Greenhouse Gases

Introduction

Certain gases in Earth's atmosphere—particularly carbon dioxide (CO₂), methane (CH₄), and water vapor—trap energy from solar radiation and thus, keep Earth warmer than it would be otherwise. These gases are termed greenhouse gases and the warming they create is termed the greenhouse effect or greenhouse warming. About 65% of greenhouse warming is caused by CO₂, the most abundant greenhouse gas.

A certain amount of greenhouse warming occurs naturally; without it, Earth would be too cold to sustain most life. However, the intensity of the greenhouse effect is being increased by human activities that add greenhouse gases to the atmosphere. For example, burning fossil fuels (coal, petroleum, and natural gas) was adding over 6 billion metric tons of carbon dioxide to the atmosphere every year as of 2006, with world usage of such fuels increasing rapidly. About half of the CO₂ released each year remains in the atmosphere and intensifies Earth's natural greenhouse effect; the rest is absorbed by plants and the oceans. The atmospheric concentration of CO₂ is also increased by deforestation, because living trees absorb CO₂ while burning and decaying wood emits it.

Because of anthropogenic (human-caused) intensification of Earth's greenhouse effect, the global climate is warming. This is causing secondary changes such as a rise in sea level, changed patterns of precipitation, shifted ranges for plant and animal populations, and more frequent extreme weather. Because the effects of global warming are various, scientists prefer the phrase global climate change, reserving the phrase global warming to refer to the average global temperature increase as such, apart from other changes.

Without the greenhouse effect, Earth's surface temperature would average about 0°F (–18°C), well below the freezing point of water. By slowing the rate at which Earth loses energy, the greenhouse effect helps maintain Earth's surface at an average temperature of about 59°F (15°C), allowing the oceans to be liquid and life to flourish.

Historical Background and Scientific Foundations

The Greenhouse Effect

Greenhouses gases alter Earth's temperature by altering its energy budget. The energy budget of any system, such as Earth, is the sum of the energy entering the system, the energy leaving the system, and any difference between the two, which corresponds to energy that is internally transformed or stored. Almost all of the energy entering Earth's climate system comes from the sun, which, like all objects, emits electromagnetic energy at a rate and of a spectral character determined by its surface temperature, about 10,800°F (6,000°C). Because of this high temperature, about one-half of the sun's emitted energy is radiated as visible light, that is, light rays having wavelengths between 400 and 700 nanometers (nm). Most of the other half of the sun's energy output is radiated in the near-infrared wavelength range between about 700 and 2,000 nm.

There is a nearly even balance between the amount of electromagnetic energy coming to Earth and the amount eventually radiated back to space. The myriad ways in which the incoming energy is absorbed, dispersed, transformed, stored, and re-radiated make up the details of Earth's energy budget. At present, because Earth is warming, it is radiating slightly less energy than it is absorbing: the difference is going to heat the atmosphere, land, and (mostly) oceans.

Incoming solar radiation is termed insolation. On average, 35% of insolation is reflected to space by Earth's atmosphere (7%), surface (4%), and clouds (24%). About 18% of insolation is absorbed by atmospheric gases (16%) and clouds (2%); 47% of insolation is absorbed by Earth's surface. Upon absorption by matter, light's energy is usually transformed into heat, that is, random molecular

motion. Land, sea, and atmosphere re-radiate most of this energy as longer-wavelength infrared radiation; a small fraction drives winds, currents, and chemical processes. Some re-radiated infrared energy escapes to space, but some is re-absorbed by the planet's surface or atmosphere. Re-absorption in the atmosphere is accomplished by greenhouse gases and is the basis of the greenhouse effect.

The term “greenhouse effect” is used because this mechanism is analogous to a glass-enclosed greenhouse heated by sunlight. A greenhouse's glass panels are transparent to insolation, but they prevent warm interior air from rising and mixing with the outside air, slowing heat loss and causing the structure to achieve equilibrium or balance with its environment at a higher temperature than it would otherwise. Similarly, when greenhouse gas concentrations in Earth's atmosphere are increased, Earth warms until it achieves a new, warmer equilibrium with its environment. As greenhouse gases are being continually added to the atmosphere, however, Earth cannot achieve thermal (heat) equilibrium, but continues to warm as long as the gases are added. If at some point greenhouse-gas concentrations were to level off, Earth would eventually, at some point, cease to get warmer.

