2. Black Carbon science and impacts: Needs for India
Carbonaceous particles containing black carbon, BC, or refractory, light-absorbing carbon (commonly called “soot”), are formed from incomplete combustion in flames. They also contain organic carbon, OC, a multitude of organic chemical compounds. BC has a unique and important role in the Earth’s climate system because it strongly absorbs solar radiation, influences cloud processes, and alters the melting of snow and ice cover. An important property of BC is its ability to strongly absorb higher-energy shortwave radiation of the ultra-violet and visible spectrum. BC is a short-lived atmospheric constituent with a mean atmospheric lifetime of about a few days to weeks.
There are important uncertainties in the science of atmospheric BC, particularly relevant to regional climate change in South Asia, which must be addressed:
- In India, BC emission rates appear to be underestimated from both energy-use (residential cooking, using biomass fuel chulhas, residential lighting using kerosene lamps, diesel transport and industry, particularly brick production in traditional kilns) and the open burning of agricultural residue in fields following harvest.
- The atmospheric abundance of BC and its mixing with other aerosol constituents, like OC or sulphate, must be better understood to accurately estimate atmospheric warming from BC.
- BC removal from the atmosphere, particularly wet removal, needs further study, which is governed by water uptake by emitted particles.
- Non-homogeneous heating and cooling by BC aerosols can change regional circulation patterns. The effects of BC on liquid clouds, from cloud absorption and heating, could alter cloud lifetime. BC could induce changes in precipitation patterns and the Indian monsoon on various time scales.
- The deposition of absorbing aerosol particles including BC and dust decreases reflectivity of snow and ice surfaces. Recently, climate model simulations have shown such snow-pack changes to propagate to greater depths, resulting in larger heating over snow surfaces, in the Himalaya and Arctic regions.
A national programme to address these knowledge gaps must focus on large-scale field measurements and modelling needed to better estimate sectoral BC emission magnitudes and their uncertainty, better infer BC source strength from in-situ and satellite observations and receptor modelling and better constrain BC climate impacts from historical and future climate model simulations.
Uncertainty in BC climate effects raises the question, “what action can we take now?” Attempts to bring BC into climate assessment frameworks, using metrics common to greenhouse gases, have led to estimates of 100-year global-warming-potential (BC-GWP-100) of 900 (140 to 1700 range) including all forcing mechanisms. Caution is needed since BC and CO2 emission amounts, which exert equivalent 100-year GWPs, have different impacts on temperature and rainfall, with different timing of these impacts. Also, mitigating BC addresses short-term climate change, but mitigating carbon dioxide is required to address long-term climate change. Yet, addressing BC
provides an opportunity to slow climate change, as highlighted by international groups such as the Climate and Clean Air Coalition. For BC-rich sectors, discussed earlier in the Indian context, climate forcing by short-lived species (particulate matter and ozone precursor gases) is substantial (up to 75%) in comparison with their co-emissions of long-lived greenhouse gases (e.g., CO2 and CH4).
Addressing select BC sources could yield immediate reduction in short-term atmospheric warming. Importantly, any BC-rich source category is a promising candidate for reducing particle matter concentrations, with important co-benefits of reduced adverse public health impacts of air pollution. Other arguments relate to poverty alleviation. It is generally argued that increasing income leads to increasing energy consumption, concurrent CO2 emissions, and an increasing climate footprint. However, at the lowest societal income levels, increase in income would lead to a shift away from high-BC-emitting energy technologies, such as biomass cooking stoves and kerosene wick lamps, leading to reduced climate footprint. Thus, climate mitigation from addressing BC emissions could be linked to strong development benefits.
Chandra Venkataraman
IIT Bombay
E-Mail: chandra@iitb.ac.in
