Popular Kheti Volume -1, Issue-3 (July-September), 2013 Available online at www.popularkheti.info © 2013 popularkheti.info ISSN:2321-0001 Global Dimming and Its Effect on Agriculture R. K. Meena*, R. Fagodiya, S. K. Meena, R. Kumar, Raju and R. S. Bana Indian Agricultural Research Institute, New Delhi -110 012 *Email of corresponding author: [email protected]
Climate change includes both global warming and global dimming. Reduction in the solar radiation reaching the earth surface is known global dimming. Global dimming is caused due to increased cloud thickness, cloud cover and suspended particulate matter in air (aerosol). These reduced solar radiations due to global dimming adversely affects photosynthesis by plant thereby having negative effect on agriculture.
Introduction contributor to global dimming (O’Dowd et The sun is the only source of energy for the al. 2013). It is the result of tiny airborne soot global ecosystem. Essential condition for particles, ash and sulphur aerosols living things on earth is the amount of compounds which reflecting the sun sunlight that penetrates through the radiation. The pollutants that lead to global atmosphere and reaches earth’s surface. dimming also lead to various human and Long term measurements of the solar environmental problems, such as smog, radiation from the 1960s to the early 1990s, respiratory problems, and acid rain. backed up by a wide range of data and a number of independent studies, showed that Evidence of Global Dimming there were substantial declines in the amount 1. Decadal changes in surface solar of the sun's energy reaching the Earth's radiation: Early studies carried out in the surface (Stanhill and Cohen 2001). This 1990s reported a general decrease of solar phenomenon of declines in solar radiation radiation at widespread locations over the reaching the earth is known as global earth between the late 1950s and the late dimming. 1980s (Wild, 2012). Increasing air pollution Global dimming was observed at the and associated increase in aerosol Earth’s surface during the 1950–1980 concentrations are considered as major cause periods. This decrease is consistent with the of the decline of surface solar radiation. impacts of anthropogenic aerosols on cloud Changes in cloud amount and optical properties, water vapour and cloud feedbacks properties, which may or may not have been due to global warming. Increases in cloud linked to the aerosol changes, have also been thickness, cloud cover and scattering proposed to contribute. A direct assessment aerosols enhance the diffuse component of of the origins of the decline of surface solar the radiation reaching the surface (Wild, radiation is complicated by the lack of 2012). adequate long-term observational datasets of The air pollution due to major influencing factors such as clouds and anthropogenic activity is the major aerosols (Wild, 2009). Clouds intercept both
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shortwave radiation from the sun and long clouds influence, in addition to total PAR wave radiation from the Earth. Their effects and diffuse PAR fraction, many other are complex and vary in time with location, atmospheric factors such as temperature, and altitude. moisture, turbulent heat fluxes and precipitation, which also have a direct or 2. Change in pan evaporation: In 1990s indirect influence on ecosystem carbon Europe, Israel, and North American assimilation (Wild et al. 2012). scientists noticed that the rate of evaporation was decreasing, although they had expected 2. Effect on Irrigation Resources: it to increase due to global warming. The Important issue is the potential impact of similar trend was also observed in China dimming on the water cycle. Solar radiation over a similar period. A decrease in solar changes effectively alter the energy available irradiance is responsible for decrease in at Earth’s surface to drive evaporation and its evaporation (Roderick and Farquhar 2002). energy equivalent. Since on a global level evaporation equals precipitation, any SSR- Impact of Global Dimming induced change in evaporation will change 1. Effect on plant photosynthesis and the intensity of the water cycle. Only if carbon uptake: Reduced atmospheric ocean gets warm, it will have the tendency to transmissivity could have resulted from initiate the rain formation. Global dimming changes in the short-wave radiative might have caused the droughts in Ethiopia properties of the sky and/or of the clouds, as in the 1970s and 80s where millions died, well as from an increase in cloud cover. because the northern hemisphere oceans These changes could be the outcome, either were not warm enough to allow rain directly or indirectly, of greater formation (Wild et al. 2012). concentrations of aerosols in the atmosphere. The direct effect relates to the increased 3. Effect on agricultural crop production: short-wave absorption by aerosols under The solar radiation reaching the Earth’s both clear and overcast sky conditions, while surface is the primary driver of plant an indirect effect refers to the role of aerosols photosynthesis. Photosynthesis increases as cloud condensation nuclei leading to nonlinearly with incident photosynthetically increased cloud cover. active radiation (PAR), saturating at light One of the study of vegetation levels that are often exceeded on bright days canopy photosynthesis revealed that the during the growing season. Under clear-sky interception of direct radiation is strongly conditions, a fraction of the plant canopy is determined by the leaf inclination angle. A illuminated by direct solar radiation number of studies relate changes in net consisting of bright ‘sunflecks’ (increases in primary production to changes in cloudiness: solar irradiance) with the remaining portion clouds, like scattering aerosols, reduce solar of the canopy being in the shade. The surface radiation and increase diffuse sunlight fraction of the canopy has leaves irradiance. However, it has to be noted that that are often light saturated and therefore
