CHAMPAIGN, Ill. — Illinois corn and soybean yields have trended steadily upward for more than 60 years. The question is, will that trend continue as carbon dioxide levels in the atmosphere continue to rise along with global temperatures and changing rainfall patterns?
Emerson Nafziger, University of Illinois crop sciences professor emeritus, looked at climate change and its impact on Illinois crop yields at U of I Extension’s Crop Management Conference.
“We’ve been hearing about climate change for quite while. Some people have been very skeptical about it. Some people still are very skeptical about it, and we’re not going to spend a lot of time trying to convince people that the climate is changing. But the evidence is pretty clear that it is,” Nafziger said.
“As we look forward, I read tales about how our crop yields are going to take a nosedive. And basically if you believe some people, there’s a pretty good chance we’d be out of business producing the crops in Illinois that we produce if the climate changes as expected over the next 50 to 75 years. We’re not going to come to that conclusion.”
Over the last 40 years, Illinois corn yields have increased overall by 2.2 bushels per acre per year. Northern Illinois yields have increased 2.35 bushels per acre per year on average in 40 years, the central region has seen a 2.36 bushel per acre increase per year and the southern region increase has been 1.96 bushels per acre per year.
Soybean yields in Illinois overall have increased 0.58 bushels per acre per year. Northern Illinois increased by 0.54 bushels per acre per year, central Illinois was up per year by 0.61 bushels per acre and southern Illinois increased by 0.58 bushels per acre per year over 40 years.
“There’s a little bit less variability around the soybean trends lines compared to what we saw with corn — on a percentage basis, not too much different,” Nafziger noted.
As yields increased from 1980 to 2019, June through August rainfall in the Prairie State has increased about 2 inches in all regions.
Temperature trends in Illinois over the last 40 years were consistent among regions, but at different levels. The maximum daily summer temperature — June through August — from 1980 to 2019 has decreased about 0.27 degrees per decade on average across Illinois and in southern Illinois, decreased less than that in central Illinois and there was no decrease in northern Illinois.
“There’s not a very strong trend because there’s so much variability. The earth has certainly warmed some of the last 40 years, but here in Illinois it would show that at least the maximum daily temperature had not risen over that period of time,” Nafziger said.
The minimum daily growing season temperature has increased by about 0.25 degrees per decade from 1980 to 2019 in all Illinois regions.
“That’s actually not too far from the increase in the earth’s temperature over at least part of that period. It’s also highly variable, but that does show a different trend than maximum temperature,” Nafziger added.
Carbon Dioxide Emissions
“When I first learned about carbon dioxide and crops, which was about 1970, we were talking about 325 or so parts per million, and today it’s 415 parts per million. Just in the last 50 years, it’s gone up by that amount,” Nafziger said.
“This is one that there isn’t any debate about. We can argue about what affect that has on weather, but we certainly can’t argue that carbon dioxide has not increased — and some people are even willing to argue that now.”
Carbon dioxide and other gases in the air act as a “greenhouse” to trap heat from the sun in the earth’s atmosphere. The earth’s air temperature has been rising since about 1970 and is now about 1.8 degrees higher than the 1950 to 1980 baseline. This change is not distributed uniformly around the globe.
Changing air temperatures have affected rainfall, as well. This is not uniform as rainfall in the Corn Belt has shown no real trend. Rainfall distribution patterns have changed in some regions of the world, though, with larger storm events and more extended dry periods.
Carbon dioxide is not the only greenhouse gas.
Water vapor and clouds are present in huge quantities compared to other greenhouse gases, but are highly variable. Evaporation cools the air.
Methane from flooded land such as swamps and rice production fields, thawing tundra, ruminants and escapes of natural gas all contribute methane, and it’s a major source of greenhouse gas.
Nitrous oxide, which is mostly released by microbes from nitrogen applied as fertilizer — estimated at about 1% of the amount of fertilizer N applied — is also a greenhouse gas source.
Each greenhouse gas is assigned a rating based on how long it lasts in the air and how well it traps heat. The global warming potential of carbon dioxide has a 1 rating, and the 20-year global warming potential for methane, because it stays in the air longer than a year, is 84 and 264 is the rating for nitrous oxide.
“These are not released as bigger quantities as is carbon dioxide, but they have a lot more effect on the warming than carbon dioxide does. That’s why we fret and try to figure out how to reduce the amount of nitrous oxide that’s released from cornfields,” Nafziger said.
“Climate change is not uniformly distributed throughout the world. The U.S. Corn Belt has warmed less than many regions in the world. This is expected to continue. From 1990 to 2019, there have been large increases in the Arctic Circle, Africa, Europe and the Atlantic Ocean off of the U.S. eastern seaboard compared to what we have in the Corn Belt.”
Predicting the effect of warming on weather in a region is done by atmospheric scientists using high-powered models that use expected global warming potential and carbon dioxide changes to predict changes in weather.
Yield trend model predictions vary a lot, so they are averaged over a number of models to give a “best guess.” Nafziger used 32 models in the data.
These models make predictions under several scenarios, Representative Concentration Pathways, that differ on how much greenhouse gas will increase in the atmosphere over the coming decades.
One model projects a medium rate of change, with global peak temperatures late in the 21st century — only if the amount of carbon dioxide emitted decreases.
The second model projects a faster rise in temperature over a longer period that likely if the greenhouse gas emission rates remain high. These models were used to project temperature and yields from 2020 to 2075.
Illinois rainfall prediction models for 2020-2075 using Representative Concentration Pathways indicate very small changes in rainfall.
The maximum daily predicted temperatures for 2020-2075 model project a rise of 0.7 degrees per decade for a total increase of 3.9 degrees if the amount of carbon dioxide emitted decreases.
Should greenhouse gas emission rates remain high, temperatures from 2020 to 2075 are projected to increase by 1.4 degrees per decade to a total of 7.8 degrees warmer by 2075.
Minimum temperatures are showing a similar pattern with an increase of 0.6 degrees per decade if there is a reduction in greenhouse gas emitted and a 1.2 degree increase emissions continue to rise.
The average corn yield from 2015 to 2019 was 185 bushels per acre and soybeans came in at 54 bushels per acre.
One way to predict future U.S. yields is to take current trend lines and extend it out to 2075. This would indicate average corn and soybean yields of 307 and 86 bushels per acre, respectively, by 2075.
Using 32 weather variability models for predicting Illinois corn yields for 2020-2075 — the middle eight models were used for determining an average — the trend line is a 2.2 bushel per acre increase each year to 312 by 2075.
Under the assumption that the amount of carbon dioxide emitted decreases, the average corn yield is projected to increase by 1.63 bushels per acre per year through 2075. If greenhouse gas emissions continue to increase, the annual average yield increase would be 1.3 bushels per acre through 2075.
Using the same weather models for Illinois soybeans, using a trend line increase of 0.58 bushels per acre per year, yields would go from 56.2 in 2020 to 88 bushels per acre trend line in 2075.
With medium warming, yields are projected to increase by 0.48 bushels per acre per year to 79 bushels per acre by 2075.
Under the scenario where greenhouse gas emissions continue to rise, soybean yields would increase by 0.43 bushels per acre per year to an average of 76 bushels per acre by 2075.
“The bottom line is it makes sense that yields will continue upward for many years, even if the rate of increase begins to slow,” Nafziger said.
“You’re going to continue to see projections if you choose to look that show us yields really declining, and I just think over the next 50 years in Illinois that’s not a very realistic picture. Even if our climate changes, I don’t think yields are going to be less in 2090 than they are today.”