Atmospheric structure and evolution

Earth’s atmosphere is a blanket that keeps the planet a beneficial 35° Celsius warmer than it would be otherwise. Human activities, however, are adding GHGs to the atmosphere that may raise the temperature enough to alter the climate.

Background

The atmosphere is an ocean of gases held to the Earth and compressed by gravity. If greenhouse gases (GHGs) were newly introduced into the atmosphere, less energy would leave the Earth than strike it, so the planet would warm until those two rates balanced. (Infrared radiation from a warmer Earth has a shorter wavelength and is therefore more likely to escape into space.) GHGs keep Venus 500° Celsius, Earth 35° Celsius, and Mars 7° Celsius warmer than each planet would be without its atmosphere. Without the greenhouse effect, much of Earth would be permanently covered with snow and ice.

Earth’s First Atmosphere

If Earth’s atmosphere had formed along with Earth itself, its composition would be expected to reflect the relative abundance of solar elements extant at the time and present in gases heavy enough to be retained by Earth’s gravity. Hydrogen and helium are the two most abundant gases, but Earth’s gravity is not strong enough to hold them, so they gradually escape into space. Oxygen is the next most abundant gas, but it is so chemically active that, without plant life to replenish it, it would soon disappear from Earth’s atmosphere. Neon is next in abundance, is chemically inert, and is heavy enough to be retained by Earth’s gravity. The fact that it is not the most abundant gas in Earth’s atmosphere provides evidence that Earth’s primordial atmosphere escaped into space, probably during a flare-up of the young Sun or during the late, heavy bombardment following the Earth’s formation.

Scientists believe that the current atmosphere consists of gases released as rocks and minerals inside the Earth were heated by the energy from radioactive decay. These gases were subsequently emitted by volcanoes. Water vapor is the most abundant volcanic gas. It condensed to form the oceans. The next most abundant volcanic gases are carbon dioxide (CO₂), nitrogen, and argon. CO₂ is removed from the atmosphere when it dissolves in the oceans, where most of it eventually combines with calcium oxide ions, precipitates out, and forms carbonate rocks (limestone). Sulfur dioxide and sulfur trioxide are chemically active and do not stay in the atmosphere very long. Thus, if the Earth’s atmosphere came from volcanoes, it should be dominated by nitrogen, followed by CO₂ and then argon. The atmospheric CO₂ would soon be depleted as it dissolved in the oceans. The atmospheres of Venus and Mars are richer in CO₂ than is Earth’s atmosphere, because those planets have no oceans to remove their CO₂.

In the absence of oxygen, iron dissolves in water, but when oxygen is dissolved in the same water, iron oxide precipitates out and sinks to the bottom. Ore deposits of this iron compound first appeared about 2.6 billion years ago and peaked about 1.8 billion years ago. This probably reflects the increasing abundance of plants that released oxygen into the atmosphere and the increasing atmospheric concentraton of oxygen that resulted. Earth’s atmospheric oxygen concentration reached a maximum of 30 percent 300 million years ago and a minimum of about 12 percent 200 million years ago. It now stands at about 21 percent.

The Faint Early Sun Paradox

Stars such as the Sun roughly double in brightness over their lifetimes as normal stars. The Sun is already 30 percent brighter than it was when it first became a normal star, 4.56 billion years ago. Based on contemporary values of Earth’s albedo and its atmosphere, Earth should not have had liquid water before about 2 billion years ago. There is, however, abundant geological evidence that liquid water has existed on Earth for at least 3.8 billion years. Mars exhibits a similar paradox, since it seems to have had abundant 3.8 billion years ago.

If the Sun had been born 7 percent more massive, the Earth would have been warm enough for liquid water. Because at normal rates the Sun can have lost only 0.05 percent of its mass since it was born, however, this is probably not the case. A sufficiently large amount of CO₂ in the atmosphere could have made the early Earth warm enough for water, but geological evidence for that much atmospheric CO₂ is lacking. However, Philip von Paris and his colleagues at the Aerospace Centre in Berlin developed a plausible computer model that allows the early Earth to have developed liquid water with only 10 percent of the CO₂ previously thought necessary, an amount not ruled out by geological evidence.

Structure of the Atmosphere

The atmosphere has settled into a series of layers, one on top of the other, like the layers of an onion. The lowest layer of the atmosphere is called the troposphere. It extends from the ground up to about 15 kilometers over the equator and slants down to just 8 kilometers in height over the poles. The word “troposphere” is based on the Greek word tropos, which means “turning” or “mixing.” Storms are a result of the mixing that takes place in this layer, so storms and almost all clouds are confined to the troposphere. In the troposphere, temperature decreases by about 8° Celsius with each 1 kilometer of altitude. Tourists are often surprised to find that the South Rim of the Grand Canyon, in Arizona, is 12° Celsius cooler than the inner gorge, 1.5 kilometers below.

The atmospheric layer above the is the stratosphere (from the Latin stratum, meaning “horizontal layer”). It extends from the troposphere up to an altitude of about 50 kilometers. Commercial airliners usually cruise in the lower stratosphere, where the reduced air density produces less drag and where they are above clouds and storms. The ozone layer, which protects Earth life from most of the Sun’s ultraviolet radiation, is located in the stratosphere, mostly between 20 and 40 kilometers high.

Above the lies the mesosphere (from the Greek mesos, meaning “middle”). The mesosphere extends from about 50 kilometers up to 80 or 90 kilometers in altitude. Incandescent trails may be observed in the mesosphere. These trails result when meteoroids the size of sand grains or pebbles strike Earth from space: They are heated by the friction of their swift passage through the mesosphere until they disintegrate.

