The gases that make up the Earth's atmosphere and the way in which energy passes through or is absorbed by these gases play a crucial role in regulating the temperature of the planet. The atmosphere, made up mostly of molecular nitrogen (78%) and oxygen (21%), contains small amounts of particular gases referred to as radiatively active gases. Prominent among the radiatively active gases are water vapor (H₂O) and carbon dioxide (CO₂), both of which exist in relatively minute quantities. These gases allow most sunlight, primarily visible radiation, to pass through the atmosphere to the planet's surface, where about 70% of the energy is absorbed, raising the temperature of the Earth. The Earth then emits thermal (infrared) radiation to space, thereby maintaining an energy balance: the amount of energy entering the Earth/atmosphere system equals the amount leaving.

As this thermal radiation makes its way out of the atmosphere, it is intercepted by radiatively active trace gases. They absorb the outgoing radiation, increasing in temperature as they do so. This interplay between thermal radiation emission and absorption by the atmosphere raises the overall temperature of the Earth and the atmosphere system above what it would be if there were no atmosphere present. In fact, without the presence of radiatively active gases in the atmosphere, the Earth would only be 1.4 degrees Fahrenheit! (-17 degrees Celsius.) Because of the energy absorbed by the atmosphere, the global average temperature is instead a comfortable 59 degrees Fahrenheit (15 degrees Celsius). This insulating ability has come to be known as the "greenhouse effect" because the process is much like that in a greenhouse, where visible light passes through the panes of glass in the ceiling, but heat is retained within through absorption of infrared radiation by the glass.

Unfortunately, human activities such as the burning of fossil-fuels, large-scale fertilizer use, cattle production, and deforestation have begun to directly increase the amount of "greenhouse gas" in the atmosphere above natural levels. This rise in greenhouse gas concentration is expected to increase the global average temperature of the planet to levels that may disrupt atmospheric, oceanic, ecological, and ultimately human systems and well-being. It is this enhancement of the natural greenhouse effect that is referred to as "global warming."

The principal greenhouse gases, in order of their estimated contribution to global warming are: carbon dioxide, methane, halocarbons, and nitrous oxide. Measurements taken at remote locations around the globe reveal the unmistakable increase in concentration of these gases in the atmosphere. Some, like carbon dioxide, are both natural and anthropogenic gases. Others, like some halocarbons, are purely man-made.

The Principal Greenhouse Gases

Carbon Dioxide (CO₂): CO₂ is by far the greatest contributor to climate change, accounting for about 64% of estimated current global warming. The primary sources of carbon dioxide emissions to the atmosphere are the production, transportation, processing, and consumption of fossil fuels (86%), tropical deforestation and other biomass burning (12%), and miscellaneous sources (2%), such as cement manufacturing and oxidation of carbon monoxide. Once emitted, a specific molecule of carbon dioxide cycles through the atmosphere and the biota before being permanently removed by oceanic processes or long term increases in terrestrial biotic storage (i.e., uptake by plants). The amount of time in which about 63 percent of the emissions of a gas are removed from the atmosphere is called its effective residence time. There is often a considerably uncertainty in this crucial parameter, which is important for calculating the climatic effects of a greenhouse gas. When the rate of emission of a greenhouse gas is greater than the rate of removal, then its atmospheric concentrations increase. For carbon dioxide, this has been happening over the last century or more. The estimated range for effective residence time of carbon dioxide is 50 to 200 years.

Methane (CH₄): Methane has both natural and anthropogenic sources of which the latter is derived primarily from fuel production, enteric fermentation (e.g. cattle), rice cultivation, landfill emissions, and deforestation (mainly biomass burning and decay of excess organic matter). Accounting for an estimated 20% of current global warming, methane emissions are a significant source of greenhouse gases. Molecule for molecule, methane is about 21 times more effective a greenhouse gas than CO₂. Methane is principally removed from the atmosphere by reacting with the hydroxyl radical (OH).¹ Because many hydrocarbons and halocarbons (including many ozone-depleting compounds) also are removed from the atmosphere through reaction with OH, higher methane concentrations can have significant effects on the general ability of the atmosphere to remove greenhouse gases. There are some indications that methane and other pollutants have caused a reduction in OH concentrations. About 30% of the increase in methane concentration in the atmosphere is due to the reduced capacity of the atmosphere to absorb it.

