Tuesday, March 15, 2011

Climate Change and Agriculture: Predicting Food Yield Changes by Degree of Temperature Increase

From a new report issued by the National Research Council predicting effects of climate change according to the number of degrees of temperature rise, I have excerpted the parts of the report which relate to agriculture.

Note that normally I try to avoid making predictions or featuring predictive scenarios on this blog as many issues are too complex to do it with certainty and I include climate change in that category. It seems that more can be gained by carefully monitoring what IS happening to detect trends, and so far, we are continuing to turn out near record-level crops here in the U.S. and elsewhere. Nevertheless, this subject is so important to agriculture, that I feel it is my duty to present what the experts are predicting.

That said, I see the following key points from the report not making sense, which serves to illustrate my point to avoid making predictions so as not to discredit a subject.
  • Yields of corn in the United States and Africa, and wheat in India, drop by 5-15% per degree of warming
  • If 5°C (9°F ) of global warming were to be reached, most regions of the world experience yield losses, and global grain prices potentially double.
So let's take these two conclusions from the report and say corn yields in the U.S. and Africa, and wheat yields in India all drop by 50% at a point when the temps have increased five degrees Celsius, then these commodities are to the "potentially price doubling" point? So corn and wheat will only be $14/bushel after their production is cut in half and global warming has occurred to the tune of five degrees Celsius? When I see that logic, I wonder about the rest of the logic presented in the report. If temps did rise that much, the state of global economics and all else would be largely changed under that scenario, that is for certain.

Overall, the agricultural yield predictions made in this report are quite dire, especially if we face a more rapid rate of temperature increases, as some of the climate scientists are currently thinking.
K. McDonald

Above Figure: Yields of corn in the United States and Africa, and wheat in India, are projected to drop by 5-15% per degree of global warming. This figure shows projected changes in yield as a function of average global temperature increase for those crops as well as for U.S. soybeans and Asian rice. The expected impacts on crop yield are from both warming and carbon dioxide increases, assuming no crop adaptation. Solid lines show best estimates, and shaded regions show ranges of projections. Values of global temperature change are relative to the preindustrial value; current global temperatures are roughly 0.7°C (1.3°F) above that value.

What does it mean? The U.S. Corn Belt is projected to experience a loss of 11% corn production per degree of warming, while at the same time global cereal demand is expected to rise by roughly 1.2% per year. That means that farmers would have to work every year to overcome shortfalls that would, under today’s conditions, take 9 years to materialize.


All else aside, increasing the amount of CO2 in the atmosphere favors the growth of many plants. Leaf pores, or stomata, shrink in response to the added CO2, which helps plants to conserve water. The enhanced CO2 also stimulates photosynthesis in plants with a C3 photosynthetic pathway, such as rice and wheat, although not in plants with a C4 pathway, such as maize (i.e., corn). C3 yields could increase by an average of more than 10% if CO2 were doubled over preindustrial values, all else being equal.

Enhanced plant growth from elevated CO2 does not necessarily translate into more food, however, because climate changes caused by CO2 can reduce yields in many regions. Crops tend to grow more quickly in warmer temperatures, leaving less time to produce grains. In addition, a changing climate will bring other hazards, including greater water stress and the risk of higher temperature peaks that can quickly damage crops. Modeling indicates that the CO2-related benefits for some crops will largely be outweighed by negative factors if global temperature rises more than 1.0°C from late 20th-century values.

The true risks may be even greater, as some potentially negative changes—including the likelihood of longer, more widespread drought and the potential for weeds, insects, and crop pathogens to spread—are not yet incorporated in most crop models. Growers in prosperous areas may be able to partially or completely adapt to these threats, for example, by varying the crops they grow and the times at which they are grown. Adaptation may be less effective where local warming exceeds 2°C, however. Its use will also be limited in the tropics, where the growing season is restricted by moisture rather than temperature.


