Shadow of greenhouse gases on farming
Milkha Singh Aulakh
IN recent years, with increasing public awareness, global warming and climate change are becoming important issues, and coordinated efforts are being made through global summits to reduce the emission of greenhouse gases (GHGs). It is thus important to examine the GHG sources, sinks, impact and mitigation options.
Carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and chlorofluorocarbons (CFCs) in the atmosphere are mainly responsible for the ‘greenhouse effect’. While CO2, CH4 and N2O are naturally present in the atmosphere, the CFCs are the product of industrial and aerial activities. Human activities increased their atmospheric concentrations enormously during the 20th century.
The Intergovernmental Panel on Climate Change (IPCC) has estimated that the composite effect of agricultural activities is approximately one-fifth of the anthropogenic greenhouse effect, whereas terrestrial ecosystems’ soils can also significantly sequester CO2 from the atmosphere; hence agriculture acts both as a source and a sink of GHGs.
After their emission into the atmosphere, these gases envelop the earth. They are nearly transparent to the visible and near-infrared wavelengths in sunlight, but they absorb and re-emit downward a large fraction of the longer infrared radiation emitted by the earth. As a result of this heat-trapping, the atmosphere radiates large amounts of long-wavelength energy downward to the earth’s surface. This phenomenon is just like a ‘greenhouse’, which is made chiefly of transparent material such as glass for trapping heat to maintain warmer climatic conditions. Consequently, long-wavelength radiant energy received on earth is increased. Water vapours account for about 50%, clouds (25%), CO2 (about 20%) and the rest of the earth’s greenhouse effect is caused by other GHG flux and aerosols. When the concentrations of GHGs rise, air temperatures go up and more water vapour evaporates into the atmosphere, which then amplifies the earth’s heating.
The concentration of CO2 in the earth’s atmosphere is nearly 412 parts per million (ppm), and rising. This represents a 47% increase since the beginning of the industrial age, when the concentration was around 280 ppm, and an 11% increase since 2000, when it was 370 ppm. Among agricultural activities, CO2 is primarily emitted from land use changes such as deforestation and cropping patterns, and correspond to 21% of the total emission, whereas the sequestration of CO2 by field crops, soil and forests from the atmosphere is around 24%.
Burning of crop residue and forest fires contribute to the emission of CO2 and other toxic gases in a short but intensive manner. Annual GHG flux data reveals that in India, 98% of CO2 emissions are from urban, industrial, vehicular and other transport activities, whereas agriculture contributes around 2%, of which about 90% is emitted by the burning of agricultural residue. Among the crop residue, rice straw is predominantly burned in open fields in north-western parts of the country.
Methane ranks only next to CO2, and its atmospheric concentration has increased from 0.8 ppm in pre-industrial times to 1.78 ppm. More recent data indicates a rate of 0.5-0.9% increase per year over the last decade. As per IPCC estimates, agricultural activities such as rice cultivation, rearing of domestic animals and biomass burning account for 22-46% of the global CH4 budget. Among these activities, rice fields are the most important source, accounting for 15-20% of the world’s total anthropogenic CH4 emission.
Rice is one of the world’s most important food crops, and the area under it has increased enormously. Globally, wetland rice covering 131 million hectares includes irrigated (53.4% of the total area), rainfed (27.3%) and deepwater (7.7%) rice systems. Irrigated rice grown on about 50% of the total harvested area contributes about 70% towards total production. Furthermore, rice production systems are being intensified through double and triple cropping in tropical areas using short-duration cultivars. Thus, increased soil tillage and puddling, inter-culture operations, prolonged soil submergence and planted period, greater inputs of nutrients, etc. could lead to greater GHG flux. For example, investigations have demonstrated that total CH4 emissions from continuously submerged rice fields of West Bengal, Odisha and Bihar are 55-95% higher than intermittently irrigated rice crops in Punjab and Haryana.
The atmospheric N2O is a GHG with current atmospheric lifetime of 116 ± 9 years, and its concentration has increased by more than 20% from 270 parts per billion (ppb) in 1750 to 331 ppb in 2018. The N2O flux generated by agriculture is associated with fertiliser nitrogen’s application, domestic animals and biomass burning. In addition to global warming, N2O leads to the destruction of the stratospheric ozone layer that protects humans and animals from UV rays.
Cause and victim
Agriculture is both a cause and a victim of global warming. According to a UN climate report, the Himalayan glaciers that are the principal dry-season water sources of Sutlej, Beas, Ravi, Yamuna and Ganga rivers, could disappear at a faster rate as temperatures rise and people living in the drainage basin of these rivers could experience floods followed by severe droughts in the coming decades. The El Niño Southern Oscillation will affect monsoon patterns more intensely as climate change warms up the ocean’s water.
There is a considerable impact of global warming on agriculture due to the rise in temperature, melting of Himalayan glaciers affecting availability of irrigation, and changed pattern of rainfall leading to increased frequency and intensity of inter- and intra-seasonal droughts. Already, we have been witnessing less rainfall during the kharif season. In recent years, wheat crop suffered huge yield losses due to high temperature during December. Further, from January-end to February, crops were severely damaged due to frost.
Due to the rise in temperature, crop water requirements also increase, and hence dwindling water resources would be stretched more. Many states, including Punjab, Haryana and Uttar Pradesh, are already facing the problem of a fall in the groundwater level at alarming rates, and would be further impacted as the flow in rivers will recede due to shrinking size of Himalayan glaciers. This will aggravate the problem of GHG emissions due to escalated energy/power requirements of the agriculture sector. Increasing temperature and humidity also increase populations of crop pathogens and insects, resulting in yield loss.
Researchers have been identifying individual sources of the GHGs and their source strengths, and crop management options to mitigate the GHG flux in agriculture such as irrigation water management, nutrient management, crop residue management, conservation tillage technology, restoration of cultivated organic soils and livestock management. Adoption of promising crop-specific and integrated management practices would go a long way to protect human and animal health, minimise environmental pollution, global warming, climate change, depletion of glacial water reserves and sustain food security for our ever-increasing population. Governments should provide incentives for minimising agriculture-induced activities such as burning of rice straw.
Regional contribution
Farm gate production and land use contributed differently to emissions due to agriculture in the regions analysed. In 2018, crops & livestock production contributed two-thirds or more in North America (66%), Asia (69%), Europe (73%) and Oceania (80%).
The author is founder expert member, UN Intergovernmental Technical Panel on Soils
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