‘cores’ of wood from tree trunks.
Alison O’Donnell, Author provided

A history of megadroughts

One of the most pressing questions for the wheatbelt is whether the decline in autumn-winter rainfall observed in recent decades is unusual or extreme. Our extended record of rainfall lets us answer this question.

Yes, rainfall since 2000 was below the 668-year average — but it was not extremely low.

The last two decades may seem particularly bad because our expectations of rainfall in the wheatbelt are likely based on memories of higher rainfall. But this frequent wet weather has actually been the anomaly. Our tree rings revealed the 20th century was wetter than any other in the last 700 years, with 12% more rain in the autumn-winter seasons on average than the 19th century.




Read more:
500 years of drought and flood: trees and corals reveal Australia’s climate history


Before the 20th century, the wheatbelt saw five droughts that were longer and more severe than any we’ve experienced in living memory, or have recorded in instrumental records. This includes two dry periods in the late 18th and 19th centuries that persisted for more than 30 years, making them “megadroughts”.

While the most recent dry period has persisted for almost two decades so far, rainfall during this period is at least 10% higher than it was in the two historical megadroughts.

This suggests prolonged droughts are a natural and relatively common feature of the wheatbelt’s climate.

An aerial view of the tree-ring site, home to trees that can live up to 1,000 years.
Hannah Etchells, Author provided

So how does human-caused climate change play into this?

It’s likely both natural climate variability and human-caused climate change contributed to the wheatbelt’s recent decline in rainfall. Unfortunately, it’s also likely their combined influence will lead to even less rainfall in the near future.

What happens now?

Our findings have important implications for assessing the risk of drought. It’s now clear we need to look beyond these instrumental records to more accurately estimate the risk of droughts for the wheatbelt.

But currently, proxy climate records like tree rings aren’t generally used in drought risk models, as there aren’t many of them in the regions scientists want to research.

Improving risk estimates leads to better informed decisions around preparing for and managing the effects of droughts and future natural disasters.




Read more:
To help drought-affected farmers, we need to support them in good times as well as bad


Our findings are a confronting prospect for the future of farming in the wheatbelt.

Australian farmers have shown tremendous innovation in their ability to adapt in the face of drought, with many shifting from livestock to crops. This resilience will be critical as farmers face a drier, more difficult future.The Conversation

Alison O’Donnell, Research Fellow in Dendroclimatology, The University of Western Australia; Edward Cook, Ewing Lamont Research Professor, Director Of Tree-Ring Lab, Columbia University, and Pauline Grierson, Director, West Australian Biogeochemistry Centre, The University of Western Australia

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Early sowing can help save Australia’s wheat from climate change



File 20190224 195873 57g5o.jpg?ixlib=rb 1.1
Timing is of the essence when it comes to growing wheat.
Author provided

James Hunt, La Trobe University

Climate change has already reduced yields for Australian wheat growers, thanks to increasingly unreliable rains and hostile temperatures. But our new research offers farmers a way to adapt.

By sowing much earlier than they currently do, wheat growers can potentially increase yields again. However, our study published today in Nature Climate Change shows that to do this they need new varieties that allow them more leeway to vary their sowing dates in the face of increasingly erratic rainfall.




Read more:
Changing climate has stalled Australian wheat yields: study


Sowing wheat is a matter of delicate timing. Seeds of current varieties need to be planted at just such a time so that, months later, the plants flower during a window of just 1-2 weeks, known as the optimal flowering period.

In Australia’s wheat belt this window is generally in early spring. At this time the soil is moist after the cool, wet winter; days are getting longer and sunnier; maximum temperatures are still relatively low; and frosts are less frequent. If crops flower outside the optimal window, yields decline sharply.

Crops and colonies

When Europeans first started trying to grow wheat in Australia, they used varieties that were suited to the cool, wet climate of northern Europe, where the optimal flowering period is in summer. These varieties were much too slow to flower in Australian conditions, and yields were very low. Wheat breeder William Farrer used faster-developing wheats from India to create the Federation variety, which revolutionised wheat production in Australia, earning Farrer the ultimate honour of having a pub named after him.

