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

June 28, 2010, by Michael Gillenwater

[Editor’s Note:  This blog post was originally published on June 28th, 2010.  The post, in its current form, has since been edited to include updated content as of April 1st, 2015.]

This question is not as silly as it may seem. This blog post is consistently the most frequently visited page on our website. It 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 the same background), I am updating our primer on the topic.

But first you should read my previous blog post on greenhouse gases, which has also been updated.

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, you can refer to the latest IPCC assessment report published in 2013 — see Chapter 8 of the Working Group I report.)

Global Warming Potentials (GWPs) are a quantified measure of the globally averaged relative radiative forcing impacts of a particular greenhouse gas. It is defined as the cumulative radiative forcing – both direct and indirect effects – integrated over a period of 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 reductions of different gases.

So 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).

There are three key factors that determine the GWP value of a GHG:

We typically only use GWP values for gases that have a long atmospheric lifetime (i.e., in years). Because 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. So if a gas is short-lived and does not have a global concentration because it is destroyed too quickly to even mix throughout the atmosphere, then it can’t really have a GWP.

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., NOx, and NMVOCs), and tropospheric aerosols (e.g., SO2 products and black carbon) vary spatially, and consequently it is difficult to quantify their radiative forcing impacts.

Some GWP values may account for indirect as well as direct effects. Indirect radiative forcing occurs when chemical transformations involving the original gas produce a gas(es) that is/are also a GHG, or when a gas influences other radiatively important processes such as the atmospheric lifetimes of other gases.

In sum, the higher the GWP value the more infrared radiation the gas will tend to absorb over its lifetime in the atmosphere, leading to more warming. Now, there are three more complications to this story.

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

The second complication is one that occasionally trips people up. Remember above when we defined GWP by saying “cumulative radiative forcing…integrated over a period of 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 (the latest report quit publishing values for 500 years).

Now, to be clear, everyone pretty much universally uses 100 year GWP values, so you often never see the time period even cited. It is just assumed you know it is 100 years. But occasionally, someone will use something different, not realizing that they are breaking convention. It is also possible to compute an infinite time horizon GWP value, which would basically mean that accounted for every bit of radiative forcing of every molecule of gas as long as it existed in the atmosphere.

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 people are using and making commitments based on GWP values while these revisions are taking place. So, say a company or a country commits to reduce its emissions by 10% and achieves that goal. Then all of a sudden 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 three times, in 2001, 2007, and 2013. The result has been a proliferation of GWP values out there that leads to a lot of confusion.

The major causes for the IPCC’s updates to GWP values involved new laboratory or radiative transfer results, improved atmospheric lifetime estimates, and improved calculations of CO2 radiative forcing and CO2 response function. When the radiative forcing of CO2 is updated, then the GWPs of 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 and ethers and other trace gases). As you can see in this table, each gas has number of GWP values that you could choose.

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

Until recently, we also almost always used the values published by the IPCC because they were adopted by the UNFCCC and Kyoto Protocol. However, now with the Kyoto Protocol effectively over, the UNFCCC has now adopted the IPCC 2007 AR4 values for international reporting. Yet, we now have newer values from the IPCC 2013 AR5 report.

So, after many years of stability based on the 1996 IPCC values, we have entered a period of confusion surrounding what vintage of GWP values should be universally applied so all climate change programs and policies around the world are consistent in their emissions accounting.

I’ve highlighted in red the values currently adopted by the UNFCCC national emissions reporting. And highlighted in green are the most recent values from the IPCC AR5. I wish I could tell you which one to use. As a default, I would recommend you use the red 2007 values to be consistent with the UNFCCC. But recognize that an agreement later this year in Paris could adopt the new 2013 AR5 values.

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

Table: Global Warming Potential Values from the IPCC for some key GHGs

Lifetime

(years)

GWP time horizon

 

 

Report Reference

20 years

100 years

500 years

Carbon dioxide (CO2)

Complex

1

1

1

1

1

1

1

1

NA

1

1

1

IPCC 2013 – AR5

IPCC 2007 – AR4

IPCC 2001 – TAR

IPCC 1996 – SAR

Methane (CH4)

12.4

12

12

12

84

72

62

56

28

25

23

21

NA

7.6

7

6.5

IPCC 2013 – AR5

IPCC 2007 – AR4

IPCC 2001 – TAR

IPCC 1996 – SAR

Nitrous oxide (N2O)

121

114

114

120

264

289

275

280

265

298

296

310

NA

153

156

170

IPCC 2013 – AR5

IPCC 2007 – AR4

IPCC 2001 – TAR

IPCC 1996 – SAR

HFC-23

222

270

260

264

10,800

12,000

9,400

9,100

12,400

14,800

12,000

11,700

NA

12,200

10,000

9,800

IPCC 2013 – AR5

IPCC 2007 – AR4

IPCC 2001 – TAR

IPCC 1996 – SAR

HFC-134a

13.4

14

13.8

13.8

3,710

3,830

3,300

3,400

1,300

1,430

1,300

1,300

NA

435

400

420

IPCC 2013 – AR5

IPCC 2007 – AR4

IPCC 2001 – TAR

IPCC 1996 – SAR

CF4 (PFC)

50,000

50,000

50,000

50,000

4,880

5,210

3,900

4,400

6,630

7,390

5,700

6,500

NA

11,200

8,900

10,000

IPCC 2013 – AR5

IPCC 2007 – AR4

IPCC 2001 – TAR

IPCC 1996 – SAR

Sulfur hexafluoride (SF6)

3,200

3,200

3,200

3,200

17,500

16,300

15,100

16,300

23,500

22,800

22,200

23,900

NA

32,600

32,400

34,900

IPCC 2013 – AR5

IPCC 2007 – AR4

IPCC 2001 – TAR

IPCC 1996 – SAR

Nitrogen trifluoride (NF3)

500

500

740

740

12,800

12,300

7,700

NA

16,100

17,200

10,800

NA

NA

20,700

13,100

NA

IPCC 2013 – AR5

IPCC 2007 – AR4

IPCC 2001 – TAR

IPCC 1996 – SAR

NA: Not available

Row 1: 2013 IPCC AR5(See Chapter 8 of Working Group I report)

Row 2: 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 4: 1996 IPCC SAR (See Chapter 2 of the Working Group I report)

 

To wrap things up for the sake of being thorough, the relationship between mass of a gas and mass of CO2 Eq. 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

So 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.” 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 actually talking about.

And in case you were wondering, according to the IPCC, GWPs typically have an uncertainty of roughly ±35 percent, though some GWPs have larger uncertainty than others.

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.

 


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7 comments on “What is a Global Warming Potential? And which one do I use?

  1. Tom Wigley on said:

    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.

  2. 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.

  3. 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?)

  4. 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)?

  5. Pablo Berrutti on said:

    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)?

  6. Charissa Rigano on said:

    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

  7. Michael Gillenwater on said:

    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:
    http://ghginstitute.org/2010/07/13/what-is-different-about-methane-ch4-emissions-the-forgotten-co2-in-ghg-emissions-accounting/

    michael

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