Earth Systems

Global health effects of future atmospheric mercury emissions

Abstract: Mercury is a potent neurotoxin that poses health risks to the global population. Anthropogenic mercury emissions to the atmosphere are projected to decrease in the future due to enhanced policy efforts such as the Minamata Convention, a legally-binding international treaty entered into force in 2017.

Here we report the development of a comprehensive climate-atmosphere-land-ocean-ecosystem and exposure-risk model framework for mercury and its application to project the health effects of future atmospheric emissions. Our results show that the accumulated health effects associated with mercury exposure during 2010-2050 are $19 (95% confidence interval: 4.7-54) trillion (2020 USD) realized to 2050 (3% discount rate) for the current policy scenario.

Our results suggest a substantial increase in global human health cost if emission reduction actions are delayed. This comprehensive modeling approach provides a much-needed tool to help parties to evaluate the effectiveness of Hg emission controls as required by the Minamata Convention.

Global air quality and health impacts of domestic and international shipping

Abstract: Shipping activities contribute to degraded air quality and premature mortalities worldwide, but previous assessments of their health impact have not yet differentiated contributions from domestic and international shipping at the global level. The impacts of domestic shipping can affect different populations, and domestic and international shipping emissions are governed under different regulatory systems. Thus, a consistent global analysis comparing the health impacts from domestic and international shipping could inform policy making in attempts to coordinate policies across multiple scales to address the health burden of shipping emissions.

In this study, we conduct an integrated analysis of global ship emissions and their PM2.5-related health impacts, in which we differentiate the contributions from domestic and international shipping. We create bottom-up global ship emission inventories based on ship activity records from the Automatic Identification System (AIS), and then apply the GEOS-Chem model and Global Exposure Mortality Model to quanitfy shipping-related PM2.5-concentrations and associated moralities. We also quantify the public health benefits under different control scenarios including the 2020 0.5% Sulphur Cap, a post-2020 0.1% Sulphur Cap, and a post-2020 Tier III NOx Standard.

We find that 94,200 (95% confidence interval: 84,800 - 103,000) premature deaths were associated with PM2.5 exposure due to maritime shipping in 2015, of which 83% were associated with international shipping activities and 17% with domestic shipping. Although the global health burdens of ship emissions are dominated by international shipping, the fraction varies by region: 44% of shipping-related deaths in China come from domestic shipping activities. We estimate about 30,200 (27,200 - 33,000) avoided deaths per year under a scenario consistent with a 2020 0.5% Sulphur Cap. We additionally examine two post-2020 control scenarios: we find that a Tier III NOx Standard would have greater benefits than a 0.1% Sulphur Cap, with the two policies reducing annual shipping-attributable PM2.5-related premature deaths by 33,300 (30,100 - 36,400) and 5,070 (4,560 - 5,540), respectively.

Moving ecological and biogeochemical transitions across the North Pacific

Abstract: In the North Pacific Ocean, nutrient rich surface waters flow south from the subpolar gyre through a transitional region and into the subtropics. Along the way, nutrients are used, recycled, and exported, leading to lower biomass and a commensurate change in ecosystem structure moving southward. We focus on the region between the two gyres (the Transition Zone) using a coupled biophysical ocean model, remote sensing, floats, and cruise data to explore the nature of the physical, biogeochemical, and ecological fields in this region.

Nonlinear interactions between biological processes and the meridional gradient in nutrient supply lead to sharp shifts across this zone. These transitions between a southern region with more uniform biological and biogeochemical properties and steep meridional gradients to the north are diagnosed from extrema in the first derivative of the properties with latitude. Some transitions like that for chlorophyll a (the transition zone chlorophyll front [TZCF]) experience large seasonal excursions while the location of the transitions in other properties moves very little. The seasonal shifts are not caused by changes in the horizontal flow field, but rather by the interaction of seasonal, depth related, forcing with the mean latitudinal gradients. Focusing on the TZCF as a case study, we express its phase velocity in terms of vertical nutrient flux and internal ecosystem processes, demonstrating their nearly equal influence on its motion.

This framework of propagating biogeochemical transitions can be systematically expanded to better understand the processes that structure ecosystems and biogeochemistry in the North Pacific and beyond.

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