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have low light-use efficiency, whereas leaves and the increase in the diffuse fraction (or in the shade are more light-use efficient but vice versa) that have opposite effects on suffers from a lower exposure to incoming photosynthesis. For the tall and relatively radiation. In contrast, under cloudy or dense forest canopies, the increase in aerosol-laden skies, sunlight is more photosynthesis with increasing diffuse scattered and incoming radiation is more fraction is the dominating effect that has diffuse, producing a more uniform irradiance been verified in numerous theoretical and of the canopy with a smaller fraction of the observational studies. Opposite canopy likely to be light saturated. characteristics with a dominance of the total Global irradiance is one important radiation effect have been found for environmental parameter influencing grassland with low canopies. This means that stomatal opening and closing, as also of the grassland cannot profit from increased changing CO 2 concentration of the diffuse ratios, but rather responds with atmosphere. Changes in air temperature will decreased productivity under reduced light also interact with change in solar radiation conditions as prevalent under global by altering the relative importance of the dimming. The impact of changes in total radiation balance which increases with air radiation and the diffuse fraction on temperature (Penman, 1948). Crop water agricultural crop photosynthesis (and yields) balance and evapotranspiration, unlike crop is in the majority similar as for grassland, but productivity, are closely relate to solar is supposed to depend on the individual radiation. (complex) stand characteristics (Wild et al. Several studies document the 2012). In addition, Changes in productivity enhanced forest net CO 2 uptake with associated with changes in radiation are often increased levels of diffuse PAR for crops. outweighed by the increase in crop yield There is clear evidence that increasing air associated with fertilization, improved pollution (aerosols) and cloudiness alter the varieties and more efficient management partitioning of surface solar radiation practices. towards a higher relative portion of diffuse light, in addition to a reduction of the total Strategies for Minimizing Effect of Global amount of sunlight. This diffuse light can Dimming on Agriculture enhance photosynthesis particularly in tall 1. Photo insensitive crop variety : Develop and dense vegetation canopies, as it is photo insensitive crop variety which can distributed more efficiently within the perform better under low light intensity and canopy compared with direct solar radiation, are efficiently able to harvest as much as which is only accessible for the outermost light energy as it reaches to crop canopy. layers (Wild et al. 2012). 2. Control environment pollution: Global The net impact of surface solar dimming can be reduced by cleaning up the radiation change on photosynthesis occurs emissions from vehicles and burning fossil through both the reduction in total radiation fuels. Another way to reduce the emission of
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particulate matter by using clean fuels and References renewable energy sources (Solar, wind, biogas, bio fuels etc.) O’Dowd C, Margaret O, Mahony, Micheal, 3. Improved management practices: Y and Ray, JB. 2013. Air Pollution: Cultivation of crops under green houses. Obscuring the Full Extent of Global Warming. Ria climate change sciences Improved crop and grazing land management committee (11). to increase soil carbon storage, restoration of Penman HL. 1948. Natural evaporation from cultivated peaty soils and degraded lands, open water, bare soil and grass. better management of waste land, reduced Proceedings of the Royal Society desertification, livestock and manure 193 :120-145. management to reduce CH 4 emissions, Roderick ML and Farquhar GD. 2002. "The improved nitrogen fertilizer application Cause of Decreased Pan Evaporation over the Past 50 techniques to reduce N 2O emissions, Years". Science 298 (5597): 1410- dedicated energy crops to replace fossil fuel 1411. use and improved energy efficiency. Stanhill G and Cohen S. 2001. Global
Conclusion dimming: a review of the evidence for a widespread and significant reduction Global dimming caused due to increased in global radiation with discussion of particulate matter and cloudiness has a its probable causes and possible significant impact on agriculture as it affects agricultural consequences. Agricultural photosynthesis. The forest ecosystem has and Forest Meteorology;107:155–278. positive benefit due to dimming while the Wild M. 2012. Enlightening global dimming and brightening: a review. Res. Lett., grassland and crop lands are negatively 2012; DOI:10.1175/BAMS-D-11- affected due to reduced photosynthesis. 00074.1. Therefore from the agricultural perspective Wild M. 2009: Global dimming and the global dimming is harmful and may brightening: A review. Journal of results into substantial reduction in food Geophysical Research 114, D00D16,10.1029/2008JD011470. production. Thus climate change which Wild M, Roesch A and Christof A. 2012. includes both global warming and global Global dimming and brightening- dimming might have negative impact on evidence and agricultural implications. agriculture. Hence global dimming should CAB-Reviews also be given due consideration, to minimise 2012(7):003:Doi:10.1079/PAVSNNR2 012(7). or nullify its impact on agriculture.
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