Integrated assessment models of climate and the economy provide estimates of the social cost of carbon and inform climate policy. We create a variant of the Regional Integrated model of Climate and the Economy (RICE)-a regionally disaggregated version of the Dynamic Integrated model of Climate and the Economy (DICE)-in which we introduce a more fine-grained representation of economic inequalities within the model's regions. This allows us to model the common observation that climate change impacts are not evenly distributed within regions and that poorer people are more vulnerable than the rest of the population. Our results suggest that this is important to the social cost of carbon-as significant, potentially, for the optimal carbon price as the debate between Stern and Nordhaus on discounting.T he most prominent debate on cost-benefit evaluation of climate policy has been on the discount rate (see, e.g. refs. 1-4).* One of the important principles this debate has highlighted is that the effect of climate impacts are discounted when they are borne by more affluent-future-generations. † However, despite Schelling's early remarks (9) that this principle should also apply across contemporaries with different levels of affluence, the interaction between climate impacts and economic inequality has only been studied by looking at inequality between regions (10, 11). This is potentially an important oversight, because in leading cost-benefit integrated assessment models (IAMs) much of the poverty associated with high levels of vulnerability is masked by regional averaging of economic variables. ‡ In light of this, we modify a leading climate-economy model, Regional Integrated model of Climate and the Economy (RICE), to include what is known about economic inequality within regions and countries. This representation of economic inequality allows us to investigate the effect on optimal policy of different assumptions about the distribution of damages by economic strata. § When subregional differences are modeled in this way, several policy-relevant aspects of the model can change dramatically even when other assumptions and parameters from RICE are held constant. As we show below, even when RICE regional damage functions are used to establish the damage level of each region, the distribution of damage within regions can cause some members of future generations to be less affluent than their current counterparts. { If the distribution of damage is less skewed to high incomes than the distribution of consumption, then weak or no climate policy will result in sufficiently large damages on the lower economic strata to eventually stop their welfare levels from improving, and instead cause them to decline. This paints a different picture from the standard narrative in leading cost-benefit IAMs, where regional average consumptions continue to grow even under business-as-usual (BAU).The implications for policy recommendations are striking, both by the standard utilitarian metric and by metrics of sustainability (18) and just...

The health co-benefits of CO 2 mitigation can provide a strong incentive for climate policy through reductions in air pollutant emissions that occur when targeting shared sources. However, reducing air pollutant emissions may also have an important co-harm, as the aerosols they form produce net cooling overall. Nevertheless, aerosol impacts have not been fully incorporated into cost-benefit modeling that estimates how much the world should optimally mitigate. Here we find that when both co-benefits and co-harms are taken fully into account, optimal climate policy results in immediate net benefits globally, overturning previous findings from cost-benefit models that omit these effects. The global health benefits from climate policy could reach trillions of dollars annually, but will importantly depend on the air quality policies that nations adopt independently of climate change. Depending on how society values better health, economically optimal levels of mitigation may be consistent with a target of 2 °C or lower.

Existing estimates of optimal climate policy ignore the possibility that carbon tax revenues could be used in a progressive way; model results therefore typically imply that near-term climate action comes at some cost to the poor. Using the Nested Inequalities Climate Economy (NICE) model, we show that an equal per capita refund of carbon tax revenues implies that achieving a 2°C target can pay large and immediate dividends for improving well-being, reducing inequality and alleviating poverty. In an optimal policy calculation that weighs the benefits against the costs of mitigation, the recommended policy is characterized by aggressive near-term climate action followed by a slower climb towards full decarbonization; this pattern-which is driven by a carbon revenue Laffer curve-prevents runaway warming while also preserving tax revenues for redistribution. Accounting for these dynamics corrects a longstanding bias against strong immediate climate action in the optimal policy literature.