Major Greenhouse Gases

Water vapor and CO₂ are the most important greenhouse gases in Earth's atmosphere; CO₂ also has important greenhouse effects on Mars and Venus. Methane (CH₄), nitrous oxide (N₂O), ozone (O₃), and chlorofluorocarbons and related compounds (used mostly as refrigerants) play a significant role. On a per-molecule basis, most of these gases cause more warming than CO₂, but they cause less warming overall because their atmospheric concentrations are much lower. Compared with carbon dioxide, a molecule of methane is 21–40 times more effective at absorbing infrared wavelengths, nitrous oxide is 200–270 times more effective, ozone 2,000 times, and CFCs and related compounds 3,000–15,000 times more effective.

Other than water vapor, the atmospheric concentrations of all of these gases have increased in the past century because of emissions associated with human activities. Prior to 1850, the concentration of CO₂ in the atmosphere was about 280 parts per million (ppm), while in 1994 it was 355 ppm, and by 2006 it had risen to 379 ppm. During the same period, CH₄ increased from 0.7 ppm to 1.7 ppm, N₂O from 0.285 ppm to over .3 ppm; and CFCs from zero to 0.7 ppb. These increased concentrations of greenhouse gases have been the main cause of the observed warming of Earth's climate in recent years. CO₂ causes about 60% of the anthropogenic greenhouse effect, CH₄15%, N₂O 5%, O₃8%, and CFCs 12%. Water vapor is a special case because it does not remain in the atmosphere very long; its atmospheric concentration is controlled by the warming effect of longer-lived greenhouse gases, and so it is termed a feedback (as opposed to a forcing) factor in global warming.

Other Processes Powered by Insolation

Not all energy from absorbed light, whether visible or infrared, is re-radiated at once as infrared light. Energy can also be transformed or stored by various physical processes. A few that are important to climate are as follows:

• Some thermal energy causes water to evaporate from plant and water surfaces or melts ice and snow. Evaporation causes local climatic cooling; condensation of water vapor into snow or rain releases heat. Both of these effects are important to regional climate and weather.
• A small amount (less than 1%) of the absorbed solar radiation generates mass-transport processes in the oceans and lower atmosphere, which disperse some of Earth's unevenly distributed thermal energy. The most important of these processes are winds and storms, ocean currents, and waves on the surface of the oceans and lakes.
• A small but ecologically critical quantity of solar energy (averaging less than 1% of the total) is absorbed by plant pigments, especially chlorophyll. This energy is used to drive photosynthesis, resulting in temporary storage of energy in the chemical bonds of biological compounds. Notably, plants use energy from sunlight to separate the carbon and oxygen in CO₂, keeping the carbon to build their tissues and releasing the oxygen.

Scientific Disputes and Progress

Globally, all of the warmest years since instrumental measurements of surface temperature began to be recorded (around 1880) have occurred since the late 1980s. Typically, these warm years have averaged about 1.5–2.0°F (0.8–1.0°C) warmer than those during the decade of the 1880s. As of 2007, the average temperature of Earth's atmosphere near the surface had risen about 1.33°F (0.74°C) since 1906. This warming closely matches increases in greenhouse gases emitted by human activities.

When human activity was first cited as the cause of increasing greenhouse gas concentrations in the late 1950s and 1960s and global warming was attributed to increasing concentrations, some scientists questioned these conclusions. They based their skepticism on several apparent deficiencies in the argument. These included: 1) air temperature is variable in time and space, making measurements from many locations over long time periods necessary to detect long-term trends; 2) older data are less accurate than modern instrumental records; 3) many

weather stations are in urban areas that are warmed by increasing quantities of buildings and pavement; and 4) climate can change for reasons other than increased greenhouse gases, including cyclical changes in solar radiation and volcanic emissions. When satellite data became available in the late 1970s, there were apparent inconsistencies between ground-level instrumental data and satellite observations.

However, by the early 2000s, all these causes of skepticism had been resolved to the satisfaction of the great majority of scientists. For example, atmospheric CO₂ and surface temperature had been linked by many independent bodies of proxy data, that is, natural biological or geological records such as tree rings, snow layers, and ocean-floor sediment layers that preserve information about ancient climate. An important form of proxy evidence for ancient temperature changes—and of direct evidence for ancient CO₂ concentrations—are ice cores from Antarctica and Greenland. These cylinders of ice, retrieved by drills from deep inside Earth's two major ice caps, preserve bubbles of ancient air which reveal ancient CO₂ concentrations and, by their ratios of oxygen isotopes, air temperatures at the time the snow that compacted to form the ice fell. Observed changes in CO₂ and surface temperature are matched, each increasing when the other increases, suggesting that either increased CO₂ causes warming, or warming causes increased CO₂, or both.

