Carbon Dioxide (CO2)

Carbon dioxide is probably the most important of the greenhouse gases, as it accounts for the largest proportion of the 'trace gases' and is currently responsible for 60% of the "enhanced greenhouse effect". It is thought that it's been in the atmosphere for over 4 billion of the Earth's 4.6 billion year geological history and in much larger proportions (up to 80%) than today.

Most of the carbon dioxide was removed from the atmosphere as early organisms evolved photosynthesis. This locked away carbon dioxide as carbonate minerals, oil shale and coal, and petroleum in the Earth's crust when the organisms died. This left 0.03% in the atmosphere today.

The CO₂ cycle

Atmospheric carbon dioxide comes from a number of natural sources, mainly the decay of plants, volcanic eruptions and as a waste product of animal, including human, respiration.

It is removed from the atmosphere by photosynthesis in plants and by dissolving in water, especially on the surface of oceans.

Carbon dioxide stays in the atmosphere for approximately 100 years.

The amount of carbon dioxide taken out of the atmosphere by plants is almost perfectly balanced with the amount put back into the atmosphere by respiration and decay. Small changes as a result of human activities can have a large impact on this delicate balance.

The impact of human activities

Burning fossil fuels releases the carbon dioxide stored millions of years ago. We use fossil fuels (petrol, diesel and kerosene) to run vehicles, heat homes, businesses and power factories. Deforestation releases the carbon stored in trees and also results in less carbon dioxide being removed from the atmosphere.

The statistics

The concentration of carbon dioxide in the atmosphere has increased more in the northern hemisphere where more fossil fuel burning occurs. Since the Industrial Revolution the concentration globally has increased by 30%.

The future

The best case scenario for the increase in carbon dioxide emissions predicts that the concentration of carbon dioxide in the atmosphere will reach double the level of before the Industrial Revolution, in 2100. The worst case scenario brings this forward to 2045.

Methane (CH4)

The importance of methane in the greenhouse effect is its warming effect. Even though it occurs in lower concentrations than carbon dioxide, it produces 21 times as much warming as CO₂. Methane accounts for 20% of the 'enhanced greenhouse effect'.

The CH₄ cycle

Methane is generated naturally by bacteria that break down organic matter, it is found in the guts of termites and other animals and in natural gas deposits. The livestock sector in general (primarily cattle, chickens, and pigs) produces 37% of all human-induced methane. All plants produce methane, and as the climate warms they produce more. In fact, of the 600 million metric tons of methane produced per year, 225 of those produced by plants.

Methane remains in the atmosphere for 11-12 years - less time than most other greenhouse gases.

Methane is removed from the atmosphere through a natural process to form water.

Currently little is known about the volume of methane that passes in and out of the atmosphere, but it is thought that this is in balance.

The impact of human activities

An increase in livestock farming and rice growing has led to an increase in atmospheric methane. Other sources are the extraction of fossil fuels, landfill sites and the burning of biomass.

The statistics

Since the Industrial Revolution, the level of Methane in the atmosphere has increased by two and a half times.

The future

The rise in methane started more recently than the rise in carbon dioxide, and the process of removal from the atmosphere is difficult to predict. However, without technological change further increases in concentrations are inevitable.

Nitrous Oxide (N2O)

Nitrous oxide, dinitrogen oxide, or dinitrogen monoxide, is used in surgery and dentistry for its anaesthetic and analgesic effects, where it is commonly known as "laughing gas" due to the euphoric effects of inhaling it.

Nitrous oxide makes up an extremely small amount of the atmosphere - it is less than one-thousandth as abundant as carbon dioxide. However, it is 200 to 300 times more effective in trapping heat than carbon dioxide.

The N₂O cycle

Nitrogen is removed from the atmosphere by plants and converted into forms such as ammonia, which can then be used by the plants. This is called nitrogen fixation. At the same time, micro-organisms remove nitrogen from the soil and put it back into the atmosphere - denitrification - and this process produces nitrous oxide.

Nitrous oxide also enters the atmosphere from the ocean.

