| Summary: | The Paris Agreement established that global warming should be limited to "well below" 2?C and encouraged efforts to limit warming to 1.5?C. Achieving this goal presents a significant challenge, especially given the presence of (i) economic inertia and adjustment costs, which penalize a swift transition away from fossil fuels, and (ii) climate uncertainty that, for example, hinders the ability to predict the amount of emissions that can be emitted before a given temperature target is passed, which is often referred to as the remaining carbon budget. This paper presents a modeling framework that explores optimal decarbonization investment strategy when both delayed learning about the remaining carbon budget and adjustment costs are present. The findings show that delaying learning about the remaining carbon budget impacts investment in three ways: (i) the cost of policy increases, especially when adjustment costs are present; (ii) abatement investment is front-loaded relative to the certainty policy; and (iii) the sectoral allocation of investment changes to favor declining investment pathways rather than bell-shaped paths. The latter effect is especially pronounced in hard-to-abate sectors, such as heavy industry. Each of the effects can be traced back to the carbon price distribution inheriting a "heavy tail" when the remaining carbon budget is learned later in the century. The paper highlights how climate uncertainty and adjustment costs combined result in a more aggressive least-cost strategy for decarbonization investment
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