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Climate change has many 'recognizable negative effects' on crops | Fact check

Kate S. Petersen   Paste BN
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The claim: Climate change has only had 'positive effects' on global food production

An Oct. 20 Facebook post (direct link, archive link) includes a graph that shows global wheat, rice and coarse grain production has increased along with global temperatures and atmospheric CO2 levels since the 1960s.

"World grain production and amount harvested per acre show that crop and food production has steadily increased, with only positive effects from our changing climate," reads part of the post's caption. "If more CO2 and warmer weather were going to cause a decline in world-wide food production, should there not have been some recognizable negative effects by now?"

The post was shared more than 2,000 times in six weeks.

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Our rating: False

Studies show that climate change has had "recognizable negative effects" on global food production. While CO2 fertilization can increase crop yields in some species, increases in global grain production are mostly due to changes in agricultural technologies and an increase in cultivated land area, researchers told Paste BN.

Clear negative impacts of climate change on global food production

Greenhouse gases generated by human activity have been accumulating in the atmosphere for decades and have caused changes in Earth's climate, including warmer temperatures and shifting weather patterns. Humans have increased the amount of CO2 in the atmosphere by 50% since the 1700s, according to NASA.

Global grain production has also increased over the last 50 years, Toshichika Iizumi, a researcher at the National Institute for Agro-Environmental Sciences in Japan, told Paste BN.

But rather than being driven by climate change, the growth in total grain production is mainly due to the use of new high-yield crop varieties and synthetic fertilizers, the expansion of irrigation systems and lands under cultivation and other changes in technique, Ehsan Eyshi Rezaei, a scientist at Leibniz Centre for Agricultural Landscape Research in Germany, told Paste BN.

"The fact that total global production has not declined does not mean climate change has had no negative effects," he said. "The negative effects are measurable and well-documented, particularly at regional scales and among vulnerable populations, and they are projected to worsen as climate change continues."

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For several important crops, climate change has likely reduced global agricultural production growth compared to the amount of growth that would have occurred without climate change, according to multiple studies.

Other studies have reported negative regional effects. For instance, one study linked climate change to stagnating and decreasing crop yields in Eastern Europe since the 1980s.

Some researchers report mixed effects. For instance, an Our World in Data analysis reported that climate change likely slowed global production of soybean and maize, but may have also slightly increased wheat production as CO2 fertilization offset losses from warming.

Climate change can negatively impact agriculture by increasing the likelihood and severity of drought, heat waves and other damaging weather events. For example, a drought believed to be influenced by climate change has reduced the amount of planted land area in Chile.

Warming temperatures can also increase crop disease severity while simultaneously driving new pest and disease patterns. Inland saltwater intrusion from climate change-driven sea level rise has also caused farmland degradation and abandonment.

"The negative impacts of climate change on crop yield are masked by technological improvements," Iizumi said.

Impacts of CO2 fertilization varies by crop, can be negative

While it is not true that climate change has "only positive effects" on agriculture, CO2 fertilization can have a positive effect on crop yields, Rezaei said. However, the effect of CO2 enrichment varies by species and depends on the availability of other nutrients, such as nitrogen, according to Lamont-Doherty Earth Observatory.

For instance, most coarse grains, such as maize and sorghum, do not "directly profit from CO2 fertilization − unless under slight drought conditions," Christoph Müller, a global agriculture and land use researcher at the Potsdam Institute for Climate Impact Research in Germany, told Paste BN.

Rice and wheat do respond to CO2 fertilization, "but breeding progress, increased use of inputs − fertilizers, pesticides, water − drive the majority" of the increase in production, he said. He added that increases in rice and maize production are also attributable to an expansion of land used to cultivate those crops.

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Even if CO2 fertilization increases a crop yield, it can have other negative effects on agriculture, Lewis Ziska, an agriculture and plant biology researcher at Columbia University, told Columbia News. For instance, agricultural weeds also benefit from CO2 fertilization.

"In crop/weed competition, weeds are the winners, and herbicides used to control weed growth become less effective" when CO2 increases, he told the outlet.

Further, elevated growth rates due to CO2 fertilization can come at the cost of nutrition, Lamont-Doherty Earth Observatory reports. Studies of crop responses to elevated CO2 levels have reported decreases in protein, phosphorus, calcium, magnesium, zinc, iron and vitamins B1, B2, B5 and B9.

The Facebook user who shared the post did not provide evidence to support the claim.

AAP also debunked the claim.

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