A 2005 ice-core study by National Aeronautics and Space Administration (NASA) scientists found that atmospheric carbon dioxide was already 27% higher than the highest CO₂ level found during the last 650,000 years. All data confirmed a step-wise correlation between atmospheric carbon dioxide concentrations and the warming of the global climate. Geological data show that atmospheric CO₂ was much higher at much more remote times, on the order of hundreds of millions of years ago, when Earth's climate was radically different than today's.

The other main scientific grounds for doubting the reality of anthropogenic climate change have also dissolved. The urban heat island effect has been taken into careful account in analyzing temperature records; more careful analysis of astronomical influences on climate shows that increased energy output from the sun is not causing today's warming; analytic errors that had created apparent disagreement between satellite and surface data have been discovered and fixed. Satellite data, improved surface data, proxy data, and computer models have all converged to support the conclusion the world is indeed warming as greenhouse gas concentrations increase. Since we know that human culture, not warming climate, has caused the expansion of fossil-fuel burning since 1750, on this occasion it is definitely increased greenhouse-gas concentrations that have caused global warming, not the other way around. Modern climate warming may, however, cause accelerated releases of some greenhouse gases, such as methane, enhancing warming still further.

Computer models are used to analyze the many complex factors that affect climate change. The most sophisticated simulations are three-dimensional general circulation models (GCMs), which are run on supercomputers. GCM models simulate the complex interactions of clouds, ocean currents, atmospheric circulations, seasons, biological processes, greenhouse gas emissions, and atmospheric chemistry to create a mathematical picture or model of how Earth's climate works.

WORDS TO KNOW

1750 VALUE: Concentrations of greenhouse gases in the atmosphere are often compared to their values in 1750. That year is just before the beginning of the Industrial Revolution, when burning of fossil fuels began to greatly increase, changing atmospheric gas concentrations. The number 1750 is arbitrary: any earlier year in the last several thousand years would do just as well, since greenhouse gas concentrations were stable during that period.

CHLOROFLUOROCARBONS: Members of the larger group of compounds termed halocarbons. All halocarbons contain carbon and halons (chlorine, fluorine, or bromine). When released into the atmosphere, CFCs and other halocarbons deplete the ozone layer and have high global warming potential.

DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosynthesis are no longer present and contributing to carbon storage.

ELECTROMAGNETIC ENERGY: Energy conveyed by electromagnetic waves, which are paired electric and magnetic fields propagating together through space. X rays, visible light, and radio waves are all electromagnetic waves.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

INSOLATION: Solar radiation incident upon a unit horizontal surface on or above Earth's surface.

METHANE: A compound of one hydrogen atom combined with four hydrogen atoms, formula CH₄. It is the simplest hydrocarbon compound. Methane is a burnable gas that is found as a fossil fuel (in natural gas) and is given off by rotting excrement.

PHOTOSYNTHESIS: The process by which green plants use light to synthesize organic compounds from carbon dioxide and water. In the process, oxygen and water are released. Increased levels of carbon dioxide can increase net photosynthesis in some plants. Plants create a very important reservoir for carbon dioxide.

SPECTRAL: Relating to a spectrum, which is an ordered range of possible vibrational frequencies for a type of wave. The spectrum of visible light, for example, orders colors from red (slowest vibrations visible) to violet (fastest vibrations visible) and is itself a small segment of the much larger electromagnetic spectrum.

URBAN HEAT ISLAND EFFECT: Warming of atmosphere in and immediately around a built-up area. Occurs because pavement and buildings absorb solar energy while being little cooled by evaporation compared to vegetation-covered ground. Skeptics of global climate change at one time argued that the expansion of urban heat islands near and around weather stations has caused an illusion of global warming by biasing temperature measurements. Although urban heat islands do exist, the argument that they produce an illusion of global warming has been discredited.

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

In 2007, the United Nations' Intergovernmental Panel on Climate Change (IPCC) issued its Fourth Assessment Report on anthropogenic emissions of greenhouse gases and climate change. The IPCC found that, despite ratification by most of the world's nations of the Kyoto Protocol, which calls for reductions in greenhouse emissions, most nations' emissions were increasing. In 2005, CH₄ abundance was up to 1,774 parts per billion, more than twice its 1750 value; N₂O abundance was at 319 parts per billion, 18% higher than its 1750 value.