Nitrous oxide has one of the longest atmosphere lifetimes of the greenhouse gases, lasting for up to 150 years.

The impact of human activities

Burning fossil fuels and wood is one source of the increase in atmospheric nitrous oxide, however the main contributor is believed to be the widespread use of nitrogen-based fertilisers. Sewage treatment plants may also be a major source of this gas.

The statistics

Since the Industrial Revolution, the level of nitrous oxide in the atmosphere has increased by 16%.

The future

Due to the long time it spends in the atmosphere, the nitrous oxide that we release today will still be trapping heat well into the next century.

Ozone (O3)

Ozone is an everyday part of the atmosphere and is constantly being created and destroyed. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals. Ozone in the upper atmosphere filters potentially damaging ultraviolet light from reaching the Earth's surface. Ozone does function as a greenhouse gas, but its strength compared to carbon dioxide is yet to be calculated.

The O₃ cycle

Ozone is created and destroyed by ultraviolet light from the Sun. It is created from oxygen by high energy rays, while low energy rays destroy it.

The impact of human activities

Some ozone is man-made by various kinds of air pollution, which then reacts in sunlight.

Water Vapour (H2O)

Gaseous water represents a small but environmentally significant constituent of the atmosphere. Human activities have little impact on the level of water vapour in the atmosphere: about 0.28% of the greenhouse effect is caused by human activity, if water vapour is taken into account - about 5.53%, if not. Water vapour, in fact, is the biggest (accounting for about 95%), and most variable, contributor to the Earth's "natural greenhouse effect".

Most water vapour is contained in the troposphere. Besides accounting for most of Earth's natural greenhouse effect, which warms the planet, gaseous water also condenses to form clouds, which may act to warm or cool, depending on the circumstances. In general terms, atmospheric water strongly influences, and is strongly influenced by weather. Cold air can hold little water, so the atmosphere over the polar regions contains very little water vapour. In contrast, air over the tropics is very humid, and the atmosphere can contain up to 4% water vapour.

It is this 'positive feedback' that makes water vapour important in climate change, as a small increase in global temperature would lead to a rise in global water vapour levels, thus further enhancing the greenhouse effect.

Human activites contribute slightly to water vapour concentrations in the atmosphere through farming, manufacturing, power generation, and transportation. However, these emissions are so dwarfed in comparison to emissions from natural sources that even the most costly efforts to limit human emissions would have a very small, perhaps undetectable, effect on global climate.

Halocarbons

Halocarbon compounds are chemicals in which one or more carbon atoms are linked with one or more halogen atoms: fluorine, chlorine, bromine or iodine. The best known in this group of gases are CFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons) and the newer substitutes HFCs (hydroflurocarbons). While the concentration of halocarbons are much lower than those of the other greenhouse gases, the warming effect that they produce ranges from 3000 to 13000 times that of carbon dioxide.

Natural halocarbons

These gases very rarely occur naturally. A few halocarbons, including methyl chloride, are produced in large amounts by natural interactions between halogen salts and debris from plants and animals, but most are created in anything more than minuscule traces only through human efforts. A large amount of the naturally occurring halocarbons are created by wood fire, dioxine for example, or vulcanic activities. A second large source are marine algae. 

The impact of human activities

CFCs were used as spray can propellents, solvents, cleaners and coolants until the mid 1970s. Many of the world's nations agreed to control the use of CFCs in 1987 when they signed the Montreal Protocol on Substances that depleted the ozone layer. The substitute HFCs, while less damaging to the ozone layer, still trap heat in the atmosphere and are adding to the greenhouse effect.

Once these gases are in the atmosphere, they resist breakdown and do not disappear for many decades. They can remain in the atmosphere for up to 400 years.

The statistics

While the concentration of CFCs is stabilizing due to the emission controls mentioned above, levels of the longer lasting gases are increasing.

The future

Some halocarbons that are effective in trapping heat are not restricted under the Montreal Protocol neither are the CFC substitutes. Due to their long atmospheric lifetimes they will continue trapping heat for centuries to come.