Greenhouse Gases and the Greenhouse Effect

The Greenhouse Effect

When the sun’s radiation reaches the Earth’s atmosphere, about 30% is reflected back into space. The remainder is absorbed by land and the oceans and heats the Earth. The heat created by this process is radiated back from the Earth’s surface toward space. Greenhouse gases, which are naturally present in the Earth’s atmosphere, trap some of that re-radiated heat and prevent it from escaping the atmosphere. This is referred to as the “greenhouse effect” because it resembles the heat trapping characteristics of its namesake. In a greenhouse, plants can be grown in colder weather because the materials that are used to construct the greenhouse allow more heat to enter than they allow to exit.

This “greenhouse effect” is what keeps the planet warm enough to support life. The greater the concentration of greenhouse gases in the atmosphere, the more heat is trapped and the warmer the planet becomes. Thus, any greenhouse gases released as a result of human activity trap more heat than would otherwise be trapped by natural processes alone, leading to progressively higher global temperatures.

The Primary Man-Made Greenhouse Gases

Carbon Dioxide (CO2) - CO2 is produced naturally through the respiration and decomposition of animal and plant matter. It is also produced by humans through the burning of fossil fuels, cement making, and the burning of biomass, including forest fires. Once a molecule of CO2 is released, its time in the atmosphere varies. While natural processes remove some CO2 in 200 years or less, a portion of the emissions are likely to stay in the atmosphere for thousands of years.

Methane (CH4) – CH4 is produced naturally through the digestion of food and the decay of plant material in low oxygen environments such as swamplands. It is also produced by humans through agricultural and livestock practices, decaying waste in landfills, and leakage during the extraction and transport of fossil fuels. Once emitted, methane’s lifetime in the atmosphere is relatively short at about 12 years.

Nitrous Oxide (N2O)– N2O is produced naturally through microbial processes in soil and water. It is also produced by humans through agricultural processes and the burning of fossil fuels. Once emitted, it remains in the atmosphere for more than 100 years.

Fluorinated Gases – These are not produced naturally. They are only produced by humans through the use of refrigerants, aerosols, and similar synthetic products. Depending on the type, fluorinated gases remain in the atmosphere for hundreds or even thousands of years after release.

Global Warming Potential

The Global Warming Potential (GWP) of a greenhouse gas refers to the heat-trapping and warming potential of the gas over a defined time period, usually 100 years. GWP allows us to compare the impact different gases have on global temperatures.

CO2 is used as the baseline against which other gases are measured. Therefore, CO2 by definition has a GWP of 1. CH4 has a GWP of about 25, meaning it is significantly more heat-trapping than CO2. Another way to think about this is that just 1 ton of CH4 emissions would have the same warming impact over 100 years as 25 tons of CO2 emissions.

N2O is far more potent still, with a GWP of almost 300. At the top of the scale, fluorinated gases have GWPs in the thousands or even tens of thousands.

If CO2 has a much lower GWP than other man-made greenhouse gases, why is so much focus placed on CO2 when discussing climate change? The answer primarily relates to the amount of CO2 produced relative to the other gases. In 2018, the amount of CO2 emitted through human activity was more than four times the amount for the other gases combined. This, combined with CO2’s relatively long atmospheric lifespan, drives its importance in the climate discussion.

Sources:
“Greenhouse Effect”, Australian Government Department of Agriculture, Water and the Environment.
“What is the Greenhouse Gas Effect?”, NASA.
“Overview of Greenhouse Gases”, United States Environmental Protection Agency.
“Greenhouse Gases”, National Oceanic and Atmospheric Administration.
“Understanding Global Warming Potentials”, United States Environmental Protection Agency.