How it works

Broekhoff, D., Gillenwater, M., Colbert-Sangree, T., and Cage, P. 2019. “Securing Climate Benefit: A Guide to Using Carbon Offsets.” Stockholm Environment Institute & Greenhouse Gas Management Institute. Offsetguide.org/pdf-download/

What are Greenhouse Gases?

A greenhouse gas (GHG) is any gas that absorbs heat reflected from the Earth’s surface, in the form of infrared radiation, trapping it in the atmosphere. The heat contributes to the heating of the plant and is known as the greenhouse effect. Carbon dioxide (CO2), methane and water vapour are the most abundant GHGs, along with surface-level ozone, nitrous oxide (N2O) and fluorinated gases. N2O is an especially potent greenhouse gas, trapping far more infrared radiation than both CO2 and methane, despite only a tiny concentration being present in the atmosphere.

Other gases, such as nitrogen oxides (NOx), act as indirect greenhouse gases by producing the greenhouse gas ‘ozone’ via photochemical reactions in the atmosphere. At the same time, however, the photochemical reaction reduces the level of methane in the atmosphere, thereby partially counterbalancing its negative impact. Consequently, NOx emissions are not as bad as direct GHG emissions.

Fully addressing climate change will require reducing emissions of all GHGs.

Scientists and policy-makers have established ‘global warming potentials’ (GWPs) to express the heat-trapping effects of all GHGs in terms of CO2-equivalents (CO2e). This makes is easier to compare the effects of different GHGs and to denominate carbon offset credits in units of CO2e emission reductions.

Concentrations of GHGs  have varied substantially during Earth’s history and these variations have driven substantial climate changes at a wide range of timescales. In general, GHG concentrations have been particularly high during warm periods and low during cold periods.

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