Understanding Greenhouse Gases
In this post from NET-ZERO:
What are the major greenhouse gases and where do they come from? Emissions of Carbon Dioxide, Methane, Nitrous Oxide, and Halocarbons.
What is Global Warming Potential? Why different greenhouse gases have different warming impacts on the planet over different timescales.
Why is carbon dioxide the most important greenhouse gas? How each greenhouse gas has contributed to warming so far and why CO2 is centre stage.
Identifying the Greenhouse Gases
The thicker the blanket of CO2 surrounding the planet, the hotter the surface temperature becomes to balance the incoming and outgoing energy. But it’s not just CO2 which increases the tog: other problem gases include methane, nitrous oxide, and a class of industrial chemicals called halocarbons.
One of the first major jobs undertaken by climate scientists was to figure out the impact of each gas and understand where these emissions were coming from.
Carbon Dioxide: Chemical formula CO2 is the by-product of the combustion of carbon based fossil fuels used for energy in transport (22%), amenities (26%), industry (26%) and agriculture (4%), further CO2 emissions come from burning or rotting vegetation during land clearing (13%) and as the by-product of fertiliser, cement, steel, and certain chemical reactions (10%).
Methane: Chemical formula CH4 is emitted from leaks in the oil and gas network (50%) and released from livestock enteric fermentation, or more commonly referred to as cow and sheep burps, (25%), paddy rice farming (5-10%), landfill (5-10%), and coal mining (5-10%).
Nitrous Oxide: Chemical formula N2 0 is mostly released from the use of fertilisers, manure, and soil management in agriculture (>70%), and the remaining emissions are from internal combustion vehicles.
Halocarbons: Chemicals containing a mix of carbon, fluorine, chlorine, bromine, hydrogen, or oxygen. Halocarbons are used as the cooling or heating fluids (refrigerants) in air conditioning, heat pumps, and refrigeration (70%) or in industrial processing and semiconductor fabrication (30%).
Scientist John Tyndall’s setup for measuring radiant heat absorption by gases (Diagram published in 1861).
The Global Warming Potential of CO2 and other GHGs
Greenhouse gases all act to warm the planet, and many have a bigger impact than carbon dioxide. Methane, nitrous oxide, and halocarbons have a much lower concentration in the atmosphere so their ability to absorb specific infrared frequencies is less saturated than CO2. This means small concentration increases have an outsized impact on warming.
Global Warming Potential of methane and nitrous oxide compared to CO2 over 100 years (IPCC AR5)
The Global Warming Potential (GWP) is gauged by the ability of each gas to increase radiative forcing (the incoming energy imbalance) and warm the planet for a given weight of emissions when compared to CO2.
Every one-tonne of methane is the equivalent of adding 28 tonnes of CO2 into the atmosphere. Every one tonne of nitrous oxide is the equivalent of 265 tonnes of CO2. To help simplify the data, greenhouse gas emissions are often quantified in carbon dioxide equivalents or CO2e to compare like for like warming potential for different GHG emissions.
But to complicate the matter further the warming impact must be compared over a given timescale. The warming potential of methane is 28 times more powerful than CO2 on a 100-year basis, but it is 84 times more powerful over 20 years. This is because methane only lasts about ten years in the atmosphere before it reacts with oxygen to form CO2.
Top: Total CO2 equivalent emissions per year including energy supply and non-energy emissions of all greenhouse gases.4 Bottom Left: Annual CO2 emissions from energy generation. Bottom Right: CO2 and other greenhouse gas emissions (in CO2e) not from energy generation. As of 2020.
Quantifying the Global Warming Impact of Each Greenhouse Gas to Date
The planet is already on average over 1⁰C hotter compared to pre-industrial times (year 1800). Scientists can multiply the increase of each gas by it’s warming potential to estimate the relative contribution to global warming so far.
Although CO2 is the least powerful of our greenhouse gases on a per tonne basis, the sheer quantity we release from the combustion of fossil fuels makes it responsible for more than half of all human induced global warming over the last 200 years.
Methane is responsible for one quarter of warming so far, nitrous oxide and halocarbons 10%, and the remainder is mostly due to soot or black carbon (which reduces the reflection of the sun’s energy back to space rather than acting as a greenhouse gas).
So CO2 takes centre stage in climate analysis not only because it is the largest contributor to global warming so far, but also because CO2 persists in the atmosphere for centuries and, without intervention, will create irreversible change over human timescales.