Federation wheat is a “spring wheat”, moving rapidly through its life cycle regardless of when it is planted. If you sow it earlier, it flowers earlier. For more than a century Australian wheat breeders have bred spring wheats, allowing growers to adjust their sowing time to get their crops to flower during the optimal period. Anzac Day has traditionally been the start of sowing season, after autumn rains have wet the soil enough for seeds to germinate.

Here is where climate change is causing a problem. If farmers sow later than mid-May, the wheat is likely to miss its spring flowering window. But southern Australia has experienced declining April and May rainfall, making it harder for growers to sow and establish crops at the right time. This in turn means crops flower too late the following spring, meaning yields are reduced by drought and heat.

Growers could start sowing earlier, and use stored soil water from summer rain (which hasn’t declined and has even increased at some locations), but current spring wheat varieties would flower too early to yield well. For farmers to sow earlier, they need a different sort of wheat in which development is slowed down by an environmental cue. One such environmental cue is called vernalisation. Plants that are sensitive to vernalisation will not flower until they have experienced a period of cold temperatures. These strains are thus called “winter wheats”.

Ironically enough, the wheat varieties that Europeans first brought to Australia were winter wheats, but they were further slowed by sensitivity to day length which made them too slow to reach the earlier flowering times needed in the hotter, drier colony.

But this problem can be sidestepped by using a “fast winter wheat”, which is sensitive to vernalisation but not to day length. Our previous research showed that this type of wheat was very suited to Australian conditions – it can be sown early but still flower at the right time. In fact, the vernalisation requirement means that this wheat can be sown over a much broader range of dates and experience fluctuating temperatures, and still flower at the right time.

Yielding results

In our new research, we developed different lines of wheat that varied in their response to vernalisation and day length, and grew them across the wheat belt to compare which ones would yield best at earlier sowing times.

We found that a fast winter wheat performed best over most of the wheatbelt, and on average yielded 10% more than spring wheat when they flower at the same time.

We then used computer simulations to investigate how these crops would perform at the scale of an entire farm. Our results showed that if Australian growers had access to adapted winter varieties in addition to spring varieties, they could start sowing earlier in seasons where there was an opportunity. If the rains come early, farmers can use the winter wheat; if they come late they can switch to the spring wheat, which yields better than winter wheat at late sowing times.

This would mean that more area of crop would be planted on time, and yields would increase as a result. If realised, this could increase national wheat production by about 20%, or roughly 7.1 million tonnes.




Read more:
Australia’s farming future: can our wheat keep feeding the world?


The main hurdle is that growers do not currently have access to suitable winter wheats. Breeding companies have started work on them, but it will be several years before suitably high-quality varieties become available.

Australian growers urgently need to keep pace with climate change. Although Australia only produces 4% of the world’s wheat, it accounts for 10% of exports and is thus important in determining global supply and price. If global wheat supply is low, prices rise, and it becomes unaffordable for many of the world’s poorest people, potentially causing malnutrition and civil unrest. Steeply rising wheat prices were among the factors behind the food riots that broke out in more than 40 countries in 2007-08, which helped to trigger the Arab Spring uprisings of 2010-12.

The world’s poorest people deserve to be able to buy wheat. But Australian wheat farmers also need to earn a decent living and stay internationally competitive. The only way to meet all these needs is to keep production costs low – and increasing yields by sowing the right wheat cultivars for Australia’s changing climate is one way to go about it.The Conversation

James Hunt, Associate Professor, La Trobe University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Changing climate has stalled Australian wheat yields: study


Zvi Hochman, CSIRO; David L. Gobbett, CSIRO, and Heidi Horan, CSIRO

Australia’s wheat yields more than trebled during the first 90 years of the 20th century but have stalled since 1990. In research published today in Global Change Biology, we show that rising temperatures and reduced rainfall, in line with global climate change, are responsible for the shortfall.

This is a major concern for wheat farmers, the Australian economy and global food security as the climate continues to change. The wheat industry is typically worth more than A$5 billion per year – Australia’s most valuable crop. Globally, food production needs to increase by at least 60% by 2050, and Australia is one of the world’s biggest wheat exporters.

