1 Introduction

Energy supply sector accounts for ~35% of global greenhouse gas (GHG) emissions [Intergovernmental Panel on Climate Change (IPCC), 2014], with electricity and heat production accounting for ~42% of carbon dioxide (CO 2 ) emissions [International Energy Agency, 2014]. In addition, it is a major source of other air pollutants [European Environment Agency, 1999]. It is therefore evident that the energy strategies we choose during the coming decades have a crucial role in determining our success in solving two major global challenges: mitigation of climate change and improvement of air quality. To ensure sustainable development and significant greenhouse gas and air pollution emission reductions, very likely, all technological options—nuclear, many types of renewables, carbon capture and storage, and smart grids and new transport technologies—are needed in the near future [IPCC, 2014].

Nuclear power (fission) can be considered a clean energy source with respect to climate and air quality, as it produces significantly less greenhouse gases and traditional air pollutants than, e.g., energy production from fossil fuels [Lenzen, 2008]. It has been calculated that GHG emissions from the full life cycle of hard coal and natural gas technologies are 410–950 g CO 2 ‐e/kWh el (CO 2 ‐equivalent GHG emissions for every kWh of electricity generated) [IPCC, 2014]. On the other hand, the actual energy production with fission does not produce greenhouse gas emissions but there are indirect emissions originating from the nuclear fuel cycle. The sources for these emissions are uranium mining, milling, enrichment, fuel fabrication, reactor construction and operation, decommissioning, fuel reprocessing, nuclear waste storage and disposal, and transport. Thus, for the whole cycle, greenhouse gas emissions from nuclear power are 4–110 g CO 2 ‐e/kWh el , i.e., roughly 1 to 2 orders magnitude lower than for combustion of coal. As a comparison, the corresponding emissions from wind turbines and hydroelectricity are 7–56 g CO 2 ‐e/kWh el and 10–30 g CO 2 ‐e/kWh el , respectively. From solar photovoltaic and concentrated solar power the emissions are 18–180 g CO 2 ‐e/kWh el and 9–63 g CO 2 ‐e/kWh el , respectively [IPCC, 2014].

However, a major concern with nuclear power is the (however small) possibility of release of radioactive material into the atmosphere, hydrosphere, or soil. In 2011, this risk was demonstrated in the nuclear accident in Fukushima, Japan. Several countries are thus facing the question whether the economic and environmental benefits of nuclear energy outweigh the risks related to radioactive contamination [Wolf, 2015]. In Japan the nuclear power plants were progressively shut down following the Fukushima accident but the government has recommenced nuclear power generation even though the majority of public opinion is opposing [Ipsos, 2011]. The first nuclear reactor was restarted in August 2015 (World Nuclear Association (WNA), world‐nuclear.org/info/Country‐Profiles/Countries‐G‐N/Japan/). On the other hand, in Germany the government is planning a complete phase out of nuclear energy by the year 2022, and the fraction of electricity from nuclear energy has already dropped from 18% in 2011 to the current 16% [AG Energiebilanzen, 2015]. Although the reduction in nuclear power has been replaced with renewable energy to some extent (mainly wind power, biomass burning, and solar power), the fraction of coal combustion also increased up to 2013 [AG Energiebilanzen, 2015]. Furthermore, when the overall energy demand recovers, the use of coal might increase again. As global GHG emissions are rising at an alarming rate, it is important to investigate the effects of these significant shifts with state‐of‐the‐art modeling tools.

In this study, we investigate what would be the climate and PM‐induced air quality consequences if all nuclear reactors worldwide were closed down and replaced by coal combustion. In a way, this presents a “worst‐case scenario” since less polluting energy sources are available. However, coal combustion is one of the cheapest sources of energy [U.S. Energy Information Administration, 2015] and its usage can increase the national energy supply security in countries with abundant national supplies [Sims et al., 2007]. Currently, approximately 25% of the energy consumption worldwide is produced by burning coal, constituting of over 40% share of anthropogenic CO 2 emissions [Smith et al., 2013]. In addition, burning of coal produces a significant amount of pollutants; thus, the largest air quality hazard from coal power arises from inhalation of the combustion products emitted to the atmosphere [Krewitt et al., 1998; Rashad and Hammad, 2000]. Of the emitted and inhaled combustion products, the most hazardous are fine particles (PM 2.5 , aerosol particles smaller than 2.5 µm in diameter) that may have several adverse effects on human health, such as lung cancer and cardiopulmonary diseases [Markandya and Wilkinson, 2007]. In a very recent study, Apte et al. [2015] found that of the 3.2 million annual deaths globally attributed to ambient PM 2.5 from all emission sources, several hundred thousands could be avoided with pollution prevention according to the World Health Organization (WHO) guidelines. However, the same aerosol particles emitted from coal combustion, which are responsible for the adverse negative health effects, mask a part of the positive radiative forcing caused by greenhouse gases [IPCC, 2014]. Therefore, PM‐targeted air pollution controls can be expected to lead to regional warming in many parts of the world [Pietikäinen et al., 2015]. All these challenges are especially important in Asia, where both the population and the average standard of living are increasing fast, and significant investments to the energy system are needed in order to meet the climate and the air quality goals [van Vliet et al., 2012].

Our work was very much motivated by the study by Kharecha and Hansen [2013], who investigated the contribution of nuclear power generation on preventing air pollution‐related deaths and greenhouse gas emissions during the years 1971–2009, when compared to a world in which the equivalent amount of energy would have been produced with coal and natural gas. They concluded that during this time nuclear power has been responsible for preventing an average of 1.84 million deaths and 64 Gt of CO 2 ‐e emissions cumulatively. Their estimates were based on simple mortality and GHG emission factors, in deaths/TWh and CO 2 ‐e/TWh, respectively. While these factors took into account all stages of the fuel cycle, from fuel extraction to electricity transport, the climate impacts of particulate matter were not considered. The climate (but not health) impacts of PM were investigated by Shindell and Faluvegi [2010] who used the Goddard Institute for Space Studies general circulation model which incorporates aerosol chemistry to study the radiative forcing from global coal‐fired power plant emissions. They found that the combined forcing of coal combustion emitted CO 2 , ozone, and aerosol precursors from 1970 to 2000 was strongly positive and that imposing air quality pollution controls will likely accelerate warming rates in the future.