Greenhouse gases and greenhouse effect

The greenhouse effect is the physical mechanism by which the atmosphere helps to maintain Earth's surface temperature within a range comfortable for organisms and ecological processes. The greenhouse effect is largely a natural phenomenon, but its intensity may be changing because of increasing concentrations of carbon dioxide and some other gases in the atmosphere. These increased concentrations are occurring as a result of human activities, especially the burning of fossil fuels and the clearing of forests . A probable consequence of an intensification of Earth's greenhouse effect will be a significant warming of the atmosphere. This could likely result in important secondary changes, such as a rise in sea level, variations in the patterns of precipitation , and large and difficult ecological and socio-economic adjustments.

Earth's greenhouse effect is a well-understood physical phenomenon. Scientists believe that in the absence of the greenhouse effect, Earth's surface temperature would average about −0.4°F (−18°C), which is colder than the freezing point of water , and more frigid than life could tolerate long term. By slowing the rate at which the planet cools itself, the greenhouse effect helps to maintain Earth's surface at an average temperature of about 59°F (15°C). This is about 59.5°F (33°C) warmer than it would otherwise be, and is within the range of temperature that life can tolerate.

An energy budget is a physical analysis of all of the energy coming into a system, all the energy going out, and any difference that might be internally transformed or stored. Almost all of the energy coming to Earth from outer space has been radiated by the closest star, the Sun . The Sun emits electromagnetic energy at a rate and spectral quality determined by its surface temperature—all bodies do this, as long as they have a temperature greater than absolute zero, or −459°F (−273°C). Fusion reactions occurring within the Sun maintain an extremely hot surface temperature, about 10,800°F (6,000°C). As a direct consequence of this surface temperature, about one-half of the Sun's emitted energy is so-called "visible" radiation with wavelengths between 0.4 and 0.7 µm (this is called visible radiation because it is the range of electromagnetic energy that the human eye can perceive), and about one-half is in the near-infrared wavelength range between about 0.7 and 2.0 µm. The Sun also emits radiation in other parts of the electromagnetic spectrum , such as ultraviolet and cosmic radiation. However, these are relatively insignificant amounts of energy (although even small doses can cause biological damage).

At the average distance of Earth from the Sun, the rate of input of solar energy is about 2 cal cm⁻² min⁻¹, a value referred to as the solar constant. There is a nearly perfect energetic balance between this quantity of electromagnetic energy incoming to Earth, and the amount that is eventually dissipated back to outer space. The myriad ways in which the incoming energy is dispersed, transformed, and stored make up Earth's energy budget.

On average, one-third of incident solar radiation is reflected back to space by the earth's atmosphere or its surface. The planet's reflectivity (or albedo) is strongly dependent on cloud cover, the density of tiny particulates in the atmosphere, and the nature of the surface, especially the cover of vegetation and water, including ice and snow.

Another one-third of the incoming radiation is absorbed by certain gases and vapors in Earth's atmosphere, especially water vapor and carbon dioxide. Upon absorption, the solar electromagnetic energy is transformed into thermal kinetic energy (that is, heat, or energy of molecular vibration). The warmed atmosphere then re-radiates energy in all directions as longer-wavelength (7–14 µm) infrared radiation. Much of this re-radiated energy escapes to outer space.

The remaining one-third of the incoming energy from the Sun is transformed or dissipated by the following processes:

Absorption and radiation at the surface

Much of the solar radiation that penetrates to Earth's surface is absorbed by living and non-living materials. This results in a transformation to thermal energy, which increases the temperature of the absorbing surfaces. Over the medium term (days) and longer term (years) there is little net storage of energy as heat. This occurs because almost all of the thermal energy is re-radiated by the surface, as electromagnetic radiation of a longer wavelength than that of the original, incident radiation. The wavelength spectrum of typical, re-radiated electromagnetic energy from Earth's surface peaks at about 10 µm, which is within the long-wave infrared range.

