June 28, 2010 in Inside the Institute by Michael Gillenwater

What is a Global Warming Potential? And which one do I use?

[Editor’s Note: This blog post was originally published on 28 June 2010. The post, in its current form, has been updated as of 8 May 2023 to reflect the final published version of AR6, and again on 17 October 2024 to address new insights on methane.]

This question is not as silly as it may seem. This blog post is the most frequently visited page on our website. It is a topic so fundamental to carbon management that many practitioners are probably afraid to seek clarification out of fear of looking uninformed. Since not everyone working on managing greenhouse gas (GHG) emissions has studied atmospheric chemistry (I admit I have but wouldn’t expect the range of folks working on these issues to have this background), we are updating our primer on the topic.

But first, you should read my previous blog post on greenhouse gases.

I’m going to skip over the underlying physics and chemistry because it is not necessary to engage at that level of scientific technicality to be an intelligent user of GWP values. If you want to dig into the science more, I suggest you refer to the 5^th IPCC Assessment Report (AR5) — see Chapter 8 of the Working Group I report for a scientific explanation.

Global Warming Potentials (GWPs) are a quantified measure of the globally averaged relative radiative forcing impacts of particular greenhouse gases in the atmosphere. It is defined as the cumulative radiative forcing – both direct and indirect effects – integrated over time from the emission of a unit mass of gas relative to some reference gas (IPCC 1996). Carbon dioxide (CO₂) was chosen by the IPCC as this reference gas and its GWP is set equal to one (1). GWP values allow you to compare the impacts of emissions and emission reductions/removals of different gases.

To be clear, GWP values are applied to units of mass (e.g., kilograms, pounds, metric tons, etc.), not to units of volume (e.g., cubic meters, cubic feet, liters).

Three key factors determine the GWP value of a GHG:

the ability of the gas to absorb infrared radiation,
where along the electromagnetic spectrum (i.e., what wavelengths) the gas absorbs radiation, and
the atmospheric lifetime of the gas.

We typically only use GWP values for gases that have a long atmospheric lifetime (i.e., in years), as only these gases last long enough in the atmosphere to mix evenly and spread throughout the atmosphere to form a relatively uniform concentration. GWP values are meant to be “global,” as the name implies. If a type of gas is short-lived and does not have a global concentration because it is destroyed too quickly to mix evenly throughout the atmosphere, then it typically is not assigned a GWP value.

Specifically, the gases with relatively long atmospheric lifetimes that tend to be evenly distributed throughout the atmosphere, and therefore have global average concentrations, are CO₂, CH₄, N₂O, HFCs, PFCs, SF₆, and NF₃. (There are numerous other more obscure chemicals you can investigate in the IPCC AR5 report chapter). The short-lived gases such as water vapor, carbon monoxide, tropospheric ozone, other ambient air pollutants (e.g., NO_x, and NMVOCs), and tropospheric aerosols (e.g., SO₂ products and black carbon) vary spatially. Consequently, it is difficult to quantify their radiative forcing impacts relative to CO₂ on a per unit mass basis.

Some GWP values may also account for indirect as well as direct effects. Indirect radiative forcing occurs when chemical transformations involving the original gas produce a type of gas or multiple gases that are also GHGs. Indirect effects can also occur when a gas influences other radiatively important processes, such as the atmospheric lifetimes of other greenhouse gases.

In sum, a higher GWP value means that the gas will absorb more infrared radiation will be absorbed by the gas and more energy will be added to the atmosphere, leading to more warming. Now, there are three more complications to this story.

The first is that gases absorb certain wavelengths of radiation. GHGs each absorb in a given “window” of the radiation spectrum. The more that window is filled, the less there is to absorb. So, as concentrations of certain gases increase they can saturate a wavelength, leaving no more radiation for additional concentrations of gas in the atmosphere to absorb.

Chart: Radiation Transmitted by the Atmosphere

The second complication is one that occasionally trips people up. Remember above when we defined GWP by saying “cumulative radiative forcing…integrated over time”? Well, that means that we have to define a time period for the integration to occur. You have to know what the integration period is to make sure you are using the correct GWP. The typical periods that the IPCC has published are 20, 100, and 500 years (in the Fifth Assessment Report (AR5), 500-year values were not published).

