Putting a value on short-lived greenhouse gases

Some greenhouse gases (GHGs), like methane, remain active in the atmosphere for a relatively short period of time compared to CO2. Scientists, economists and policy makers disagree on whether it would pay to increase efforts to reduce emissions of these short-lived GHGs. New research highlights how uncertainty about the impacts of GHGs on the climate determines the cost-effectiveness of abatement approaches.

The Kyoto Protocol allows countries to meet their emissions targets by reducing six different greenhouse gases (GHGs): carbon dioxide (CO2), methane, nitrous oxide and three groups of potent, fluorinated GHGs (HFCs, PFCs and SF6).

The Kyoto Protocol uses global warming potentials (GWPs) to set limits on each gas. GWPs are an estimate of how much a given amount of greenhouse gas contributes to global warming, compared with CO2. On this scale, CO2 has a value of GWP1and methane is currently set at GWP100.

Some economists suggest that to meet temperature targets cost-effectively, short-lived GHGs like methane should have a lower value. This would have the effect of reducing the attractiveness of cutting emissions of short-lived GHGs, such as methane. They argue that reducing short-lived GHGs reduction today is less important than tackling long-lived GHGs because long-lived GHGs have an impact on meeting climate targets in the more distant future. Only when the targets draw closer, should more emphasis be placed on reducing emissions of short-lived GHGs.

Swedish researchers created a model to investigate these issues. The model deals with how our uncertainty surrounding the climate's sensitivity to GHGs is resolved. Assuming a time-scale that runs from 1990-2200, with abatement starting in 2000, the model uses emissions forecasts from the Intergovernmental Panel on Climate Change (IPCC). They investigated a number of scenarios using the model, including: a scenario where there is gradually less uncertainty about climate change and a scenario where there is full understanding of the extent of climate sensitivity for prediction purposes.

When there is uncertainty and subsequent learning about the climate sensitivity, cost-effective near-term abatement of methane increases, compared to a deterministic case in which we know the true value of the climate sensitivity. This effect is stronger for the short-lived GHG, and so its relative GWP value should increase and more emphasis should be put on near-term methane abatement.

Attempts to delay major investments in the energy sector by focusing short-term efforts on non-CO2 greenhouse gases could save money but prove disastrous if the climate proves to be more sensitive to GHGs than expected. Alternatively, the financial costs of methane abatement might be attractive in a scenario where the climate proves less sensitive to GHGs than expected.

A key question that remains for policy makers is the GWP100 value for methane. Taking uncertainty about climate sensitivity into account puts methane close to its current GWP100 value - so renegotiating this value is probably not necessary. The additional benefits of targeting emissions of methane, ozone precursors and aerosols are still highly relevant and should be considered when choosing a policy response.

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