Integrated assessment of air quality and climate change for policy-making: highlights of IPCC AR5 and research challenges

Recently, air quality in China has received significant attention, especially since the unprecedented heavily polluted haze episodes were observed in populated areas of central and eastern China [1]. Major atmospheric air pollutants include tropospheric ozone (O3) and aerosols (small liquid or solid particles, also called PM2.5 when referring to particles with diameters of 2.5μm or less), which result mainly from emissions of methane (CH4), carbon monoxide (CO), nitrogen oxides (NOx), nonmethane volatile organics (NMVOCs), sulfur dioxide (SO2), ammonia (NH3), black carbon (BC), organic carbon (OC), and dust from human activities, such as energy production, industry, transportation, and agricultural and residential activities [2]. Emissions from natural sources, such as lightning NOx, NOx from soil, biogenic hydrocarbons, dimethyl sulfide (DMS) and sea salt from the oceans, as well as emissions from wild fires, also contribute to the formation of air pollutants.


PERSPECTIVES GEOSCIENCES
Integrated assessment of air quality and climate change for policy-making: highlights of IPCC AR5 and research challenges Hong Liao 1,2, * and Wenyuan Chang 1 Recently, air quality in China has received significant attention, especially since the unprecedented heavily polluted haze episodes were observed in populated areas of central and eastern China [1]. Major atmospheric air pollutants include tropospheric ozone (O 3 ) and aerosols (small liquid or solid particles, also called PM 2.5 when referring to particles with diameters of 2.5 μm or less), which result mainly from emissions of methane (CH 4 ), carbon monoxide (CO), nitrogen oxides (NO x ), nonmethane volatile organics (NMVOCs), sulfur dioxide (SO 2 ), ammonia (NH 3 ), black carbon (BC), organic carbon (OC), and dust from human activities, such as energy production, industry, transportation, and agricultural and residential activities [2]. Emissions from natural sources, such as lightning NO x , NO x from soil, biogenic hydrocarbons, dimethyl sulfide (DMS) and sea salt from the oceans, as well as emissions from wild fires, also contribute to the formation of air pollutants. It should be noted that following advances in global modeling of the formation and removal of chemical species in the atmosphere, the IPCC WGI AR5 presents for the first time emission-based estimates of radiative forcing values. It also provides a number of metrics to quantify and communicate the relative and absolute contributions to climate change of emissions of different substances, and of emissions from various regions/countries, sources, and sectors. Furthermore, the IPCC AR5 presents for the first time the radiative forcing by emissions of short-lived gas-phase species of NO x , CO, and NMVOCs. These species do not participate in radiative transfer in the atmosphere, but influence the concentrations of many greenhouse gases and aerosols through chemical processes, reflecting the strong link between air quality and climate change.

ROLE OF AIR POLLUTANTS IN CLIMATE CHANGE
Air pollutants are short-lived species that have a lifetime from just days up to about a decade. Different air pollutants have either a warming or a cooling effect on climate depending on their chemical and physical characteristics. Emissions of CO and NMVOCs are virtually certain to have induced a positive radiative forcing on climate because they lead to increases in the concentrations of CO 2 , CH 4 , and O 3 through chemical reactions. NO x is estimated to have a net negative radiative forcing through altering the concentrations of nitrate aerosol, CH 4 , and tropospheric O 3 (Fig. 1). Among the major anthropogenic aerosol species in the atmosphere, sulfate, nitrate, ammonium, and OC have a cooling effect through aerosol-radiation and aerosol-cloud interactions, whereas BC has a warming effect via the absorption of sunlight. Air quality controls that target tropospheric O 3 or aerosols (or particulate matter) might lead to complex effects on the climate (Fig. 2). Air quality controls might also target specific sectors of anthropogenic activity, such as transportation or energy production. Thus, co-emitted species within the targeted sector could lead to a complex mix of chemistry and climate perturbations.

REDUCTIONS OF AIR POLLUTANTS TO MITIGATE CLIMATE CHANGE
To protect human health, strategies for air pollution abatement must be pursued aggressively. However, in the meantime, rapid climate change is of worldwide concern. In the 2009 Copenhagen Accord, many nations agreed to limit the increase in global temperature since preindustrial times to below 2 • C by initiating significant cuts in global emissions of greenhouse gases [4]. Because of the long lifetime of CO 2 in the atmosphere, reductions in CO 2 emissions are essential for any climatological long-term benefits. Compared with CO 2 , short-lived species, such as CH 4 , tropospheric O 3 , and BC aerosol, persist for just a short time in the atmosphere and have positive radiative forcing values (Fig. 1). Urgent action to reduce the concentrations of these short-lived species in the atmosphere will improve air quality (reducing risks to health and crop yields) and combat short-term accelerated warming (reducing risks of crossing a critical temperature threshold) [  achieved by control of the short-lived climatic forcers, whereas reductions in CO 2 emissions are required to limit long-term climate change.

