Greenhouse gases

Gases that trap heat in the atmosphere are called greenhouse gases (GHGs). The trapping of heat is called the greenhouse effect. A majority of the world’s scientific community and most of the world’s political community now agree that man’s activities in releasing greenhouse gases are causing raised concentrations of these gases in the atmosphere, resulting in an increase in the earth’s average temperature that causes climate change. Common greenhouse gases include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases. Greenhouse gases are emitted through various natural causes and man-made activities. 

Greenhouse gases trap heat within the earth’s atmosphere —
heat comes in, but doesn't radiate back out

The impact of each gas on the greenhouse effect depends on three factors: the concentration of the gas, the amount of time the gas stays in the atmosphere, and how strongly it acts to trap heat. For each GHG, a global warming potential (GWP) can be calculated to combine the effects of how long it remains in the atmosphere and how strongly it traps heat. Gases with a higher GWP contribute more to warming the earth. Here are GWPs of common greenhouse gases:

Carbon dioxide (CO2): By definition CO2 is the reference gas, so its GWP is 1.

Methane (CH4): Has a GWP of 21-34 over 100 years.

Nitrous oxide (N2O): Has a GWP of 265-310 over 100 years.

Fluorinated gases: Have GWPs in the thousands or tens of thousands depending on the specific gas.

Since CO2 is used as the reference gas, GHG emissions are measured in terms of tons (U.S.) or tonnes (most of the rest of the world) of CO2 equivalent (abbreviated as CO2e or CDE). 

Source: U.S. EPA using data in the Intergovernmental Panel on
Climate Change (IPCC) 2014 assessment

Sources of man-made gases in the energy industry include combustion of fossil fuels for generation of electricity and releases of un-combusted methane during production and transport of natural gas. 

Source: U.S. EPA using data from IPCC 2014
Source: International Energy Agency (IEA)

Greenhouse gas emissions in the energy industry can be reduced by transitioning electric generation to low-carbon (natural gas instead of coal) or zero carbon (nuclear, hydropower, and renewable) resources, improving energy efficiency, transitioning transportation from fossil fuel to electric (if the source of electricity is low-carbon), transitioning building heating from fossil fuel to electric heat, and reducing fugitive emissions of methane in natural gas and oil production and transport. The impact of the clean energy transition in developed economies can be seen in the following graph:

Source: International Energy Agency (IEA)

As of 2018, here are the top 15 emitters of GHGs by country according to the Union of Concerned Scientists:

Rank Country Emissions (GT of CO2) % of Global Emissions
1 China 10.06 28%
2 United States 5.41 15%
3 India 2.65 7%
4 Russia 1.71 5%
5 Japan 1.16 3%
6 Germany 0.75 2%
7 Iran 0.72 2%
8 South Korea 0.65 2%
9 Saudi Arabia 0.62 2%
10 Indonesia 0.61 2%
11 Canada 0.56 2%
12 Mexico 0.47 1%
13 South America 0.46 1%
14 Brazil 0.45 1%
15 Turkey 0.42 1%

As you can see, China and the U.S. alone are responsible for nearly 43% of GHGs. The top 15 countries listed above are responsible for 74% of GHGs while the rest of world accounts for the remaining 26%. 

As we move forward, efforts to control GHG emissions will likely focus initially on electric generation and transportation. We can expect to see ongoing growth of renewable generation, battery storage that helps manage the variability of renewable output, and the beginnings of a transition to electric vehicles (EVs). It now appears that reducing output of GHGs is one of the key forces that will drive transition in the energy industry.