In this short paper, Gerard Fox and George Yarrow argue that, in the context of climate change policies, the nature and significance of any potentially problematic economic externalities are functions of strategic policy choices: that is, they vary according to the particular policy strategy chosen. The traditionally identified externality – that the benefits of carbon abatement efforts by any one country are mostly enjoyed by other countries – comes from strategies that are conceptualised in terms of determining quantities (of carbon emissions or abatement), an approach to economic policy that was adopted in Soviet-style central planning. By leading to external effects that then call for difficult-to-achieve correction, in effect the quantitative planning system establishes self-created obstacles to attaining that which is desired. Science and technology policy approaches based on sharing of knowledge and know- how are shown to have very different implications for the nature and significance of any associated externalities. The development of the Oxford/AstraZeneca Covid vaccine is given as an example of the possible, alternative, strategic approaches.