Greenhouse effect

The greenhouse effect is the process through which gases in a planet's atmosphere trap heat, creating a warmer temperature on the planet's surface. The greenhouse effect is a natural phenomenon that has helped make Earth habitable. Without the greenhouse effect, Earth’s surface would be too cold to host most life forms. However, changes that accelerate this process have the potential to increase Earth’s temperature enough to cause significant climate change, which can negatively impact humans and other species.

The atmospheric gases that cause the greenhouse effect are known as greenhouse gases (GHGs), and include water vapor, carbon dioxide, methane, ozone, and others. GHG levels in the Earth's atmosphere have varied throughout the planet's history, and the phenomenon has influenced significant natural climate changes, including ice ages. However, humans have also greatly influenced the process by releasing GHGs into the atmosphere by burning fossil fuels and other actions. Human-induced global warming through the greenhouse effect is considered a key driver of climate change, and scientists have called for reducing GHG emissions from human activity to avoid severe ecological disruption.

History

By the early 1800s, scientists had discovered that Earth’s atmosphere affected its climate. However, they did not identify the actual phenomenon known as the greenhouse effect until later. Several scientists have been credited with discovering the greenhouse effect. Early considerations of the concept appeared in the 1820s, and experimental evidence began to emerge in the following decades.

In 1856, Eunice Newton Foote published a paper about her research into how sunlight heated different types of gases. She found that glass cylinders filled with moist air or "carbonic acid" (carbon dioxide) became warmer and retained heat longer than cylinders filled with dry air. Based on those findings, she theorized that atmospheric water vapor and carbon dioxide contributed to the Earth's surface temperature and that shifts in the levels of those gases could lead to climate change. However, Foote's pioneering work was largely forgotten until being rediscovered in the twenty-first century. Meanwhile, Irish scientist John Tyndall conducted similar experiments beginning in 1859, with his findings including more detail about the heating effects of solar radiation rather than simply sunlight. He also theorized that the climate in northern Europe had changed over time, possibly because of the greenhouse effect. Tyndall’s work gained more mainstream attention than Foote's and eventually formed the foundation of what would become climate science.

In the late 1800s, Swedish scientist Svante Arrhenius hypothesized that burning fossil fuels would eventually cause global temperatures to increase because of carbon dioxide in the atmosphere. Arrhenius even predicted how much global temperatures would rise if the carbon dioxide concentrations doubled. However, the larger scientific community did not notice and consider Arrhenius’s work for several decades.

In 1958, American scientist Charles David Keeling began measuring atmospheric carbon dioxide levels from a monitoring station in Hawaii. His work helped scientists understand that concentrations of GHGs increased over time. It also helped to verify Arrhenius’s hypothesis about human activity causing an increase in the amount of carbon in the atmosphere. Keeling went on to monitor GHG levels for the next fifty years, and other scientists also joined him in tracking atmospheric levels of carbon and other GHGs.

Overview

The greenhouse effect is a process that traps heat close to Earth’s surface. It involves radiation, or energy, traveling to and from Earth. Solar radiation provides the light, heat, and energy that most living organisms need to survive. As solar radiation travels toward Earth, it either reflects into space or travels through Earth’s atmosphere to its surface. About 30 percent of the sun’s energy reflects into space without ever reaching Earth’s surface, and so does not affect the planet’s temperature.

However, about 70 percent of the sun’s energy passes through Earth’s atmosphere and is absorbed by Earth’s surface. The surface then reemits the absorbed energy as heat. Without the atmosphere, all the reemitted energy would travel away from Earth’s surface and into space. While some of the absorbed energy is indeed released from the atmosphere back into space, certain gases in the atmosphere reflect some of the radiation back toward the Earth's surface. In effect, these gases trap the heat, preventing it from leaving the atmosphere entirely.

Earth’s atmosphere is comprised of layers of gases that stay in place around Earth because of gravity. The atmosphere includes many gases, but the three most common—nitrogen, oxygen, and argon—do not reflect thermal radiation. The gases that do trap heat energy are called greenhouse gases (GHGs) because they are responsible for the greenhouse effect. They include carbon dioxide, methane, nitrous oxide, ozone, various chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs), and others. Although sometimes considered separately, water vapor is also a major contributor to the greenhouse effect.

Earth’s surface temperature is determined by the energy entering and leaving the planet. Because the greenhouse effect causes energy in the form of heat to be trapped close to Earth’s surface, it plays an important role in determining Earth’s temperature. Without the greenhouse effect, Earth would be much colder and most likely inhospitable to life. Earth’s average temperature would be roughly –2 degrees Fahrenheit (–19 degrees Celsius) instead of the approximately 57 degrees Fahrenheit (14 degrees Celsius) observed throughout the twentieth century.

