13.2: Trends in greenhouse gases

Concentrations of greenhouse gases in the atmosphere affect Earth’s climate by determining the strength of the greenhouse effect. Briefly, some of the radiation from the sun is absorbed at Earth’s surface and radiated as infrared radiation, which we feel as heat. Some of this infrared radiation is lost to space but much of it is absorbed by greenhouse gases in the atmosphere and re-radiated in all directions, including back to Earth’s surface. This re-radiation of infrared radiation by greenhouse gases constitutes the greenhouse effect. Global average temperature is about 33°C warmer than it would be without this effect. As concentrations of greenhouse gases increase, the Earth system warms, affecting patterns of energy transfer through the atmosphere and oceans that define climate.

There are several greenhouse gases in the atmosphere. This chapter focuses on three of the most influential: carbon dioxide, methane, and nitrous oxide. Concentrations of all three of these gases follow an overall increasing trend as a result of human activities (Fig. \(13.3\)). Relative to atmospheric levels in 1750, near the beginning of the Industrial Revolution, concentrations measured in 2019 were higher by 47% for carbon dioxide, 156% for methane, and 23% for nitrous oxide (IPCC, 2021).

As these increases have occurred, the Earth has warmed. The United Nations Intergovernmental Panel on Climate Change (IPCC) recently concluded that the likely range of total human-caused warming is 0.8 to 1.3°C from 1850-1900 to 2010-2019 (IPCC, 2021). Most of this warming is from the increased concentration of carbon dioxide, which has by far the highest rate of emissions among greenhouse gases emitted from human activities. However, warming from the other gases is still significant because they are more potent as greenhouse gases than carbon dioxide. The 100-year warming potentials of methane and nitrous oxide are, respectively, about 27-30 times and 273 times that of carbon dioxide (US EPA, 2022).

Various human activities are causing these increases. The biggest source of carbon dioxide is fossil fuel use, followed by cement production and land use effects such as agricultural activity and deforestation (Friedlingstein et al., 2022; IPCC, 2021). Drivers of increased methane levels are not as well understood as those for carbon dioxide, but likely contributors include expansion of oil and gas production, gas distribution systems, and rising emissions from landfills, livestock herds, and wetlands, including those used for rice cultivation (Plant et al., 2022; Turner et al., 2019). Lastly, increases in nitrous oxide levels are thought to be driven primarily by increased applications of nitrogen fertilizer for food production (Park et al., 2012; Tian et al., 2020) though other sources include industrial emissions and combustion of fossil fuels.

Human impacts to greenhouse gases are coupled with natural biogeochemical cycles and the microbial activities within those cycles. For example, nitrous oxide production from fertilizer primarily occurs via nitrification and denitrification, which are catalyzed by microorganisms. Humans are effectively setting the table for growth in the populations of these microorganisms by adding nitrogen fertilizers to crop soils over broad regions. In other words, we are providing more nourishment for nitrous oxide-producing microbes, and as a result nitrous oxide levels in the atmosphere are increasing. More details about the roles of microorganisms are included below for each greenhouse gas.

Lastly, within the overall increasing trends, you may have noticed that concentrations of each greenhouse follow a repeating pattern of increasing and decreasing each year (Fig. \(13.4\)). These patterns reflect seasonal variation in the proportions of gas production and consumption. For carbon dioxide, global atmospheric levels decrease during spring and summer in the Northern Hemisphere, when carbon dioxide uptake by plants is high relative to carbon dioxide emissions from respiration and other sources. However, during cool months in the Northern Hemisphere, carbon dioxide uptake by plants decreases and the trend reverses. For methane, the cycle reflects seasonality in its destruction by photochemical reactions (Dlugokencky et al., 1994). For nitrous oxide, the seasonal cycle is relatively weak and reflects mixing of air in the atmosphere and seasonality in nitrous oxide production (Jiang et al., 2007).

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