Within the context of our current research and understanding of climate change, decisionmakers are particularly concerned with the extent of future climate change, its comprehensive impact, and the types of socioeconomic pathways available with respect to mitigation and adaptation. Among the factors contributing to these important issues, the role of air pollution in global and regional climate warming remains as one of the largest uncertainties. On the basis of understanding of the IPCC Fifth Assessment Report, specifically, in the role of air pollution in climate change, scenarios establishment, and relationship between the Shared Socioeconomic Pathways (SSPs) and Representative Concentration Pathways (RCPs). Weaknesses and reflections were discussed here especially in strengthening impact, adaptation and mitigation research that related with changes in air pollution and climate. In the future, there are needs to in-depth understand how and why the air pollution in China is so serious and changing; to understand the likely future changes in air pollution and climate; to strengthen comprehensive impact research and selective reduction strategies related to changes in air pollution and climate. Furthermore, this study outlines the needs to develop strategies to close the loop of differential impacts and costs; to establish co-benefits and sustainable development goals, to identify the crucial risks and options for synergies/trade-offs; to integrate sector-specific details with macro-economics, and to integrate the assessments of the various policy instruments. All these focus areas will help to facilitate the transition of economic development patterns towards green and low-carbon development.
Air pollution and climate change ; SSPs and RCPs ; Impact, adaptation and vulnerability ; Future Earth
Since the mid-20th centenary, climate has been experienced an obvious change in China and in the world characterized by warming, which has been unprecedented over decades to millennia, especially associated with strength of extreme weather and climate events or both. It is extremely likely that human activities caused more than half of the observed increase in global mean surface temperature from 1951 to 2010 (Bindoff et al., 2013 ). In consequence, decisionmakers are particularly concerned with the extent of future climate change (Collins et al., 2013 and Kirtman et al., 2013 ) in specific times and regions. Particularly, the questions include when we can expect the global mean surface temperature to increase by 2 °C; what is the likelihood for reduced carbon emissions and development to affect national efforts to remain under the 2 °C threshold; what socioeconomic pathway options are available for mitigation; and what impacts are to be estimated from the various aspects of socioeconomic development. All these concerns relate to fundamental issues in national plans for future development, including development patterns and micro socioeconomic policy orientation.
The largest uncertainties for better understanding of these important issues mentioned above are from further quantitatively recognition of role of air pollution in growing warming of climate; better estimation to what extent for the influence of air pollution on these changes both regionally and globally (Kirtman et al., 2013 ). This is because, unlike greenhouse gases (GHGs), the constituent components of air pollution, such as ozone, various aerosol constituents and their induced cloud, have different climatic effects with respect to warming or cooling the climate system. Uncertainties about their overall climatic effects remain unclear (Boucher et al., 2013 ). In the case of aerosols, they have natural or anthropogenic sources, including inorganic species (such as sulfate, nitrate, ammonium, sea salt), organic species (also termed organic aerosol or OA), black carbon (BC, a distinct type of carbonaceous material formed from the incomplete combustion of fossil and biomass based fuels under certain conditions), mineral species (mostly desert dust) and primary biological aerosol particles (PBAP). Fossil fuel and biofuel emissions contribute to the effective radiative forcing due to aerosol-radiation interactions that takes rapid adjustments into account (ERFari) via sulphate aerosol: −0.4 (−0.6 to −0.2) W m−2 , black carbon aerosol: +0.4 (+0.05 to +0.8) W m−2 , and primary and secondary organic aerosol: −0.12 (−0.4 to +0.1) W m−2 . Additional ERFari contributions occur via biomass burning emissions: +0.0 (−0.2 to +0.2) W m−2 , nitrate aerosol: −0.11 (−0.3 to −0.03) W m−2 , and mineral dust: −0.1 (−0.3 to +0.1) W m−2 , although the latter may not be entirely of anthropogenic origin, while there is robust evidence for the existence of rapid adjustment of clouds in response to aerosol (Boucher et al., 2013 ). Different species show either positive or negative radiative effects, and the uncertainty estimate is wider based on multiple lines of evidence from models, remotely sensed data, and ground-based measurements. Local emissions, combined with background levels and meteorological conditions, conducive to the formation and accumulation of ozone and aerosol particle pollution, are known to produce extreme pollution episodes at local and regional scales (Zhang et al., 2013 ). There is low confidence in our ability to project changes in the occurrence of meteorological blocking associated with extreme episodes. All else being equal, warmer temperatures are expected to trigger positive chemical and local emission feedbacks to further enhance pollution levels (Kirtman et al., 2013 ).
Among the aerosol components that are related to air pollution, some are also relevant to what some researchers called short-lived climate pollutants, which are substances with relatively short life spans (from days to dozens of years) in the air and that also probably cause global climate warming by their own (Shoemaker et al., 2013 ). The short-lived climate pollutants mainly include black carbon, ozone, and methane in the troposphere, and some hydrofluorocarbons (HFCs). Not only are short-lived climate pollutants considered to be harmful air pollutants that adversely impact public health, agriculture, and environmental ecosystems, but also they are considered as one of the culprits in the growing global warming effects.
