The article in Nature Sustainability is here: https://www.nature.com/articles/s41893-018-0136-7

Traditionally, air pollution, climate change, and water scarcity have been separately addressed by scientists in their respective fields.  However, both energy production and consumption can simultaneously affect regional air quality, local water stress, and the global climate.  Thus, efforts to reshape the energy system need to capture the underlying interactions to avoid unintended consequences and/or a failure to achieve co-benefits. 

The Science, Technology and Environmental Policy (STEP) program at Princeton University and the International Institute for Applied Systems Analysis (IIASA) are both great institutions in which to conduct interdisciplinary work. The first author, a doctoral scandidate in STEP when conducting the project, was fortunate to have the opportunity to integrate the resources of these two platforms through her Young Scientist Summer Program Fellowship at IIASA. Jointly with her coauthors, her research goal was to analyze the underlying air quality-carbon-water synergies and trade-offs in the energy industry, something which has not been widely studied. 

We chose to analyze China’s energy system because it has both one of the world’s largest economies and the largest population. As a result, its energy plans have important domestic as well as global implications for sustainable development. At the same time, China’s energy transition sets an example for many developing countries which are focused on economic growth. Without proper design, development of new energy systems will result in the lock-in of multiple environmental problems including domestic air pollution, local water scarcity, and global climate change. However, opportunities exist, with strategic development of energy resources, to minimize the environmental problems and maximize co-benefits.

As is widely known, China has a coal-dominated primary energy structure (~64%), with the cleanest fossil fuel (natural gas) accounting for only ~6% of China’s total primary energy consumption. This number is far below the global average of ~16%. Primarily to address severe air pollution problems, China has been pushing for an end-use coal-to-natural gas switch by increasing natural gas supplies from a variety of sources and substituting natural gas for coal in various end-uses. 

To capture the underlying interactions and to identify the multiple environmental impacts of a coal to natural gas conversion, we couple a lifecycle analysis with an integrated assessment. We examine the impacts of substituting natural gas (from six different sources in China) for coal and compare the resulting population-weighted PM_2.5 surface concentrations, carbon emissions, and water stress-weighted water consumption. Our work is designed to explore the implications of various combinations of gas supply and end-uses on three environmental targets in turn: air quality improvement, carbon mitigation, and water stress alleviation.

We find that, despite the benefits of coal-based synthetic natural gas (SNG) for air quality, the use of SNG results in substantial dis-benefits -- increased carbon emissions and water demand, particularly in regions already suffering from high per capita carbon emissions and severe water scarcity.  We also find that, for gas sources other than SNG, end-uses play a dominant role in affecting the magnitude of the environmental impacts, especially effects on air quality. Thus, more attention on energy deployment strategies, in addition to current extensive discussions on clean energy source choices, is needed. 

Also, we identify notable air quality-carbon co-benefits and air quality-water trade-offs in term of the degree of environmental improvement due to sectoral and regional natural gas end-uses. These trade-offs result from sectoral differences affecting environmental impacts and the geographic mismatch between regions of high air pollution and regions of high water stress. These inherent air quality-water trade-offs identified here not only exist for coal substitution with natural gas, but also for coal substitution with renewables. For instance, substituting wind power for coal-fired power plants equipped with end-of-pipe control technologies will bring water savings but only small air quality benefits. Displacing residential and industrial coal uses with natural gas or electrifying these two sectors will be more effective at achieving desired air quality improvements.  

The trade-offs and co-benefits we identified in China may also exist in other countries where regions of high air pollution differ from those with high water stress (i.e., India). Thus, in future efforts to improve air quality, reduce water stress, and reduce carbon emissions, analyses of energy end-use strategies will be an important complement to efforts to increase supplies of clean energy.