The GWP metric: which timescale reflects the real climate impact of HFCs?

The GWP metric: which timescale reflects the real climate impact of HFCs?

The benefits of basing policies on Global Warming Potential (GWP) of hydrofluorocarbons (HFCs) calculated over a 20 year periodThe GWP metric: which timescale reflects the real climate impact of HFCs? rather than on the current standard of a 100 year period were discussed at a side event this week at the 31st Open-ended Working Group (31 OEWG) meeting of the Parties to the Montreal Protocol on Substances that Deplete the Ozone Layer, in Montreal, Canada.

Janos Maté of Greenpeace International and David Kanter of Princeton University presented a paper titled “The Benefits of Basing Policies on the 20 Year GWP of HFCs”.

The authors argue that the 20 year GWP index (GWP20) is better suited to reflecting the climate impact of HFCs, as the average lifetime of HFCs in use today is 21.7 years. “20 years is the real effect of the climate impact of HFCs. This we hope can help give a sense of urgency to the debate and thereby speed up action to deal with HFCs”, explained Kanter.

Eventual adoption of the 20 year GWP index would have immediate policy implications, among which:

  • Redefining low GWP substances: While GWP100 values of some substances may seem deceptively attractive to some policy makers, the same substances measured using GWP20 become much less appealing. For example is HFC-32, with a GWP of 675 over 100 years but a GWP of 2,330 over 20 years.
  • Eventually taxing HFCs according to their real climate impact across their atmospheric lifetime

Rationale for 20 year GWP (GWP20) metric for HFCs

GWP metrics measure the potency of greenhouse gases (GHGs) over a specific period of time, relative to carbon dioxide (CO2), which has a GWP of 1.

The timescale chosen is particularly important due to variability in lifetime of different GHGs.
“For the evaluation of short-term effects, a time horizon of a few decades could be taken […]”, notes the first Assessment report by the Intergovernmental Panel on Climate Change (IPCC, 1990, p.58). “For the evaluation of sea-level rise, the commitment to greenhouse warming over a 100 year or longer time horizon may be appropriate”.”

GWP20 highlights the benefits of action more clearly than GWP100

By highlighting the benefits of action more clearly than GWP100, GWP20 provides an even greater incentive for the Parties to the Montreal Protocol to enact measures that progressively limit an eventually limit the use of HFCs.

To illustrate how the choice of the timeframe reinforces the urgency to speed up action on phasing out HFCs, the authors presented a graph from the Velders et al. 2009 study (titled “The large contribution of projected HFC emissions to future climate forcing), that compares emissions projections based on GWP20 and GWP100. “[…] using GWPs with a 100- year time horizon yield and HFC consumption of 6.4-9.9 GtCO2-eq per year in 2050. If, instead, a 20-year time horizon is used, the consumption increases to 12.6-20 GtCO2-eq per year”, reads the Velders study.

“The climate forcing significance of a given time series of HFC emissions is highly sensitive to the time-horizon assumed because the HFC lifetimes are short compared with CO2 lifetime”, notes the same study.

Basing Montreal Protocol policies on the short-term climate impact

“Given that the effects of climate change are already being felt, it is prudent to base Montreal Protocol policies on the short-term climate impact of ODS (ozone depleting substances) alternatives”, reads the paper.

“The whole purpose of this paper is to promote discussion”, said Maté already at the outset of the presentation, in recognition of the fact that getting the Montreal Protocol delegates to change over from the 100 year to the 20 year metric would be a difficult endavour.

However, the choice of the GWP timeframe is a matter of political will, as also rightly points out a 1994 IPCC report: “the choice of time horizon will depend on policy considerations” (IPCC 1994, p.26).

happy wheels
Share this