Greenhouse Effect and Climate Change

Although the greenhouse effect is vital for life as it exists on Earth, it is also a key factor in global warming and climate change. Certain human activities, especially burning of fossil fuels and deforestation, greatly increase greenhouse gas emissions, strengthening the greenhouse effect and therefore increasing the Earth's average surface temperature. Such anthropomorphic climate change has important implications for humans and other living things. Scientists believe that the effects of climate change will become more impactful over time.

Carbon dioxide is the gas that is most often associated with climate change. It is the most abundant GHG released by human activity. It is not the most potent GHG, but its dramatic increase in Earth’s atmosphere because of human activity has significantly altered Earth’s climate. Carbon dioxide makes up only a small percentage of the overall atmosphere, but humans nearly doubled the amount of carbon dioxide in the atmosphere between the early 1800s and the 2020s.

Methane is another important GHG. It is much more potent than carbon dioxide, meaning that each particle of methane will trap more heat in Earth’s atmosphere than each particle of carbon dioxide. Methane makes up a small portion of the atmosphere but is still an important GHG because of its strength.

Nitrous oxide is another GHG that scientists have blamed for contributing to climate change. This gas is emitted during agricultural and industrial activities. It is also released when people burn fossil fuels. It stays in the atmosphere for about one hundred years, which is longer than some other GHGs.

Fluorinated gases are a group of potent GHGs. Natural processes do not generally release these gases, so human activity is responsible for their presence in the atmosphere. Examples include hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride. While these gases are found in very low concentrations in the atmosphere, they can trap large amounts of heat, so they still contribute to climate change.

Water vapor is technically the most abundant GHG gas. However, it is also unique because it is the only GHG with a concentration that is not directly affected by human activities. Human actions do not directly increase levels of water vapor in the atmosphere like they increase other GHGs. However, warm air holds more water than cold air. Therefore, as human actions increase the levels of other GHGs and, thereby, increase temperatures, the amount of water vapor in the air increases. In turn, this further enhances the greenhouse effect.

The amount of time that a GHG remains in the atmosphere after it is released ranges from a few years to hundreds of years. The longer a GHG remains in the atmosphere, the more likely it is to build up and further contribute to the greenhouse effect. Scientists track the levels of each GHG because each type of gas has a different potential for influencing climate change. Scientists determine the global warming potential (GWP) of each GHG by measuring how much energy one ton of the gas will absorb compared to how much energy one ton of carbon dioxide absorbs during the same time. They use carbon dioxide as a baseline because this gas has had the most significant impact on climate change. Because of the nature of the measurement, carbon dioxide’s GWP is 1. Methane has a GWP of 27 to 30, indicating that it can absorb much more energy than carbon dioxide. Nitrous oxide’s GWP is even higher at 273. Fluorinated gases have GWPs that are hundreds or thousands of times higher than carbon dioxide. These measurements help scientists make predictions about how much impact GHG emissions will have on the greenhouse effect—and therefore climate change—in the future.

Bibliography

“The Greenhouse Effect.” British Geological Survey, www.bgs.ac.uk/discovering-geology/climate-change/how-does-the-greenhouse-effect-work/. Accessed 11 Sept. 2024.

“The Greenhouse Effect.” UCAR Center for Science Education, scied.ucar.edu/learning-zone/how-climate-works/greenhouse-effect. Accessed 11 Sept. 2024.

Jackson, Roland. “Who Discovered the Greenhouse Effect?” Royal Institute, 17 May 2019, www.rigb.org/explore-science/explore/blog/who-discovered-greenhouse-effect. Accessed 23 Aug. 2022.

“Overview of Greenhouse Gases.” US Environmental Protection Agency, 11 Apr. 2024, www.epa.gov/ghgemissions/overview-greenhouse-gases. Accessed 11 Sept. 2024.

Rosen, Julia. “The Science of Climate Change Explained: Facts, Evidence, and Proof.” New York Times, 6 Nov. 2021, www.nytimes.com/article/climate-change-global-warming-faq.html. Accessed 23 Aug. 2022.

“What Is the Greenhouse Effect?” American Chemical Society, www.acs.org/content/acs/en/climatescience/climatesciencenarratives/what-is-the-greenhouse-effect.html. Accessed 22 Aug. 2022.

"What Is the Greenhouse Effect?" NASA, science.nasa.gov/climate-change/faq/what-is-the-greenhouse-effect/. Accessed 11 Sept. 2024.