Carbon Capture And Storage In Meeting Climate Change Targets
Climate change is a multifaceted and challenging problem while there is no comprehensive sure-shot solution. Among the most promising and potential solutions to mitigate climate change, the best strategy would be carbon (C) capture and long-term storage, known as C sequestration. The increase in atmospheric concentration of CO2 by 31% since 1750 from fossil fuel combustion and land use change requires identification of strategies to mitigate the threats of the associated global warming. The perilous impact of excess C released in the environment upon the future of mankind is being hotly debated in almost all corners of the world at the moment. But in the era of rapid urbanization and industrialization the fossil fuels provide the vast majority of energy. The unregulated emission of C and other gases by industries and the excessive consumption of fossil fuels lead to the greenhouse effect, which in turn causes global climate change. To mitigate this problem the measurement of the total amount of C generation of a defined population, system or activity, considering all relevant sources, sinks and storage within the spatial and temporal boundary of the population, system or activity of interest are obligatory. Also improvement in monitoring and verification protocols for C sequestration are needed for a quantitative economic and policy analysis. Thus, broadly speaking, for the existence of the human race we have to move forward to a civilization that is ecologically sustainable.
Wetlands as Carbon Storehouse
Wetlands play two distinct roles in mitigating the effects of climate change- the management of greenhouse gases and physically buffering the climate change impacts. Among the terrestrial ecosystems, wetlands have the highest C density and it stocks 40% of global terrestrial C. Wetlands only cover 2- 6% (3.5*106 km2) in proportion to the earth’s land surface, yet wetlands contain a large amount of the world’s C (535 Gt in 100cm top soil in a rate of 44.6 Tg C yr-1) which is 20–25% of the world’s organic soil C. However, the long-term stability of stored C depends on several biogeochemical, physical and physiological interactions in water columns and sediments.
Kolkata – Ecologically subsidized metropolis
West Bengal, an exceedingly populated states of India, with a population of 9.13 crores (2011 census), is blessed with around 54 natural and 9 human-made major wetlands, covering a total area of about 3,44,527 ha. These wetlands are silently rendering myriad services for the wellbeing of the state. However, these wetland systems are under severe threats of extinction for enormous anthropogenic pressures of different kinds. Even before knowing fully the economic values and ecosystem services rendered by these wetlands, we are losing them fast. Creaking under the loads of encroachments, the only Ramsar site (No. 1208) of this state, East Kolkata Wetland (EKW; 12500ha) ecosystem is considered the largest natural effluent treatment plant (ETP) of Kolkata city wastes since past century. Interestingly, the Kolkata does not have a sewage treatment plant, but has the EKW ecosystem at its eastern boundary that has been ameliorating all the wastes of the city, for which staggering Rs 4,680 million year-1 as sewage treatment costs is being saved. Hence, Dr. Dhurobojyoti Ghosh, an eminent environmentalist and a UN Global 500 Laureate, has termed Kolkata as ecologically subsidized city. Moreover, this EKW ecosystem is the world largest “wastewater-fed agri-piscicultural system” and it is a unique example of “wise use of wetlands”. EKW takes care of nearly 0.71 million m3 composite wastewater from tanning and leather processing industries in three prominent agglomerates, other medium and small scale industries, municipal sewage and run off and ≃ 4000 MT solid waste daily. Nearly 50,000 agro-workers- “the traditional ecologist”- produces ≃150 tons d−1 vegetables in garbage farms irrigated by wastewater and 18000 tons yr−1 fish in wastewater-fed fisheries.
Lock down Carbon
Focus of various research works during last 50 years was on different ecotoxicological aspects to inquire the ameliorative efficacy of EKW ecosystem and sustainability of sewage-fed agri-piscicultural practices at EKW. Different publications have amply pointed out how these systems of generation of ‘wealth from wastes’ are thriving sustainably since past several decades. However, the investigation on the efficiency of C sequestration at EKW ecosystem has been started only lately since 2010 by the author of this article under the supervision of Dr. SK Mukhopadhyay, GCELT and Prof. Siddartha Datta, Jadavpur University. To investigate the efficiency of EKW in C sequestration, leaving aside aerial C species, we have identified two big carbon sources i.e. tannery industry waste and composite wastewater from Kolkata city. From tannery industry 40.54 ± 3.22 kg C in the forms of liquid wastes and 279.63 ± 13.23 kg C in the form of solid waste are discharged daily into EKW ecosystem. Whereas, composite wastewaters, which pass thorough a web of canals in the EKW, contain 348.1 ± 154.9 mg L-1 total organic carbon (TOC) and 95.7 ± 47.1 mg L-1 total inorganic carbon (TIC) of which EKW traps 63% of TOC and 71% of TIC. This was possible for the quintessential flow-rate of wastewater in the carrying canals, long residence time in the wastewater-fed fish ponds and repeated use of wastewater in irrigating agricultural fields at the EKW ecosystem. Therefore, the efficiency of EKW in locking down more than 60% carbon from wastes, highlight the importance of EKW to provide yet another remarkable ecosystem service by sequestering C in a cost effective way.
