Paper Proposes Two-Value Reporting Standard for Global Warming Potential

Critical for making informed decisions about climate policy

May 4, 2017
Stacy MacDiarmid, 512-691-3439,


A new Policy Forum paper, “Unmask temporal trade-offs in climate policy debates,” was published today in Science. Led by Ilissa Ocko of Environmental Defense Fund (EDF), the paper asserts that both short- and long-term global warming impacts should be considered when making decisions about climate policy. It  proposes the adoption of a very simple two-valued reporting standard that maintains the familiarity of the widely-used global warming potential (GWP) but includes both 20- and 100-year timescales to provide a more complete picture of the climate impacts of specific decisions.

GWPs have become an essential element of climate policy, but a fundamental problem with the way they are currently reported has led to confusion and some unproductive debates within the policy community. Because use of GWP relies on the selection of a single timescale, trade-offs between near- and long-term climate effects are hidden. The most common GWP is GWP100, which addresses the warming potential over a 100-year time scale. But this discounts the short-term warming impacts of several important short-lived climate pollutants, including methane, which is 84 times more powerful a warming agent than carbon dioxide over 20 years. Some groups advocate for a 20-year time scale, GWP20, but this, used alone, can overinflate the role of methane and other short-term climate pollutants in long-term climate considerations.

“It is imperative that both the near- and long-term climate impacts of policies be transparent to a decision-maker,” said Ocko. “We are not saying that one timescale is more important than the other, just that the decision-maker should be able to easily consider the climate impacts on all timescales.”

Ocko and her co-authors propose GWP20/100, which would always include a two-valued pair with both 20-year and 100-year time horizons. The authors contend that a two-number pair wouldn’t be difficult to adopt, and as with other number pairs such as blood pressure or SAT scores we need to build the expectation that a presentation of a single time frame is understood to represent incomplete information.

“Different gases have widely different lifetimes in the atmosphere after emission and affect the climate in different ways over widely different timescales,” said Michael Oppenheimer of Princeton University, who is a co-author on the paper. “The paired approach creates a more comprehensive picture of the nature of climate change and the effects of various policies to stem its consequences.”

“We need global climate institutions like the UNFCCC and IPCC to adopt the GWP20/100 system in reporting warming implications of different policies or actions,” said Steven Hamburg, Chief Scientist at EDF. “While there are often ethical tradeoffs when making policy decisions, a two-value reporting standard will make those tradeoffs explicit, ensuring better climate policy outcomes.”

Funding Disclosure:

Financial support for this work was provided by the Environmental Defense Fund (EDF), the Robertson Foundation, the Kravis Scientific Research Fund, and the High Meadows Foundation.

# # #

Environmental Defense Fund (, a leading international nonprofit organization, creates transformational solutions to the most serious environmental problems. EDF links science, economics, law and innovative private-sector partnerships. Connect with us on EDF Voices, Twitter and Facebook.