Atmosphere and climate change

Earth’s climate system has five components: the atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere. Among these components, the atmosphere is the most sensitive to changes in Earth’s climate. Thus, current data suggesting global warming are mostly atmospheric, observed as an increase of near-surface air temperature over the last hundred years.

Background

The atmosphere is a layer of gas surrounding the Earth. It is a mixture of several components, mostly gas-phased molecules. The total weight of Earth’s atmosphere is about 5.08 quadrillion metric tons. Its existence makes possible life on Earth, and changes in its composition may destroy life on Earth.

The atmosphere’s various physical properties affect many aspects of human existence. Its optical properties make the sky blue and create rainbows and auroras. It carries sound waves, making aural communication possible. It also propagates heat, and seasonal changes in atmospheric thermal properties. This results in cooler and warmer temperatures at different times of year. More generally, the weather is a function of the atmosphere: wind, precipitation, humidity, and storm systems are all atmospheric phenomena.

Atmospheric Structure

Because of the Earth’s gravity, most atmospheric molecules are distributed very close to the Earth’s surface. That is why the atmosphere is relatively thin. Earth’s atmosphere stretches from the surface of the planet up to as far as 10,000 kilometers (6,214 miles) above. More than 90 percent of Earth’s atmosphere lies within 32 kilometers of the planet’s surface, and the lower atmosphere begins to shade into the upper atmosphere and outer space at an altitude of about 80 kilometers. This distance is only a fraction of the Earth’s radius, which is about 6,400 kilometers.

The atmosphere’s density and pressure decrease steadily with altitude. Altitudinal variations in temperature, on the other hand, are more complex. The temperature decreases with altitude from the surface to about 10-16 kilometers high. From that height to about 20 kilometers, the temperature remains relatively constant. From 20 to 45 kilometers, the temperature increases with height, and at altitudes of 45-50 kilometers it again remains constant. Above 50 kilometers, the temperature again begins to decrease with altitude, continuing until reaching a height of about 80 kilometers. From that height to about 90 kilometers, temperature is constant. From 90 kilometers to the end of Earth’s atmosphere, temperature increases with altitude. Based on these altitudinal temperature variations, scientists divide the atmosphere into sections: the troposphere, tropopause, stratosphere, stratopause, mesosphere, mesopause, and thermosphere.

The atmosphere can also be classified according to its composition. In the lower atmosphere (below 80 kilometers), weather convection and turbulence mixing keep the composition of air fairly uniform. This area is therefore known as the homosphere. Above the homosphere, collisions among atoms and molecules are infrequent, and the air is unable to keep itself well mixed. This layer is therefore called the heterosphere. Furthermore, above the stratosphere, large concentrations of ions and free electrons exist, in a layer known as the ionosphere.

The Energy Budget and the Greenhouse Effect

The Sun is the ultimate energy source for the Earth. Solar energy reaches the Earth via solar radiation. During daytime, the Earth is warmed by the Sun, while during nighttime the Earth cools. The sunlight that the Earth receives is a form of electromagnetic radiation with a relatively short wavelength, called “shortwave radiation.” When the Earth cools during night, the heat it releases into space is another form of electromagnetic radiation with a relatively long wavelength—“longwave radiation,” or infrared radiation (IR).

If Earth did not have an atmosphere, it would simply receive shortwave solar radiation during the day and give out longwave radiation at night. When an was reached (meaning that the amount of energy received was equal to the amount of energy given out), the Earth would be balanced at an equilibrium temperature. This temperature would be much lower than is the actual temperature on Earth: Some scientists have estimated it at about -18° Celsius. Earth’s surface temperature is thus much warmer than it would be without an atmosphere.

This difference in temperature is a result of the greenhouse effect. Shortwave solar radiation can mostly penetrate Earth’s atmosphere and reach the its surface. Infrared radiation, by contrast, is intercepted by the atmosphere, and some of this longwave radiation is reflected back to Earth’s surface, increasing the equilibrium temperature of the planet. The strength of the and the warmth of Earth’s equilibrium temperature are dependent on the specific composition and properties of the atmosphere. Changes in can significantly alter Earth’s and equilibrium temperature.

Atmospheric Composition, Chemistry, and GHGs

The atmosphere is composed of nitrogen (about 78 percent by volume), oxygen (21 percent), and various trace gases (totaling about 1 percent). If all trace gases were removed, these percentages for nitrogen and oxygen would remain fairly constant up to an altitude of about 80 kilometers. At the planet’s surface, there is an approximate balance between the destruction (output) and production (input) of these gases.

The two most plentiful components of the atmosphere, nitrogen and oxygen, are of significance to life on Earth. Humans and animals cannot live without oxygen. By contrast, the trace noble gases, such as argon, neon, and helium, are not very active chemically. Water vapor is distributed inconsistently in the lower atmosphere; its concentration varies greatly from place to place and from time to time, and it can constitute from 0 to 4 percent of local air. This variable concentration is one reason that water vapor is so important in influencing Earth’s weather and climate.

Water vapor provides the main physical substance of storms and precipitation, and its into liquid water generates the large amount of energy (latent heat) necessary to initiate powerful and violent storms. Water vapor is also a greenhouse gas (GHG): It strongly absorbs longwave radiation and reemits this radiation back to the Earth, causing global warming. Clouds, which are generated from water vapor, also play an extremely important role in climate and climate change.

Another very important GHG is carbon dioxide (CO₂). Observations indicate that the concentration of CO₂ in the atmosphere has been rising steadily for more than a century. The increase of CO₂ concentration indicates that CO₂ is entering the atmosphere at a greater rate than its rate of removal. This rise is largely attributable to the burning of fossil fuels, such as coal and oil. Deforestation also contributes to the increase in atmospheric CO₂ concentration. Estimates project that by sometime in the second half of the twenty-first century, CO₂ levels will be twice as high as they were early in the twentieth century. Other GHGs include methane, nitrous oxide, and chlorofluorocarbons.