Now, to be clear, everyone pretty much universally uses 100 year GWP values, so you will hardly ever see the time period cited. It is just assumed you know that it is 100 years. But occasionally, someone will use something different, not realizing that they are breaking convention. It is increasingly common to see a 20-year methane GWP quoted because it results in a much higher GWP value for that gas. It is also possible to compute an infinite time horizon GWP value, which would account for every bit of radiative forcing of every molecule of gas as long as it existed in the atmosphere (i.e., the gas’s entire atmospheric lifetime).

The last complication relates to the fact that the IPCC keeps updating its GWP values with each of its scientific assessment reports. It makes sense to update GWP values as our scientific understanding improves. However, the problem is that governments, companies, and others are using and making commitments based on GWP values while these revisions are taking place. So, say a company or a country commits to reducing its emissions by 10% and achieves that target. Then suddenly GWP values change and now they no longer make the goal using the new GWP values (due to the mix of different GHGs they emit and reduce). It would be like moving the net after you already kicked the ball towards the goal.

For this reason, the Kyoto Protocol fixed the use of GWP values published by the IPCC in 1996 in its Second Assessment Report. Since then the IPCC has updated its GWP values four times, in 2001, 2007, 2013, and 2021. The result has been a proliferation of GWP values out there that leads to a lot of confusion. To help clarify this confusion, the Conference of the Parties serving as the meeting of the Parties to the Paris Agreement (CMA) has agreed to begin with the AR5 values and update the GWP values used as they are released. Specifically, the language states:

“Each Party shall use the 100-year time-horizon global warming potential (GWP) values from the IPCC Fifth Assessment Report, or 100-year time-horizon GWP values from a subsequent IPCC assessment report as agreed upon by the CMA, to report aggregate emissions and removals of GHGs, expressed in CO₂ eq.”

The major reason for the IPCC’s repeated updates to GWP values involves new laboratory or radiative transfer results, improved atmospheric lifetime estimates, and improved calculations of CO₂ radiative forcing and CO₂ response function. When the absolute radiative forcing of CO₂ is updated, then the GWPs of all the other gases relative to CO₂ also change. The IPCC has also added numerous new, and rarely used, gases to its list of GWPs.

The result of the varying time periods and the regular updates by the IPCC is a complicated state of affairs. This table presents GWP values for the most common GHGs (there are many more if we listed all the HFCs, PFCs, halogenated alcohols, ethers, and other trace gases). As you can see in this table, each gas has several GWP values that you could choose.

But the truth is, contrary to what a layperson might expect, we typically only use 100-year values, even though some gases have lifetimes of thousands of years.

In the past, we also almost always used the 1996 Second Assessment Report (SAR) values published by the IPCC because they were adopted by the UNFCCC and Kyoto Protocol. However, with the Kyoto Protocol effectively over, the UNFCCC has now adopted the IPCC 2013 AR5 values for international reporting and intends to update to the latest GWP values as they are released in new IPCC Assessment Reports.

So, there is rightly some confusion surrounding what vintage of GWP values should be universally applied so all climate change programs and policies around the world are comparable in their emissions accounting. As new UNFCCC agreements are established there is often pressure to update the GWP values applied, but such changes are complicated when past commitments and agreements were based on earlier GWP values.

I’ve highlighted in red the values currently adopted by the UNFCCC national emissions reporting guidelines. And highlighted in green are the most recent values from the IPCC AR6. I wish I could tell you which one to use. As a default, I would recommend you use the red 2013 values to be consistent with the UNFCCC. But recognize that a future decision under the UNFCCC will adopt the new 2021 AR6 values. Whatever you do, it is essential that you transparently disclose what set of GWP values you apply.

And if you are still using the old 1996 SAR values, it is probably time to update.

