Greenhouse effect

The "greenhouse effect" is the process by which the atmosphere warms a planet. Mars, Venus and other planets have a greenhouse effect too, but for simplicity the rest of this article will refer to the case of Earth.

When solar radiation reaches Earth's atmosphere, some is reflected back and some is absorbed, but much of it passes through and reaches the surface. There, most of the radiation is absorbed, which warms the surface. The surface radiates heat back at longer (infrared) wavelengths, and the atmosphere absorbs some of this radiation. This warms the atmosphere, and it eventually passes some of the energy back to the surface. [1] .

The composition of the atmosphere means that it absorbs more infrared radiation than visible sunlight. The atmosphere's effect of sending energy radiated from the surface back down outweighs its effect of reducing the amount of sunlight which reaches the surface. The result is that the surface of the earth is warmer than it would be in the absence of the atmosphere.

It is commonplace for over-simplistic descriptions of the "greenhouse" effect to assert that the same mechanism warms greenhouses (e.g. [2] ), but the effect is different: see below. For this reason the term is often written in quotes; such usage will be dropped from here on.

The term greenhouse effect may be used to refer to two different things in common parlance: the total greenhouse effect (see also climate change), or more loosely the additional (anthropogenic) greenhouse effect (see also anthropogenic global warming). The former is accepted by all; the latter is a matter of dispute. This page is about the former.

Controlling factors

Water vapor (H₂O) causes about 60% of Earth's naturally-occurring greenhouse effect, with others carbon dioxide (CO₂) (about 26%), methane (CH₄), nitrous oxide (N₂O) and ozone (O₃) (about 8%) [3] , collectively known as greenhouse gases. This "greenhouse effect" occurs naturally in our atmosphere and is responsible for the earth's surface temperature which allows life on Earth.

Visible light from the Sun is partially able to pass through the atmosphere and reach the planet's surface where much of it is absorbed, thereby warming the surface [4] . The actual amounts absorbed at any place and time depend strongly on the atmosphere (primarily the clouds), the surface albedo (snow being reflective, oceans absorbing) and latitude (higher latitudes have a longer atmospheric path length and thus more scattering and absoption). Some of the heat is radiated back at longer infrared wavelengths (the rest, assuming no long-term temperature changes, is moved within the atmosphere or oceans; there is a net flux of absorbed energy from the equator to the poles) and the greenhouse gases in the atmosphere absorb some of this radiation, thereby warming up and eventually passing some of the energy back to the surface. The wavelengths of light that a gas absorbs is a function of the quantum mechanically-determined energy levels that are characteristic of the different gas molecules. It so happens that tri- (and more) atomic gases absorb strongly in infra-red wavelengths, which is why H₂O and CO₂ are greenhouses gases but the major atmospheric constituents (N₂ and O₂) are not.

The degree of the greenhouse effect is dependent primarily on the concentration of greenhouse gases in the planetary atmosphere. Of the planets Venus, Earth, and Mars the dense, carbon dioxide-rich atmosphere of Venus causes a runaway greenhouse effect with surface temperatures hot enough to melt lead, the atmosphere of Earth causes a greenhouse effect of habitable temperatures, and the thin atmosphere of Mars causes a minimal greenhouse effect.

The use of the term runaway greenhouse effect to describe the situation obtaining on Venus, emphasises the interaction of the greenhouse effect with other processes in feedback cycles. Venus is sufficiently strongly heated by the Sun that water is vaporised and so carbon dioxide is not reabsorbed by the planetary crust. As a result, the greenhouse effect has been progressively intensified by positive feedback. On Earth there is a substantial hydrosphere and biosphere which respond to higher temperatures by recycling atmospheric carbon more quickly (in geologic terms; the timescale for the ocean/biosphere to remove a CO₂ perturbation is of the order of several hundred years). The presence of liquid water thus limits the increase in the greenhouse effect through negative feedback. This state of affairs is expected to persist for at least hundreds of millions of years, but, ultimately, the warming of an aging Sun will overwhelm this regulatory effect.