Evaporation and melting of water

Some of the electromagnetic energy that penetrates to Earth's surface is absorbed and transformed to heat. Much of this thermal energy subsequently causes water to evaporate from plant and inorganic surfaces, or it causes ice and snow to melt.

Winds, waves, and currents

A small amount (less than 1%) of the absorbed solar radiation causes mass-transport processes to occur in the oceans and lower atmosphere, which disperses of some of Earth's unevenly distributed thermal energy. The most important of these physical processes are winds and storms, water currents, and waves on the surface of the oceans and lakes .

Photosynthesis

Although small, an ecologically critical quantity of solar energy, averaging less than 1% of the total, is absorbed by plant pigments, especially chlorophyll. This absorbed energy is used to drive photosynthesis, the energetic result of which is a temporary storage of energy in the inter-atomic bonds of biochemical compounds.

If the atmosphere was transparent to the long-wave infrared energy that is re-radiated by Earth's atmosphere and surface, then that energy would travel unobstructed to outer space. However, so-called radiatively active gases (or RAGs; also known as "greenhouse gases") in the atmosphere are efficient absorbers within this range of infrared wavelengths, and these substances thereby slow the radiative cooling of the planet. When these atmospheric gases absorb infrared radiation, they develop a larger content of thermal energy, which is then dissipated by a re-radiation (again, of a longer wavelength than the electromagnetic energy that was absorbed). Some of the secondarily re-radiated energy is directed back to Earth's surface, so the net effect of the RAGs is to slow the rate of cooling of the planet.

This process has been called the "greenhouse effect" because its mechanism is analogous to that by which a glass-enclosed space is heated by solar energy. That is, a green-house's glass and humid atmosphere are transparent to incoming solar radiation, but absorb much of the re-radiated, long-wave infrared energy, slowing down the rate of cooling of the structure.

Water vapor (H₂O) and carbon dioxide (CO₂) are the most important radiatively active constituents of Earth's atmosphere. Methane (CH₄), nitrous oxide (N₂O), ozone (O₃), and chlorofluorocarbons (CFCs) play a more minor role. On a per-molecule basis, these gases differ in their ability to absorb infrared wavelengths. Compared with carbon dioxide, a molecule of methane is 11–25 times more effective at absorbing infrared, nitrous oxide is 200–270 times, ozone 2,000 times, and CFCs 3,000–15,000 times.

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 ppm, while in 1994 it was 355 ppm. During the same period CH₄ increased from 0.7 ppm to 1.7 ppm, N₂O from 0.285 ppm to 0.304 ppm; and CFCs from zero to 0.7 ppb. These increased concentrations are believed to contribute to a hypothesized increase in the intensity of Earth's greenhouse effect, an increase attributable to human activities. Overall, CO₂ is estimated to account for about 60% of this enhancement of the greenhouse effect, CH₄ 15%, N₂O 5%, O₃ 8%, and CFCs 12%.

The physical mechanism of the greenhouse effect is conceptually simple, and this phenomenon is acknowledged by scientists as helping to keep Earth's temperature within the comfort zone for organisms. It is also known that the concentrations of CO₂ and other RAGs have increased in Earth's atmosphere, and will continue to do so. However, it has proven difficult to demonstrate that a warming of Earth's surface or lower atmosphere has been caused by a stronger greenhouse effect.

Since the beginning of instrumental recordings of surface temperature around 1880, it appears that almost all of the warmest years have occurred during the late 1980s and 1990s. Typically, these warm years have averaged about 1.5–2.0°F (0.8–1.0°C) warmer than occurred during the decade of the 1880s. Overall, Earth's surface air temperature has increased by about 0.9°F (0.5°C) since 1850.