• Individual land species experience shifts in timing of flowers and breeding cycles, in geographic ranges, and in population
• Zones of depleted oxygen (dead zones) become larger and longer-lasting
• 5–15% reduced yield of US corn, African corn, and Indian wheat per degree


• 5–10% less rainfall per degree in Mediterranean, SW North America, southern Africa dry seasons
• 5–10% more rainfall per degree in Alaska and other high latitude NH areas
• 3–10% more heavy rain per degree in most land areas

• 5–10% less streamflow per degree in some river basins, including the Arkansas and Rio Grande

FOR 5°C Warming
• Yield losses in most regions and potential doubling of global grain prices


Food and human health
• Yields of corn in the United States and Africa, and wheat in India, drop by 5-15% per degree of warming
• If 5°C (9°F ) of global warming were to be reached, most regions of the world experience yield losses, and global grain prices potentially double
• Crop pests, weeds, and disease shift in geographic range and frequency
• Risk of heat-related illness and death increases
• The timing and geographic range of allergens and vectorborne diseases shifts

Some impacts that can be quantified per degree of warming include:
• 5-15% reductions in the yields of crops as currently grown
• 3-10% increases in the amount of rain falling during the heaviest precipitation events
• 5-10% decreases in streamflow in some river basins, including the Arkansas and the Rio Grande
• 200-400% increases in the area burned by wildfire in parts of the western United States


Source: Warmıng world - impacts by degree. Based on the National Research Council report, Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia (2011) [PDF]


  1. Hi Kay,
    You may want to take a look at this paper, which I found very interesting and disturbing. http://www.pnas.org/content/106/37/15594.abstract

    The reason it stands out to me is because it uses actual county level data on yields and temperatures to develop a new crop model. In other words, there have been "natural experiments" for certain places in certain years over the vast midwest that give us a window into what would happen if the average temperatures were so high.

    This suggests that the response to temperature change is very non-linear. Instead of saying that each 1 C of rise leads to a 5-15% decline in yields, the effect is much more pronounced towards the higher range of temperature increase.

    The response of plants to changes in temperature is very hard to appreciate because of so many compounding factors, but those who appreciate plant physiology can readily come up with good reasons why high temperatures would show this non-linear response. Pollination issues are a real concern, such as the pollen tube in corn, as well as night time respiration, and photo oxidation.

    Thanks for highlighting this review piece, and I agree that the price model in that report looks odd and under responsive. Could be the authors are downplaying the risk, which is not uncommon in these sorts of reports.

  2. Jason,

    Thanks for your valuable insight and link. I will review it and add it to my growing list of references under the "climate change" tab on this blog.

    The abstract from your link:
    The United States produces 41% of the world's corn and 38% of the world's soybeans. These crops comprise two of the four largest sources of caloric energy produced and are thus critical for world food supply. We pair a panel of county-level yields for these two crops, plus cotton (a warmer-weather crop), with a new fine-scale weather dataset that incorporates the whole distribution of temperatures within each day and across all days in the growing season. We find that yields increase with temperature up to 29° C for corn, 30° C for soybeans, and 32° C for cotton but that temperatures above these thresholds are very harmful. The slope of the decline above the optimum is significantly steeper than the incline below it. The same nonlinear and asymmetric relationship is found when we isolate either time-series or cross-sectional variations in temperatures and yields. This suggests limited historical adaptation of seed varieties or management practices to warmer temperatures because the cross-section includes farmers' adaptations to warmer climates and the time-series does not. Holding current growing regions fixed, area-weighted average yields are predicted to decrease by 30–46% before the end of the century under the slowest (B1) warming scenario and decrease by 63–82% under the most rapid warming scenario (A1FI) under the Hadley III model.

  3. You really made a mistake by not allowing CCP to repost.

  4. ianash
    I spent a great deal of time trying to politely educate Tenney, author of Cliimate Change Psychology about the ethics of using other people's work in full without their permission. Both of us work without pay, basically, so that's not an issue. But, we do all rely upon the google search engines if we are to be successful as a blogger. If there were no ethics whatsoever honored, so that any blogger took any post from another whenever, the system would crash. I give posts in full to a few sites, because we have an agreement, they don't take without asking permission, etc.

    If you are an "aggregator" then you need to: 1) ask permission before taking 2) know which sources you can use in full (most govt sources, for example). Beyond that, you just put up daily links, or at most the first paragraph or two with a "read more" linking to the author's page. For good ethical and classy examples from those who do this well and are highly successful trusted sources of providing links, and I did send a couple examples to Tenney, too, are: 1) http://economistsview.typepad.com/economistsview/2011/03/links-for-2011-03-25.html and 2) http://ricefarmer.blogspot.com/.

    I hope you see the difference. I think the ideas I presented to Tenney were new to her, and I think they will take some time for her to process, but we had a balanced and enjoyable email exchange and I know her motives are good, so I have hope that over time she will do the right thing.