Sources and impacts of greenhouse gases

Take away ideas and understandings:

1. Understand the balance between incoming solar and outgoing terrestrial radiation.
2. Understand the principle physics underlying the greenhouse effect.
3. Know the dominant greenhouse gases and their sources.
4. Understand the concept of global warming potential.
5. Understand the relative radiative impacts of greenhouse gases.
6. Understand the concept of "selective absorber" and its relevance to greenhouse warming.
7. What are the relative warming contributions of the different greenhouse gases? What compounds offset some of this warming?
8. What other factors are partly responsible for the warming observed over the past century?

1.0 Introduction to the greenhouse effect: Radiative Forcing

The temperature of the Earth's surface and atmosphere is determined by the balance between incoming energy and outgoing energy. Surface temperatures rise when when more energy is received than lost. The Earth surface receives about 50% of the incoming solar radiation (after some losses by atmospheric absorption and reflection) and this energy heats the surface. The warmed surface reradiates heat back out at longer infrared wavelengths. This radiation is known as terrestrial radiation, as opposed to solar radiation coming in from the sun.

The greenhouse effect is a result of the partial absorption and reradiation back to earth of this outgoing infrared radiation. A change in the net radiative energy available to the global Earth-atmosphere system is termed a radiative forcing and changes in the concentrations of greenhouse gases in the troposphere (such as CO2, N2O, H2O, etc) is one such forcing. Increases in the concentrations of greenhouse gases will reduce the efficiency with which the Earth’s surface radiates to space. More of the outgoing terrestrial radiation from the surface is absorbed by the atmosphere and re-emitted at higher altitudes and lower temperatures. This results in a positive radiative forcing that tends to warm the lower atmosphere and surface. Because less heat escapes to space, this is the enhanced greenhouse effect – an enhancement of an effect that has operated in the Earth’s atmosphere for billions of years due to the presence of naturally occurring greenhouse gases: water vapor, carbon dioxide, ozone, methane and nitrous oxide.

Different greenhouse gases have differing abilities to warm the atmosphere. The net wariming from an ensemble of greenhouse gases depends on the size of the increase in concentration of each greenhouse gas, the radiative properties of the gases involved (indicated by their global warming potential, see below), and the concentrations of other greenhouse gases already present in the atmosphere. Further, many greenhouse gases reside in the atmosphere for centuries after being emitted, thereby introducing a long-term commitment to positive radiative forcing.

Some manmade substances in the atmosphere such as soot and other airborne particples (not gases) can reflect incoming solar radiation and actually have a cooling effect. This cooling effect depends on the size, composition, and concentration of these particles and unfortunately the full radiative effect of these particles is not well known. This is issue is big point of contention between scientists arguing the reality of future global warming. In addition, changes in aerosol concentrations can alter cloud amount and cloud reflectivity through their effect on cloud properties and lifetimes. In most cases, tropospheric aerosols tend to produce a negative radiative forcing and a cooler climate. They have a much shorter lifetime (days to weeks) than most greenhouse gases (decades to centuries), and, as a result, their concentrations respond much more quickly to changes in emissions. Volcanic activity can inject large amounts of sulphur-containing gases (primarily sulfur dioxide) into the stratosphere, which are transformed into sulfate aerosols. Individual eruptions can produce a large, but transitory, negative radiative forcing, tending to cool the Earth’s surface and lower atmosphere over periods of a few years.

When radiative forcing changes, the climate system responds on various time-scales. The longest of these are due to the large heat capacity of the deep ocean and dynamic adjustment of the ice sheets. This means that the transient response to a change (either positive or negative) may last for thousands of years. Any changes in the radiative balance of the Earth, including those due to an increase in greenhouse gases or in aerosols, will alter the global hydrological cycle and atmospheric and oceanic circulation, thereby affecting weather patterns and regional temperatures and precipitation.

