Waste disposal results in the  direct and indirect emissions of greenhouse gases (GHGs) The main gases being  carbon dioxide  (CO2), methane (CH4), nitrous oxide (N2O) and non-methane hydrocarbons (NMHCs) .

Composting helps divert  vegetable and other organic waste  matter from landfill.  Not only does composting keep landfills from filling up prematurely, it minimizes the pollution they create. Plant matter buried with other waste in landfill creates   an anaerobic environment.

This anaerobic fermentation in landfill and open stockpiles of waste such as manure piles.  — produces  significant amounts of  methane into the atmosphere unless contained and used as a fuel using processes such as anaerobic digestion. The global warming effect is mitigated  where of methane produced by from landfill sitescan is  used to produce electricity, with carbon dioxide as a by-product, which has a weaker global warming effect but this is still a greenhouse gas. 

 The Government of Western Australia in Composting to Avoid Methane Production (https://www.agric.wa.gov.au/climate-change/composting-avoid-methane-production) states that " Global emissions from waste have almost doubled since 1970 and now produce 3% of anthropogenic (human origin) emissions (IPCC 2014). About half of these emissions come from the anaerobic fermentation of solid waste disposed of on land. However, the aerobic process of composting does not produce methane because methane-producing microbes are not active in the presence of oxygen. Composting could be  one method to reduce methane emissions from organic waste currently stockpiled or sent to landfill. Composting practices that minimise anaerobic conditions and maximise aerobic conditions will be the most effective in reducing greenhouse gas emissions." This applies to home composting just as it does to large scale commercial composting 

 Agriculture is a significant contributor to climate change  producing  at least 14% of global greenhouse. Large quantities of methane are emitted as a result of activities such as: ploughing, which  releases carbon dioxide from the soil,  and keeping  livestock. Farming is a significant user of fossil fuels and, of course, fertilizers

However, while much  attention has been focused on the problems caused by farting cows  farming can help reduce climate change by  carbon sequestration

Making aerobic compost can contribute to the efforts to reduce the rate of global warming as  aerobic composting reduces or prevents the release of methane during organic matter breakdown. Methane is a significant contributor to global greenhouse gas emissions being  26 times more potent than carbon dioxide  

The use of composting in farming, horticulture and  home gardening contributes to the removal of  atmospheric carbon through a process known as  soil carbon sequestration through the storage of compost carbon.  There is also an indirect contribution  due to enhanced plant growth resulting from the use of compost on the soil.  

If as a  result of applying compost fewer chemical fertilisers and pesticides are used, and the increased organic matter in the soil results in better water retention leading to  reduced irrigation there will be a further reduction of greenhouse gas emissions.

Spreading compost and using a non-dig method can boost the soils carbon storage ability.  Agriculture animal husbandry is a significant  contributor  to global warming but research has shown that if compost is spread on  grazing lands it could capture a significant amount of greenhouse gas emissions with carbon storage utilising  humus formation. The beneficial  effect  would continue into the future  resulting in improvement of the overall greenhouse gas balance.

  The Intergovernmental Panel on Climate Change and the European Commission has recognised as Carbon sequestration in soil one of the measures through which greenhouse gas emissions can be mitigated. Composting originated in agriculture land and it has now been recognised that there is a role  it could play in carbon sequestration of  agriculture land with some large industrial  farms using large-scale composting techniques to deal with  animal waste.

Home composting has the potential to reduce Greenhouse gas  emissions through  lower on-site emissions  from the compost bins and the minimal need for  transportation and processing. , when compared with kerbside collections and othe  municipal waste  systems. This includes allof the current processes i.e. centralised composting, AD, incineration and landfill,   has the potential to reduce GHG emissions through both lower on-site emissions and the minimal need for  transportation and processing.

Three types of methods are common for the recycling of organic wastes, namely aerobic composting, vermicomposting  and anaerobic digestion.

1.  In aerobic composting the waste iscontinually  aerated by air flowing  through the organic material  and by intermittent physical turning of the waste by the composter. There are two forms of composting hot and cool depending on the temperatures reached during the process of decomposition. This is the main composting domestic system.
2. Vermicomposting is an aerobic compostingsystem butwith the composting and aeration processes  assisted  by composting  worms. The composting material  is at, or about, ambient temperature with little temperature veriation in the heap, 
3.  Anaerobic  digestion is less commonly used and is carried out in the oxygen deprived environment. The material being composted shows only a very slight incease in temperature.. 

By aerobic  composting at home, on the allotment, in businesses and at school everyone can  play a part in  reducing the quantity of methane released into the atmosphere.

One of the environmental  reasons for composting at home and on the allotment is to avoid sending organic waste to landfill where it will decompose to produce methane, a major greenhouse gas. which forms under the  anaerobic conditions found inside a landfill site. Methane is  twenty-six  times more potent than carbon dioxide The methane can  be collected and burnt  to generate electricity but often it is just lost to the atmosphere.

Anaerobic composting is commonly used to treat human effluent and livestock waste on farms and from houses in hotter countries as well as food and other organic waste as an alternative to landfill.  Anaerobic digestion (AD) involves the fermentation of the waste under anaerobic micro-organisms . Methane and other biogases produced are used for power heating or power generation or heating and inorganic solids. The  remaining bio-solids can be used as a  fertilizer. 

In addition to the benefits that that have long been associated with aerobic composting e.g.  the   recycling of  plant nutrients, improvement to soil structure, and the recycling of  minerals  the  gaseous emissions are composed mainly of water (H2O) and carbon dioxide.  Aerobic composting  does not produce methane as the microbes involved in aerobic composting utilise some of the  carbon in the organic waste  for energy and lock the remaining  carbon as in the soil as humus.(carbon sequestration).

