Declining sunshine hours across India threaten solar energy forecasts

Yash Pal Ralhan

(ralhan.yashpal@gmail.com)

Sunshine hours in India are declining every year during the three decades from 1988 to 2018, a new alarming study reveals, and this study delves into the trend of sunshine reaching the earth’s surface, both temporally and spatially.

Solar radiation is one of the most significant unconventional energy sources. It has emerged as one of the most promising strategies to mitigate ill effects of the conventional energy processes. Environmental degradation, energy security, climate change, compromised health, etc., can be addressed by such non-conventional energy generation methods. In recent decades, however, various regions across the globe have shown significant fluctuations in observed solar radiation. 

The study covered nine geographically diverse regions, including 20 stations in India, spanning the years 1988 to 2018. The monthly sunshine hours (SSH) analysis concluded significant increments from October to May followed by significant drops from June to July in six regions, except the northern inland and Himalayan regions that showed comparatively opposite monthly trends. The trend analysis depicted an annual negative trend in all geographical regions with different rates. 

 Several studies in India reported a persistent solar dimming in the 21st century. One of the most accepted reasons for such dimming is transmissivity change of the earth’s atmosphere due to an increase in aerosol number density?. During the 1990s, India’s economic growth drove urbanization, land-use changes, and industrialization, leading to increased fossil fuel consumption, vehicular emissions, and biomass burning. These activities significantly raised aerosol concentrations in the atmosphere and thus reduced solar radiation.

Sunshine hours are dependent on the concurrent weather conditions that can obscure the solar incidence. The presence of clouds is one of the most prevalent ways that can obstruct the incoming sunlight, although such conditions can also be achieved by the presence of fog or dust storms. Hygroscopic aerosols, acting as cloud condensation nuclei (CCN), play important roles in the modulation of cloud properties and the hydrological cycle, including the lifetime of clouds. As per the theory of condensation for warm cloud systems, bigger cloud droplets are formed when the natural balance of aerosols and humidity persists, and the formation of perceptible clouds is achieved. For a higher number of aerosols in the same amount of humidity, however, a greater number of smaller-sized cloud droplets are developed, and the cloud lifetime is prolonged. 

The annual SSH trend across India depicts maximum decline in SSH, observed in the northern inland region, particularly over Amritsar and Kolkata, as well as in the Himalayan region and West Coast, specifically at Mumbai. However, clear seasonal variation was found in SSH. In the winter season, the whole north inland and east and west coasts show a significant decline in the SSH trend. Whereas, comparatively fewer but smaller negative trends were found in the central inland, Deccan Plateau, Himalayan region and northeast region. The pre-monsoon season also shows an overall negative trend, with a few stations, namely Bangalore, Kolkata, and Mumbai, showing comparatively higher declining rates of SSH. In monsoon and post-monsoon seasons, the northeastern station of Dibrugarh shows a positive trend, in contrast to all other regions. 

 The west coast saw sunshine hours fall by 8.6 hours per year, while the northern plains recorded the steepest drop at 13.1 hours a year. The east coast and Deccan plateau also showed dipping trends of 4.9 and 3.1 hours per year, respectively. What this means is that India, the world’s third-largest solar power market and a predominantly agrarian society, is, in all probability, entering an age of solar scarcity.

For a country that has bet its climate strategy on solar energy, reduced solar incidence translates directly to lower power generation. Researchers at the Indian Institute of Technology, Delhi, estimate that air pollution between 2001 and 2018 cut India’s utilisable solar irradiance by nearly 30%, which would require tens of billions worth of extra investment to meet India’s solar energy needs. Then there is the impact on agriculture. In states such as Punjab, Haryana and Uttar Pradesh, where the dimming is strong, farmers report sluggish crop growth and prolonged moisture on leaves – a recipe for fungal disease and resultant crop losses.

The human mind and the body are at risk as well. Diminished sunlight alters hormone levels, blunts alertness, and disrupts sleep cycles.

India’s single time zone compounds the effect. Psychologists note higher rates of fatigue, anxiety and seasonal depression when sunlight drops below certain thresholds. Vitamin D deficiency – already widespread in Indian cities – rises with loss of sunlight. The consequences for flora and fauna are equally worrying. To fight the dimming, India must first address its cause: dirty air. That means treating aerosol pollution not merely as a health hazard but as an economic threat. The National Clean Air Programme aims to reduce particulate pollution by 20-30%. This could yield an additional 6-16 terawatt-hours of solar energy annually, worth hundreds of millions of dollars. China moved from decades of sunlight dimming to a recent phase of brightening by strictly implementing environmental norms. India too can win this war against the approaching west coast, which saw sunshine hours fall by 8.6 hours per year, while the northern plains recorded the steepest drop at 13.1 hours a year. The east coast and Deccan plateau also showed dipping trends of 4.9 and 3.1 hours per year, respectively. What this means is that India, the world’s third-largest solar power market and a predominantly agrarian society, is, in all probability, entering an age of solar scarcity.