Halocarbons: Halocarbons are a class of chemical compounds, both human-made and natural, containing carbon and one or more atoms belonging to the halogen group of elements, such as fluorine and chlorine.² The most abundant halocarbons in terms of their contribution to global warming are chlorofluorcarbons (CFCs, also known by the trade name, Freon); specifically CFC-11 and CFC-12. Though existing in relatively trace amounts in the atmosphere, these chemical compounds exhibit powerful radiative trapping abilities in addition to their well-known ozone depleting properties. Halocarbons account for about 10% of current global warming, but the atmospheric concentration of these compounds has begun to fall as a result of an international ban on their production and consumption. Measurements of similar chemicals used as substitutes for CFCs -- hydrochlorofluorcarbons (HCFCs) and hydrofluorcarbons (HFCs) -- are now showing concentration increases. Should concentrations continue to rise, these substitute chemicals may contribute significantly to global warming in the future.

Nitrous Oxide (N₂O): Like CO₂, nitrous oxide is present naturally in the atmosphere. However, the extensive use of artificial nitrogen fertilizer and fossil-fuel combustion account for the majority of anthropogenic emissions of nitrous oxide. N₂O levels account for about 6% of current global warming.

Characteristics of the Principal Greenhouse Gases
greenhouse gas	primary sources	present concentration in atmosphere (ppmv)	% annual increase	Atmospheric Increase*	effective residence time in atmosphere	sinks and reservoirs*
carbon dioxide	production of commercial energy; deforestation; other biomass burning.	360	0.4%	~7.1 billion metric tons/yr3	50-200 years	Atmospheric reservoir; ocean uptake; uptake by N. Hemisphere forest growth. (Occurs over a few years.) Transfer to soils and to the deep ocean (Occurs on century time scale)
methane	natural gas production and transmission; enteric fermentation (e.g., cattle); rice cultivation, landfill emissions, deforestation	1.7	0.5%	~37 million metric tons/yr.	12.5 years	Main removal process: tropospheric hydroxyl radical (OH)4; also: stratosphere; soils
halocarbons Most abundant are CFC-11 and CFC-12	solely of human origin: used in industrial processes and end-use products like air-conditioners and refrigerators (as coolants and insulation)	CFC-11 = 270 CFC-12 = 500	Falling due to ban on use. Substitutes (HCFCs, and HFCs) are showing increases.	CFCs: currently ~0 should decrease slowly due to Montreal Protrocol; HCFCs, HFCs: recently showing an increase	ranges from a few years to a few thousand years	Atmospheric reservoir; removed mainly through breakdown by sunlight (photolysis) in the stratosphere
nitrous oxide	mainly from use of fertilizer and fossil-fuel combustion	315	0.25%	3-8 million metric tons/yr.	120 years	Removed mainly through breakdown by sunlight (photolysis) in the stratosphere
* "Atmospheric increase" and "sinks and resevoirs" from Intergovernmental Panel on Climate Change, Climate Change 1995 (Cambridge University Press, 1996), pp. 15-19. CFCs = chlorofluorocarbons HFCs = hydrofluorocarbons HCFCs = hydrochlorofluorocarbons ppmv = parts per million by volume

Measuring and Modeling Global Warming

Temperature data collected over the last century show a statistically significant rise in global mean temperature of between 0.3 and 0.6 degrees Celsius since the late 19th century. While there are some uncertainties as to the extent this is attributable to increases in greenhouse gases, the temperature increases recorded so far are broadly consistent with global warming theory. This evidence, coupled with that of greater occurrences of extreme climatic events, has led the Intergovernmental Panel on Climate Change (IPCC) to conclude that "the balance of evidence suggests that there is a discernible human influence on the global climate."⁵

In order to estimate what future changes in climate might occur as a result of greenhouse gas increases, models of climate, called general circulation models have been developed with various assumptions about the workings of the physical climate. Though there are uncertainties with the projection (mainly associated with the role of increased evaporation and cloud formation in redistributing radiation and thermal energy) the near-consensus estimate is that the average global temperature would increase 1.0 to 3.5 degrees Celsius with a doubling of pre-industrial carbon dioxide-equivalent. Under present trends, this is expected to occur near the year 2100. Regionally, temperatures could increase as much as 10 degrees Celsius in the polar regions and possibly not at all in equatorial areas.

