What is Global Warming Potential?
In climate science, “global warming potential” (GWP) is a loose measurement of the comparative contribution made by individual greenhouse gases (GHGs) to the greenhouse effect in the lower atmosphere.
To be specific, GWP measures how much heat energy a unit of a particular greenhouse gas will absorb, over a given period of time, compared to a unit of carbon dioxide (CO2).
Global warming is a well-documented feature of Earth’s climate system and there are numerous scientific parameters which can help us understand it. Greenhouse gas emissions, notably man-made emissions, are a major cause of climate change, but possess a wide range of different characteristics. Fortunately, scientists have devised certain metrics to help us understand the effect of emissions on Earth’s temperature and the greenhouse effect.
“Global warming potential” is one such metric which can be used to quantify the warming effect of varying substances. Other metrics in this area of climate change include Radiative Forcing (RF) and Global Temperature Potential (GTP).
- What is Global Warming Potential?
- Definition of Global Warming Potential
- What is Radiative Forcing?
- A Simple Description of Global Warming Potential
- GWP First Proposed During UN Earth Summit Negotiations
- Greenhouse Gases Trap Heat
- Global Warming Potential Varies from Gas to Gas
- Lifetime of Greenhouse Gases
- 100-Year Global Warming Potential of the Main GHGs
- Global Temperature Potential (GTP)
Definition of Global Warming Potential
Technically speaking “Global Warming Potential” (GWP) is defined as: a time-integrated radiative forcing caused by an emission of a given component (greenhouse gas), relative to an emission of an equal mass of carbon dioxide (CO2).
What is Radiative Forcing?
Radiative Forcing (RF) is basically a disruption to earth’s energy balance between incoming and outgoing solar energy. It’s an energy imbalance imposed on the climate system either externally or by human activities – an enforced heating up or cooling down.
For example, an external RF could be caused by a change in the amount of solar energy from the sun. Quasi-external RF factors include major volcanic emissions. Human activities that can cause RF include: emissions of greenhouse gases, aerosols, deforestation or inappropriate land use change.
A Simple Description of Global Warming Potential
“Global warming potential” helps us to understand the relative impact of different greenhouse gases on Earth’s temperature. It converts one unit of any gas into its “carbon dioxide equivalent”, thus acting like a greenhouse gas exchange rate, with carbon dioxide as its basic reference. For example, the basic GWP of methane is 28. This means one unit of methane (CH4) has the same warming effect as 28 units of carbon dioxide (CO2).
Knowing the GWP of a gas enables us to analyze complex emission policy options or compare the relative effects of certain gases or combinations of gases.
Here’s a simple example: suppose we want to know which is worst for our climate over the next century – (A) 2,000 tons of methane emissions or (B) 49,000 tons of CO2 emissions. Knowing the global warming potential of methane (28) gives us the answer straightaway – 2,000 tons of methane x 28 = 56,000 tons of carbon dioxide equivalent. So, A is worse.
GWP First Proposed During UN Earth Summit Negotiations
The first mention of “global warming potential” appeared in the IPCC’s First Assessment Report (Houghton et al. 1990). Lacking detail, it was worked up during UN climate negotiations prior to the UNFCCC treaty agreed at the Rio “Earth Summit” (June 1992), and made operational in the 1997 Kyoto Protocol.
It was proposed as a concept comparable to “Ozone Depleting Potential”. Since then, “global warming potential” has become much more significant in climate change debates than has “ozone depleting potential” within ozone debates. 1
We’ll go into more detail about GWP in a moment. First, let’s look at what happens to solar energy that reaches Planet Earth from the sun and how it ends up being trapped in the troposphere by the so-called ‘greenhouse effect’. And also, how man-made greenhouse gases have destabilized the system and caused the planet to overheat.
Greenhouse Gases Trap Heat
It takes sunlight about 8 minutes and 20 seconds to reach Earth. On arrival, just under half this solar energy passes straight through the atmosphere and strikes Earth’s surface, where it is absorbed by land, vegetation and water.
In accordance with the laws of physics, some of this heat energy is re-radiated back into the atmosphere from the Earth’s surface in the form of infrared radiation. But as this infrared energy rises into the air, most of it is absorbed by greenhouse gases (GHGs) which redirect much of it back down to the surface. This is the Greenhouse Effect.