There is some good news, though. So far, despite poorer conditions for growing wheat, farmers have managed to improve farming practices and at least stabilise yields. The question is how long they can continue to do so.

Worsening weather

While wheat yields have been largely the same over the 26 years from 1990 to 2015, potential yields have declined by 27% since 1990, from 4.4 tonnes per hectare to 3.2 tonnes per hectare.

Potential yields are the limit on what a wheat field can produce. This is determined by weather, soil type, the genetic potential of the best adapted wheat varieties and sustainable best practice. Farmers’ actual yields are further restricted by economic considerations, attitude to risk, knowledge and other socio-economic factors.

While yield potential has declined overall, the trend has not been evenly distributed. While some areas have not suffered any decline, others have declined by up to 100kg per hectare each year.

We found this decline in yield potential by investigating 50 high-quality weather stations located throughout Australia’s wheat-growing areas.

Analysis of the weather data revealed that, on average, the amount of rain falling on growing crops declined by 2.8mm per season, or 28% over 26 years, while maximum daily temperatures increased by an average of 1.05℃.

To calculate the impact of these climate trends on potential wheat yields we applied a crop simulation model, APSIM, which has been thoroughly validated against field experiments in Australia, to the 50 weather stations.

Climate variability or climate change?

There is strong evidence globally that increasing greenhouse gases are causing rises in temperature.

Recent studies have also attributed observed rainfall trends in our study region to anthropogenic climate change.

Statistically, the chance of observing the decline in yield potential over 50 weather stations and 26 years through random variability is less than one in 100 billion.

We can also separate the individual impacts of rainfall decline, temperature rise and more CO₂ in the atmosphere (all else being equal, rising atmospheric CO₂ means more plant growth).

First, we statistically removed the rising temperature trends from the daily temperature records and re-ran the simulations. This showed that lower rainfall accounted for 83% of the decline in yield potential, while temperature rise alone was responsible for 17% of the decline.

Next we re-ran our simulations with climate records, keeping CO₂ at 1990 levels. The CO₂ enrichment effect, whereby crop growth benefits from higher atmospheric CO₂ levels, prevented a further 4% decline relative to 1990 yields.

So the rising CO₂ levels provided a small benefit compared to the combined impact of rainfall and temperature trends.

Closing the yield gap

Why then have actual yields remained steady when yield potential has declined by 27%? Here it is important to understand the concept of yield gaps, the difference between potential yields and farmers’ actual yields.

An earlier study showed that between 1996 and 2010 Australia’s wheat growers achieved 49% of their yield potential – so there was a 51% “yield gap” between what the fields could potentially produce and what farmers actually harvested.

Averaged out over a number of seasons, Australia’s most productive farmers achieve about 80% of their yield potential. Globally, this is considered to be the ceiling for many crops.

Wheat farmers are closing the yield gap. From harvesting 38% of potential yields in 1990 this increased to 55% by 2015. This is why, despite the decrease in yield potential, actual yields have been stable.

Impressively, wheat growers have adopted advances in technology and adapted them to their needs. They have adopted improved varieties as well as improved practices, including reduced cultivation (or “tillage”) of their land, controlled traffic to reduce soil compaction, integrated weed management and seasonally targeted fertiliser use. This has enabled them to keep pace with an increasingly challenging climate.

What about the future?

Let’s assume that the climate trend observed over the past 26 years continues at the same rate during the next 26 years, and that farmers continue to close the yield gap so that all farmers reach 80% of yield potential.

If this happens, we calculate that the national wheat yield will fall from the recent average of 1.74 tonnes per hectare to 1.55 tonnes per hectare in 2041. Such a future would be challenging for wheat producers, especially in more marginal areas with higher rates of decline in yield potential.

While total wheat production and therefore exports under this scenario will decrease, Australia can continue to contribute to future global food security through its agricultural research and development.

The Conversation

Zvi Hochman, Senior Principal Research Scientist, Farming Systems, CSIRO; David L. Gobbett, Spatial data analyst, CSIRO, and Heidi Horan, Cropping Systems Modeller, CSIRO

This article was originally published on The Conversation. Read the original article.