Abstract Air transport Greenhouse Gas (GHG) emissions estimates differ greatly, depending on the calculation method employed. Among the IPCC, ICAO, DEFRA, and BrighterPlanet calculation methods, the largest estimate may be up to 4.5 times larger than the smallest. Such heterogeneity – and ambiguity over the true estimate – confuses the consumer, undermining the credibility of emissions estimates in general. Consequently, GHG emissions estimates do not currently appear on the front page of flight search-engine results. Even where there are differences between alternative flights’ emissions, this information is unavailable to consumers at the point of choice. When external considerations rule out alternative travel-modes, the relative ranking of flight options’ GHG emissions is sufficient to inform consumers’ decision making. Whereas widespread agreement on a gold standard remains elusive, the present study shows that the principal GHG emissions calculation methods produce consistent rankings within specific route-structure classes. Hence, for many consumers, the flight identified as most GHG efficient is not sensitive to the specific calculation method employed. But unless GHG emissions information is displayed at the point of decision, it cannot enter into consumers’ decision making. A credible and ambiguity-free alternative would thus be to display GHG ranking information on the front page of flight search-engine results.