Ensuring that offsets and other internationally transferred mitigation outcomes contribute effectively to limiting global warming

Abstract

Ensuring the environmental integrity of internationally transferred mitigation outcomes, whether through offset arrangements, a market mechanism or non-market approaches, is a priority for the implementation of Article 6 of the Paris Agreement. Any conventional transferred mitigation outcome, such as an offset agreement, that involves exchanging greenhouse gases with different lifetimes can increase global warming on some timescales. We show that a simple 'do no harm' principle regarding the choice of metrics to use in such transactions can be used to guard against this, noting that it may also be applicable in other contexts such as voluntary and compliance carbon markets. We also show that both approximate and exact 'warming equivalent' exchanges are possible, but present challenges of implementation in any conventional market. Warming-equivalent emissions may, however, be useful in formulating warming budgets in a two-basket approach to mitigation and in reporting contributions to warming in the context of the global stocktake.

Keywords: Paris Agreement; greenhouse gas metrics; internationally transferred mitigation outcomes; offsets; voluntary carbon markets.

Figure 1.

Impact on global mean surface temperature of transfers involving ‘offsetting’ the emission of methane with avoided emission or removal of CO₂. Left panel shows the impact of a one-off transfer occurring in year 0, while the right panel shows the impact of a sustained transfer offsetting a constant rate of emission of methane with a constant rate of avoided emission or removal of CO₂, starting in year 0. Green, purple and yellow lines show impact on global temperature when the amount of CO₂ is calculated using GWP₂₀, GWP₁₀₀, & GTP₁₀₀, respectively, blue lines using warming-equivalent emissions calculated using GWP* and red lines using linear warming-equivalent emissions. Grey lines show warming caused by methane emissions without any CO₂ offsetting. Based on the ‘do no harm’ principle proposed here, GWP₂₀ would be the recommended conventional metric for this class of transaction. All calculations performed using the standard AR5 impulse response model with thermal response parameters scaled to give an equilibrium climate sensitivity of 2.75 °C (original model was 3.9 °C) [6].

Figure 2.

As figure 1, but for transfers involving offsetting emission of CO₂ with avoided emission of methane. GTP₁₀₀ would be the recommended conventional metric for this class of transaction under a ‘do no harm’ principle.

Figure 3.

Warming-equivalent emissions of CO₂ giving the same forcing response to (a) a pulse emission of methane in year 0 and (b) a sustained constant emission of methane starting in year 0, calculated using the GWP* approximation in blue and exact linear warming equivalent (LWE) emissions (multiplying the forcing response to methane emissions by the inverse of the CO₂ absolute global forcing potential (AGFP) matrix—see section 6) in red.

Figure 4.

Warming-equivalent emissions of methane giving the same forcing response to (a) a pulse emission of CO₂ in year 0 and (b) a sustained constant emission of CO₂ starting in year 0, calculated using the GWP* approximation in blue and exact LWE emissions in red.

Figure 5.

(a) Annual emissions of CO₂ (grey) and methane (other colours) under various metrics for a representative 1.5 °C-consistent scenario. Thin lines show metric-equivalent methane emissions using GWP₁₀₀ (purple), GWP₂₀ (green) and GTP₁₀₀ (yellow). Thick red line shows exact LWE emissions obtained by inverting the AR5 linear response model, while thin blue line shows the GWP* approximation. (b) CO₂-induced (grey dotted), methane-induced (red dotted) and combined (black dotted) warming calculated with the AR5 linear impulse-response model compared with cumulative emissions under the various metrics. Cumulative totals from both gases are shown aggregated using GWP₁₀₀ (dashed black), LWE (thick black), GWP* (thin black), GWP₂₀ and GTP₁₀₀ (upper and lower dash-dot black).