FULL CLAIM: "This satellite image shows the photosynthesis of America's 100 million acre Corn crop. Further research found the US corn crop, at its peak, produces 40% more oxygen than the Amazon rainforest. Thank you farmers."
A series of Facebook posts (such as this one) claiming that the United States Corn Belt produces 40% more oxygen than the Amazon rainforest went viral in September 2021. This claim is based on a 2014 study in Proceedings of the National Academy of Sciences that received wide attention at the time (see here, here, or here).
The study attempted to understand how a new remote sensing technique could improve the knowledge we have of how croplands around the globe are growing and incorporating carbon from the air.
This technology, called Sun-induced Chlorophyll Fluorescence, measures the emission of a small amount of near-infrared light by the chlorophyll molecule when it receives sunlight. This infrared radiation is emitted in all directions and can be detected from space by a satellite, giving the scientists an overview of how much photosynthesis is occurring on the ground at a given place and a given time. With this technique, scientists can estimate gross primary productivity, which is the amount of carbon that plants take from the atmosphere through photosynthesis to grow and develop.
As the authors of the study wrote:
“Here we demonstrate that new space-based observations of chlorophyll fluorescence, an emission intrinsically linked to plant biochemistry, enable an accurate, global, and time-resolved measurement of crop photosynthesis, which is not possible from any other remote vegetation measurement. Our results show that chlorophyll fluorescence data can be used as a unique benchmark to improve our global models, thus providing more reliable projections of agricultural productivity and climate impact on crop yields.”
In addition to the main conclusion that this technique works well, the scientists noticed that the productivity of certain large cropland areas, such as the US Corn Belt or the Indo-Gangetic Plains in India, was underestimated in previous studies. In their study, the maximum value of sun-induced chlorophyll fluorescence worldwide is achieved in the US Corn Belt in July, at approximately 5 g CO2/m²/d. In contrast, South American croplands are estimated to produce a maximum of about 2.9 g CO2/m²/d in February (Southern hemisphere summer).
Climate Feedback reached out to the first author of the study, Luis Guanter, who refuted the claim:
“Our study had nothing to do with oxygen, but with plant gross primary production (GPP, amount of carbon absorbed by the plants through photosynthesis). During the June-July period the US Corn Belt shows in general higher rates of greenness (leaf chlorophyll content x plant biomass) and hence chlorophyll fluorescence (a proxy for photosynthesis) per unit surface than the Amazon forest, as it is shown in Figure 1. It is probably true that the US Corn Belt is fixing more carbon from the atmosphere in June-July than the entire Amazon forest for the same time period, since the US Corn Belt represents a large extension of high and dense plants growing at the same time.”
Figure 1 – Global map of maximum monthly sun-induced chlorophyll fluorescence for 2009. From Guanter et al. (2014)
The claim that the US Corn Belt is overall more productive than the Amazon is inaccurate and misleading as it wrongly implies that the study concluded that the US Crop Belt was more productive year-round than the entire Amazon and that this productivity would result in the largest amount of oxygen emissions.
The paper determines whether the new Chlorophyll Fluorescence technique could be useful for remotely measuring cropland gross primary productivity. Using this technique, the authors observed that the US Crop Belt exhibited the highest level of productivity worldwide in July 2009. Based on this result, they concluded that the US Corn Belt holds the world record for gross primary productivity, specifically during that month.
However, the paper did not study oxygen production, and there’s no evidence that these plants produce more oxygen throughout the entire year compared to other areas like the Amazon rainforest. For these reasons, the claim that the US Corn Belt is producing more oxygen than the Amazon is inaccurate and unsupported by the research it cites.
[Guanter is the first author of the scientific article on which the Facebook meme relied]
“Our study had nothing to do with oxygen, but with plant gross primary production (GPP, amount of carbon absorbed by the plants through photosynthesis). Based on our analysis, the area-integrated GPP of the crops in the US Corn Belt is huge during their growing season (June-July), as a huge amount of highly productive plants are growing quickly and at the same time. During this June-July time, the GPP peak for the US Corn Belt seems to be higher than that of the Amazon for the same time period. Of course, this is not true anymore if we averaged for the whole year (not only June-July), and in any case, the atmospheric carbon absorbed by the crops in summer is returned to the atmosphere later through respiration.”
“What we meant is that during that June-July period the US Corn Belt shows in general higher rates of greenness (leaf chlorophyll content x plant biomass) and hence chlorophyll fluorescence (a proxy for photosynthesis) per unit surface than the Amazon forest, as it is shown in this figure. It is probably true that the US Corn Belt is fixing more carbon from the atmosphere in June-July than the entire Amazon forest for the same time period since the US Corn Belt represents a large extension of high and dense plants growing at the same time. Apparently, the drop in atmospheric CO2 associated with this growth can also be seen in time series of CO2 data produced with satellites measuring concentrations of greenhouse gases. All this carbon is returned to the atmosphere after the summer.
But again, there is no mention of the Amazon region in our article.”
- Guanter et al. (2014). Global and time-resolved monitoring of crop photosynthesis with chlorophyll fluorescence. Proceedings of the National Academy of Sciences