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What are the greenhouse gases

The table below shows the major greenhouse gases.

Greenhouse gasMajor sources
Carbon Dioxide (CO2)Fossil fuel combustion; Deforestation; Cement production
Methane (NH4)Fossil fuel production; Agriculture; Landfills
Nitrous Oxide (N20)Fertilizer application; Fossil fuel and biomass combustion; Industrial processes
F-gassesRefrigerant; Electricity transmission; Semiconductor manufacturing
The major greenhouse gasses; Fifth Assessment Report (Intergovernmental Panel on Climate Change IPCC, 2014)

CO2e and Global Warming potential

Global warming potential (GWP) is the heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide (CO2). GWP is 1 for CO2. For other gases it depends on the gas and the time frame.

Greenhouse gasChemical formula100-year potential
Carbon dioxideCO21
Nitrous oxideN2O
Global warming potential (GWP) for the 3 major Greenhouse gasses; source: Global warming potential by Wikipedia

The effect of greenhouse gases

The gasses contribute differently, this is partly due to the concentration and partly due to the effect. The figure below shows the relative contribution of each of the gases.

Licensed under CC-BY by the author Hannah Ritchie

The effect of CO2 emissions on temperature rise

In order to determine the effect of CO2 on temperature increase, we first need to consider which scenario to look at. Scientist have agreed on a list of standardized scenarios.

Shared Socioeconomic Pathways (SSPs) are scenarios of projected socioeconomic global changes up to 2100. They are used to derive greenhouse gas emissions scenarios with different climate policies.

The scenarios are:

  • SSP1: Sustainability (Taking the Green Road)
  • SSP2: Middle of the Road
  • SSP3: Regional Rivalry (A Rocky Road)
  • SSP4: Inequality (A Road divided)
  • SSP5: Fossil-fueled Development (Taking the Highway

For more information please refer to The Shared Socio‐Economic Pathways (SSPs): An Overview by UNFCC.

Each scenario represets a given cummulative CO2 emission which makes it possible to estimate the temperature increase by scientific models.

Source: IPCC AR6 report

It is important that a distinction is made between stopping greenhouse gas emissions and reducing emissions to net-zero. What would happen if we just stopped producing carbon dioxide, the prevailing greenhouse gas, but made no effort to remove it from the atmosphere? As carbon sinks, our lands and oceans would absorb some of the CO2 in the atmosphere, thus reducing atmospheric temperatures over time. However, the CO2 left un-absorbed by the sinks would linger in the atmosphere for around 300-1000 years. As for the ocean, it would likely continue to warm until it reached the same temperature as the atmosphere, which would bring the earth back into radiative equilibrium. Radiative equilibrium refers to a scenario where incoming solar energy is balanced by an equal amount of energy being radiated back into space. At this point, global temperature is the most stable it can be. Studies predict that reaching radiative equilibrium (after stopping carbon emissions) would call for around 0.5ºC of further warming.

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