This trapping of heat trying to escape into space is what keeps the average surface temperature of Planet Earth at a cosy around 59°F (15°C), instead of the chilly minus 18°C (zero degrees Fahrenheit) that it would otherwise be.
The heat-trapping is done not by the most abundant atmospheric gases, like nitrogen and oxygen, but by more complex molecules of chemicals that are found in smaller quantities. Water vapor, for instance, is the most common greenhouse gas, while carbon dioxide (CO2) is the second-most common. Methane (CH4), nitrous oxide (N2O) and several F-gases also contribute.
Unfortunately, over the past 250 years, we have built our modern energy system on coal and peat, oil and petroleum, as well as natural gas – which between them emit billions of tonnes of carbon dioxide, annually. As a result, we have completely unbalanced this previously stable greenhouse effect mechanism and caused rising temperatures in every corner of the globe.
Global Warming Potential Varies from Gas to Gas
As we have seen, “global warming potential” measures the heat absorption power of an individual greenhouse gas, as compared with that of carbon dioxide. GWP largely depends on two factors: the gas’s capacity to absorb energy and re-radiate it (radiative efficiency), and how long it remains in the atmosphere (its ‘lifetime’).
Based on these factors, each gas is given a GWP rating, which is a measure of how much heat energy it can absorb over a set period of time, compared to 1 ton of carbon dioxide (CO2). The higher a gas’s GWP rating, the higher its contribution to global warming compared to CO2 over the set period.
The most common time period used in GWP assessments is 100 years, although 20 years is also used and is especially informative for short-lived GHGs.
Why the 100-year GWP was originally chosen as the set period, as opposed to a shorter or longer timeframe, is unclear. One participant in the UNFCCC negotiations states that the 100-year period was chosen simply because it was the middle value in a table compiled by the IPCC. 1
In any event, as we have seen, “global warming potential” allows comparisons of the differing warming impacts of different gases. GWPs enable policymakers to calculate total emissions from multiple sources, in order to set up a national GHG inventory, for instance, or to compare the emission reduction benefits of different climate change mitigation strategies.
Lifetime of Greenhouse Gases
The active lifespan of GHGs varies enormously.
Carbon dioxide is much a longer-lived greenhouse gas. Researchers calculate that about 70 percent of CO2 emissions are removed by carbon sinks in the first 100 years. Thereafter, absorption slumps, with only another 10 percent being removed during the next 300 years. The remaining 20 percent stays active in the atmosphere for tens of thousands of years before being removed. The mean lifetime of CO2 is between 30,000 and 35,000 years. 5
Obviously, the longer the lifetime of the gas, the greater its atmospheric concentration. This is why carbon dioxide is so potent: it stays active for a very long time. So even if man-made CO2 emissions were to cease completely, atmospheric temperatures are not likely to decrease significantly for thousands of years. 6
Figure 1. Lifetime & Global Warming Potential of Sample GHGs
|Gas||Lifetime (yrs)||GWP (20yrs)||GWP (100 yrs)|
|Water Vapor||4-9 days||N/A||N/A|
|Nitrogen trifluoride NF3||550||12,800||16,100|
|Sulphur hexafluoride SF6||3200||17,500||23,500|
100-Year Global Warming Potential of the Main GHGs
Water vapour absorbs a large amount of heat energy in the air, but it doesn’t accumulate in the atmosphere like other greenhouse gases. This is due to its very short atmospheric lifetime – from a few hours to a few days – caused by its rapid removal as precipitation (rain/snow). See: Water Cycle.
The amount of water vapor in the atmosphere typically increases as the temperature rises. This is because more heat leads to more evaporation. So, water vapor is not generally regarded as a direct cause of warming, but as part of a positive climate feedback loop. The atmosphere becomes warmer, so more water evaporates. And since water vapor is a greenhouse gas, it causes the temperature to rise further. This leads to more water vapor…and so on.
As a result, the GWP value for water vapor has yet to be calculated.
Carbon dioxide (CO2) accounts for 72.6 percent of the warming caused by man-made emissions. (Source: UN Emissions Gap Report 2020.) As we have seen it has an average atmospheric lifetime of around 30-35,000 years. The “global warming potential” of CO2 is always 1, because all other GWPs are calculated in comparison to it. Its tropospheric concentration has increased by roughly 45 percent since 1800.
