Black Carbon Particles Needs Attention in Climate Change Mitigation Policies

Climate change poses enormous challenges to human civilization in food security, water security, and health security. Anthropogenic emissions of Greenhouse Gases (GHGs) are made responsible for climate change. The climate change mitigation agreements and treaties, from the Kyoto Protocol (1997) to the Paris Agreement (2015), are mainly focusing on emission reduction of GHGs. The Copenhagen Accord (2009) set the target of emission reduction of GHGs to the level of 1990, intending to keep the global warming below 2-degree centigrade (°C) above the temperature level of the pre-industrial era. The Paris Agreement (2015) further pursued efforts to limit the temperature increase to 1.5°C by reducing emissions of GHGs to 40 Gigatonne (Gt CO-eq) by 2030. However, assuming the countries will achieve the target of emission reduction of GHGs by 2030, the target of keeping global warming below 1.5°C is unlikely to achieve because the Paris Agreement (2015) has not included emission reduction of black carbon (BC) particles in the intergovernmental negotiation. The BC particles are strong climate warming agents whose climate forcing is more than half of that of carbon dioxide (CO 2 ) – the main GHG. This article argues for the inclusion of BC mitigation measures in the climate change mitigation measures. As BC also causes severe health impacts, BC mitigation will bring multiple co-benets for health and environment, including a quick xing of climate change problems in a few weeks, since the residence time of BC in the atmosphere is about a week.


Introduction
Changing climate of the Earth system is signi cantly affecting the global temperature, ecosystem, agriculture production, water cycle, and water resources, including the melting of glaciers, which are creating enormous challenges to human civilization in water security, food security, and health security (Kang et  Assuming the countries will achieve the target of emission reduction of GHGs of 40 Gt by 2030. Yet, the target of holding the increase in global average temperature to below 2°C or even limiting the temperature rise to 1.5°C may notbe achieved because the Paris Agreement (2015) has not included the emission reduction of black carbon (BC) particles.
This article presents a policy-based assessment of BC mitigation and argues that special attention is needed for inclusion of the BC mitigation measures in the climate change mitigation measures or policies, because without that the target of holding the increase in global average temperature to below 2°C may not be possible. This article also highlighted that mitigation of BC will bring multiple co-bene ts for health and the environment, including a quick xing of climate change problems in a few weeks since the residence time of BC is about a week in the atmosphere. Radiative forcing (RF, also referred to as climate forcing), global warming potential (GWP), Global Temperature-change Potential (GPT) are the key parameters to check a potentiality of a chemical species to in uence the climate (cooling or warming effects). RF is the measurement of the capacity of a gas or particle to affect the Earth's energy balance. If the RF of a forcing agent is positive, it absorbs solar radiation in the atmosphere, results in warming effects. Else, if the RF is negative, it scatters or re ects solar radiation, causes cooling effects. While GWP is a relative measure of the amount of heat traps by a certain mass of   (Fig. 1a). Thus, the RF of BC is more than half of that of CO2 -the main GHG, making the BC the second most climate-warming agent after the CO 2 . In the case of GWP, the GWP of BC_direct and BC_total are 2100 and 3200 times higher than that of CO 2 , respectively, in 20 years of time horizon (Fig. 1b) and 590 and 910 times higher than that of CO 2 , respectively, in 100 years of time horizon (Fig. 1c). Whereas, in the case of GTP, the GTP of BC_direct and BC_total are 600 and 925 times higher than that of CO 2 , respectively, in 20 years of time horizon (Fig. 1d) and 80 and 130 times higher than that of CO 2 , respectively, in 100 years of time horizon (Fig. 1e). The comparison of RF, GWP, and GTP of BC_direct and BC_total with other GHGs (e.g., CH 4 , halocarbons, and N 2 O) has been shown in Fig. 1(a -e).

Bc Particles And Their Climate Impacts
The climate impacts of the BC are not as straightforward as those of GHGs. The GHGs create a heat-trapping blanket in the atmosphere, which resulted accumulation of heat and causes an increase in temperature on the Earth's surface. On the contrary, the impacts of BC particles are manifolds. BC absorbs incoming solar radiation in the atmosphere as a direct effect and causes indirect effects, such as affecting cloud properties, which affects patterns and level of precipitation or rain (Koch et al., 2010;Hansen et al., 1997). BC heat the air and decreases relative humidity in their immediate proximity, resulting in a positive RF known as the semidirect effect. This lowers cloud cover at the altitude of the aerosols while rising cloud covers below by inhibiting convection. This effect induces a decrease in cloud cover at su ciently low altitudes without a compensating rise at much lower levels. As a result, BC has a strong warming effect at the surface but a minor warming effect higher up. The elevation of the e ciency of BC forcing is mainly due to this effect.
Over the past decades, the Asian monsoon is weakening, and these changes in the Asian monsoon have been attributed to higher BC emissions in the Asian region ( (Bond et al., 2013). Whereas the RT of GHGs, for example, CO 2, is a decade to a century, 114 years for N 2 O, and 10-12 years for CH 4 . Also, BC particles with co-emitted other air pollutants cause severe health impacts and globally cause over 7 million premature deaths annually (WHO, 2014). Thus, emission reduction of BC particles could provide an opportunity for quick xing of the climate change in the near term and with multiple co-bene ts in terms of health, environment, and climate.

Conclusion
Studies suggested that without incorporating BC emission reduction measures in the climate change mitigation policies, the target of 2 o C is unlike to be achieved. However, implementing the identi ed total mitigation measures for BC and CH 4 could avoid global temperature rise by 0.5 o C (Ramanathan and Xu, 2010;Shindell et al., 2012;Steven and Andrew, 2013). An opportunity was lost in December 2015 when the COP21 made the Paris Agreement. BC particles emission reduction could have been included in the Agreement. Since BC's climate warming effects are quite large, a legally binding agreement is needed among the countries for BC emission reduction. BC emissions reduction will provide multiple co-bene ts in terms of health and the environment, including a quick xing of climate change problems in a few weeks since the RT of BC is about a week in the atmosphere. The uncertainty in emission inventories is complicated by physical uncertainties, especially in indirect impacts, resulting in a broad range of estimates of black carbon's exposure to Earth's radiative budget. Improved global emission inventories, standardized reporting of the simulation model, and more research into aerosol and cloud microphysics and aerosol transport to and deposition on snow and ice-covered regions, both would help to reduce critical uncertainties. The mitigation community needs to engage with countries on this issue and jointly think of other enabling conditions to put in place to support the resiliency of BC emission and climate change mitigation policy.