However, the temperature data on which these apparent changes are based suffer from some important deficiencies, including: (1) air temperature is variable in time and space, making it difficult to determine statistically significant, longer-term trends; (2) older data are generally less accurate than modern records; (3) many weather stations are in urban areas, and are influenced by "heat island" effects; and (4) climate can change for reasons other than a greenhouse response to increased concentrations of CO₂ and other RAGs, including albedo-related influences of volcanic emissions of sulfur dioxide, sulfate, and fine particulates into the upper atmosphere. Moreover, it is well known that the interval 1350 to 1850, known as the Little Ice Age, was relatively cool, and that global climate has been generally warming since that time period.

Some studies have provided evidence for linkages between historical variations of atmospheric CO₂ and surface temperature. Important evidence comes from a core of Antarctic glacial ice that represents a 160,000-year time period. Concentrations of CO₂ in the ice were determined by analysis of air bubbles in layers of known age, while changes in air temperature were inferred from ratios of oxygen isotopes (because isotopes differ in weight, their rates of diffusion are affected by temperature in predictably different ways, and this affects their relative concentrations in the glacial ice). Because changes in CO₂ and surface temperature were positively correlated, a potential greenhouse mechanism is suggested. However, this study could not determine whether increased CO₂ might have resulted in warming through an intensified greenhouse effect, or whether warming could have increased CO₂ release from ecosystems by increasing the rate of decomposition of biomass, especially in cold regions.

Because of the difficulties in measurement and interpretation of climatic change using real-world data, computer models have been used to predict potential climatic changes caused by increases in atmospheric RAGs. The most sophisticated simulations are the so-called "three-dimensional general circulation models" (GCMs), which are run on supercomputers. GCM models simulate the extremely complex, mass-transport processes involved in atmospheric circulation , and the interaction of these with variables that contribute to climate. To perform a simulation experiment with a GCM model, components are adjusted to reflect the probable physical influence of increased concentrations of CO₂ and other RAGs.

Many simulation experiments have been performed, using a variety of GCM models. Of course, the results vary according to the specifics of the experiment. However, a central tendency of experiments using a common CO₂ scenario (a doubling of CO₂ from its recent concentration of 360 ppm) is for an increase in average surface temperature of 1.8–7.2°F (1–4°C). This warming is predicted to be especially great in polar regions, where temperature increases could be two or three times greater than in the tropics.

One of the best-known models was designed and used by the International Panel on Climate Change (IPCC). This GCM model made assumptions about population and economic growth, resource availability, and management options that resulted in increases or decreases of RAGs in the atmosphere. Scenarios were developed for emissions of CO₂, other RAGs, and sulfate aerosols, which may cool the atmosphere by increasing its albedo and by affecting cloud formation. For a simple doubling of atmospheric CO₂, the IPCC estimate was for a 4.5°F (2.5°C) increase in average surface temperature. The estimates of more advanced IPCC scenarios (with adjustments for other RAGs and sulfate) were similar, and predicted a 2.7–5.4°F (1.5–3°C) increase in temperature by the year 2100, compared with 1990.

It is likely that the direct effects of climate change caused by an intensification of the greenhouse effect would be substantially restricted to plants. The temperature changes might cause large changes in the quantities, distribution, or timing of precipitation, and this would have a large effect on vegetation. There is, however, even more uncertainty about the potential changes in rainfall patterns than of temperature, and effects on soil moisture and vegetation are also uncertain. Still, it is reasonable to predict that any large changes in patterns of precipitation would result in fundamental reorganizations of vegetation on the terrestrial landscape.

Studies of changes in vegetation during the warming climate that followed the most recent, Pleistocene, glaciation , suggest that plant species responded in unique, individualistic ways. This results from the differing tolerances of species to changes in climate and other aspects of the environment, and their different abilities to colonize newly available habitat. In any event, the species composition of plant communities was different then from what occurs at the present time. Of course, the vegetation was, and is, dynamic, because plant species have not completed their post-glacial movements into suitable habitats.

