Ground-level ozone is an invisible air pollutant that causes significant damage to crops worldwide. Tackling methane emissions, one of the precursors, could be the key to curbing rising background levels of ozone.
Methane is primarily known as a potent greenhouse gas that drives global warming. However, its role in the formation of ground-level ozone is less widely recognized. Ground-level ozone (O₃) forms through a chemical reaction where methane (CH₄), carbon monoxide (CO), and volatile organic compounds (VOCs) react with nitrogen oxides (NOx) under the influence of sunlight’s UV radiation. This reactive gas is not only harmful to human health but also causes extensive damage to plants and ecosystems.
In this article, I delve into the widespread damage ozone inflicts on agricultural crops —and it’s no small matter. According to the World Meteorological Organization (WMO), global crop losses due to ozone damage between 2010 and 2012 were estimated at around 12.4% for soybeans, 7.1% for wheat, 4.4% for rice, and 6.1% for maize. Amounting to a total 227 million tonnes in losses for these four staple crops, this has a substantial impact both on local economies and global food security. [1]
The problem is global. The extent of its impact is a combination of climatic conditions, regional pollution levels, and type of agricultural production. WMO has identified several particularly vulnerable regions. In the US, multiple states along the West Coast, as well as central and northeastern regions, are heavily affected, along with southern Canada. Eastern China, Korea, Japan, and parts of India are also hotspots. In Europe, the Mediterranean area is the most affected, though ozone damage also significantly impacts other regions, less so for Ireland, northern Scandinavia, and the Baltics. In a 2019 study by EIONET that assessed wheat crop losses [2], nearly half of the EU member states experienced losses exceeding five percent. Greece was the hardest hit, with 9% of its wheat yield lost due to ozone damage. Quantitatively, the greatest losses occurred in large-producing countries such as France, Germany, and Poland. And this wasn’t an extreme case – 2019 was a year with relatively low ozone levels, according to the report authors.
Plant sensitivity to ozone has been known since the 1950s [3]. Researchers in California observed plant damage linked to local smog containing ozone. Plants absorb ozone through their stomata while simultaneously taking in carbon dioxide for photosynthesis. Once inside plant cells, ozone can cause significant damage, especially when the plant’s natural antioxidants, which act as a first line of defence, are depleted. This leads to impaired photosynthesis, reduced growth, and often smaller seeds with lower nutritional content. In legumes, nitrogen fixation ability also declines, which may explain why soybeans are the most sensitive to ozone among major crops. Ozone thresholds are typically expressed in AOT40, but PODY is another indicator that is becoming more widely used (see box).
In recent decades, ground-level ozone levels have shown varied trends [1]. Thanks to reduced emissions of nitrogen oxides and volatile organic compounds, the most extreme ozone peaks have levelled off. At the same time, background ozone levels have increased globally, largely due to rising methane concentrations in the atmosphere and climate change. Methane, with an average atmospheric lifespan of about 12 years, is considered a short-lived greenhouse gas. However, compared to other ozone-forming substances that have lifespans of only days or weeks, methane persists much longer, giving it a global impact on ozone levels, unlike NOx and other precursors, which mainly have local-regional effects.[4]
The environmental costs of methane emissions have typically been calculated based on their impact on climate and public health, a concept known as the “social cost of methane”. A 2023 study shows that if ozone damage to crops is also factored in, the marginal cost of methane emissions could increase by an additional 15–30%, or 423–556 USD per ton [4].
Ground-level ozone is somewhat of an under-the-radar issue when it comes to air pollution, and its impact on crops is mostly recognised by a few researchers and experts in government agencies. Most farmers do not associate lower crop yields with the colourless gas formed on sunny days. Brown spots on leaves are often mistaken for drought stress or pest damage.
Likewise, farmers are unaware that improved air quality reduces such damage. Let’s return to California, where the problem was first identified. Between 1980 and 2015, air quality improved significantly there, and according to one study, this cleaner air led to an annual increase in fruit and nut yields worth 600 million USD [5]. In a local newspaper [6], one of the researchers behind the study, Steven Davis, was interviewed: “A lot of California farmers may not appreciate that air quality standards have had such a benefit on their ability to grow crops,” he said, continuing, “The irony is that by fighting against certain environmental regulations, these folks may be damaging their own earning capacity”.
[1] World Meteorological Organization (2023). The Impacts of Tropospheric Ozone Pollution on Crop Yield: Mechanisms, Quantification and Options for Mitigation https://library.wmo.int/idurl/4/68654
[2] ETC/ATNI Report 17/2021: Wheat yield loss in 2019 in Europe due to ozone exposure. https://www.eionet.europa.eu/etcs/etc-atni/products/etc-atni-report-17-2...
[3] Agathokleous, Evgenios & Saitanis, Costas. (2023). Effects of Ozone on Forests. 10.1007/978-981-15-2760-9_24.
[4] Sampedro, Jon & Waldhoff, Stephanie & Sarofim, Marcus & Van Dingenen, Rita. (2023). Marginal Damage of Methane Emissions: Ozone Impacts on Agriculture. Environmental and Resource Economics. 84. 1-32. 10.1007/s10640-022-00750-6.
[5] Hong, Chaopeng & Mueller, Nathaniel & Burney, Jennifer & Zhang, Yang & AghaKouchak, Amir & Moore, Frances & Qin, Yue & Tong, Dan & Davis, Steven. (2020). Impacts of ozone and climate change on yields of perennial crops in California. Nature Food. 1. 10.1038/s43016-020-0043-8.
[6] UC San Diego Today, March 19, 2020, California’s Strict Air Quality Regulations Help Farmers Prosper. https://today.ucsd.edu/story/californias-strict-air-quality-regulations-...
Methods to assess crop loss from ozoneThere are several methods for assessing the risk of ozone damage to crops, with the most widely used being AOTX (Accumulated Ozone Exposure over a Threshold of X ppb). This method calculates ozone exposure during daytime hours over the course of a growing season. In Europe, the growing season for crops is typically defined as May through July. The most common threshold, referred to as AOT40, is set at 40 ppb. Under the EU's air quality directive, there are two specific targets for ozone exposure on agricultural land:
In 2022, 10 member states met the target for all agricultural land, and only four member states (Finland, Ireland, Latvia, and Lithuania) met the long-term goal. [7] A more biologically accurate method for assessing risks is to measure how much ozone plants actually absorb. This measure is called Phytotoxic Ozone Dose (PODY), and it takes into account how a plant’s ozone uptake varies depending on the size of its stomata, weather conditions, and other environmental factors. Different threshold values (Y) are used for different plant types and vegetation; for example, Y=6 is used for wheat. [8] ICP Vegetation [9], which developed the POD method, has also established specific critical threshold values for different species and vegetation types. The critical level for wheat (where yield is reduced by 5%) is set at an ozone uptake of 1.3 mmol/m² during a growing season, while the corresponding value for potatoes is 3.6 mmol/m². The EU has not formulated any targets based on PODY. However, member states are required to assess and report exceedances of critical PODY levels for crops like wheat, tomatoes, and potatoes. [10, 11] In summary: The advantage of the AOTX method is that it is simple and flexible since all you need are hourly ozone measurements. To calculate PODY, meteorological data is also required, but it provides more accurate estimates of the risk of ozone damage. [7] European Environment Agency, 24 June 2024, Exposure of Europe’s ecosystems to ozone https://www.eea.europa.eu/en/analysis/indicators/exposure-of-europes-eco... |