The IPCC's 2007 report on climate change also presented revised predictions for how much the global climate might warm if emissions of greenhouse gases continue. Depending on whether human societies reduce their emissions or continue with business as usual, by 2100 world climate is likely to warm by several degrees, with a minimum of 1.98°F (1.1°C) and a maximum, barring unforeseen feedbacks or tipping points, of 11.52°F (6.4°C).

Under the auspices of the United Nations, various international negotiations have been undertaken to try to get nations to agree to decisive actions to reduce their emissions of greenhouse gases, beginning with the United Nations Framework Convention on Climate Change (1992). A major modification to that treaty came out of a meeting held in Kyoto, Japan, in 1997. Most industrialized countries (with the notable exceptions of the United States and Australia) agreed to reduce their CO₂ by 5–7% or more below their 1990 levels by the year 2012. Australia announced plans to ratify in late 2007. Kyoto was a first step, and its shortcomings have been much debated; discussions to design a follow-up treaty to take effect after Kyoto's expiration began in late 2007.

Keeping greenhouse gases out of the atmosphere is the most reliable, least hazardous way to mitigate global warming. Researchers are also testing ways to sequester, or store, carbon dioxide emitted by large centralized sources such as power plants, preventing it from entering the atmosphere. Some scientists are considering geoengineering efforts to deflect sunlight from Earth or otherwise alter the planet's energy budget. Replanting tropical forests and decreasing their destruction, adjusting agriculture practices, generating energy from affordable, rapidly deployable low-carbon sources, ceasing to manufacture CFCs and related compounds, and other measures can reduce greenhouse-gas emissions and eventually stabilize Earth's climate in a less-altered state than will occur if emissions continue to grow.

In 2007, UN experts announced that the most cost-effective measure of all—the most greenhouse-gas abatement achieved per dollar spent—was increased energy efficiency. Much, probably most of the energy consumed by modern industrial society is wasted, and most energy is generated by methods that add greenhouse gases to the atmosphere; decreasing usage of energy and materials reduces greenhouse-gas emissions and mitigates global climate change.

IN CONTEXT: POST-INDUSTRIAL RISE IN GREENHOUSE GASES

The IPCC asserts that “The post-industrial rise in greenhouse gases is ‘unprecedented’ and does not stem from natural mechanisms.”

“Current concentrations of atmospheric CO₂ and CH₄ far exceed pre-industrial values found in polar ice core records of atmospheric composition dating back 650,000 years. Multiple lines of evidence confirm that the post-industrial rise in these gases does not stem from natural mechanisms….”

“The total radiative forcing of the Earth's climate due to increases in the concentrations of the LLGHGs [Long-lived greenhouse gases] CO₂, CH₄ and N₂O, and very likely the rate of increase in the total forcing due to these gases over the period since 1750, are unprecedented in more than 10,000 years.”

IPCC scientists further assert that it is “very likely” that the rate of increase “over the past four decades is at least six times faster than at any time during the two millennia before the Industrial Era….”

Note: Emphasis shown above exists in the original published text.

SOURCE:Solomon, S., et al., eds. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press. 2007.

Greenhouse gases already in the atmosphere commit the world to a certain amount of future climate change regardless of how successful society may be at reducing emissions. Even given sudden stabilization of greenhouse-gas concentrations, it would still take centuries for the Earth's atmosphere to get into thermal equilibrium with the waters of the deep oceans, and during this period, climate would continue to change, albeit more slowly than today. Reducing greenhouse-gas emissions cannot halt global climate change, but would slow it and reduce its future extent.

See Also Global Warming; Greenhouse Effect.

BIBLIOGRAPHY

Books

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. Emmaus, PA: Rodale Press, 2006.

Morganstein, Stanley. The Greenhouse Effect. Cheshire, UK: Trafford, 2003.

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Wuebbles, Donald J., and Jae Edmonds. Primer on Greenhouse Gases. Boca Raton, FL: CRC, 1991.

Periodicals

Karl, Thomas R., and Kevin E. Trenberth. “Modern Global Climate Change.” Science 302 (2003): 1,719–1,723.

Web Sites

“Climate Change.” U.S. Environmental Protection Agency (EPA), November 19, 2007. < www.epa.gov/climatechange/> (accessed November 25, 2007).

“Greenhouse Gases: Frequently Asked Questions.” National Oceanic and Atmospheric Administration (NOAA). < http://www.ncdc.noaa.gov/oa/climate/gases.html> (accessed November 21, 2007).

Larry Gilman