The key to realize the role of air pollution and short-lived climate pollutants related constitutes in growing global warming effects is the establishment of future change scenarios (Ebi et al., 2014a ). These scenarios, however, greatly depend on judgments about the future adaptation and mitigation policies to be established and applied, including the technological, economic, sectoral, and institutional requirements of various alternative mitigation pathways, and feedback from these policies (O'neill et al., 2014 ). For instance, within stabilization scenario groupings, the extent to which carbon dioxide removal is applied before and after 2050 changes with all other conditions, e.g., the timing of mitigation measures and the underlying policy assumptions. Lower carbon dioxide removal application rates before 2050 generally imply its enhanced application from 2050 to 2100. Across stabilization scenario groupings, the extent to which carbon dioxide removal is applied is closely correlated with various policy assumptions, and in some scenarios, more closely than with the stabilization level (Clark et al., 2014 ). To achieve better scenario results, careful judgments must be made to explore the requirements of alternative policy pathways and the wider sustainable development implications of climate policies, and to advance public finance analyses of carbon pricing and other policies (IPCC, 2014c ). Technological limitations can also increase mitigation costs (Clark et al., 2014 ), and mitigation steps will require change throughout the economy. Systemic approaches are expected to be most effective (Edenhofer et al., 2014 ), but the lack of available carbon capture and storage technologies will necessitate increased mitigation efforts in other sectors, particularly in agriculture, forestry, and other land uses, and have related objectives, means, and implications for climate policy (IPCC, 2014c ).
The Shared Socioeconomic Pathways (SSPs) provide some insights for communities concerned with climate change impact, adaptation and vulnerability (IAV) (Kriegler et al., 2014 ), but a number of limitations must be overcome to actively integrate IAV in SSP scenarios (Wilbanks and Ebi, 2014 ). Thus far, there has been limited use of SSPs and new climate scenarios by IAV research communities due to the challenge of integrating most IAV research within the SSP framework at the outset. IAV research communities, in general, are not easily involved in processes such as the development of new scenarios for climate research and assessment. One reason is the lack of access to integrated assessment model (IAM) applications outside the existing SSP group, especially in developing countries (O'neill et al., 2014 ). Another problem is that the SSPs were not finalized in time for the publication of the IPCC Fifth Assessment Report (AR5). There was no systematic assessment of the socioeconomic uncertainties and no consistent link to impacts in the AR5 (van Vuuren et al., 2014 ), and the integrated models in AR5 only report a few relevant indicators (IPCC, 2014b ).
The development of SSPs (Rozenberg et al., 2014 ) also needs to quantitatively relate them with Representative Concentration Pathways (RCPs) (Ebi et al., 2014b ). RCPs are highly related to forcing, concentrations, emissions, and land use, mainly using by Earth-system model simulations. An representative global model intercomparison project is the fifth phase of the Coupled Model Intercomparison Project (CMIP5) (Taylor et al., 2012 ). A CMIP6 is also planned, which will include an AerChemMIP theme to explore the relationship between air pollution and future climate change, focusing on the influence of aerosols, in addition to GHGs, on climate warming (Meehl et al., 2014 ). This theme will primarily diagnose the forcings and feedbacks that involve near-term climate forcers and chemically reactive well-mixed GHGs, determine past and future changes in the chemical composition of the atmosphere, and estimate the global-to-regional climate response from these changes.
On the basis of SSPs and RCPs, and the application of Earth-system model and integrated assessment model simulations, it particularly needs to investigate the emission drivers, mitigative capacities, socioeconomic pathways, exposures, sensitivities, and adaptive capacities. It also needs to combine the research and activities from both the physical and social scientists (van Ruijven et al., 2014 ), and then co-design, co-deliver, and co-produce ideas and output from the beginning of project designs to their final implementation. For instance, if the social scientists and natural scientists cooperate at the design stage of the study on climate change and the role of air pollution, this co-design activities will let us to have better understanding of the relation between variety of scenarios, as well as the result of the SSPs and concentration-radiative forcing of pollutants; this will also have better connection between scenarios derived mainly from future social and economic situation and radiative forcing estimation used by Earth-system model. Moreover, if co-design, co-deliver, and co-produce ideas and output from the beginning of project designs to their final implementation among the social, natural scientists and decisionmakers, it will have more accurately estimation of the effects and feedback of institutional framework in the future; and it will be helpful to disseminate the research results to the decisionmakers effectively in short time when research outputs occurred constantly. IAV and mitigation investigations should concentrate on the influence on sustainable development pathways, particularly in China. A new framework for developing climate policies in a broader sustainable development context, including the social perspective, market perspective, and governmental intervention, has been touched upon by IPCC (2014b) . Other studies also intend to determine the effect of mitigation on social development requires and identify synergies and tradeoffs between multiple objectives; to explore the multiple externalities; and to evaluate the interaction between different policy instruments.