GHG	Lifetime (years)	20 year GWP	100 year GWP	500 year GWP	Report Reference
Carbon dioxide (CO₂)	Complex	1 1 1 1 1	1 1 1 1 1	1 NA 1 1 1	IPCC 2021 – AR6 IPCC 2013 – AR5 IPCC 2007 – AR4 IPCC 2001 – TAR IPCC 1996 – SAR
Methane (CH₄) Applicable to: fossil – combustion & non fossil*	11.8 12.4 12 12 12	81.2 84 72 62 56	27.9 28 25 23 21	7.95 NA 7.6 7 6.5	IPCC 2021 – AR6 IPCC 2013 – AR5 IPCC 2007 – AR4 IPCC 2001 – TAR IPCC 1996 – SAR
Methane (CH₄) (fossil – fugitive & process)*	11.8 12.4	82.5 85	29.8 30	10 15	IPCC 2021 – AR6 IPCC 2013 – AR5
Nitrous oxide (N₂O)	109 121 114 114 120	273 264 289 275 280	273 265 298 296 310	130 NA 153 156 170	IPCC 2021 – AR6 IPCC 2013 – AR5 IPCC 2007 – AR4 IPCC 2001 – TAR IPCC 1996 – SAR
HFC-23	228 222 270 260 264	12400 10800 12000 9400 9100	14600 12400 14800 12000 11700	10500 NA 12200 10000 9800	IPCC 2021 – AR6 IPCC 2013 – AR5 IPCC 2007 – AR4 IPCC 2001 – TAR IPCC 1996 – SAR
HFC-134a	14 13.4 14 13.8 13.8	4140 3710 3830 3300 3400	1530 1300 1430 1300 1300	436 NA 435 400 420	IPCC 2021 – AR6 IPCC 2013 – AR5 IPCC 2007 – AR4 IPCC 2001 – TAR IPCC 1996 – SAR
CF4 (PFC)	50000 50000 50000 50000 50000	5300 4880 5210 3900 4400	7380 6630 7390 5700 6500	10600 NA 11200 8900 10000	IPCC 2021 – AR6 IPCC 2013 – AR5 IPCC 2007 – AR4 IPCC 2001 – TAR IPCC 1996 – SAR
Sulfur hexafluoride (SF₆)	3200 3200 3200 3200 3200	18300 17500 16300 15100 16300	25200 23500 22800 22200 23900	34100 NA 32600 32400 34900	IPCC 2021 – AR6 IPCC 2013 – AR5 IPCC 2007 – AR4 IPCC 2001 – TAR IPCC 1996 – SAR
Nitrogen trifluoride (NF₃)	569 500 500 740 740	13400 12800 12300 7700 NA	17400 16100 17200 10800 NA	18200 NA 20700 13100 NA	IPCC 2021 – AR6 IPCC 2013 – AR5 IPCC 2007 – AR4 IPCC 2001 – TAR IPCC 1996 – SAR
*Methane from fossil fuels and methane from biogenic sources affect the atmospheric stock of CO₂ differently. NA: Not available

Row 1: 2021 IPCC AR6 (See Chapter 7 Supplementary Material of Working Group I report)

Row 2: 2013 IPCC AR5 (See Chapter 8 of Working Group I report)

Row 3: 2007 IPCC AR4 (See Chapter 2 of Working Group I report)

Row 4: 2001 IPCC TAR (See Chapter 6 of Working Group I report)

Row 5: 1996 IPCC SAR (See Chapter 2 of the Working Group I report)

You will note that there are two separate sets of GWP values for methane (CH₄). The reason is that the fate of CH₄ in the atmosphere is to be oxidized to CO₂. If this methane originates from fossil fuels, then it results in an addition of CO₂ to the atmosphere. In contrast, if the methane originates from a biogenic source, then it just returns CO₂ that was previously in the atmosphere. To obtain guidance on which of these two methane GWP values to use for a given emission source category estimation, this post will explain.

To wrap things up for the sake of being thorough, the relationship between the mass of gas and the mass of CO₂ equivalents can be expressed as follows:

mass CO₂ Eq. = (mass of gas) x (GWP)

Where:

mass CO₂ Eq. = mass (e.g., metric tons) of Carbon Dioxide Equivalents

GWP = Global Warming Potential

The calculation is easy. Just multiply the mass of your gas by its GWP value to get CO₂ equivalent emissions. Be sure to label the resulting emissions not as CO₂, but as “CO₂-equivalents” (CO₂e or CO₂eq). Note, that this is not carbon, but CO₂. The ratio of carbon to CO₂ is 12/44. So, if you hear someone talking about carbon emissions make sure you have them clarify what they are talking about. Many errors in calculations occur that are simply a failed correction involving a factor of 3.667 (i.e., 44/12).

And in case you were wondering, there are uncertainties in GWP values. The uncertainty ranges of major GHGs are presented in the table below. There is no uncertainty in the GWP value for CO2 because it is 1 by definition.

