# NEFDB Phase 3 Aviation Radiative-Forcing Multiplier ## TL;DR - **Factor key:** `ng.aviation.domestic.rf_multiplier` - **Value:** `1.9` - **Unit:** `multiplier` - **Source:** Lee et al. (2021), *Atmospheric Environment*, 244, 117834 - **Exact source location:** Table F.1a, row **"Contrail cirrus (km basis)"**, column **GTP₅₀**, value **1.9** (PDF page 57 of the accepted manuscript). ## Full citation Lee, D.S., Fahey, D.W., Skowron, A., Allen, M.R., Burkhardt, U., Chen, Q., Doherty, S.J., Freeman, S., Forster, P.M., Fuglestvedt, J., Gettelman, A., et al. (2021) The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018. *Atmospheric Environment*, 244, 117834. DOI: ## Where the value 1.9 appears in Lee et al. 2021 In the accepted manuscript (file: `lee_et_al_2021_aviation_rf.pdf`), **page 57**, Appendix Table F.1a lists emission metrics and corresponding CO₂-equivalent emissions for the ERF components of 2018 aviation emissions and cloudiness, calculated with the CO₂ impulse-response function (IRF) **without C-cycle feedbacks** (Gasser et al., 2017) and the climate IRF from Boucher and Reddy (2008). The relevant excerpt is: ``` Table F.1a. Emission metrics and corresponding CO₂-equivalent emissions for the ERF components of 2018 aviation emissions and cloudiness using CO₂ IRF without C-cycle feedbacks from Gasser et al. (2017), and climate IRF from Boucher and Reddy (2008). Metrics ERF term GWP20 GWP50 GWP100 GTP20 GTP50 GTP100 ───────────────────────────────────────────────────────────────────────────────── CO₂ 1 1 1 1 1 1 Contrail cirrus (Tg CO₂ basis) 2.39 1.15 0.68 0.70 0.11 0.10 Contrail cirrus (km basis) 40 19 11 12 1.9 1.6 Net NOx 637 216 122 -231 -75 14 Soot emissions 4409 2125 1252 1295 210 177 SO₂ emissions -856 -412 -243 -251 -41 -34 Water vapor emissions 0.22 0.11 0.06 0.07 0.01 0.009 ``` The value **1.9** sits in the **"Contrail cirrus (km basis)"** row under the **GTP₅₀** column. ## What the multiplier means Lee et al. calculate **emission-equivalency metrics** (GWP and GTP over 20-, 50-, and 100-year horizons) that express each non-CO₂ aviation forcing term as a CO₂-equivalent "multiplier" relative to CO₂ emissions alone. The GTP₅₀ value of **1.9 for contrail cirrus on a per-km basis** indicates that, under the GTP₅₀ metric, the climate-temperature response from contrail-cirrus forcing associated with aviation distance flown is about **1.9 times** the response from the CO₂ emitted on that same flight distance. NEFDB adopts **1.9** as the optional **radiative-forcing multiplier** for domestic aviation non-CO₂ climate effects. It is a single, conservative reference multiplier for users who want to broaden a CO₂-only footprint estimate to include non-CO₂ impacts, without requiring a separate contrail-cirrus model. ## Caveats 1. **Metric- and time-horizon-dependent.** The 1.9 value is tied to **GTP₅₀** and to the **contrail-cirrus (km basis)** component. Using GWP₂₀, GWP₅₀, GWP₁₀₀, GTP₂₀, or GTP₁₀₀ gives very different numbers (see Table F.1a). A different metric or a different component could yield a multiplier anywhere from below 1 to above 40 for specific terms. 2. **Lee et al. total multipliers span 1.0–4.0.** In Table 5, the ratio of total CO₂-equivalent aviation emissions (km basis) to CO₂-only emissions ranges from **1.0** (GTP₅₀) to **4.0** (GWP₂₀), with a GWP\*₁₀₀ value of about **3.0**. The choice of 1.9 therefore represents a narrower, GTP₅₀-oriented contrail-cirrus anchor rather than the full envelope of possible aviation multipliers. 3. **Does not capture all non-CO₂ effects.** The 1.9 value is derived from contrail cirrus only; it does not separately incorporate NOₓ net effects (which can be positive or negative depending on metric and time horizon), soot and sulfate aerosol effects, or stratospheric water vapor. 4. **Scenario-dependent.** Lee et al. note that a constant multiplier should not be applied to future scenarios that deviate substantially from the recent growth trajectory of aviation emissions. Under constant or declining emissions, non-CO₂ multipliers decline toward unity because short-lived climate forcers maintain a roughly constant level of warming rather than accumulating like CO₂. 5. **Optional add-on.** This multiplier is **not** embedded in the base route CO₂ emission factors. It is an optional, additive climate-adjustment layer. ## Usage in the NEFDB API - The base aviation route factor returns **CO₂ only** by default. - To include the non-CO₂ RF multiplier, pass the optional flag `rf: true` (or equivalent API parameter). The API multiplies the base CO₂ result by `ng.aviation.domestic.rf_multiplier = 1.9` and returns the adjusted total. - Because the multiplier has no physical unit, it carries the NEFDB unit `multiplier`. - Documentation of the choice and these caveats should travel with every response that uses the multiplier so that downstream reports can show the non-CO₂ adjustment transparently. ## Audit trail - **Phase:** 3 - **Derived from:** `lee_et_al_2021_aviation_rf.pdf`, page 57, Table F.1a, "Contrail cirrus (km basis)", GTP₅₀. - **Derivation date:** 2026-06-19 - **NEFDB file:** `docs/transport_aviation/rf_multiplier_phase3.md`