The uncertainties and weaknesses in the links between the integrated assessment model results and many other policy objectives remains a challenge, and AR5 has only established a qualitative link. However, the consistency of the temperature projections and likelihood qualifiers in Working Group III (WGIII) of the IPCC AR5 and across all WGs constitutes a major advance (IPCC, 2014c ). Closing the loop to include impacts remains the main challenge (IPCC, 2014a ) for IPCC AR6. The establishment and simulation of RCP7.3, RCP3.5, and RCP1.5 also represent future international research needs for different SSP patterns. A comparison of results has been made for some key indicators, such as the identification of energy reduction as a key mitigation strategy. WGIII mostly considered middle-of-the-road scenarios, and could not systematically assess socioeconomic uncertainties (IPCC, 2014b ).
In addition to the above research needs, IAV research must be strengthened (Wilbanks and Ebi, 2014 ), especially in developing countries like China. IAV research covers a wide range of disciplines and domains, from model analyses of the possible impacts of climate change on specific systems, such as cereal crop yields, to practitioners working at the community level to increase resilience to climate variability, such as reducing the regional impact of extreme weather. IAV research is typically relatively localized in space and/or sectors, because impacts and adaptations occur in diverse contexts according to hazard, location, range, affected systems, affected populations, coping capacities, stakeholders, decisionmakers, and academic disciplines of researchers. The tendencies toward fragmentation are made worse by the fact that IAV research has generally not received significant funding support, especially in developing countries like China. This has led to small research teams conducting small research projects, which rarely involve any quantitative modeling like integrated assessment model. There are exceptions, of course, in data-rich sectors with modeling traditions, such as ecology, water, and agriculture.
The historical focus on generally small geographic regions and short time scales has meant that very little IAV research has used scenarios for a starting point. The scenarios from Special Report on Emissions Scenarios (SRES) and RCPs have tended to be used in vulnerability assessments, issue identification, and for overall framework development. In any system, over the next couple of decades, climate change will likely to be assigned lesser importance than socioeconomic development. And in those systems where climate change may be seen as more critical, system sensitivities to climate indexes are rarely well-known. Until these sensitivities can be estimated with confidence, it will be difficult to use scenarios that project climate parameters as a starting point. At the same time, IAV researchers eagerly anticipate the time when their science will support more scenario-oriented analyses. There is also no IAV equivalent of the CMIP. When connections are made between the new scenario process and key IAV individuals, the results will be positive.
In this paper, we would like suggest a number of future research needs between climate change and the role of air pollution (details in Fig. 1 ). The general suggestions include strengthening the research on the comprehensive impacts and selective reductions related to changes in air pollution and climate. The loop between differential impacts and differential costs suggested to be closed. Co-benefits and sustainable development goals should be identified, in addition to the crucial risks, synergies/trade-offs, and options. Sector-specific details and macro-economics should also be integrated in order to integrate our assessment of policy instruments. In addition, the WGI RCPs should be better harmonized with the ∼1200 WGIII scenarios from the SRES; the scope of future IPCC reports should somewhat be expanded, and the three working groups can work together to synthesize this scope at the very outset of future IPCCs. Experts can also be chosen to work with every WG to generate global scenarios as well as regional and national scenarios, and to fit national scenarios into the global framework using the same methodologies.
Summary of relationship of components and research needs between the future climate change and the role of air pollution.
China is a very important global actor in future air quality and climate change studies, because of vast territory area, largest emissions in CO2 and aerosols in the world; future energy revolution both for energy conservation and CO2 emission reductions aiming to mitigation of climate change, and pollutant emission control in combating air pollution; change the way of development to low carbon and sustainable development with large scale of urbanization. Each of these issues provides many important directions for future research efforts. Co-design, co-delivery and co-produce are needed in researches in national countermeasures, governance, trade-offs, synergies, side-effects related with future air pollution and climate change. The objectives are suggested to identify the potential obstacles to establishing strong air pollution control and management mechanisms, and to the development of institutional frameworks for green and low-carbon technologies. Researchers can facilitate the transition of economic development patterns towards green and low-carbon technologies, which are integral to ecological civilization development. Inevitable choices should also be made for overall planning with respect to international and national environmental protection and climate change.
Pollutants linked with Environmental and Climate Change (PECC) is one theme of the China National Committee for Future Earth (CNC-FE). Implementation of the PECC is the first CNC-FE strategy target. A national panel of experts has prepared a CNC-FE theme implementation plan, including the PECC plan. PECC-air pollution and climate change is one of the three PECC topics. The scope of PECC-air pollution and climate change includes dynamic pollutants, national development, and transformation towards sustainability. These PECC-air pollution and climate change objectives can be summarized as seeking: an in-depth understanding of how and why air pollution in China is so serious and rapidly changing; an understanding of likely future changes in air pollution and climate, the close links between air pollution and climate change and the development of an effective mitigation policy. The objectives also include to evaluate the interaction between different aspects of mitigation; to evaluate the combined effects of air pollution and climate change, and to form national countermeasures for air pollution and climate mitigation toward sustainable development and responsible environmental management; to effectively influence decision-makers and disseminate findings to the public; and finally to identify the impact of and link between air pollution and climate change.
This research was supported by grants from National Key Project of Basic Research (2011CB403401 ).