Anthropogenic climate change

The term anthropogenic climate change refers to the impact of humans on the global climate. Since the Industrial Revolution, two types of human activity have contributed significantly to changes in Earth’s climate: modifications of the planet’s surface and the invention and deployment of new energy technologies. Scientists have identified global warming due to human-induced greenhouse gas emissions, in particular, as a key driver of climate change and a serious threat to people and biodiversity.

Background

Large-scale human impact on the Earth extends back many centuries. However, at the beginning of the Industrial Revolution, in the mid-eighteenth century, the pace of human effects on the natural environment increased. The invention of power sources such as the steam engine, the internal combustion engine, and systems for delivering electric power sped anthropogenic environmental alterations in three broad and interrelated ways. First, these technologies made it easier to modify the landscape. Second, they created a vast demand for energy to fuel them. Finally, greenhouse gas emissions from energy sources such as burning wood and fossil fuels altered the composition of the atmosphere.

How Human Changes to Earth’s Surface Affect Climate

When humans modify the landscape, or surface, of Earth, they can trigger climate change in a number of ways. For example, by replacing forest with cleared land for use as farmland or pasture, or simply by felling timber for wood or fuel, they can increase the albedo of Earth’s surface: More sunlight is reflected back out to space and less is absorbed and retained by the Earth. Old forests, especially conifer forests, are generally dark, whereas younger growth or cleared land is much lighter. Overall, however, human changes to the planet's albedo are thought to have a relatively minor effect on global cooling or warming trends.

Many human changes to the landscape can affect climate variability at the local scale. One of the most familiar examples is the urban-heat-island effect. Cities impede the flow of air, trapping heat, and they also contain large areas of materials such as asphalt and concrete that absorb heat. As a result, cities tend to be significantly warmer than nearby countryside. Other effects associated with urban heat islands include increased rainfall and stalling of intense rainstorms.

A more extreme example of climatic effects associated with human changes to Earth’s surface can be seen in the drying of the Aral Sea in Central Asia. Beginning in the 1960s, diversion of water for agriculture steadily reduced the area of the Aral Sea (actually a vast lake); it was almost completely dried up by the 2010s. The original size of the Aral Sea was about 68,000 square kilometers, making it the fourth-largest lake in the world—large enough to have significant moderating effects on the regional climate. These effects have almost entirely disappeared with the drying of the lake, and the regional climate has become more continental, with hotter summers, colder winters, and much less rainfall. Similarly, increased drought has also been linked to some other instances of large-scale landscape changes due to human diversion of natural water features, such as the vast Three Gorges Dam hydroelectric project in China.

Indirect Human Modifications of the Atmosphere

Human changes to the landscape also affect the atmosphere. Human activities may create dust and smoke, for example, which can reflect sunlight back into space or block sunlight from reaching Earth’s surface. Such activities thus increase Earth’s albedo and cool the planet, but they can also prevent radiation from being reflected back into space from the surface and thus trap heat, warming the planet. For example, studies of condensation trails, or contrails, of aircraft have shown that they reflect sunlight back to space but also prevent heat from the surface from escaping, so that their overall effect is to warm the Earth through the greenhouse effect. Tiny particles, or aerosols, can also serve as nuclei for the condensation of water droplets and affect fog, cloud cover, or precipitation.

The degree to which human-induced dust and smoke alter visibility is remarkable. In preindustrial times, it was normal in most places for visibility to exceed 100 kilometers. Persistent haze unrelated to local weather was so unusual in preindustrial times that it was recorded by chroniclers and has been used by geologists to pinpoint the dates of large volcanic eruptions in remote areas of the world. By contrast, visibility in heavily populated contemporary industrial regions is often only a few kilometers, and even many remote areas are threatened with diminished air quality. Some studies have suggested that global dimming—the reduction of sunlight reaching the surface of the Earth by dust, haze, and smoke—may have even partially masked the effects of global warming.

Human changes to the landscape often release greenhouse gases (GHGs). These changes generally result in the destruction of biomass, either by burning or by decay, thereby adding carbon dioxide (CO₂), one of the six major GHGs, to the atmosphere. Drainage of wetlands for agriculture can also result in the decomposition of organic material and also adds CO₂ to the atmosphere.

