Gases that trap heat in the atmosphere are called greenhouse gases or GHGs. When sunlight reaches the Earth’s surface, it can either be reflected back into space or absorbed by Earth. Once absorbed, the planet releases some of the energy back into the atmosphere as heat (also called infrared radiation). GHGs like water vapor (H2O), carbon dioxide (CO2) and methane (CH4) absorb energy, which slow or prevent the loss of heat in to space. This process is commonly referred to as the “greenhouse effect”, whereby GHGs act like a blanket, making the Earth warmer than it would otherwise be.
Since the Industrial Revolution began around 1750, human activities have contributed substantially to climate change by adding CO2 and other heat-trapping gases to the atmosphere. These GHG emissions have increased the greenhouse effect, leading to rises in the Earth’s surface temperatures. According to the National Research Council (Advancing the Science of Climate Change, 2010), atmospheric CO2 concentrations have increased by almost 40% since pre-industrial times, from approximately 280 parts per million by volume (ppmv) in the 18th century to 390 ppmv in 2010. The current CO2 level is higher than it has been in at least 800,000 years. The primary human activity affecting the amount and rate of climate change is greenhouse gas emissions from the burning of fossil fuels for electricity, heat, and transportation.
The main GHGs directly emitted by humans include CO2, CH4, nitrous oxide (N2O), and several others:
- Carbon dioxide (CO2): CO2 is absorbed and emitted naturally as part of the carbon cycle through animal and plant respiration, volcanic eruptions, and ocean-atmosphere exchange. Human activities, such as the burning of fossil fuels and changes in land use, release large amounts of carbon to the atmosphere, causing CO2 concentrations in the atmosphere to rise.
- Methane (CH4): Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills.
- Nitrous oxide (N2O): Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and solid waste.
- Fluorinated gases or F-gases: Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6) are synthetic, powerful GHGs that are emitted from a variety of industrial processes. F-gases are often used in coolants, foaming agents, fire extinguishers, solvents, pesticides, and aerosol propellants. F-gases are also sometimes used as substitutes for stratospheric ozone-depleting substances. These gases are typically emitted in smaller quantities, but because of their potency, they are sometimes referred to as “High Global Warming Potential” gases. F-gases have a long atmospheric lifetime and some of these emissions will affect the climate for many decades or centuries.
- Tropospheric ozone (O3): Tropospheric ozone has a short atmospheric lifetime, but it is a potent GHG. Chemical reactions create ozone from emissions of nitrogen oxides and volatile organic compounds from automobiles, power plants, and other industrial and commercial sources in the presence of sunlight. In addition to trapping heat, ozone is a pollutant that can cause respiratory health problems and damage crops and ecosystems.
- Water vapor: This is the most abundant GHG and significant in terms of its contribution to the natural greenhouse effect, despite having a short atmospheric lifetime. While some human activities can influence local water vapor levels, the concentration of water vapor on a global scale is controlled by temperature which influences overall rates of evaporation and precipitation. As a result, the global concentration of water vapor is not substantially affected by direct human emissions.
The effect of GHGs on climate change depends on three main factors: (i) the concentration of GHGs in the atmosphere; (ii) the length of time that GHGs stay in the atmosphere; and (iii) the impact of GHGs on global temperatures.
The concentration of GHGs in the atmosphere is measured in parts per million, parts per billion, and sometimes parts per trillion. One part per million is equivalent to one drop of water diluted into about 13 gallons of liquid.
With respect to the length of time that GHGs stay in the atmosphere, each GHG can remain in the atmosphere for different amounts of time, ranging from a few years to thousands of years. All of these gases remain in the atmosphere long enough to become well mixed, meaning that the amount that is measured in the atmosphere is roughly the same all over the world, regardless of the source of the emissions.
In terms of the impact of GHGs on global temperatures, the two most important characteristics are how well the gas absorbs energy (preventing it from immediately escaping to space) and how long the gas stays in the atmosphere. Some GHGs have a stronger impact than others on global temperatures. For each GHG, a Global Warming Potential (GWP) has been calculated to reflect how long it remains in the atmosphere, on average, and how strongly it absorbs energy. The GWP for a gas is a measure of the total energy that a gas absorbs over a particular period of time (usually 100 years), compared to CO2. Gases with a higher GWP absorb more energy, per pound, than gases with a lower GWP, and thus contribute more to changes in global temperatures. For example, methane’s 100-year GWP is 21, which means that methane will cause 21 times as much warming as an equivalent mass of carbon dioxide over a 100-year time period.
Accurate reporting and monitoring of GHG emissions is fundamental to reducing greenhouse gases and taking meaningful action to combat climate change. After all, you cannot manage what you do not measure.