Greenhouse gas emissions warm the atmosphere and can have a significant impact on weather patterns and the amount of rainfall globally. The 2015 Paris Agreement set the reduction of greenhouse emissions as a critical step in achieving the goal of limiting the increase in global average temperatures to 1.5 to 2° Celsius above pre-industrial levels.
An international team of scientists used 20 different climate models to examine how rainfall could be affected by climate change around the world. The team combined the models with greenhouse gas emission scenarios to predict the extent of the areas affected and how quickly the precipitation changes would become detectable or the “new normal” for crop growing regions. The timeline for this discernible change to rainfall is what scientists refer to as the “time of emergence”.
The study, published in the journal Proceedings of the National Academy of Sciences, warns that up to 14 per cent of land dedicated to wheat, maize, rice and soybean could have less rainfall, while up to 31 per cent may see increases in rainfall. However, in a low-emission scenario, the percentages are three per cent and one per cent respectively.
Study co-author Professor Andrew J. Challinor, from the Priestley International Centre for Climate at Leeds University in the UK, said: “Changes in rainfall patterns have been challenging to predict in the past, making it difficult to offer advice on how growing conditions may change. This is the first study to overlay predicted time of emergence on croplands and growing seasons.” Challinor notes that wheat, maize, rice and soybean represent roughly 40 per cent of global caloric intake, and he explains that the new findings show that limiting greenhouse gas emissions can contribute to preserving the rainfall patterns vital for their growth.
The study shows that while low-emissions scenarios still showed some effect on rainfall patterns in certain regions, the higher the amount of greenhouse gas emissions, the higher the percentage of land affected by drier conditions in key crop growing areas, such as South Western Australia and Southern Africa.
The scientists warn that the greenhouse gas mitigation measures needed to achieve climate targets, such as the one set by the Paris Agreement, will go a long way to helping reduce the risk of future droughts or flood conditions and possibly avoid a global food crisis.
Under a high emission scenario, France, Australia and Turkey – three of the world's top 15 wheat producers – would see 26 per cent, 28 per cent and 88 per cent respectively of their wheat growing land affected by reduced rainfall. In France and Turkey this is reduced to zero per cent under the lowest emission scenario, and in Australia, it drops to four per cent.
Argentina and, Brazil are the three biggest producers of soybean. With a high emission scenario, 70 per cent and nine per cent of Argentinian and Brazilian soybean area, respectively, would be affected by reductions in rainfall. In both cases this drops to zero under a low emissions scenario.
China and India are the world’s two biggest rice producers globally and are among the countries predicted to have wetter conditions for all four crops included in the study, even in a scenario with low levels of emissions. However, curtailing greenhouse gas emissions from high to low levels reduces the area affected from 11 per cent down to six per cent for China, and 80 per cent to 17 per cent for India.
The study warns that without significant reductions in greenhouse gas emissions, patterns of increased precipitation in high latitudes, including areas in North America and Europe, could emerge as early as the 2020s and in some areas may have already altered due to climate change.
Patterns of decreased precipitation in areas such as the Mediterranean, western Mexico, Chile, South Africa and Australia could emerge by the 2050s.
It has been suggested that more precipitation may mean higher crop production. However the authors warn that when coupled with rising sea levels, higher temperatures and increased potential for flooding, higher production is not assured.
Maisa Rojas, Fabrice Lambert, Julian Ramirez-Villegas, Andrew J. Challinor: “Emergence of robust precipitation changes across crop production areas in the 21st century”; Proceedings of the National Academy of Sciences (PNAS), March 2019.
Link to CIAT world map "Dramatic rainfall changes for key crops expected even with reduced greenhouse gas emissions"