Guest essay by Eric Worrall
A PNAS study claims that crop yields will fall by up to 7% for each degree celsius of global warming, assuming no CO2 fertilisation and no adaption measures.
Climate change will cut crop yields: study
August 15, 2017
Climate change will have a negative effect on key crops such as wheat, rice, and maize, according to a major scientific report out Tuesday that reviewed 70 prior studies on global warming and agriculture.
“Each degree Celsius increase in global mean temperature is estimated to reduce average global yields of wheat by six percent,” said the report.
Rice yields would be cut by 3.2 percent, and maize by 7.4 percent for each degree of Celsius warming (almost two degrees Fahrenheit), it added.
“Estimates of soybean yields did not change significantly.”
The abstract of the study;
Wheat, rice, maize, and soybean provide two-thirds of human caloric intake. Assessing the impact of global temperature increase on production of these crops is therefore critical to maintaining global food supply, but different studies have yielded different results. Here, we investigated the impacts of temperature on yields of the four crops by compiling extensive published results from four analytical methods: global grid-based and local point-based models, statistical regressions, and field-warming experiments. Results from the different methods consistently showed negative temperature impacts on crop yield at the global scale, generally underpinned by similar impacts at country and site scales. Without CO2 fertilization, effective adaptation, and genetic improvement, each degree-Celsius increase in global mean temperature would, on average, reduce global yields of wheat by 6.0%, rice by 3.2%, maize by 7.4%, and soybean by 3.1%. Results are highly heterogeneous across crops and geographical areas, with some positive impact estimates. Multimethod analyses improved the confidence in assessments of future climate impacts on global major crops and suggest crop- and region-specific adaptation strategies to ensure food security for an increasing world population.
Read more (paywalled): http://www.pnas.org/content/early/2017/08/10/1701762114
This is a good example of climate hype.
The authors of the study did the right thing, they explained their study ignored real world factors such as adaption, genetic improvement and CO2 fertilisation. There is a place in science for careful studies which seek to adjust just one factor, to study the impact of that adjustment. But their study has been spun into a narrative of failing crop yields.
In the real world, any deficit is more than compensated by the factors the study excluded.
CO2 fertilisation has a dramatic effect on plant growth. The slight rise in CO2 levels to date has measurably greened the world. Commercial greenhouses take this a lot further; they burn vast quantities of natural gas and discard the heat, just to produce enough CO2 for their plants to maximise growth – usually around 1000ppm, more than double current atmospheric levels.
Genetic improvement, production of species such as dwarf rice, can have a huge impact on yield. The world may be on the cusp of truly decoding the genetic blueprint, of an unprecedented level of understanding and control over crops and farm animals. There is plenty of scope for further advances.
As for adaption – down here on the edge of the tropics, we have a simple adaption we use to grow temperate vegetables which can’t tolerate our tropical Summers; We plant them in Autumn. The vegetables grow happily through our very mild winters, and fruit in Spring, before the Summer heat kills them.
I suspect a lot more global warming would be required to allow temperate Northern Hemisphere farmers to plant tomatoes unprotected outdoors in Autumn.
Edit (EW): The following image demonstrates the dramatic effect of CO2 fertilisation on plant growth.