Projections: How may the source-receptor relationships change over the next 20 to 40 years due to changes in emissions and climate change?

In Europe, changes in emissions outside Europe and global methane concentrations will largely drive future annual average O3 levels. Without additional controls, global methane emissions are expected to grow, increasing O3 mortality in Europe in 2050 by up to 8,000 additional premature deaths compared to 2010 levels. Implementation of mitigation policies, largely outside of Europe, can decrease methane emissions overall and decrease O3 mortality in Europe by up to 2000 premature deaths per year compared to 2010 levels, a difference of 10,000 deaths per year between the highest and lowest global CH4 emissions scenarios.

In North America, the difference between the highest and lowest global CH4 emissions scenarios corresponds to a difference of up to 5,000 deaths per year in 2050. The sectors with substantial mitigation potential are fossil fuel production, waste and wastewater management, and agriculture, with the largest emissions in China, followed by Latin America, Africa, India, and North America [vanDingenen 2018].

Shipping makes a significant contribution to both O3 and PM2.5 levels in Europe and North America, particularly in coastal regions. The impact of planned emissions control policies under the International Maritime Organization have been examined on the regional scale. However, the impact of the emission controls on intercontinental transport has not. TF HTAP is organizing analysis to examine the global impact of controls in each of the IMO designated Emission Control Areas.

Air pollution in South Asia is expected to continue to increase largely because of changes in emissions within the region. Implementation of clean technologies and climate change mitigation policies could substantially decrease pollution levels with large benefits for human health, crops, and ecosystems. The benefits of emissions decreases in South Asia would mostly accrue to the region itself, and then downwind regions in East Asia and the Middle East. However, the global implications of emissions reductions in South Asia should be further explored.

Since 2012, TF HTAP has organized a series of workshops with TFIAM and CIAM to explore future global scenarios and the potential for mitigation. IIASA’s work under the ECLIPSE project has contributed significantly to these discussions. Sessions have focused on improving our understanding of emissions and mitigation potential for marine shipping; residential heating, cooking, and lighting; agricultural burning; transportation; electricity production; and various sources of methane.

The HTAP1 multi-model experiments also examined the potential impacts of changes in meteorology and transport patterns expected as a result of climate change on O3 concentrations using a set of three models that simulated climate changes between the periods 2000 and 2100. Future changes in climate are expected to increase the effect of O3 precursor emissions over source regions and reduce the effect over downwind receptor regions. However, the magnitude of these effects is relatively small, and is driven mostly by changes in atmospheric chemistry and not by changes in transport patterns. The effect of natural emission changes and wider climate-related feedbacks are potentially important, but have not been evaluated fully yet.

Since 2012, TF HTAP has organized a series of workshops with TFIAM and CIAM to explore future global scenarios and the potential for mitigation. IIASA’s work under the ECLIPSE project has contributed significantly to these discussions. Sessions have focused on improving our understanding of emissions and mitigation potential for marine shipping; residential heating, cooking, and lighting; agricultural burning; transportation; electricity production; and various sources of methane.

Mercury

Two recent studies have developed global emission projections for anthropogenic Hg emissions in the years 2020 and 2050. One of the studies was based on the IPCC Special Report on Emissions Scenarios (SRES), used in previous IPCC assessments. The other study was developed for the United Nations Environment Programme’s (UNEP’s) Mercury Programme. Both studies conclude that significant increases, up to 25% in 2020 and 100% in 2050 as compared to 2005, in global Hg emissions can be expected if no major changes in emission controls are introduced and economic activity continues to increase. In both studies, the largest increase in emissions is projected from coal combustion for electricity generation in Asia. However, the implementation of available emission control technology could stabilize or decrease these emissions.

As part of the HTAP multi-model experiments, three of the scenarios for 2020 were simulated by the participating models. The scenarios ranged from a 25% increase to a 65% decrease in global emissions over 2005 levels. The participating models predicted corresponding increases in deposition of 2-25% and decreases in deposition of 25-35% in four source regions, respectively, in annual simulations. In remote regions, such as the Arctic, the changes were predicted to be smaller – ranging from 1.5-5% increases to 15-20% decreases.

The intercontinental source-receptor relationships under these future scenarios are not significantly different from the source-receptor relationships estimated for current emissions. The large contribution of natural sources and re-emitted legacy Hg to deposition dampens the relative response of Hg deposition to changes in new anthropogenic emissions, reinforcing the long term benefit of decreasing the amount of Hg re-circulating in the environment by decreasing Hg emissions globally now.

The impacts of climate change on intercontinental transport of Hg were not explicitly addressed in the HTAP multi-model experiments, and there is large uncertainty about how climate change will effect natural and recycled emissions of Hg, as well as the atmospheric chemistry and transport of Hg. Climate change effects on temperatures, frequency of forest fires, plant growth and decomposition will significantly impact the terrestrial-atmospheric exchange of Hg. Likewise, ocean- atmosphere exchange of Hg will be affected by changes in temperature, wind speeds, storm frequency, as well as changes in atmospheric oxidant and aerosol concentrations. The net effect of these changes, however, has not been adequately studied yet.

Persistent Organic Pollutants

Over the next 40 years, emissions and patterns of intercontinental transport of some POPs will continue to decrease and shift as a result of national and international regulations. For those POPs whose use has been banned or strictly limited, geographic shifts in transport may occur as re-emission of legacy pollution results in migration or dispersion of the pollutants. For POPs that are still in use as chemicals or are unintentionally released from combustion or other industrial processes, differences in regulations or economic activity may lead to shifts in the spatial distribution of emissions. In Europe and North America, full implementation of the LRTAP POPs Protocol, the Stockholm Convention, and other national legislation is expected to decrease emissions by more than 90% for hexachlorobenzene (HCB) and PCB, more than 60% for pentachlorodibenzodioxins and furans (PCDD/Fs), and 30-50% for PAHs.

For POPs that are still in commerce and have yet to be addressed by national and international regulations, intercontinental flows are expected to increase as continued emissions contribute to the stock of the pollutant circulating in the environment. Climate change may further alter the magnitude and patterns of emissions and intercontinental transport of POPs. Similar to Hg, climate change may significantly alter the exchange of POPs among the atmosphere and water, soil, vegetation, sediments, snow, and ice. There is evidence that climate change phenomena, e.g., elevated temperatures and sea-ice reduction, and extreme climate change-induced events, such as forest fires, flooding and glacial melting, will remobilize POPs previously deposited in sinks, e.g., forest soils and vegetation, ocean and lake sediments and glaciers. Climate change may also alter the exposures of individuals and populations and their vulnerability to chemical exposures.

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