Human activities release other GHGs, particularly methane. Modification of the land can release methane trapped in the soil. Agriculture increases the amount of methane in the atmosphere in several ways. Livestock produce large amounts of methane in their digestive tracts, so increased numbers of cattle lead to increased methane emissions. Clearing of forest lands for agriculture reduces the ability of soils to absorb and oxidize methane. Certain types of agriculture, notably rice production, create oxygen-poor conditions for the decay of organic materials and thus emit methane. Finally, burial of waste rather than incineration results in methane emission.

According to a controversial theory by William Ruddiman, if it were not for human activities, the Earth would already have passed the peak of the present interglacial period and would be on the way to the start of the next glacial advance. Ruddiman argues that, while clearing forests for agriculture increased the CO₂ content of the atmosphere, increased methane production—especially that due to rice cultivation—is the more important climatic change agent. His "early anthropocene" hypothesis suggests that anthropegenic climate change began thousands of years ago with the rise of large-scale agriculture, rather than during the Industrial Revolution.

Direct Human Changes to the Atmosphere

Beginning with the Industrial Revolution, human activities began modifying the composition of the atmosphere directly on a large scale. The burning of fuels—first wood and then fossil fuels—released increasing amounts of smoke and gases directly into the atmosphere. Among the most important emissions were CO₂, nitrogen oxides, sulfur dioxide, and ozone-depleting chemicals.

CO₂ is considered the most important anthropogenic GHG contributing to the enhanced greenhouse effect. The latter effect supplements Earth’s already significant natural greenhouse effect, in the context of which water vapor is the single most important GHG in Earth’s atmosphere.

Nitrogen oxides are the result of high-temperature combustion during which atmospheric nitrogen and oxygen combine. Ordinary fires are not hot enough to cause reactions between nitrogen and oxygen, but at temperatures above 1,600° Celsius the two gases can react. Lightning is a natural source of nitrogen oxides, but human activities also create large amounts, especially in internal combustion engines. Nitrogen oxides react with hydrocarbons to cause smog as well as ozone in the lower atmosphere. Ozone high in the atmosphere protects Earth’s surface from ultraviolet light, but at ground level ozone is a pollutant that contributes to respiratory problems. Finally, nitrogen oxides combine with water vapor to form nitric acid and contribute to acid precipitation.

Sulfur dioxide is emitted naturally by volcanoes but also is produced by smelting of sulfide ores or burning fossil fuels that contain sulfur. Sulfur dioxide is a GHG, but its most important environmental effect is that it combines with water vapor to create sulfuric acid. Tiny droplets or aerosols of sulfuric acid can aggravate respiratory problems, contribute to atmospheric haze, and make rain and snow more acidic, contributing to acid precipitation.

Ozone-depleting chemicals include a large number of synthetic chemicals, mostly organic chemicals containing chlorine or bromine. Both of these elements are highly effective at destroying ozone, and ozone-depleting chemicals are extremely stable, enabling them to survive long enough to reach high altitudes. International controls on ozone-depleting chemicals, such as the Montreal Protocol and the Kyoto Protocol, have slowed the depletion of stratospheric ozone, but the existing chemicals in the atmosphere will continue to have an effect for a long time. Although ozone-depleting chemicals also act as GHGs, in most respects global warming and ozone depletion are separate problems.

Key Concepts

• aerosols: tiny particles suspended in Earth’s atmosphere
• albedo: the fraction of incident light reflected from a body such as Earth
• anthropogenic: deriving from human sources or activities
• fossil fuels: energy sources such as coal, oil, and natural gas that were formed by the chemical alteration of plant and animal matter under geologic pressure over long periods of time
• global dimming: a reduction in the amount of sunlight reaching the surface of the Earth
• greenhouse gases (GHGs): atmospheric gases that trap heat within a planetary system rather than allowing it to escape into space
• urban heat island: a spot on Earth’s surface that is significantly warmer than the surrounding area as a result of human alterations to the landscape

Bibliography

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Ruddiman, William F. Plows, Plagues, and Petroleum: How Humans Took Control of Climate. Princeton University Press, 2005.

Sagan, Carl, Owen B. Toon, and James B. Pollack.“Anthropogenic Albedo Changes and the Earth’s Climate.” Science 206, no. 4425 (December 21, 1979): 1363-1368.

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