RESEARCH CHALLENGES
The important scientific question is whether the policies intended to reduce air pollutants can also benefit the climate. The integrated assessment of air quality and climate change is a challenge and one that requires the coupling of socio-economic models with advanced atmospheric chemistry-climate models. Model predictions depend critically on the assumed future emission scenarios [7-9]. Therefore, interaction among economic analysts, emissions experts, and atmospheric scientists will be paramount for the creation of decision-support tools to evaluate policy options for air quality and climate. The climatic effects of reducing air pollution over the next few decades, including those that result from the implementation of specific policies designed to mitigate climate change, need to be understood and communicated to those who will bear the burden. Integrated scientific research on air quality and climate should emphasize both the long-term climatic benefits and, more importantly, the immediate benefits for air quality and energy security.
During the past decade, important advances have been made with regard to air pollutants and their roles in climate change in China. Emission inventories of tropospheric O 3 , aerosol precursors, and aerosols have become available for the China domain, which allow the simulation of air pollutant concentrations and the estimation of their climatic effects using numerical models. Increasing numbers of ground measurements of air pollutants in China, together with satellite measurements of aerosol optical properties and cloud properties, have provided data sets to constrain the simulated climatic effects of short-lived species. However, estimates of the climatic effects of air pollutants in China are still subject to large uncertainties. The fundamental requirements of science pertaining to the integrated assessment of air quality and climate policy, to achieve sustainable PERSPECTIVES development and a low carbon society, are as follows.
(1) Continued improvement in economics, emissions, and policy cost models for analysis of the interaction of human activities with climate processes. Simultaneous changes in the emissions of air pollutants and greenhouse gases need to be developed based on economic analyses of different sectors and regions. A flow chart presenting the integrated assessment of air quality and climate to support policy making is given in Fig. 3. (2) Nationwide long-term measurements of concentrations of well-mixed greenhouse gases and pollutants are needed. Ground measurements of speciated aerosol mass concentrations and size-resolved aerosol number concentrations are important for evaluation of emission inventories and for improving the representations of chemical species in climate models. Satellite measurements have excellent spatiotemporal coverage, which are useful for analyses of the physical/chemical/optical characteristics of air pollutants and for source attribution.
(3) Improved understanding of aerosolcloud interactions. How aerosol particles influence clouds is one of the most difficult challenges in climate simulation because the microphysical processes involved are very complex. Aerosol-cloud feedbacks also influence concentrations and distributions of air pollutants through processes such as aqueous aerosol formation and the wet deposition of both gas-phase and aerosol species. Measurements and modeling of aerosol microphysical properties and cloud properties are required to reduce uncertainties associated with the quantification of air pollutants and climatic effects of aerosols.

(4) Continued development of Earth
System Models that account for coupled dynamic and chemical atmosphere, ocean, land, and natural ecosystem interactions and feedbacks.

WATER IS IMPORTANT
Water is a basic component of human existence, and people and ecosystems depend on it. It is one of the most fundamental requirements for the survival of all living things. However, water is a finite resource that has quantitative limitations and qualitative vulnerability. Water shortage is a global concern owing to increasing population, economic growth and climate change. China is facing severe water scarcity. Some experts believe that the water crisis may come before an energy crisis in China. Energy and water are inextricably and reciprocally linked; the production of energy requires large volumes of water (for example, in the USA and China, electricity production requires over 40% of all daily freshwater withdrawals [1]), and both the treatment and distribution of water depend upon readily available, low-cost energy. More recently, water management for unconventional shale gas extraction has dominated environmental debates surrounding the gas industry [2].

WATER SCIENCE ON THE MOLECULAR SCALE
Water has long been an important topic in science and technology. Water has been intensely studied, and 'water science' refers to studies on water and related issues. In the past, water science was concerned with large volumes of water such as the flooding of rivers, atmospheric water evaporation/ condensation, freezing/melting of seawater and physical/chemical properties of bulk water in industrial processes. However, modern water science is also concerned with water on the molecular scale. This is because modern science depends on the understanding of matter at the molecular level. Whether it is in the fields of life sciences, materials science or environmental science, scientists realize that answers to their questions are taking place on the molecular scale. From a molecular point of view, many bulk properties of water are still not well understood. 'Water is simple but very complex', Professor Guozhen Yang said in his opening speech at the Water Science Forum 2013, held in Beijing. It is simple because everyone knows that water is H 2 O and many scientists believe that we understand water. However, we know very little about it. Water is not simply a molecule of H 2 O, but a group of H 2 O molecules linked by hydrogen bonds, and this constitutes the most mysterious matter in the world. Water has many anomalous properties. For example, no other liquids are found simultaneously in all three phases: gas, liquid and solid; water undergoes a negative thermal expansion below 4 • C; water freezes from the top surface; water O-H stretching vibrations last longer at high temperatures; hot water freezes more rapidly than cold water; high surface tension and small surface potential co-exist in water; and anomalous magnetic and microwave radiation effects occur in water. Recently, it has been revealed that water is more mysterious when confined to an interface, as interfacial water has different properties from the bulk state. Researchers have devoted much effort to understanding interfacial water properties; however, little is known about water interfaces and the