The impact of human health co-benefits on evaluations of global climate policy

Abstract

The health co-benefits of CO₂ 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.

Fig. 1

Optimal decarbonization and temperature. a Decarbonization over time for the reference case optimal policy that excludes health co-benefits (blue line) and in the full RICE + AIR optimal policy that includes health co-benefits (red line). b Estimated global temperature rise above preindustrial levels that would occur given the decarbonization in a. Decarbonization is relative to a business-as-usual scenario without any climate action, and 100% decarbonization signifies zero net carbon emissions

Fig. 2

Costs and benefits of mitigation. Decomposition of the change in global consumption relative to the business-as-usual (BAU) scenario under a the full RICE + AIR optimal policy, and b the reference case optimal policy. Health co-benefits and benefits from avoided CO₂ damages are positive, while mitigation costs and aerosol co-harms (climate damage from the co-reduction of cooling aerosols) are negative. The black solid line displays the global net effect. a shows that the net effect on global consumption is immediately positive when health co-benefits are taken into account, in contrast to the reference case (b), which is representative of standard cost-benefit models that do not include health co-benefits and thus imply that optimal climate policy has net costs for much of this century. If health co-benefits are added to the reference policy in b—by adding the light-red bars displayed in a—the global net effect becomes immediately positive, and if health co-benefits were removed from a, the net effect would be negative for most of this century

Fig. 3

Health benefits of carbon mitigation. Life-years gained a overall and b per 100,000 population by region from the air quality improvements associated with the optimal decarbonization in RICE + AIR. c, d show the resulting monetized benefits in total, and as a percent of GDP, respectively. Note that if a region’s PM_2.5 exposure (concentration) drops below 5.8 µg/m³, health benefits no longer accrue—this threshold assumption is common in other global air quality assessments, and is tested in the sensitivity analyses

Fig. 4

Impact of independent air quality control. a Optimal decarbonization and b associated global temperature rise given different levels of autonomous air quality control. The reference and RICE + AIR cases from Fig. 1, where χ = 1, are displayed as the blue and red solid lines, respectively

Fig. 5

The influence of pure time preference. Optimal decarbonization with low (0.1%), medium (1.5%), and high (3.5%) rates of pure time preference for the reference case (that excludes health co-benefits) and in the full RICE + AIR case (that includes health co-benefits). All runs have a consumption elasticity of marginal utility of 1.5

Fig. 6

FUND + AIR results. Optimal decarbonization rates over time for the FUND reference case (that excludes health benefits) and the FUND + AIR case (that includes health benefits)

Fig. 7

Diagram of the AIR module. Flow chart illustrating how the AIR module (rectangles) links with the RICE model (gray circle) to estimate emissions of air pollutants and their impacts. The model/method underlying each step in AIR is shown within parentheses, along with the relevant equations