Home composting has a role to play. It reduces the carbon footprint associated with the collection and transport of  the organic waste by the council by eliminating the road-miles  involved in collecting and taking it  to the landfill tip or processing plant. Home composting also saves the road-miles involved  in transporting commercially produced compost to retailers and homes. It is to be welcomed that landfilling of domestic organic waste is increasingly being replaced by commercial composting, but this still involves the road-miles in collection. In addition, methane emission from home composting  is less than from commercial composting.

 While each individual home and backyard composter can only play a small role the total land that could be composted would be significant if the majority of gardens, lawns and parks were brought into use. Many County and City Councils have recognized that there is a climate emergency and may be actively promoting home composting offering subsidized compost bins, advice and training through Master Composter and similar schemes.  It might be interesting to ask whether they are adopting systems of composting and carbon sequestration in their parks and grounds using the compost made from the green waste they collect from households. Many city parks also have large grass areas.

 Most people now  live in cities or suburban areas and only have small gardens  but even these are likely to have either a lawn  or  if not, there will be a managed area of grass around the apartment block or  houses as well as round local  commercial buildings and offices. In country  towns and  villages, the garden and grassed area will be larger .

Environmentally lawns have not had a good press. They can reduce biodiversity, encourage  the use of with synthetic which can pollute the environment and  kill unintended beneficial  species. The mowing, fertilization and general  high level of maintenance necessary can  result in a  net emission of carbon dioxide and nitrous oxide both  contributors to global warming.  This could be countered bytheir carbon sequestration capability especially if managed environmentally   

But just because we are trying to compost aerobically does not mean that all the heap will be aerobic all the time. If the feedstock has a high moisture content Compost Moisture combined with a high temperature or is not adequately aerated anaerobic zones will be  formed in the “aerobic” compost heap  leading to the production of  methane  and nitrous oxide. It is recognised that aerating the feedstock (organic material) during composting speeds the decomposition process. It also helps the environment as the greater amount of oxygen available to the composting micro-organisms  the less methane, and the more  carbon dioxide, is produced during decomposition. The use of composting techniques  which maximise aerobic conditions in the home compost heap include the design and set up of the bin to allow good access of air, ensuring there are air spaces in the material, which may require the addition of a bulking agent, in addition to the ensuring correct mix of greens and browns. Traditionally  regular aeration (turning) has been recommended  as well as careful control of the moisture content so that  water does not fill the air spaces in the material. A lid or cover to the bin or heap is a key factor in controlling the moisture level to prevent the compost becoming saturated during heavy rain.

The need avoid the formation of anaerobic zones within the composting material  might be thought to favour  the use of hot composting techniques by the environmentally aware composter as the organic waste is aerated regularly when it is most active during the first week or two and careful control of the moisture content.  

However, the situation is complicated as not all the greenhouse gases produced in anaerobic areas  are emitted into  the atmosphere some  of the methane  is converted to carbon dioxide  by  aerobic methane-oxidising bacteria (Methanotroph) on reaching the surface layers of the heap. Turning or mixing of the heap to aerate all of the content to encourage hot composting and remove anaerobic areas within the bin will disrupt the activity of  these  methanotrophic aerobes as the  surface layers in which they live and work are turned to the centre of the pile  allowing any small amounts of  methane  from any anarobic areas, not broken up by the mixing, to be  emitted to the atmosphere  until the Methanotrophs are re-established at the heap. 

The evidence would appear to show that part of the price of killing pathogens and producing compost quickly by  weekly mixing is that there will be slightly  higher  emission factors when compared with cold composting techniques  with no mixing. It has been reported that hot composting  within the  temperature range of 50-60°C produces the highest methane emissions. At temperatures of over 65°C methane emissions are reduced. Monitoring the temperature so that temperatures within the 50-60°C range should not be prolonged past the time necessary to kill weeds and potential pathogens is necessary to keep the methane emissions as low as possible,

Often when using  cold composting at home  where the organic material is added in small amounts composting will take up to 18 months to a year. The above  suggests that there might be an environmental case for not  aerating the bin during the active stages of composting even if  this may allow small pockets of anaerobic activity to develop. Even careful  aeration using a commercial aerator that reaches the lower depths of the bin to encourage continuation of aerobic decomposition will result in  disturbance of the surface layers. But the bin will need aerating so a better procedure might be to  set up the composting bin to provide greater aeration. For example, the bin can be mounted on a wooden pallet or a  layer of  wood chips to provide for the entry of air at the base. Airflow within the lower part of the bin can be increased by the addition of a layer of twigs and small sticks  or a layer of criss-crossed sticks and/or a ventilation tube can be introduced to the bin. 

Home composting may produce some greenhouses gases but less than other processes and  the compost produced will when converted to humus will provide a valuable carbon sink.

Greenhouse gas emissions from home composting in practice https://www.sciencedirect.com/science/article/pii/S0960852413016295#!

Carbon sequestration by compost application

https://www.climate-policy-watcher.org/source-reduction/carbon-sequestration-by-compost-application.html#download

 Compost Can Turn Agricultural Soils into a Carbon Sink, Thus Protecting Against Climate Change  https://www.sciencedaily.com/releases/2008/02/080225072624.htm

 Green House Gases and Solid Waste Management. http://wmr.sagepub.com/current.dtl.

https://www.resilience.org/stories/2015-09-02/why-not-start-today-backyard-carbon-sequestration-is-something-nearly-everyone-can-do/

https://www.agric.wa.gov.au/climate-change/composting-avoid-methane-production

Greenhouse Gas Emissions from Food and Garden Waste Composting Effects of Management and Process Conditions https://pdfs.semanticscholar.org/9876/30128898513b27f1e4c1c7aff372417bcf7a.pdf