GHG	GWP20	GWP100	GWP500
Methane, Fossil and Non-Fossil (CH₄)	32	40	48
Nitrous oxide (N₂O)	43	47	49
CFC-11	29	37	42
PFC-14 (CF₄)	26	33	35
HFC-134a	28	38	40
HFC-32	31	38	40

Lastly, there is one more confusing issue, which I will only touch on briefly. There are numerous gases like chlorofluorocarbons (CFCs), hydrobromocarbons (e.g., methyl bromide), and halons that deplete stratospheric ozone. These gases are being phased out under the Montreal Protocol and related international agreements. They are also GHGs, although their impact on radiative forcing is even more complex because stratospheric ozone is also a GHG. So, ozone-depleting substances (ODSs) have both positive and negative radiative forcing effects. We generally do not include them in GHG emission inventories because they are being phased out, although some carbon offset projects are crediting the destruction of ODSs.

Read the previous post in this series.

Learn More

Comments

Reply

Tom Wigley

June 29, 2010 AT 4:16PM

Although built into the Kyoto Protocol, GWPs have serious flaws. Users should be aware of these flaws. The three papers listed below are my own work, but there are many other (including more recent) papers on the topic.

Wigley, T.M.L., 1998: The Kyoto Protocol: CO2, CH4 and climate implications. Geophysical Research Letters 25, 2285–2288.

Smith, S.J. and Wigley, T.M.L., 2000: Global warming potentials: 1. Climatic implications of emissions reductions. Climatic Change 44, 445–457.

Smith, S.J. and Wigley, T.M.L., 2000: Global warming potentials: 2. Accuracy. Climatic Change 44, 459-469.
Reply

Michael Gillenwater

July 11, 2010 AT 4:44PM

Tom,

I was hoping that someone would open up the debate over whether GWPs, in their current form, are the best index to use for GHG emissions accounting.

I encourage readers and members to look at the papers Tom’s references and give your thoughts on the topic.
Reply

Derik Broekhoff

July 22, 2010 AT 10:07AM

Michael,
Excellent article (as always). To amplify on Tom’s comment, here is an excerpt from the recently published National Research Council report, “Stabilization Targets for Atmospheric Greenhouse Gas Concentrations” (available at http://www.nap.edu/catalog/12877.html):
“Insofar as it is perceived that control of methane or black carbon may be technically easier or less economically
disruptive than controlling CO2 emissions, mitigation of the short-lived warming influences has sometimes been thought of as a way of “buying time” to put CO2 emission controls into place. This is a fallacy. While one does buy a rapid reduction by reducing methane or black carbon emissions, this has little or no effect on the long term climate, which is essentially controlled by CO2 emissions, because of the persistence of CO2 in the atmosphere…. The effect of mitigation of methane and black carbon is thus to trim the peak warming rather than limit the long-term warming to which the Earth is subjected. If the early action to mitigate methane emissions were done instead of actions that could have reduced net cumulative carbon emissions, the long term CO2 concentration would be increased as a consequence. Peak trimming in that case would come at the expense of an increased warming that will persist for millennia. Carbon emission control and short term forcing agent control are two separate control knobs that affect entirely distinct aspects of the Earth’s climate, and should not be viewed as substituting for one another.”
This certainly calls into question the notion of using CH4 reductions to offset CO2 emissions. My only observation is that the idea of reducing CH4 “instead of” CO2 implies a predetermined budget for emissions reductions, within which we are making this tradeoff. In reality, we don’t have such a budget (yet), and it could be reasonably argued that if we exclude CH4 offsets from a cap-and-trade program, for example, caps would be set accordingly higher, meaning there is not necessarily a 1-for-1 tradeoff. In other words, allowing CH4 (and certain other non-CO2) offsets should be seen as means to achieving short-term avoidance of peak warming, not as substiting CH4 reductions for CO2. One hopes that policymakers will explicitly recognize this, however, in setting overall emissions limits…

(By the way, the longevity of CO2 in the atmosphere also has implications for whether temporary storage of C (e.g., through sequestration in trees or soils) can be considered an offset to CO2 emissions… a topic for another post?)

Reply

jb

January 15, 2018 AT 7:59PM

I don’t think that anyone is advocating reduction of short-lived climate pollutants in lieu of CO2 reductions. The idea is to reduce BOTH.