The thermosphere (from the Greek thermos, meaning “heat”) begins about 90 kilometers above the ground and extends upward to about 600 kilometers high. Gas molecules of the thermosphere absorb solar energy and convert it into kinetic energy (energy of motion). The speed of a molecule is a measure of its temperature, and speeds corresponding to temperatures of up to 15,000° Celsius are expected in thermosphere gas molecules. While that may seem hot, an unprotected person in the thermosphere would soon freeze, because such gas molecules are few and far between, making the average temperature of this layer extremely cold. Auroras form in the lower thermosphere when high-energy particles slam into air atoms.

The International Space Station (ISS) orbits in the thermosphere, about 340 kilometers above Earth’s surface. The atmosphere there is thin enough that drag on the ISS is small, but not zero. The advantage to this location is that, although it must periodically be reboosted by a supply ship while in use, when the ISS is finally abandoned it will slowly and naturally deorbit itself.

Beginning in the lower thermosphere, air atoms are far enough apart that when ultraviolet light from the Sun drives electrons away from their parent atoms, they do not recombine for some time. The clouds of free electrons (or ions) that thus form can reflect radio waves. This region is the ionosphere. Depending upon the intensity of incident sunlight, the ionosphere can extend throughout the thermosphere and up into the exosphere. The exosphere begins 500 to 600 kilometers high and extends up to about 10,000 kilometers, where it shades into interplanetary space.

Earth’s Greenhouse Gases

Clouds and water vapor together are responsible for roughly 80 percent of Earth’s greenhouse warming. People can affect the amount of water vapor in the atmosphere locally by deforestation, for example, but the global amount is determined by Earth’s vast oceans. The global amount of atmospheric water vapor remains quite constant over time, so most atmospheric scientists conclude that it cannot be responsible for recent warming. Water vapor also plays a major role in feedback; for example, if Earth warms, more water will evaporate from the oceans, becoming vapor. More water vapor will produce more clouds, and more clouds blanketing Earth will keep it warmer, but more clouds will also reflect more sunlight back into space, keeping Earth cooler. Experts indicate that the warming effect will be greater than the cooling effect, but it is nevertheless apparent that global warming is quite a complex phenomenon.

CO₂ accounts for most of the rest of Earth’s greenhouse effect. The amount of atmospheric CO₂ is increasing by about 0.4 percent per year, with perhaps the majority of that increase coming from transportation, power generation, and other human activities. It also comes from volcanoes and burning vegetation. CO₂is removed from the atmosphere by plants and by being dissolved in the ocean. In the ocean, it may combine with calcium to make limestone. It is also removed from the ocean by sea creatures that use limestone to make their shells.

Methane is present in the atmosphere in only trace amounts, but, molecule for molecule, it is about twenty times more effective than CO₂ as a GHG. Atmospheric methane has more than doubled in the past two centuries, according to the Environmental Protection Agency (EPA). Significant amounts of it are released by coal and oil production, cattle and sheep, swamps and rice paddies, and jungle termites.

Ice Ages

Life on Earth has survived many cooling and warming cycles in the past, but scientists do not fully understand the causes of climate change. Different factors dominate changes at different times. For example, increased atmospheric CO₂ from volcanoes may warm Earth enough to end an ice age, but in other cases atmospheric CO₂ might not increase until centuries after the climate warms. A few of the other factors to consider are Milanković cycles, solar intensity, albedo, and the ability of the oceans to absorb CO₂ and to lock it in limestone.

Context

At one other time, humanity stood on the verge of having the power to change the climate. When world stockpiles of nuclear weapons were at their highest levels, some people wondered if an all-out nuclear war would result in nuclear winter—months or years of freezing temperatures and little sunlight—followed by ten thousand years of nuclear spring—temperatures several degrees above normal and dangerous levels of ultraviolet light reaching the ground. Although it is doubtful that there were ever enough warheads to cause nuclear winter, stockpiles have been reduced. Humans are again on the verge of being able to change the climates of Earth, and enough is not known to predict all of the results of changing the climate or of taking actions designed to avoid changing it.

Key Concepts

• albedo: percentage of incident light Earth reflects back into space
• greenhouse gases (GHGs): gases that allow sunlight to pass through to the ground but trap, at least partially, the infrared radiation that would otherwise escape into space
• ice age: a period during which and glaciers cover a significant fraction of Earth’s surface
• late heavy bombardment: a period about 3.9 to 4.0 billion years ago, when Earth was pummeled by debris from space at one thousand times the normal rate, heating the atmosphere and melting the crust
• Milanković cycles: recurring time periods during which the shape of the Earth’s orbit, the tilt of its axis, and the occurrence of its farthest distance from the Sun all change

Bibliography

Archer, David. Global Warming: Understanding the Forecast. Malden, Mass.: Blackwell, 2007.

Barry, Roger G. Atmosphere, Weather, and Climate. 8th ed. New York: Routledge, 2003.

"Importance of Methane." US Environmental Protection Agency (EPA), 21 Oct. 2024, www.epa.gov/gmi/importance-methane. Accessed 20 Dec. 2024. O

"O'Malley, Isabella. "Climbing Methane Levels in the Atmosphere Are Contributing to Overall Rise in Greenhouse Gases." PBS, 6 Apr. 2023, www.pbs.org/newshour/science/climbing-methane-levels-in-the-atmosphere-are-contributing-to-overall-rise-in-greenhouse-gases. Accessed 20 Dec. 2024.

Walker, Gabrielle. An Ocean of Air: Why the Wind Blows and Other Mysteries of the Atmosphere. New York: Harvest Books, 2007.