Biogeosciences Research Spotlight

Which Greenhouse Gas Does the Most Damage to Crops?

Models showed that approximately 93% of crop losses over the rest of the century could be caused by non–carbon dioxide emissions, the most damaging of these being methane.

Source: Earth's Future


Scientists and policy makers alike have recognized that agricultural yields are at risk from climate change and human-caused emissions. Carbon dioxide is the largest driver of climate change, but in a new study, Shindell found that other anthropogenic, or human-sourced, emissions cause more damage to crop yields.

Methane (mainly from natural gas production and livestock), halocarbons (used in refrigeration and air conditioning), and black carbon (from fossil and biomass fuels) all contribute to climate change to varying degrees. But until now, studies have not attempted to attribute agricultural impacts to individual pollutants; instead, many focused on how downstream processes, like increasing temperature, affect the globe.

Here the author sought to create a more complete picture of the effects of human emissions. Using climate data from his previous studies and the Intergovernmental Panel on Climate Change’s fourth and fifth assessment reports and crop yield data from previous work, the author created a model to determine the effect that individual greenhouse gases have on global temperature, precipitation, carbon dioxide, and ozone—all of which affect crop success.

The model revealed that in the short term—in the first decade after emissions are released—the greatest damage to crops per ton comes from black carbon and from gases used in refrigeration. Methane emissions are also very harmful to plants because the gas increases surface ozone that causes harmful chlorosis, or a yellowing of the leaves.

The effects of carbon dioxide are more complicated. Carbon dioxide fertilizes plants, which means as the amount of the gas in the atmosphere increases, crop yields initially increase as well. But as carbon emissions continue to contribute to climate warming, the overall impact becomes negative and will outweigh the benefits of fertilization after only 10 years.

Overall, the model suggests that approximately 93% of crop losses throughout the rest of this century will be caused by non–carbon dioxide emissions, the most damaging of these being methane. Even if the impact of surface ozone is taken out of the results, the non–carbon dioxide damage is still 9 times larger than that caused by carbon dioxide.

Finally, to see how policy and societal actions to mitigate pollutant emissions might affect crops, the author compared two future scenarios: one with low emissions and another with high emissions. If strong emission mitigation techniques are implemented, the results show that crop yields will improve by about 3% for reduced carbon dioxide, 5% for a reduction in hydrofluorocarbons, and 16% if methane is reduced. Under the high-emission scenario, crop yield losses will be about 25% greater by the end of the century, threatening global agriculture.

As the first study to look at the relative contributions of individual pollutants to climate change and crop yield losses, the findings here fill an important gap for policy makers, who can limit specific pollutant emissions. The author suggests that policy makers should strengthen efforts to reduce methane and hydrofluorocarbons in the atmosphere to help prevent severe crop loss by the end of the century. (Earth’s Future, doi:10.1002/2016EF000377, 2016)

—Alexandra Branscombe, Freelance Writer

Citation: Branscombe, A. (2016), Which greenhouse gas does the most damage to crops?, Eos, 97, Published on 15 August 2016.
© 2016. The authors. CC BY-NC-ND 3.0
  • davidlaing

    This article underscores a pernicious quality of modeling studies. In the modern arena of politicized scientific thinking, theoretical, especially elaborately mathematically theoretical, computerized modeling is generally regarded as sacrosanct and beyond questioning, regardless of whether such modeling is supported by evidence from the real world. The bottom line is that Earth and Nature are always right, and have always been so, and that scientists are all too often wrong in their conclusions and their thinking, but they allow the sophistication of their theoretical models to proxy for good science, that is, theory that is well-backed-up by solid evidence from the real world. We should be careful, here. Yes, we should make use of theory and computer models, but no, we should not do so in the absence of real data that support the assumptions that go into the models. Unfortunately, what constitutes real data is often misconstrued by the researchers themselves, and clever theory is allowed to proxy for hard evidence. What is needed here is a far greater emphasis on observed realities in the real world, and less confidence placed on the assumptions of theoreticians and model developers, Beyond the obvious fact that real observations cannot be argued with, there is the little-appreciated fact that most human beings have a “reality tunnel,” which colors their view of the real world, especially skewing it toward what the pundits agree is the most likely scenario. This is why I think its far more important to pay attention to Earth and to what’s happening on it than to what pours forth from overly-fertile human imaginations.

  • Ian Saint John

    I would not recommend taking ANY single ‘study’ as fact. It may be based on poor analysis. In this case, GW is about higher temperatures, and that doesn’t care if the GHG is methane OR CO2. Either one causes crop losses through:

    1: drought (increased evaporation leading to drier conditions overall)
    2: stomata shrink to restrict water loss, leading to less CO2 intake. This compensates somewhat for higher CO2 levels.
    3: Lower productivity of soil microbes (that prepare elements for roots) as soil dries out and water transport slows.
    4: crop failures due to climate changes in specific regions.
    5: crop risk/damage/failure due to starting up new agricultural areas or changing crops in a region.
    6: temperatures exceed tolerance for some crops in the major equatorial regions that have MOST of the agricutlural area.

    Now, yes, CO2 can have a ‘fertilizing effect’ in terms of producing more starch or woody material (mostly carbon, oxygen and hydrogen) but they tend to reduce the NUTRIENT content of food. As well, higher growth of the non-nutrient compoents of food crops tends to deplete the other soil factors so you get a ‘bumper crop’ for a few years, followed by normal or subnormal yields as, say, iron becomes the ‘limiting nutrient’.

    The biggest problem I have with this article is that it does not give ANY reasoning as to why methane would be ‘worse’ for agriculture than CO2. It references a ‘computer model’ but does not give any facts to support the model output. Nor is there any claim that the model has passed any testing such as modelling the past.

    • D Shindell

      While true that the temperature and precipitation changes don’t ‘care’ if the gas is methane or CO2, the composition does ‘care’. As the article says, “Methane emissions are also very harmful to plants because the gas increases surface ozone that causes harmful chlorosis, or a yellowing of
      the leaves.” CO2 emissions do not directly affect surface ozone, but instead increase the amount of CO2 in the atmosphere leading to plant fertilization. Hence though the climate impacts are similar, the composition changes have opposite effects, one augmenting climate-related damages whereas one offsets some of those damages. The scientific article is open access, and all the reasoning as to why methane’s impact is larger than CO2’s is given in further detail there.

      • Ian Saint John

        Can’t say that I have noticed a lot of ‘smog’ in most farm fields, either before or after AGW. In fact, I usually find air quality higher there. Mostly I find change in the temperature and precipitation. Certainly in terms of crop productivity.

        I accept that some ozone may be increased due to over-fertilisation (N2O and VOCs), but cannot see how the slight increase would have more of an effect than the effects of drought, heat stress, etc.

        It might be useful to put more effort into reducing excess nitrogen fertiliser application. Without the N2O emissions, ozone would probably not be produced so much.

        Yes, the article is available publicly and I applaud doing science on such factors. Not expecting that and most of the references are only available in abstract.

        My criticisms are mostly in relation to how the results were communicated above.