Real greenhouses

The term 'greenhouse effect' originally came from the greenhouses used for gardening, but it is a misnomer since greenhouses operate differently. A greenhouse is built of glass; it heats up primarily because the Sun warms the ground inside it, which warms the air near the ground, and this air is prevented from rising and flowing away. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature will drop considerably. It has also been demonstrated experimentally (Wood, 1909). Greenhouses thus work by preventing convection; the greenhouse effect however reduces radiation loss, not convection.

Effects of various gases

It is hard to disentangle the percentage contributions to the "GH" effect from different gases, because there are overlaps in the IR spectrum of the various gases. However, one can calculate the percentage of trapped radiation remaining, and discover:

(Source: Ramanathan and Coakley, Rev. Geophys and Space Phys., 16 465 (1978))

Water vapour effects

Water vapor is the major contributor to Earth's greenhouse effect. Its effects vary due to localized concentrations, mixture with other gases, frequencies of light, different behavior in different levels of the atmosphere, and whether positive or negative feedback takes place. High humidity also affects cloud formation, which has major effects upon temperature but is distinct from water vapor gas.

The IPCC TAR (2001; section 2.5.3) reports that, despite non-uniform effects and difficulties in assessing the quality of the data, water vapour has generally increased over the 20th Century.

Estimates of the percentage of Earth's greenhouse effect due to water vapor:

• 36% (table above)
• 60-70% Nova. Greenhouse - Green Planet [5]

Including clouds, the table above would suggest 50%. For the cloudless case, IPCC 1990, p 47-48 estimate water vapour at 60-70% whereas Baliunas & Soon estimate 88% [6] considering only H2O and CO2. For a theoretical case if no other greenhouse gases were in the atmosphere, Richard Lindzen estimated 98% (Global warming: the origin and nature of the alleged scientific consensus. Regulation, Spring 1992 issue, 87-98 [7] ).

Water vapour in the troposphere, unlike the better-known greenhouse gasses such as CO₂, is essentially passive in terms of climate: the residence time for water vapour in the atmosphere is short (about a week) so perturbations to water vapour rapidly re-equilibriate. In contrast, the lifetimes of CO₂, methane, etc, are long (hundreds of years) and hence perturbations remain. Thus, in response to a temperature perturbation caused by enhanced CO₂, water vapour would increase, resulting in a (limited) positive feedback and higher temperatures. In response to a perturbation from enhanced water vapour, the atmosphere would re-equilibriate due to clouds causing reflective cooling and water-removing rain. The contrails of high-flying aircraft sometimes form high clouds which seem to slightly alter the local weather.

Global warming

In recent years some researchers see the greenhouse effect as a significant contributing factor to the current global warming, due to the increased concentration of some greenhouse gases in the atmosphere as a result of human activity. Such climatologists are concerned that increased levels of greenhouse gases in the atmosphere might cause more heat to be trapped. The hypothesis that a man-made increase in greenhouse gas concentration would lead to a higher global mean temperature was first postulated in the late 19th century by Swedish chemist and 1903 Nobel Laureate Svante Arrhenius (see global warming), although, his peers largely rejected that theory. The theory that human greenhouse gas emissions are connected with the observed heating of Earth's atmosphere in the 20th century has steadily gained adherents in the popular community since the 1980s, to the extent that many bodies around the world have strongly endorsed it. Automobile exhausts, coal-burning power plants, factory smokestacks, and other waste vents of the industrial age now pump six billion tons of carbon dioxide and other greenhouse gases into Earth's atmosphere each year. Concentrations of human-influenced greenhouse gases in the atmosphere are currently at approximately 25% above pre-industrial values. This is considerably higher than at any time during the last 420,000 years, the period for which reliable data (from ice cores) exists. From less direct geological evidence, it is believed that values this high were last attained 40 million years ago. Since the last Ice Age, the Earth has had a lower temperature than usual, so discussion about recent warming since the Little Ice Age continues. See also:

• emissions trading
• Kyoto Protocol

References

• Kiehl, J.T., and Trenberth, K. (1997). Earth's annual mean global energy budget, Bulletin of the American Meteorological Society 78 (2), 197–208.
• Wood, R.W. (1909). Note on the Theory of the Greenhouse, Philosophical Magazine 17, p319–320. For the text of this online, see http://www.wmc.care4free.net/sci/wood_rw.1909.html