Reply

Karen Henk

November 1, 2010 AT 3:45PM

We are refining a standards document that currently includes references to the “GWP” of a given manufactured product. I have seen precedents to this, studies measuring the GWP of pavement or the GWP of various cook stoves, but this seems to be an improper use of the term. Is there any context in which it would be appropriate to measure or reference the “GWP” or “GWP impact” of a manufactured product (rather than a GHG)?
Reply

Pablo Berrutti

June 1, 2011 AT 4:14PM

This is a very helpful article and conversation thank you.

As an investor I think from a bottom-up (company level) perspective comparability trumps accuracy i.e. even where there is debate about the accuracy of GWP, I would prefer to see companies use the same GWP when reporting CO2-e than go their our own way (e.g. because they think the 2007 updates are superior). However, companies should report both CO2-e and breakdown by gas so further analysis is possible.

The discussion around choosing reductions in methane vs C02 is an interesting one and not something I have heard before. Are you suggesting that it is better to allow landfill (and other sources) to continue emitting CH4 rather than capturing the CH4 and burning it for energy use (which creates C02)?
Reply

Charissa Rigano

October 29, 2013 AT 8:34AM

I just left this question on the message board:
on IPCC AR5 and the GWP of Methane

I was reviewing IPCC 5th assessment report where there is a better description of fossil vs biogenic methane they seem to suggest that the new 100-yr GWP for methane is 34. What is everyone doing with the new information? changing all your factors? How do you interpret what IPCC wrote?

Regarding the new 100-yr GWP for methane of 34, this comes from Table 8.7 in the recently-released ‘final draft’ IPCC 5th Assessment Report. Table 8.7 contains GWP values for a subset of non-CO2 gases, showing values calculated both with and without the effect of climate-carbon feedbacks. The 100-yr GWP of methane is listed as either 28 (without climate-carbon feedbacks) or 34 (with climate-carbon feedbacks). IPCC offers the following language, which appears to recommend using the higher values (i.e., with climate-carbon feedbacks, which for methane is 34):

“While the AGWP for the reference gas CO2 included climate-carbon feedbacks, this is not the case for the non-CO2 gas in the numerator of GWP, as recognized by Gillett and Matthews (2010), Joos et al. (2013), Collins et al. (2013) and Sarofim (2012). This means that the GWPs presented in AR4 may underestimate the relative impacts of non-CO2 gases. The different inclusions of feedbacks partially represent the current-state of knowledge, but also reflect inconsistent and ambiguous definitions. In calculations of AGWP for CO2 in AR5 we use the IRF for CO2 from Joos et al. (2013) which includes climate–carbon feedbacks. Metric values in AR5 are presented both with and without including climate-carbon feedbacks for non-CO2 gases. This feedback is based on the carbon-cycle response in a similar set of models (Arora et al., 2013) as used for the reference gas (Collins et al., 2013). Though uncertainties in the carbon-cycle are substantial, it is likely that including the climate-carbon feedback for non-CO2 gases as well as for CO2 provides a better estimate of the metric value than including it only for CO2.”

Thanks
Charissa
Reply

Michael Gillenwater

October 29, 2013 AT 11:08AM

Charissa,

Great question. And you have anticipated what will be an updated version of this blog post. We just have not gotten to it yet since the AR5 is so new. I think the general points in the original blog post, though, still stand.

You might also want to look at this blog post here:
https://ghginstitute.org/2010/07/13/what-is-different-about-methane-ch4-emissions-the-forgotten-co2-in-ghg-emissions-accounting/

michael

Reply

Jesper H. Kløverpris

May 10, 2017 AT 4:44AM

Hi Micahel,

In your comment above, you seem to suggest that there will be a(n updated) blogpost to shed more light on GWPs with (or without) climate-carbon feedback. Is there any news? I think the LCA community (as well as other stakeholders, incl. policy-makers) could (still) use more guidance on this topic.

Thanks in advance.

Best regards,
Jesper

Reply

Rich

September 13, 2016 AT 9:31AM

As I understand it, GWP of CO2 is non-linear and the impact of additional CO2 in the atmosphere is less as its concentration increases. This effect is primarily due to the limited infrared bandwidth CO2 is capable of absorbing. As energy in that band decreases, additional CO2 has less of an impact.