Shortwave Radiation Budget (incoming Solar radiation)

About 50% of incoming solar radiation is actually absorbed by the Earth's surface (see below). Solar radiation entering the Earth's atmosphere (called "shortwave" radiation) can be reflected off clouds, the surface, and air molecules and dust. On a global average this accounts for about 30% of incoming radiation. This percentage is quantified as the albedo of the system. Another 19% on average is absorbed by the atmosphere, mainly by ozone in the Earth's stratosphere. The remaining 51% is absorbed by the Earth's surface.

Longwave Radiation Budget (ongoing Terrestrial radiation)

As was learned in the beginning of the semester all objects emit radiation in an amount and at a wavelength dictated by the object's temperature (Wein's Law). The 51% of shortwave radiation absorbed by the Earth's surface heats the surface. But as the surface heats it emits radiation in the infrared back into the atmosphere.

The figure below shows the annual global average exchange of energy between the Earth's surface and the atmosphere. Note the 51% of original solar radiation is absorbed, but 117% of the original solar input is emitted to the atmosphere, how can this be?

The answer makes sense when we consider that the surface of a planet receives a great deal of energy from its own atmosphere. Thus the effect of the atmosphere is to warm the surface over the temperature above that resulting from the Sun's energy.

The atmosphere warms the Earth by "trapping" radiation, allowing the surface to warm to 300°K. At that temperature, the black body surface radiation is large enough to ensure that an equilibrium condition pertains. The atmosphere traps radiation through the action of certain gases, called Greenhouse Gases. These gases (e.g., CO₂, H₂O, NO, CFCs, CO) are very good at absorbing and re-emitting infrared radiation. They intercept the IR radiation from the ground and reflect some of the energy back to the ground, warming it up more than would occur otherwise.

2.0 History of greenhouse gas accumulation in the atmosphere

3.0 Sources and projected changes in greenhouse gas emissions

What are the sources, residence times, and projected concentrations of these compounds?

The main human activities that increase greenhouse gases are energy use (automobiles, goods tranport, etc), air conditioning, and agriculture.

The US is the leading greenhouse gas emitter, comprising nearly 20% of the global average emissions. The former soviet republics comprise the next largest joint emitter at near 14% of gloabl emisions, followed by China (10%), Japan (5%), Brazil, Germany, and India (each ~4%).

Future emissions depend, in part, on the changing demography of individual nations and on adaptation of future technologies. Significantly, the US and Europe will only comprise ~3-10% of future fossil fuel usage, whereas China and India are projected to lead consumption, having nearly tripled their consumption over the last twenty years.

4.0 Greenhouse gas "Global Warming Potential"

The Global Warming Potential (GWP) of a greenhouse gas is the ratio of global warming, or radiative forcing – both direct and indirect – from one unit mass of a greenhouse gas to that of one unit mass of carbon dioxide over a period of time. Hence this is a measure of the potential for global warming per unit mass relative to carbon dioxide.

Global Warming Potentials are presented in Table 1 for an expanded set of gases. GWPs are a measure of the relative radiative effect of a given substance compared to CO₂, integrated over a chosen time horizon. New categories of gases in Table 1 include fluorinated organic molecules, many of which are ethers that are proposed as halocarbon substitutes. Some of the GWPs have larger uncertainties than that of others, particularly for those gases where detailed laboratory data on lifetimes are not yet available. The direct GWPs have been calculated relative to CO₂ using an improved calculation of the CO₂ radiative forcing, the SAR response function for a CO₂ pulse, and new values for the radiative forcing and lifetimes for a number of halocarbons. Indirect GWPs, resulting from indirect radiative forcing effects, are also estimated for some new gases, including carbon monoxide. The direct GWPs for those species whose lifetimes are well characterized are estimated to be accurate within ±35%, but the indirect GWPs are less certain.

5.0 Relative warming contributions of different greenhouse gases

6.0 The rise in atmospheric carbon dioxide

Updated April 17, 2002

©2002 P. deMenocal (LDEO, Columbia Univ.)