Ozone (O₃) is another important gas for Earth’s weather and climate. At Earth’s surface, O₃ is a major air pollutant, and it is closely monitored for its effects on air quality. However, at upper levels (about 25 kilometers high), O₃ forms a shield for Earth’s inhabitants from harmful ultraviolet solar radiation. For this reason, the loss of O₃ high in the atmosphere as a consequence of human activity has become a serious global-scale issue. One of the examples of O₃ depletion is the O₃ hole found over Antarctica. Finally, aerosols, including particulate matter, are also important constituents of the atmosphere, affecting weather formation, air quality, and climate change.

During the first decades of the twenty-first century, efforts aimed at reducing the GHG and stopping the depletion of the Ozone proved to show signs of success. The hole in the ozone layer in the stratosphere (the portion that protects Earth from the sun's UV rays) was decreasing. In 2023, the hole over Antarctica had an area of 10 million square miles (26 million square kilometers). This was significantly larger than its size in 2022, which was 8.91 million square miles (23.2 million square kilometers), and its size in 2021, which was 8.99 million square miles (23.3 million square kilometers).

Weather and Climate

Atmospheric conditions can generally be classified as either weather or climate. Weather is a particular atmospheric state at a given time and place. Climate is an average of weather conditions at a given location over a period of time.

Weather includes many atmospheric phenomena of different scales, including middle latitude cyclones (extratropical cyclones), hurricanes (tropical cyclones), heavy rains and floods, mesoscale convective systems, thunderstorms, and tornadoes. Climate includes atmospheric conditions that are millennial, centennial, decadal, interannual, or seasonal. For example, global warming can occur on a centennial or longer timescale, an El Niño or episode will generally occur on an interannual timescale, and seasonal changes occur on relatively short timescales of months, weeks, or days.

Global Warming and Climate Change

Earth’s climate has changed constantly over its history. The planet has experienced many cold periods, as well as several warm periods. For example, about ten thousand years ago, the Earth cooled during a period known as the Younger Dryas, when the average global temperature was about 3° Celsius colder than it is today. However, about six thousand years ago, the Earth reached the middle of an interglacial period, known as the Mid-Holocene Maximum. The temperature then was 1° Celsius higher than today’s norm. Some of the natural mechanisms causing this climatological variabilty include: drift of plate tectonics, volcanic activities, ocean circulations, variations in Earth’s orbit, and solar variability.

The rapid warming that has occurred in the past hundred years seems to coincide with the socioeconomic development and industrialization of humankind. During the past century, human life and societies began to depend heavily on burning fossil fuels. As a result, increasing amounts of CO₂ have been added to the atmosphere. Global temperatures and CO₂ levels evince a consistent upward trend during the same period. Therefore, many scientists believe that human activity may have contributed to global warming.

According to National Oceanic and Atmospheric Administration's (NOAA) 2023 Annual Climate Report, 2023 was the warmest year since global records began in 1850. Seven months ranked among the warmest on record, June through December. The July global temperature was likely the warmest of all recorded months, and the July, August, and September temperatures were above the long-term average of 1.8 degree Fahrenheit (1.0 degree Celsius). Ocean temperatures were also record-breaking for nine consecutive months from April to December.

Some possible consequences of this global warming include higher maximum and minimum temperatures, more hot days and heat waves, fewer cold days and frost days, more intense precipitation events, more summer drying and drought, increased tropical cyclone intensity, increased Asian precipitation variability, intensified droughts and floods associated with El Niño events, and increased intensity of midlatitude storms. Global warming will also exert some profound effects on many other social and environmental issues. For example, the distribution of water resources and farming may be changed by future warmer climates. Warming-induced sea-level rise can have significant effects on many countries’ coasts. Arctic sea-ice melting can also have geopolitical and economic consequences.

Context

The atmosphere is central to almost all aspects of human existence, constituting not only the source of vital oxygen but also the medium of movement, sound, and weather. It is also the most variable component of Earth’s climate system. Because that system is so complex and interconnected, changes in the atmosphere will inevitably result in changes to the rest of the system, some of which are extremely difficult to predict. At the same time, feedback from other components of Earth’s environment may exert a significant influence on the atmosphere, including producing positive and negative feedback loops that help alter or maintain Earth’s climate.

Key Concepts

• condensation: the transformation of a substance from its gaseous to its liquid state, accompanied by a release of heat
• convection: motion in a fluid that results in the transport and mixing of the fluid’s physical properties, such as heat
• coupled atmosphere-ocean models: computer simulations of alterations in and interactions between Earth’s atmosphere and oceans
• El Niño and La Niña: periodic warming and cooling of the eastern tropical Pacific Ocean that affects global weather patterns
• greenhouse gases (GHGs): atmospheric trace gases that allow sunlight to reach Earth’s surface but prevent heat from escaping into space
• latent heat: the heat released or absorbed by a change of state, such as condensation

Bibliography

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"Annual 2023 Global Climate Report." National Oceanic and Atmospheric Administration (NOAA),www.ncei.noaa.gov/access/monitoring/monthly-report/global/202313. Accessed 12 Dec. 2024.

Climate Change Science Program, U.S. Our Changing Planet. Washington, D.C.: Author, 2006.

Gillespie, Alison. "Antarctic Ozone Hole Slightly Smaller in 2022." National Oceanic and Atmospheric Administration, United States Department of Commerce, 26 Oct. 2022, www.noaa.gov/news-release/antarctic-ozone-hole-slightly-smaller-in-2022. Accessed 12 Dec. 2024.

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