All the other GHGs have different energy absorption bands and because their concentrations are much lower, their impact remains linear. Therefore, over time as CO2 concentrations in the atmosphere rise, the GWP of all the other gases will become higher because the denominator of the calculation (CO2 GWP) goes down.

I suggest that the climate change campaign should be split into two parts: reducing emissions of CO2 and reducing emissions of all other GHGs.

I realize that both parts are important, but reducing emissions of CO2 is a much more politically difficult and costly endeavor than reducing emissions of the other GHGs. Emission of CO2 is currently the basis of our energy supply which has been essential for economic development and prosperity. Dialing back CO2 emissions without a cost-effective energy source to replace it will mean dialing back economic development and prosperity. It’s no wonder many political factions are fighting climate change efforts.

While nations seek to reduce CO2 and develop alternative energy sources, we should work toward eliminating emission of all the other GHGs. Such and endeavor would be politically easier, require much less investment, and can be done faster. Because many of these gases have atmospheric lifetimes of thousands of years, failure to move forward with controlling these other GHGs will mean they will be in the atmosphere “forever”. The Montreal Protocol that established an agreement to end the emissions of ozone depleting gases has worked well, and would provide a good model for this effort.

Reply

Michael Gillenwater

September 16, 2016 AT 1:27PM

Rich,

You are basically correct on the physics. Obviously as the concentration of any GHG increases, eventually the “window” starts to close and they become non-linear as well, but for most of the non-CO2 gases we will be in the linear portion of the curve for awhile (hopefully forever).

As for splitting the focus on mitigation between CO2 and non-CO2, this framing of the issue has been part of the policy discussion for some time. Back during the Kyoto Protocol negotiations, during my early days as a professional, there was much debate about this exact issue. Whether to even include non-CO2 gases and whether to treat them as fungible with CO2. Still some policies focus only on CO2 (e.g., RGGI cap and trade). It is widely understood that the root problem to solve is CO2 emissions due to their atmospheric lifetime and contribution to overall radiative forcing. But that we need to attack on all fronts, which also includes non-CO2, especially the fluorinated long-lived species, which are effectively permanent in the atmosphere. Then the question could be whether to make CO2 and non-CO2 emission reductions fungible from a policy standpoint. (And it can get more complicated when you bring in things like black carbon.) I would argue that this focuses on the wrong question. Instead we should look at the nature of the emission sources (and sinks) and structure our policies around how best to promote reduction in emissions and enhancement of removals for each type of source and sink, regardless of the GHG (of course we need to account for the relative radiative forcing impact). Focusing just or primarily on structuring our policies around which gas is emitted really fails to address the more practical issues of how to achieve mitigation in the most effective way, which is more a function of the characteristics of the source or sink (e.g., technology, economics, behaviors, etc.).

Reply

Kamen Chanov

September 28, 2016 AT 11:43AM

Hello!
Thank you for this article – it truly sheds some light on the issue. I have recently taken interest in climate change – its causes and possible mitigation mechanisms. As a layman, however, I find it somewhat difficult to quantify the damage done to the atmosphere by methane and CO2. I would appreciate it if you could clarify it for me.

Here is what I cannot understand exactly:
If we use AR5 values, GWP for methane is 28 times higher than that of CO2. According to the report, the atmospheric abundance of CO2 is 390.5 ppm, whereas for methane that’s 1803.2 ppb. CO2 abundance in the atmosphere is thus greater than that of methane, but if they were the same, the priority would have then been to reduce methane emissions since its GWP is greater, is that right?

With that in mind and taking into account the findings of the AR5, how do I calculate the total GWP of the entire amount of CO2 in the atmosphere since its value is 1..? Pardon my math, it has never been my strongest suit… In other words, taking into account AR5 findings, would you say that methane is currently having greater impact on global warming or CO2?

Reply

Michael Gillenwater

October 2, 2016 AT 8:03AM

Kamen,

So first, GWPs are an index to adjust a GHG to an equivalent amount of CO2 with respect to the gas’s radiative forcing impact over a set time period (100 years typically). It is used for emissions, not for the total amount in the atmosphere, as it is calculated as the impact of a pulse to the atmosphere given current concentrations in the atmosphere. As a GHG’s contrentration increases in the atmosphere, the radiative forcing impact of a extra ton added goes down. In other words, the absorptive bandwidth begins to saturate.

