Greenhouse Gases

Continuing from a previous post, I’ll explain in a bit more detail about greenhouse gases.

Carbon Dioxide

Of the 1% of trace gasses in the Earth’s atmosphere, a large proportion of it is Carbon Dioxide which is why it’s considered as the most important gas in influencing the average global temperature, being responsible for as much as 60% of man-made global warming.

Carbon Dioxide is released into the atmosphere by a number of natural and man-made processes; whenever you exhale, if a volcano erupts, when a plant dies and breaks down. The amount of carbon dioxide produced by decaying plants, cellular respiration and volcanic eruptions is balanced by the amount absorbed by plants during photosynthesis and by the surface of the ocean (which absorbs carbon dioxide by dissolving it in the water).

Man-made atmospheric carbon dioxide is largely produced by burning fossil fuels like gas, coal, oil and their derivatives. Unlike natural processes that produce carbon, there is no counter-balancing system to absorb the carbon dioxide that we produce. This means that man-made carbon dioxide continues to build up in the atmosphere over time.

Modern life largely depends on fossil fuels. Most likely the electricity we use comes from a coal-burning power station, unless you are signed up to a green energy tariff. We use gas-powered heating, unless your hot water and heating comes from solar panels, ground source heat pumps or geothermal sources as it does in geologically active countries such as Iceland.

Factories, businesses and transport are major users of fossil fuels, either directly or indirectly in the form of a fuel for electricity and heat generation. They are also responsible for mass deforestation due to their demand for timber and paper, which is another large contributor to man-made greenhouse gas emissions.

Deforestation is a double-edged sword, because not only is carbon dioxide released when trees are cut down, but it also removes the very thing that absorbs carbon dioxide from the air.

So far, carbon dioxide levels are 30% greater now than they were before the Industrial Revolution and most of this is in the Northern Hemisphere where the majority of fossil fuel burning has occurred. This is expected to rise to 100% sometime between 2045 and 2100.

Water Vapour

Water vapour is the biggest contributor to the natural greenhouse effect. Nearly all the water vapour in the Earth’s atmosphere is naturally occurring, mostly from the evaporation of seawater. On a cloudy night during winter, temperatures will often stay a few degrees higher than normal. On a clear winter’s night, temperatures tend to drop by a few degrees and frost often results. This is a direct example of one form of water vapour in clouds raising the temperature of the air underneath.

Depending on the Earth’s temperature, the amount of water vapour in the atmosphere can vary. Warmer air holds more moisture, colder air holds less. The planet getting warmer due to extra greenhouse gas emissions would result in more water vapour in the atmosphere, which in turn would result in warmer temperatures, meaning more water in the atmosphere and even higher temperatures… creating a vicious cycle.

Because water vapour eventually condenses, producing clouds, it quickly becomes opaque. The tops of the clouds are able to reflect sunlight back into space, helping to keep the air underneath them cool. A process that cools the Earth by reflecting sunlight back into space is called an Albedo Effect. In the case of an albedo effect caused by cloud cover, it is more commonly known as Global Dimming. It is for this reason that cloudy days are usually cooler than sunny days.

At the moment, man-made water vapour is not contributing significant amounts to the global total, but that could change if Hydrogen Fuel Cells become widely used as they emit water vapour as an exhaust gas.

Methane

Methane is a very strong greenhouse gas. It is 21 times more effective than Carbon Dioxide at warming the atmosphere. Fortunately, it exists in much lower concentrations that Carbon Dioxide and it is thought to contribute to 20% of man-made global warming. It also stays in the atmosphere for about 1/10th of the time that Carbon Dioxide does.

Methane naturally occurs in the Earth’s atmosphere as a result of the biodegrading of organic material by bacteria. Livestock flatulence is thought to be a significant contributor to atmospheric methane levels. Other factors that contribute to the levels of methane in the atmosphere are agriculture (particularly Rice Paddies), Landfill, Coal Mining, Gas Production, Burning of Biomass, Termites and evaporation from oceans, freshwater and wetlands.