Methane (CH4) accounts for 18 percent of the warming caused by man-made emissions. (Source: UN Emissions Gap Report 2020.) Methane only has an average lifetime of about 12.4 years, although its warming effect per gram of gas is higher than CO2. The GWP of methane is calculated to be 28, meaning it has 28 times more heat-trapping power than carbon dioxide.
Its tropospheric concentration has increased by roughly 250 percent since 1800. In 2020, the level of methane rose to 1876 parts per billion, of which roughly 55-60 percent was due to human activity. For more, see: Why Are Methane Emissions Rising?
Nitrous oxide (N2O) accounts for around 5.4 percent of the warming caused by man-made emissions. (Source: UN Emissions Gap Report 2020.) N2O has an average lifetime of about 121 years, and a GWP rating of 265. This means it has 265 times more heat-trapping power than carbon dioxide. Its tropospheric concentration has increased by 15 percent since about 1800.
Another important category of greenhouse gases are synthetic gases created for specific uses throughout industry.
They include (a) the four F-gases: Hydrofluorocarbons (HFCs), Perfluorocarbons (PFCs), Sulfur Hexafluoride and Nitrogen Trifluoride. (b) Ozone depleting substances such as: Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), Chlorocarbons, Hydrochlorocarbons, Bromocarbons, Halons and others.
Synthetic greenhouse gases first came to attention in the early 1970s, due to the damage they caused to the ozone layer in the stratosphere, that shields Planet Earth from harmful ultraviolet radiation.
As of January 2021, they account for around 3.3 percent of the warming caused by man-made emissions. (Source: UN Emissions Gap Report 2020.)
F-gases are generally even more potent per gram than nitrous oxide, although GWPs of F-gases vary significantly – from less than a year to many thousands of years.
The perfluorocarbon compound CF4, for instance, has a lifetime of 50,000 years and a GWP of 6,630, while the fully-fluorinated Sulphur hexafluoride (SF6) has a lifetime of 3,200 years and a GWP of 23,500. F-gases are sometimes referred to as “high-GWP gases” because, gram for gram, they trap considerably more heat than carbon dioxide.
HFCs (designed in the 1990s to replace the banned CFCs and HCFCs) present a particular problem due to their widespread use and high “global warming potential”. Examples include HFC-23, with a GWP rating of 12,400; and HFC-134a (Norflurane), with a GWP of 1,300. Climate experts are concerned that their continuing success as refrigerants is boosting their atmospheric concentrations, posing a serious risk for climate change.
Global Temperature Potential (GTP)
An alternative measurement to “global warming potential” is the Global Temperature Potential (GTP).
While GWP is a measurement of the warming effect of a greenhouse gas during a given time period (relative to CO2), GTP is a measurement of the temperature change at the end of that period (also, relative to CO2).
The calculation of GTP is more complex than that for GWP, as it requires the use of climate models to calculate how much the climate system responds to rising levels of greenhouse gases (climate sensitivity) and how quickly the system responds, taking into account ocean absorption of heat. 8
- “Building an Effective Climate Regime While Avoiding Carbon and Energy Stalemate.” Prof Michael Wara – Columbia Journal of Environmental Law, 2016.
- IPCC. Fourth Assessment Report. Climate Change 2007: Working Group I: The Physical Science Basis.
- “The residence time of water in the atmosphere revisited.” van der Ent, R. J. and Tuinenburg, O. A. Hydrol. Earth Syst. Sci., 21, 779–790. (2017)
- “A revised picture of the atmospheric moisture residence time.” Laderach, A. and Sodemann, H. Geophys. Res. Lett., 43, 924–933. (2016).
- “Atmospheric Lifetime of Fossil Fuel Carbon Dioxide.” Archer, David et al (2009). Annual Review of Earth and Planetary Sciences.
- “Irreversible climate change due to carbon dioxide emissions.” Solomon S et al. (February 2009). PNAS. USA. 106 (6): 1704–09.
- “Anthropogenic and Natural Radiative Forcing.” Page 731. Myhre, G. et al. 2013: In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the IPCC Fifth Assessment Report . Cambridge University Press. PDF: www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter08_FINAL.pdf
- “Understanding Global Warming Potentials.” US EPA.