The Telegraph, Kolkata, 18 Oct 2012
New Delhi, Oct. 17: If you feel your days are not so bright any more, it could be because even sunlight has joined the ranks of industrialists, lampooners and dissenters who appear to be shunning Calcutta.
Sunlight reaching the ground has decreased across India over the past three decades, but Calcutta appears to have suffered the sharpest dip among 12 cities.
Scientists have documented a steady decline in the solar radiation reaching the ground across the country, a trend they say contrasts with brighter days observed since 1990 across western Europe, North America and parts of China.
The researchers at the Indian Institute of Tropical Meteorology (IITM), Pune, examining long-term trends in solar radiation through a series of studies, have observed declines in both bright sunshine hours and overall brightness in several cities.
One IITM study suggests that Calcutta’s days are now on average about 14 per cent less bright now than they were in the early-1970s, while the solar radiation near the ground in New Delhi has reduced by about 12 per cent during that period.
Scientists say the implications of these changes are still unclear but decreased sunlight has the potential to exacerbate vitamin D deficiency, increase the risk of allergies, pull down crop yields, and alter the hydrological cycle through reduced evaporation.
Endocrinologists say reduced sunlight hours could have implications for populations already deficient in vitamin D. “We already have indications of widespread vitamin D deficiency in Calcutta and other cities in India,” said Satinath Mukhopadhyay, an endocrinologist at the Institute of Post Graduate Medical Education and Research (IPGMER) hospital, Calcutta.
A study of a sample of healthy persons accompanying patients visiting the IPGMER hospital, Mukhopadhyay said, indicates that up to 70 per cent of the population may have some level of vitamin D deficiency.
Some medical studies suggest that sunlight, through vitamin D, can protect children from childhood asthma, while others have hinted at a role for this key vitamin in fighting infections such as tuberculosis.
“Of course, sunlight exposure doesn’t depend exclusively on the amount of solar radiation on the ground, it also depends greatly on occupations and behaviour,” an endocrinologist said.
An analysis of 35 years of near-ground solar radiation data collected from 12 cities suggests that the duration of bright sunshine hours has on average reduced by 12 minutes per decade between 1971 and 2005. Vishakhapatnam on the east coast has shown the most significant loss of bright sunshine hours -- nearly 30 minutes per decade during that period.
“What we’re seeing is not alarming but it isn’t something we can ignore,” said Govindan Pandithurai, an atmospheric physicist at the IITM who is among scientists tracking trends in solar radiation.
Researchers believe a combination of changes in cloud cover, cloud properties and tiny particles of dust, soot, and salts in the atmosphere -- collectively called aerosols -- have contributed to the reduced sunlight.
“But we still need to understand exactly how these components affect sunlight, and which of them is contributing the most at which locations,” Pandithurai told The Telegraph.
The Indian observations are consistent with climate charts generated by the Asian Turfgrass Centre in Thailand that suggest that Calcutta, among 52 cities worldwide, had the second lowest hours of sunlight in September, after Doula in Cameroon.
At the IITM, another atmospheric scientist, B. Padma Kumari, is currently exploring how the observed reduced solar radiation might affect the hydrological cycle over the subcontinent.
“Evaporation over continental India shows a significant decreasing trend -- as would be expected under the trend of decreased solar radiation,” Kumari said.
Agricultural scientists alerted by these studies have launched their own observations to determine whether reduced sunlight hours might in any way influence crop yields, mainly in the Indo-Gangetic plains.
“Any impact on crops will depend on the phase of plant growth during which sunlight is reduced,” said Vadlamudi Uma Maheshwar Rao, director of the All India Co-ordinated Project on Agricultural Meteorology, Hyderabad.
Rao and his colleagues have established 22 sunlight measuring instruments in agricultural universities in different states for daily solar radiation data to correlate sunlight with the growth phase of different cultivated crops.
Sections of atmospheric scientists say human activities such as fossil fuel emissions, biomass burning, and construction can contribute to the load of aerosols in the atmosphere that can obstruct sunlight.
“But we’re not ready yet to quantify the contribution from aerosols to the reduced sunlight,” said Vijay Soni, a scientist at the India Meteorological Department, New Delhi.
Calcutta’s diffuse radiation --sunlight reflected by particles in the atmosphere -- shows a tiny decline by 0.4 per cent per decade. If the solar radiation reduction was because of aerosols, Soni said, the diffused light should have increased.
“On the other hand, Jodhpur has seen a 3.6 per cent radiation decline per decade, but it is not an industrialised city,” Soni said. “Changes in dust load from the Thar desert are likely to be contributing to Jodhpur’s loss of sunlight.”
Kumari said a similar “dimming” trend observed across North America and western Europe during the 1980s appears to have reversed since the early-1990s. “This has been attributed to pollution control measures,” she said.
But scientists believe the properties of clouds and their interaction with aerosols in the tropical regions might be different from those in the northern latitudes. That, Pandithurai said, might also explain the contrast between India and those locations.
