EDUCATION: GLOBAL METHANE INVENTORY

The Greenhouse Effect, Greenhouse Gases, and Global Warming

By Harvey Augenbraun, Elaine Matthews, and David Sarma

Note: This page was originally written in conjunction with the 1997 Global Methane Inventory.

1. Introduction to the Greenhouse Effect

The Earth's surface spheres (atmosphere, hydrosphere, lithosphere, biosphere) interact and behave much like a living community. The interaction of these spheres with solar energy and each other results in changing conditions which we refer to as weather and climate. Before reaching the Earth's surface, solar radiation passes through clouds and atmosphere, which reflect, scatter, absorb, and transmit various amounts of energy. The Earth's surface reflects some of the incoming solar radiation and absorbs the remainder. As the surface absorbs the energy, it heats and radiates the energy back into space. When the rates of absorption and radiation are equal (radiative balance) the Earth's temperature is stable. If the atmosphere did not exist, the Earth's surface would reach radiative balance at 33° Celsius (60° Fahrenheit) below its present balance temperature. However, some gases in the atmosphere absorb some of the energy radiated from the surface. They heat and re-radiate energy back to the surface. In this way, the atmosphere maintains a higher surface temperature than the Earth would have without an atmosphere. This process is called the greenhouse effect. The following sections provide overviews of the greenhouse effect, atmospheric greenhouse gases, and sources and sinks of the gases.

2. Natural Greenhouse Effect

Much of the energy absorbed at the Earth's surface is radiated upward as infrared (IR) thermal energy. Several gases that occur naturally in the atmosphere absorb this infrared energy and re-radiate it back to the surface. Therefore, heat that would be lost to space is trapped near the surface. The atmosphere and its heat-absorbing gases warm the Earth's surface, which reaches radiative balance at a higher temperature than if there were no atmosphere, or an atmosphere without IR-trapping gases. The term "greenhouse" is used to describe this phenomenon since these gases act like the glass of a greenhouse to trap heat and maintain higher interior temperatures than would normally occur. The atmospheric gases most responsible for this effect are water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and ozone (O₃). This "greenhouse effect" occurs naturally in our atmosphere and is responsible for the Earth's moderate surface temperature.

Water vapor and carbon dioxide are the largest contributors to the natural greenhouse effect due to their overall abundance in the atmosphere. Methane, although at very low atmospheric concentrations, is a much more efficient absorber of infrared radiation than either H₂O or CO₂. Over a 100 year period, a molecule of methane can absorb about 25 times more IR energy than a molecule of carbon dioxide. A molecule of nitrous oxide can absorb about 320 times more energy than one of CO₂. Therefore, although methane and nitrous oxide occur in very low concentrations, they exert a substantial influence because they are potent absorbers.

3. Past Oscillations in Carbon Dioxide and Methane Concentrations

Natural greenhouse gas concentrations have varied over time. Figure 1-1 shows variations in CH₄ and CO₂ concentrations and temperature anomalies for the past 200,000 years as derived from analysis of the Vostok ice cores from Antarctica. The major temperature trends from warmer (140,000 yrs BP) to colder (20,000 yrs BP) are called the interglacial (warmer) and glacial (colder) periods. CH₄ varied from ~700 parts per billion by volume (ppbv) to ~300 ppbv, CO₂ varied from ~300 parts per million by volume (ppmv) to ~150 ppmv. Temperature anomalies varied about 4-5° C (Lorius and Oeschger, 1994).

Figure 1-1: Variations in CH₄ and CO₂ concentrations and anomalies as derived from analysis of the Vostok ice cores.

4. Anthropogenic Greenhouse Effect

Since the beginning of the Industrial Revolution about 200 years ago, atmospheric concentrations of greenhouse gases including CO₂, CH₄ and N₂O have risen substantially. These increases are a result of a variety of anthropogenic activities such as the production and use of fossil fuels, as well as other industrial and agricultural activities.

Figure 1-2: Trends in atmospheric CO2 (Lorius and Oeschger, 1994)

4.1 Carbon Dioxide

Naturally occurring CO₂ undergoes a seasonal cycle. Atmospheric CO₂ is taken up through photosynthesis of plants during the growing season and released through respiration throughout most of the year. These exchanges are about equal over the period of one year. This biospheric cycle is the major cause of the large seasonal oscillations in atmospheric concentrations of CO₂. In addition, naturally occurring forest fires release CO₂ when the vegetation is burned. This disturbance in the cycle is followed by a longer term uptake of CO₂ by regrowth of vegetation lasting years to decades after the fire.

Anthropogenic activities perturb the natural carbon cycle. During the last ~200 years, atmospheric concentrations of CO₂ have increased about 25% (Figure 1-2). Because CO₂ is not chemically active, terrestrial emissions either accumulate in the atmosphere or are taken up by the oceans or by the terrestrial biosphere (vegetation and/or soil). The largest single source of atmospheric CO₂ is the burning of fossil fuels which accounts for ~80% of the annual emission from the Earth to the atmosphere (Matthews 1997). CO₂ is also emitted from vegetation and soils from tropical deforestation and from savanna fires. A portion of the CO₂ emitted from the Earth accumulates in the atmosphere while some is taken up by the oceans and by the biosphere, for example during the regrowth of forests following abandonment and fires (Table 1-1).

Figure 1-3: Trends in atmospheric N2O (Lorius and Oeschger, 1994)

4.2 Nitrous Oxide

Naturally occurring terrestrial N₂O emissions are primarily due to microbial action in soils, especially in tropical regions. These processes are strongly controlled by the status of oxygen, water, and nutrients in the soils.

Anthropogenic activities have contributed to a 10% growth in nitrous oxide concentrations in the atmosphere over the last ~200 years (Figure 1-3). Sources (Table 1-1) include land clearing, biomass burning, fossil fuel combustion, and nitrogen fertilizer use. Unlike CO₂, nitrous oxide is chemically active; it is broken down in the stratosphere by sunlight (photolysis) and other chemical reactions.

4.3 Methane

Figure 1-4: Trends in atmospheric CH4 (Lorius and Oeschger, 1994)

Naturally occurring terrestrial methane emissions are dominated by wetlands where anaerobic decay of organic material results in production and release of methane. Other natural methane sources include wild fires in both forests and grasslands, and wild animals (Table 1-1).

Atmospheric concentrations of methane have approximately doubled during the last two centuries (Figure 1-4). Anthropogenic sources include rice cultivation, agricultural animals such as cattle and other ruminants, animal waste, landfills, and the mining, processing, and distribution of fossil fuels. The emission of methane by both natural and anthropogenic sources varies seasonally. For example, methane is produced only when wetlands at high latitudes are warmed and thawed or when rice fields are flooded. Since methane is chemically active, it is broken down in the troposphere by hydroxyl (OH) radicals eventually forming carbon monoxide.

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