In any region where the climate becomes drier (for example, because of decreased precipitation), a result could be a decreased area of forest, and an expansion of savanna or prairie . A landscape change of this character is believed to have occurred in the New World tropics during the Pleistocene glaciations. Because of the relatively dry climate at that time, presently continuous rainforest may have been constricted into relatively small refugia (that is, isolated patches). These forest remnants may have existed within a landscape matrix of savanna and grassland. Such an enormous restructuring of the character of the tropical landscape must have had a tremendous effect on the multitude of rare species that live in that region. Likewise, climate change potentially associated with an intensification of the greenhouse effect would have a devastating effect on Earth's natural ecosystems and the species that they sustain.

There would also be important changes in the ability of the land to support crop plants. This would be particularly true of lands cultivated in regions that are marginal in terms of rainfall, and are vulnerable to drought and desertification. For example, important crops such as wheat are grown in regions of the western interior of North America that formerly supported natural shortgrass prairie. It has been estimated that about 40% of this semiarid region, measuring 988 million acres (400 million ha), has already been desertified by agricultural activities, and crop-limiting droughts occur there sporadically. This climatic handicap can be partially managed by irrigation. However, there is a shortage of water for irrigation, and this practice can cause its own environmental problems, such as salinization. Clearly, in many areas substantial changes in climate would place the present agricultural systems at great risk.

Patterns of wildfire would also be influenced by changes in precipitation regimes. Based on the predictions of climate models, it has been suggested that there could be a 50% increase in the area of forest annually burned in Canada, presently about 2.5–4.9 million acres (1–2 million ha) in typical years.

Some shallow marine ecosystems might be affected by increases in seawater temperature. Corals are vulnerable to large increases in water temperature, which may deprive them of their symbiotic algae (called zooxanthellae), sometimes resulting in death of the colony. Widespread coral "bleachings" were apparently caused by warm water associated with an El Niño event in 1982–83.

Another probable effect of warming could be an increase in sea level. This would be caused by the combination of (1) a thermal expansion of the volume of warmed seawater, and (2) melting of polar glaciers . The IPCC models predicted that sea level in 2100 could be 10.5–21 in (27–50 cm) higher than today. Depending on the rate of change in sea level, there could be substantial problems for low-lying, coastal agricultural areas and cities.

Most GCM models predict that high latitudes will experience the greatest intensity of climatic warming. Ecologists have suggested that the warming of northern ecosystems could induce a positive feedback to climate change. This could be caused by a change of great expanses of boreal forest and arctic tundra from sinks for atmospheric CO₂, into sources of that greenhouse gas. In this scenario, the climate warming caused by increases in RAGs would increase the depth of annual thawing of frozen soils, exposing large quantities of carbon-rich organic materials in the permafrost to microbial decomposition, and thereby increasing the emission of CO₂ to the atmosphere.

It is likely that an intensification of Earth's greenhouse effect would have large climatic and ecological consequences.

Under the auspices of the United Nations Environment Program, various international negotiations have been undertaken to try to get nations to agree to decisive actions to reduce their emissions of RAGs. One recent major agreement came out of a large meeting held in Kyoto, Japan, in 1997. There, industrial countries, such as those of North America and Western Europe , agreed to reduce their CO₂ by as much as 5–7% of their 1990 levels by the year 2012. These reductions will be a huge challenge for those countries to achieve.

One possible complementary way to balance the emissions of RAGs would be to remove some atmospheric CO₂ by increasing its fixation by growing plants, especially through the planting of forests onto agricultural land. Similarly, the prevention of deforestation will avoid large amounts of CO₂emissions through the conversion of high-carbon forests into low-carbon agro-ecosystems.

See also Atmospheric chemistry; Atmospheric circulation; Atmospheric composition and structure; Desert and desertification; Earth (planet); El Niño and La Niña phenomena; Forests and deforestation; Fuels and fuel chemistry; Global warming; Ozone layer and hole dynamics; Ozone layer depletion; Petroleum, economic uses of