Warming beyond this estimate is possible given further increases in greenhouse gas concentrations. Many researchers have suggested the possibility of catastrophic, sudden increases in methane and/or carbon dioxide. Increasing temperatures could cause a sufficient melting of permafrost and frozen soil layers in the polar regions to release huge amounts of methane and carbon dioxide now trapped in them. The quantities of greenhouse gases emitted could potentially be so large and their effects on atmospheric chemistry and composition so unpredictable that no model exists that could even begin to estimate the effects with moderate confidence. We do not even know enough to calculate how likely or unlikely such catastrophic events might be. We know only that they are possible and that the resulting changes may be devastating far beyond anything projected by current models of global warming.

Aside from changes in global average temperature, a variety of other climate variables may change as a result of the increased absorption of outgoing radiation and the subsequent increase in temperature. While there are considerable uncertainties as to the specifics, the most important possible changes are:

• an increase in global precipitation, especially in mid- to high-latitude regions in winter
• a decrease in soil moisture over the mid-latitudes in summer
• diminishing global sea ice and snow cover
• an increase in tropical storm intensity
• a rise in global sea level of 50 cm (a little over 1.5 feet) by the year 2100

A host of alterations to ecological, biogeochemical, human, and animal systems could occur in response to these perturbations to the climate and hydrologic systems. They may be a result of the absolute magnitude of climate change, and/or the rapidity with which the projected changes occur. In fact, some researchers believe that the speed of temperature change and other changes is likely to be the main cause of any subsequent ecological and economic disruption since neither ecosystems nor populations will have enough time to adjust.⁶

What are the Options?

As the science has improved and uncertainties narrow, more options to mitigate global warming have emerged. Given that CO₂ emissions from fossils fuel use are the largest single source of greenhouse gases, changes in current energy production and consumption are being examined carefully. Because coal produces more CO₂ per unit energy delivered than natural gas, many proposals include shifts in electricity production towards natural gas (see below). Further reductions in CO₂ emissions can come from greater energy efficiency measures such as improved lighting, more efficient industrial processes, co-generation of electricity and heat (see "Dear Arjun"), better building insulation, and more efficient cars and trucks. Increased reliance on nuclear power has also been suggested, but it is not an environmentally or economically sound alternative (see main article).

Given the current trends in energy consumption and the expansion of electricity use in many parts of the world, a shift away from fossil-based energy appears necessary in the long-run to mitigate the projected rise in CO₂ to the extent many feel is necessary. To that end, renewable energy supplies such as solar photovoltaics, biomass, and wind energy are being considered. Natural gas could provide a good source of fuel during the transition to these energy supplies. However, it should be noted that natural gas production, transmission, and use involves small amounts of methane emission whose greenhouse effect is greatly magnified since methane is a more powerful greenhouse gas than carbon dioxide. Increased use of natural gas must therefore be accompanied by measures to reduce anthropogenic methane emissions. This can be done in a variety of ways, such as capturing and using methane emitted from landfills (emissions due to anaerobic decomposition of organic matter, such as food waste), reducing transmission losses, and converting animal manure into usable methane through anaerobic digestion.

There is also the possibility of direct removal of CO₂ from the atmosphere through net growth of plants and trees; a mitigation option referred to as carbon sequestration. Through reforestation of areas that had been converted to agriculture in the past (for example, New England) some of the rising CO₂ in the atmosphere can be permanently stored in soils or in the tissue of living things. Other sequestration schemes, such as pumping CO₂ into underground and undersea reservoirs, have also been proposed.

Limiting the emission of other greenhouse gases, such as halocarbons, N₂O (nitrous oxide), and CH₄ (methane) can also help mitigate global warming. As noted above, gains have been made through the regulation of the well-known CFCs, but compounds such as hydrofluorocarbons and hydrochlorofluorocarbons are either unregulated or are slated to be regulated decades from now.

The build-up of greenhouse gases due to human activities over the last century is an incontrovertible, established fact. The general radiative characteristics of these gases are also well known. These facts, coupled with many other laboratory experiments, observations of the Earth's temperature, and biogeochemical characteristics, have led to the general conclusion accepted by most scientists in the field that increasing greenhouse gases have already affected the climate and are likely to affect it far more if we do not act to curb them. The means to curb them are known -- the largest uncertainties revolve not around technical facts, but cost.

Kevin Gurney is an atmospheric scientist at the Donald Bren School of Environmental Science and Management at the University of California in Santa Barbara. He is also co-author with Arjun Makhijani of Mending the Ozone Hole; Science, Technology and Policy, published by MIT Press in 1995.