"The transformation of economies towards net zero greenhouse gas (GHG) emissions within very few decades will require substantial investments. However, the transition to a low-carbon economy exhibits a high degree of complexity and path dependency. Consequently, firms and investors systematically face deep uncertainty, a situation with no information about the probabilities of possible outcomes, where even the possible outcomes are not known. In this article, we present concepts and classifications of uncertainty and discuss potential sources of deep uncertainty for firms and investors, such as uncertainty about technology development and innovation, the behaviour of competitors, or climate-related policy development. Standard modelling-based decision support tools typically used by firms and investors are of limited value under deep uncertainty. Thus, deep uncertainty deters decision-making and delays low-carbon investment. This article addresses this issue by providing an overview of approaches for decision-making under deep uncertainty. We analyse their applicability for firms and investors and discuss what challenges they might face. Based on this analysis, we derive guidelines for policymakers to reduce deep uncertainty and support firms' decision-making under deep uncertainty."

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