Scenario Emissions Pathway Calculator

An emissions pathway is a time series of annual greenhouse gas flows linking a baseline year to a target year, and its shape matters as much as its endpoints because warming tracks cumulative CO2 rather than the emission rate in any single year. The linear form, E(t) = E0 - (E0 - ET) x (t/T), spreads an identical absolute cut across every year, which is easy to audit but assumes abatement cost stays flat. The exponential form, E(t) = E0 x exp(-kt) with k = -ln(1 - r)/T for a fractional reduction r over T years, holds the percentage cut constant and front-loads tonnes while emissions are still high. The logistic S-curve, centred at T/2, mirrors how technology diffusion actually behaves: slow early deployment, a steep middle phase, then a long tail of hard-to-abate residuals. For reference, the Science Based Targets initiative requires roughly 4.2 percent absolute reduction per year for 1.5 degrees C alignment, while business-as-usual growth is commonly assumed at 1 to 3 percent per year. Several limits apply. Cumulative totals here are simple sums of annual endpoint values, so they only approximate an integral. The alignment flags are threshold tests on reduction percentage and target year alone; they do not check scope coverage, base-year recalculation, or whether offsets are being counted as reductions, all of which formal validation requires. The IPCC AR6 remaining carbon budget of roughly 500 GtCO2 from 2020 for a 50 percent chance of holding 1.5 degrees C is a global constraint, so allocating a share to a single company or city requires an explicit downscaling method.

Formal emissions scenario work began with the IPCC First Assessment Report in 1990, whose SA90 scenarios were superseded by the IS92 family in 1992. The Special Report on Emissions Scenarios in 2000 introduced the A1, A2, B1 and B2 storylines that shaped a decade of climate modelling. Because those narrative-first scenarios were slow to couple with climate models, the community restructured the process around Representative Concentration Pathways, published in 2011 and labelled by radiative forcing at 2100 (RCP2.6 through RCP8.5), which were later paired with the Shared Socioeconomic Pathways describing the societal conditions under which each forcing level might arise. The pathway shapes themselves borrow from older work. Cesare Marchetti and Nebojsa Nakicenovic at IIASA showed in 1979 that historical energy source substitutions follow logistic curves, which is why S-curve trajectories are used for technology-driven transitions. Yoichi Kaya proposed in 1990 the identity decomposing emissions into population, income, energy intensity and carbon intensity, still the standard scaffolding for scenario construction. Entity-level pathway setting is much newer: the Science Based Targets initiative was founded in 2015 by CDP, the UN Global Compact, the World Resources Institute and WWF, the same year the Paris Agreement made net zero an operational goal, and its Corporate Net-Zero Standard followed in 2021.