So, you would not use a GWP to calculate the impact of all the CO2 currently in the atmosphere. The IPCC AR5 working group I report includes graphs and tables with the total radiative impact of different forcing agents (CO2, CH4, aerosols, etc.) in W/m2 units. I would suggest you refer to that data.

Reply

Badran

April 10, 2017 AT 3:52PM

GWP unit?

Reply

Michael Gillenwater

May 11, 2017 AT 10:55AM

Badran,

Great question. GWP is a unitless ratio relative to the gas CO2, which is indexed to 1.0, so it has no units.

The ratio is of the time integrated radiative forcing of a pulse release of gas in question relative to CO2.

You an find a deeper explanation of the science here:
https://en.wikipedia.org/wiki/Global_warming_potential

Michael

Reply

Anirudh

September 15, 2017 AT 8:16AM

Hey ,
Im new to LCA , I would like to understand if it is possible to compare a life cycle analysis made using GWP100 to GWP 20 or vice versa or even how accurate could be to even compare two products having the same GWP 100 impacts

Reply

Michael Gillenwater

September 28, 2017 AT 9:32PM

Anirudh,

I would not compare CO2 equivalent results using two different sets of GWP values. I can’t think of any reason to do that other than to see what the implications of using the two different assumptions (e.g., lifetime, vintage of values) would be.

As for the accuracy of comparison, that is a more complicated question to answer. There are a range of uncertainties potentially involved. The uncertainty in GWP indexes is just one you would need to be aware of. The IPCC Guidelines has a deep discussion on GHG data uncertainty…the types and how to assess it. I would recommend you look there.

Michael

Reply

Azeem

January 8, 2018 AT 2:35PM

what is the meaning of Climate-credit feedback? how can we define it? the GWP is high for CH4 (34) and N2O (298) with inclusion of climate-credit feedback. can someone please clear me what is the climate-credit feedback?

Reply

Michael Gillenwater

January 11, 2018 AT 11:27AM

Azeem,

Unfortunately, I am not familiar with the term “Climate-credit feedback”. And a internet search of the term produces zero (0) results, so it is apparently not a term that is in general use. If you can explain what you mean more clearly, we would be happy to respond.

Michael

Reply

MMM

November 1, 2019 AT 9:28AM

FYI: I think the commenter meant “climate-carbon” feedback, as in https://iopscience.iop.org/article/10.1088/1748-9326/aa61dc/meta and https://www.earth-syst-dynam.net/8/235/2017/…

Basically, the climate-carbon feedback is the fact that as rate of uptake of carbon by the soils and oceans is sensitive to temperature, and generally more carbon is emitted (or less is taken up) at higher temperatures. So, including the feedback means larger impacts. AR4 and the lower AR5 values include this feedback for CO2, but not for non-CO2 gases, artificially depressing the GWP. So adding it back in for non-CO2 gases to be consistent with the approach for CO2 increases the GWP.

Reply

Michael Gillenwater

November 14, 2019 AT 4:45PM

MMM Thank you again. That clarifies this question and makes sense. Pretty much anything that adjusts CO2’s radiative forcing propagates through all the GWPs, obviously.

Reply

Monali K

February 2, 2018 AT 4:34AM

As suggested in this article I have calculated radiative forcing of that pollutant species from IR spectra and atmospheric lifetime from its rate of reaction with OH radical, then how can I calculate a GWP of that species in 20 year time horizon?
Reply

Michael Gillenwater

February 2, 2018 AT 2:58PM

Monali,

I do not fully understand your question. Which pollutant? If you are referring to reaction with OH, are you dealing with methane? If your intention is to independently estimate your own GWP value, then I would refer to the methodological literature on those methods. You can find that literature referenced in the IPCC Working Group 1 report. My post here is intended to explain the scientific basis for GWP values. It is not intended as a methodological guide for conducting your own research for the purpose of deriving them.

Michael
Reply

Xochitl Cruz

February 6, 2018 AT 11:15AM

Hi,
I have estimated BC radiative forcing from satellite data, ground data and by using OPAC and SBDART software. I wonder how to estimate BC GWP from such a radiative forcing estimate. Thanks

Reply

Michael Gillenwater

March 9, 2018 AT 12:39PM

Xochitl,

Similar to my response to Monali above, my post here is not about how to experimentally derive new GWP values. I recommend you look into the literature for that experimental work, which is referenced in the chapter on radiative forcing in the IPCC Working Group 1 report.

