Will Climate Change Devastate U.S. Corn Yields?

by Marlo Lewis on April 3, 2014

in Blog

In which region of the world are plants most productive in photosynthesizing water and carbon dioxide into carbohydrates? If you guessed the tropical rain forest, you’d be wrong. The region with the highest gross primary production (GPP) from photosynthesis is the U.S. corn belt.

That is the finding of a new study (Guanter et al. 2014) published in Proceedings of the National Academy of Sciences (PNAS). The team of 20 researchers used satellite-based spectroscopy to monitor sun-induced chlorophyll fluorescence (SIF), an electromagnetic signal emitted as a byproduct of photosynthesis.

marlo post

Global map of maximum monthly sun-induced chlorophyll fluorescence (SIF) per 0.5° grid box for 2009.

The results of the study really shouldn’t be surprising. The U.S. leads the world in combined private-public R&D spending on agriculture and is the world’s top corn producer and agricultural exporter. Nonetheless — and this too is not surprising — the corn belt GPP reflected in satellite SIF data substantially exceeds the GPP estimated in carbon cycle models. The researchers report:

Our SIF-based crop GPP estimates are 50–75% higher than results from state-of-the-art carbon cycle models over, for example, the US Corn Belt and the Indo-Gangetic Plain, implying that current models severely underestimate the role of management.

Perhaps to appease the political-correctness guardians at PNAS, the study begins with a warning that “past advances” in agriculture are “threatened by climate change,” and the authors say their research is significant because it provides benchmark data for “more reliable projections of climate impact on crop yields.”

Clearly, though, the finding is also significant for another reason. It doesn’t fit into the fear narrative promoted by the recently-released IPCC Working Group II (WG2) report on climate impacts. Current models “severely underestimate the role of management.” That suggests current models underestimate farmers’ ability to adapt to climate change.

According to the WG2 report, 10% of climate model runs project that wheat, rice, and corn yields will decline by more than 25% during 2030-2049 compared to the late-20th century if global temperatures rise by 2ºC or more (p. 17). A scary prospect considering there may be an additional 2 billion mouths to feed by mid-century. In its presser for the report, WG2 calls climate change a threat to global food security.

On the other hand, the WG2 report acknowledges that 10% of model runs project crop yields to increase by more than 10% during 2030-2049. So there’s no ‘consensus’ among models at this point. Besides, with global warming on hold for the past 17 and half years, only a reckless gambler would wager that global temperatures will rise 2ºC or more by mid-century.

Climate blogger Anthony Watts recently posted charts showing that global crop yields continue trending upward. Here’s one constructed by climatologist Roy Spencer using USDA data:


Source: Dr. Roy Spencer plotting USDA data

Much current food security angst centers on corn yields. WG2 lead author Chris Field, for example, claims corn yields decline when summer temperatures exceed 84ºF. As previously noted on this blog, in most places where U.S. corn is grown, July daily maximum temperatures often reach 90˚F. Yet U.S. corn yields have increased six-fold since the 1930s.

Corn Yields U.S.

Source: Dr. Mark J. Perry plotting USDA data

Just as IPCC climate models project more global warming than has actually occurred, they also over-predict regional warming in the corn belt. Indeed, notes Spencer, there has been no net warming in the corn belt for decades.


Source: Dr. Roy Spencer plotting CMIP5 model data and trend (red) against US Historical Climate Network data (blue)

Based on conversations with agricultural experts and other information, Spencer reasonably concludes that technology — what Guanter et al. call the “role of management” — currently trumps climate to the point where it is impossible to discern a greenhouse signal in U.S. corn yield trends. He writes:

The IPCC claims there is a negative impact of global warming on corn, but the experts I have talked to say there is no way to get that out of the data. You would have to have accurate quantitative knowledge of the technological trend, which you don’t.

In other words, without an accurate removal of the factors leading to the huge increase in corn yield (which is not possible), you can’t back out of the data any kind of climate-related signal. (If anything, the face-value evidence is that warming leads to higher yields, not lower.)

And without that accurate quantitative knowledge (and no evidence of observed corn belt climate change anyway), they tell me there is little reason to depart from a forecast of slowly increasing corn yields in the coming years.


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