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

By: Pablo Berrutti

Pablo Berrutti — Wed, 01 Jun 2011 23:14:49 +0000

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)?

By: Karen Henk

Karen Henk — Mon, 01 Nov 2010 22:45:27 +0000

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)?

By: Derik Broekhoff

Derik Broekhoff — Thu, 22 Jul 2010 17:07:48 +0000

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?)

By: Michael Gillenwater

Michael Gillenwater — Sun, 11 Jul 2010 23:44:20 +0000

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.

By: Tom Wigley

Tom Wigley — Tue, 29 Jun 2010 23:16:02 +0000

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.