By JR on Sunday, January 29, 2017
Changing climate has stalled Australian wheat yields (?)
As Warmist articles go, the study below shows unusual statistical sophistication. But the connection to Warmism is tokenistic. The article would read much the same without reference to Warmism. And it is refreshing that the model they use has had extensive validation. Most unusual! Warmist models normally have no predictive skill whatever.
In the end, however, they find that weather has reduced potential crop yields, not mainly via warming but mainly by reduced rainfall: "lower rainfall accounted for 83% of the decline in yield potential, while temperature rise alone was responsible for 17% of the decline"
And that is a problem. Warmer seas should usually produce MORE rainfall. How come the alleged warming was accompanied by LESS rainfall? The authors do not know, or, if they do, they are not saying. So the statistics are in fact incompatible with anthropogenic global warming. A warmer globe should have produced more rainfall. But there was LESS rainfall.
All one can reasonably say in the circumstances is that there were poorly-understood local factors at work, not global ones. From any point of view, what they have to account for is the reduced rainfall and they have not done that
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.
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.
The distribution of the annual change in wheat yield potential from 1990 to 2015. Each dot represents one of the 50 weather stations used in the study. David Gobbett, Zvi Hochman and Heidi Horan, Author provided
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.