Frozen methane exists below the sea floor in shallow oceans, created by decaying organic matter. It can also be found in permafrost. These deposits are called “Methane Hydrates”. During periods of cool temperatures and high pressure, these deposits remain stable, slowly allowing some of the methane to escape. During warmer periods, these deposits can become very unstable, emitting tonnes of methane into the oceans, which then bubbles up to the surface and into the atmosphere. This is what is thought to have caused a sudden 7°C increase in global temperatures, 55 million years ago, although it is believed that the release was due to tectonic plate movements (earthquakes). The warming effect of the Methane lasted about 100,000 years.

Frozen methane hydrates could be used for cheap hydrogen production. The problem is that, as yet, no process has been developed to safely extract the methane from the frozen deposits in large quantities without making them unstable. At the moment, the best treatment appears to be releasing methane from the permafrost and burning it as it emerges, producing carbon dioxide and water vapour (which are both weaker greenhouse gasses).

Methane is eventually cleared from the atmosphere in 10-12 years. It reacts with a compound of oxygen and hydrogen, called OH radicals. The end product is Carbon Dioxide and Water, both of which are greenhouse gasses. However, only a limited amount of OH radicals exist in the atmosphere and, when these are all used up, methane could remain in the atmosphere for hundreds or even thousands of years.

The quantities of methane in the atmosphere are currently 2.5 times the amount of pre-industrial levels. It may even be cheaper and easier in the short term to concentrate on reducing methane emissions than reducing carbon dioxide emissions.

Nitrous Oxide

Nitrous oxide has the greatest potential warming effect of all the greenhouse gases, being 200-300 times more effective at trapping heat than carbon dioxide. There is nearly a thousand times more carbon dioxide in the Earth’s atmosphere than Nitrous Oxide so, whilst it has an extreme warming effect, it is not very abundant. It can remain in the atmosphere for 150 years.

Nitrous oxide is produced by the activity of micro-organisms, which remove nitrogen from the soil and put it in the atmosphere. It is also released into water by the plant life that lives in the oceans and flooded forests. The Nitrous Oxide gradually makes its way to the surface and on into the atmosphere. Plants absorb nitrous oxide from the air and fix it in the soil as ammonia. This cycle keeps the balance of nitrous oxide constant.

The main man-made source of Nitrous Oxide is through the use of nitrogen-based fertilisers. However, burning biomass and fossil fuels also produces Nitrous Oxide, as do sewage works and water treatment centres.

Since pre-industrial times, the levels of Nitrous Oxide in the Earth’s atmosphere have increase by 16%.

Ozone

Whilst it is known that Ozone is a greenhouse gas, it is not known precisely how effective it is at trapping heat. Ozone is created from oxygen by high-energy ultraviolet (UV) light from the sun. It is instantly destroyed by low-energy UV sunlight and the chlorofluorocarbons (CFCs) contained within aerosols and cooling systems. The Ozone Layer is a thin veil of ozone that exists in the upper reaches of the atmosphere.

Ozone can be created by pollution from the burning of fossil fuels reacting with sunlight. It can be very poisonous at ground level, especially to people with respiratory diseases.

Ozone also plays an important role in protecting us from the sun’s harmful ultraviolet rays, due to the way it interacts with them. The ozone layer does exactly this and is too far from the surface of the earth to cause health problems.

Halocarbons

Halocarbons are used in cooling and heat exchange systems. The most infamous of these is Chlorofluorocarbons (CFCs), which were banned in 1987 in many countries due to their role in the depletion of the ozone layer. Other types of Halocarbons include Hydrochlorofluorocarbons (HCFCs) and Hydrofluorocarbons (HFCs). The quantity of Halocarbons in the atmosphere is miniscule compared to other greenhouse gasses, but they are 3000 to 13,000 times more powerful than Carbon Dioxide and can remain in the atmosphere for hundreds to thousands of years.

These gasses do not occur naturally on Earth; they are all man-made. HFCs were designed to replace CFCs. They are not as damaging to the ozone layer, but they are just as effective at trapping heat.

Due to the amount of time they linger in the Earth’s atmosphere, CFCs will continue to warm the Earth for centuries to come, despite the fact that they are no longer being produced. There are no restrictions on HFCs and their concentrations are rising all the time.

source: Climal.com