In this second part of a three-part series, Angela Lovell looks at how a changing climate could change yields and growing conditions
Most plants respond positively to elevated carbon dioxide (CO2) concentrations and low levels of warming, but higher levels of warming often negatively affect growth and yields, says a 2009 report by the U.S. Global Change Research Program (GCRP).
As CO2 levels rise, the positive effect on plant growth is likely to be soon overtaken by the impact of other climate change factors such as temperature increases, altered precipitation patterns and extreme weather events.
“The benefits of CO2 at the global scale will eventually be outweighed by the harm from climate change induced by CO2 and other greenhouse gases,” say Lobell and Gourdji in the GCRP program. “There is considerable debate about exactly when net impacts will become negative. A likely scenario in the near-term is that warming will slow global yield growth by about 1-1/2 per cent per decade, while CO2 increases will raise yields by roughly the same amount.
Past mid century, it is likely that CO2 benefits will diminish and climate effects will be larger. It is plausible that the net effects of warming and CO2 could result in a yield decrease up to three per cent, or an increase up to two per cent per decade, depending on rates of temperature and CO2 change and the responsiveness of crop yields.
Warmer temperatures can benefit some crops in some areas. For example, growing ranges for corn and soybeans could expand.
But for other crops, such as small-grained cereals like wheat, faster plant growth under warmer conditions may mean reduced grain yields. The grain-filling period shortens as temperature rises, giving less time for plants to mature and produce grain. “Analysis of crop responses suggests that even moderate increases in temperature will decrease yields of corn, wheat, sorghum, bean, rice, cotton, and peanut crops,” says the GCRP report. Canola, oats and barley also have relatively low optimal temperature thresholds — it’s expected that yields of these crops would decline under hotter, dryer conditions.
And warming, scientists say, seems to be increasing the risk of plant damage from frost. “Mild winters and warm, early springs, which are beginning to occur more frequently as climate warms, induce premature plant development and blooming, resulting in exposure of vulnerable young plants and plant tissues to subsequent late-season frosts,” says the GCRP report.
Extreme precipitation events, such as drought or downpours, are predicted to become more common and both have negative impacts on yields through limiting plant growth
Some plants may become more water-use efficient as CO2 concentrations increase, which should also generally cause plants to grow larger. In some cases, however, this will also reduce nitrogen and protein levels, making some crops and forages in pastures and rangelands less nutritious for humans or animals and even pests. Pests will need to eat more of a crop to obtain the nutrition they need.
Most scientists agree that increased precipitation will accompany climate change, and that this will be especially true on the Canadian Prairies as the trend towards violent summer storms accompanied by heavy downpours increases.
Water is not just needed for plant growth, but also for plant cooling to prevent heat stress, which again can have a negative impact on yield and seed production. As water becomes scarce, more farmers will irrigate, until water becomes a serious limiting factor.
Weeds and Pests
Weeds benefit more than crops from higher temperatures and CO2 concentrations. Scientists are studying some general patterns of vegetation change, which are already beginning to occur. Their research seems to support what every farmer instinctively knows: weeds and invasive species are adapted to survive. They have traits, such as prolific seed production and faster growth rates, which allow them to respond better to change.
Herbicide and other pesticide use and costs are likely to go up as the climate warms and CO2 levels rise. The hunt will be on for new products, because research has also shown that glyphosate loses its efficiency against weeds that are grown at the CO2 levels predicted to occur under climate change. High temperatures are also known to reduce the effectiveness of pesticides such as pyrethroids.
Insects and diseases will thrive under warming conditions. Some of the pests and disease pathogens, which are now limited by their ability to overwinter in frigid Canadian soils, will be able to survive and proliferate quickly with milder winters and longer growing seasons. Hot, humid conditions will contribute to disease development and spread, forcing farmers to incur additional costs and risk environmental degradation through the use of more insecticides and fungicides.
For more information, “The influence of climate change on global productivity” by David B. Lobell and Sharon M. Gourdji of Stanford University is online at www.plantphysiol.org, just search “Lobell” to access a free pdf. †