Michaeal

Reply

Will C

February 23, 2018 AT 7:03AM

Hi,

Really useful article. I just wanted to look further into the time period aspect of GWP values. Is there any real reason that 100 years has become the norm, outside of its nice roundness?

It seems to me that using the 100 year figures does give some commonly used HFC’s a bit of an easy time relative to their lifetimes. If using the 20 year figure, the scale of increase of many common HFC’s compared to 100 year is far more significant than for many commonly used natural gases. Therefore using 20 year figures would act as a bigger incentive to move away from HFC’s and into natural gasses (this is coming from a refrigerant point of view).

Does anyone have any feelings on this matter (is one more scientifically sound and is it moral to engineer our GWP levels to push towards lower emissions)?

Thanks,
Will

Reply

Michael Gillenwater

March 9, 2018 AT 12:52PM

Will,

Good question. The selection of time horizon over which to integrate the cumulative radiative forcing effect of a GHG is, as you assume, fairly arbitrary. It is a policy choice. One could argue that we should use a very long timeframe, given that climate change is a long-term issue. Then we would fully consider the impact of long lived gases (e.g., PFCs). The problem there is that shorter lived species like methane, that could have a more immediate impact (and benefit if emissions are reduced) could be considered undervalued.

In contrast, going with a short horizon, like 20 years, would recklessly ignore the long-term impacts of gases like SF6.

So, the conclusion is that we need a compromise. The early creators of GWPs developed tables with 20, 100, and 500 year time horizon values. As you note, the choice patently appears to be based on reasonable round numbers, with the middle value being the obvious choice.

You will often see those that are focused on methane to push for use of 20 year GWPs, for obvious reasons. And those focused on F gases to use the 500 year values, for similar reasons.

Ultimately, in the context of the international treaty process, for which GWPs were created, a common metric was needed that all countries would use so comparisons and compliance determinations could be made.

However, you still see much debate on whether GWP is the best metric. There is still an ongoing discussion track under the UNFCCC process on metrics with advocates for Global Temperature Potential (GTP) and use of more economic impact valuation techniques.

Michael

Reply

MMM

November 1, 2019 AT 11:34AM

There have been a couple of papers which have investigated this question recently – see, e.g., Mignone & Malapragada (https://link.springer.com/article/10.1007/s10584-019-02486-7). If the “global damage potential” is considered the “true” answer (but too complex to use for policy purposes), then one can use it to calibrate the GWP time horizon relative to the assumed discount rate. This approach generally finds that the 100 year GWP is appropriate given a discount rate of about 3%. 3% is a common value used for long-term problems… though some argue it should be lower. A lower discount rate would correspond to a longer appropriate time horizon.

Reply

Michael Gillenwater

November 14, 2019 AT 4:42PM

Thanks MMM for sharing the research. It seems this debate continues under the “metrics” track. I like tying the question to the use of discount rate.

Reply

Paul Wenman

April 25, 2019 AT 7:57AM

There is much debate and graphical presentation of cumulative CO2 emissions over time, for different countries. So until 1990ish the USA, UK and Germany were biggest emitters, but Russia and then China have overtaken. Have these emissions data been (can they be) converted to represent the relative contribution of different countries to actual recorded global warming up to any given date, and for projected warming into the future?

Reply

Michael Gillenwater

May 21, 2019 AT 1:36AM

Paul, the simple answer is yes they can. And I am pretty sure some other analysts have done so in studies of cumulative historical contribution to climate change. Analysts like Richard Heede have even done so for individual companies. Similarly, there are all sorts of projections of emissions, including ones by the IPCC.

GWP values are slightly dependent on global projections of emissions, because to come up with a 20 or 100 or 500 year timeframe, one must assume future concentrations to do the radiative forcing modeling/calculations. But these assumptions need only be for global emissions in aggregate.

-Michael

Reply

Ngunjiri N. James

July 2, 2021 AT 12:22PM

Very informative
Reply

Michael+Gillenwater

September 22, 2021 AT 12:58PM

Here is a great discussion comparing the effects of CO2 and CH4, including GWPs from Gavin Schmidt at NASA.

https://www.realclimate.org/index.php/archives/2021/09/the-definitive-co2-ch4-comparison-post/

What is a Global Warming Potential? And which one do I use?

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