Green Carbon Pocket
To measure the C sequestration efficiency we choose three most potential components of EKW; these are - pisciculture system, agricultural system and wetlands macrophytes. At the primary producer level 2038.6 ± 244.8 mg C m-3 d-1, at primary consumer level 307 ± 19.3 mg C m-3 and at secondary consumer level 11531.4 ± 318.2 mg C m-3 are present at EKW ponds. These C either get transferred to the tertiary trophic levels as food or get released as dead parts and faecal products of plankton and fish and stored in the bottom sediment of the ponds. Furthermore, the rates of C accumulation by agricultural plants is 6343.7kg ha-1 C y−1 and out of that 2313.6 kg ha-1 y−1 is sequestered in agricultural soils as crop residues. This residual C not only enhances the soil C storage but also increases different nutrients which make the soil more fertile for next crop production. Thereby, the residues of the plants add a special economic benefit to the farmers, substituting the input of fertilizers. Alongside, the wetland macrophytes sequester 9.57 ton C ha-1yr-1. Moreover, rhizo-deposition add more than 200 water soluble C compounds in different forms of exudates like amino acids, organic acids, sugar, secondary metabolites and complex polymeric compounds and these are increasing the soil C concentrations in the EKW area.
EKW: Nature’s Lungs
EKW ecosystem provide O2 to the Kolkata city through photosysthesis by phytoplankters and macrophytes and takes out CO2 liberated by the city. 19.5 – 22.5 g kg-1 C are stored in upper 20 cm soil of EKW. On an average Kolkata city emits 3482.28 t CO2 day−1 from vehicular exhaust, human respiration and small-scale industries while soil respiration contributes 3.16 t CO2 m−2 day−1. However, the EKW at the backyard the Kolkata city sequester C in one hand and ameliorate toxic chemicals on other. Thus, EKW efficiently perform dual functions: functions as lungs and kidneys of the nature. Although some research works have reported that after organic matter mineralization wetland emits CO2 and CH4, but it has been estimated that on an average, CH4 emitted from wetlands is 14% of the wetland’s C sequestration. The ratio of C sequestration: CH4 is 7.14 : 1 and in respect of CO2, the ratio is equivalent to 19:5 (CO2 : CH4).
Right to Carbon credit
The C credit market in India is yet to be popularized and introduced from the base level. Farmers of the EKW areas are not aware of the fact that what an amazing work they have been performing by producing edibles depending on the wastes. They are not only providing food for the millions, they also help to sequester C in different biotic forms. However, they are yet to reap benefit from the services they provide unknowingly. Neither the ecological services rendered by the EKW ecosystem are given any importance nor are the excellent efforts of resource recovery by the village artisans acknowledged. The farmers need to be made aware of the C credit market and the ecological and monetary gains that they could foresee from their agri-piscicultural practices. In the near future, if the farmers could monetarily gain from the C credit, it would be a major step forward towards the sustainable development of the EKW areas. Century-long agri-piscicultural practices have externalized the ecological values of the wetland ecosystem for economic uplift of the rural artisans. C trapping and storing can be regarded as a potential option for Kolkata city to become an exemplary participant in mitigating the global climate change. However, the EKW areas shrink by 38.6% between 1972 and 2011. Shrinkage of water-spread area within the EKW was recorded as 71.2% between 1945 and 2010 and astonishingly from 2003 till present time this reduction is 63.9%. Such huge changes in land use and land cover surely have a slow but prolonged impact on various aspects of environment at local, regional and global scales. Motivated conservation efforts coupled with effective management strategies for wetland restoration and conservation could only resist further shrinkage of this sustainably productive and excellently effective natural ETP at Kolkata city’s backyard.
