Greenhouse gases (GHG) is a term used to describe atmospheric gases that warm the atmosphere when emitted.

These are critical to life on earth but in excess will increase global temperatures and create damaging climate changes, a situation the majority of climate scientists believe is already underway and accelerating due to human activity, principally from the use of fossil fuels. The principal GHG are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O).

Greenhouse gas (GHG) emissions have received increasing global scientific, regulatory and community attention over recent decades. The intergovernmental panel on climate change (IPCC), a United Nations expert agency involving collaboration with leading climate scientists from across the globe stated in its 2023 assessment report that “human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming, with global surface temperature reaching 1.1°C above 1850-1900 in 2011-2020.

Global greenhouse gas emissions have continued to increase, with unequal historical and ongoing contributions arising from unsustainable energy use, land use and land-use change, lifestyles and patterns of consumption and production across regions, between and within countries, and among individuals”.

Despite the IPCC panel assigning high confidence to this report of the evidence some individuals and groups dispute this consensus. High confidence is however a very strong statement in science indicating that further evidence is extremely unlikely to reduce this certainty. The warming effect is created by more of the sun’s rays being retained within the atmosphere rather than reflected into space. 

Energy use is estimated to produce 73.2% of global emissions with agriculture, forestry and land use 18.4%. The principal GHGs are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) which are reported as CO2 equivalents (CO2e) in most reporting systems with the CO2e of methane calculated as 28 times CO2 and N2O 265 times CO2.

These multipliers indicate the relative warming impact per unit of each gas when added to the atmosphere. The degree of effective warming however differs greatly with the “life” or radiative forcing impact differing greatly. CO2 can remain in the atmosphere for up to 1000 years unless removed by various carbon dioxide “sinks”.

The oceans and plants are major and critical CO2 sinks with plants utilising photosynthesis to convert CO2 to carbon and oxygen. The oxygen is released to the atmosphere and essential to animal, including human, life whereas the carbon is utilised for plant growth and exchanged for other soil nutrients by soil microbes when exuded from roots.

Once methane is produced and released to the atmosphere, the majority will be oxidized to CO2 via the process of hydroxyl oxidation in about 12 years, making it a short-lived climate pollutant (SLCP), a category of pollutants that have lifetimes much shorter than CO2 from days to a few decades. IPCC assessments primarily report global warming potential as GWP100, a calculation of the warming effect over a 100 year timeframe.