In the last 70 years, northern India has become hotter and drier, with the extent of the dry zone appearing to expand. The zone, which previously ended in western Uttar Pradesh, has now extended into the eastern parts of the state.
At the same time, much of southern India has recorded an increase in precipitation, resulting in a decline in the hot semi-arid zone — an entirely opposite phenomenon. These findings are part of a study titled ‘Identifying the Climate Zones Shifts of Indian Meteorological Subdivisions: Insights from Historical Record’, conducted by scientists from the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM), Pune.
The research reveals a gradual alteration in the country’s climate zones, which could potentially have an impact on agriculture. Climate zones are defined by temperature and rainfall. Researchers redrew India’s climate map for three successive 30-year periods — 1951-1980, 1981-2010 and 1991-2020 — using data from IMD’s observational records. The study focuses on four climate zones: hot semi-arid (characterised by low rainfall), tropical savannah (warm throughout the year with a long dry season), tropical monsoon (the wettest zone, with heavy seasonal rainfall) and temperate (characterised by hot summers and mild, dry winters).
Changes across regions
In north-western India, the hot semi-arid zone expanded from approximately 25 per cent of the region’s area during 1951-1980 to 33 per cent in 1991-2020. In contrast, the temperate zone shrank from about 25 per cent to less than 20 per cent. This shift indicates that roughly 7 per cent of the region transitioned to drier conditions as temperatures rose and rainfall declined.
Peninsular India experienced changes that were entirely opposite in nature. The hot semi-arid zone decreased from about 30 per cent of the region’s area to 16 per cent, with most of the area shifting to the tropical savannah zone, which now accounts for nearly three-quarters of the peninsula. The authors attribute this shift to increasing rainfall in southern India. Within these broader trends, Marathwada and western Vidarbha in Maharashtra have entered the hot semi-arid zone, while the tropical monsoon zone has expanded along the Konkan coast, coastal Karnataka and Kerala. “When we compared India’s climate map decade by decade from 1951 to 2020, the most noticeable change was in the north, where the hot, dry zone has pushed east into Uttar Pradesh. This area is crucial for agriculture, so it is important to closely monitor the timing of sowing and harvesting,” said Ananya Karmakar, a scientist at IMD Pune and lead author of the study published in MAUSAM, IMD’s peer-reviewed journal.
Reasons and impacts of climate change
To investigate why the northern plains have become drier, researchers compared rainfall data from Uttar Pradesh, Bihar and Gangetic West Bengal between 1981 and 2020 with ocean temperature records. They found that years of low rainfall correlate with a recurring pattern of warmer and cooler waters in the North Pacific, known as the Victoria Mode.During certain phases, this pattern diverts moisture away from northern India. The authors caution that these changes could affect paddy cultivation and necessitate closer monitoring of agricultural practices in the region. “The drying trend in the northern plains aligns with the Victoria Mode in the North Pacific, which can diminish moisture in northern India. Understanding this pattern could help us forecast rainfall deficits in the region,” explained Rajib Chattopadhyay of IITM Pune, a co-author of the study. The study comes in a year when El Niño, a periodic warming of the Pacific Ocean that often weakens the Indian monsoon, left the monsoon season 12.6 per cent below normal, the weakest performance since 2015. Drought has already been declared in large parts of Maharashtra, Karnataka, Telangana, Andhra Pradesh and Rajasthan.
While the study does not specifically address El Niño, it points to longer-term climatic changes that could exacerbate the effects of such years, particularly in regions such as Marathwada and western Vidarbha, which are now experiencing poor monsoon conditions in an increasingly drier climate. “This year’s monsoon ended 12.6 per cent below normal, primarily due to El Niño, and the drought declared across several states illustrates the severe impact such conditions can have on farmers. However, our seven decades of data also show that India’s climate zones have shifted significantly within a single lifetime. In places like Marathwada and western Vidarbha, a poor monsoon now occurs in a progressively drier climate compared to the 1960s. IMD will utilise this long-term data alongside its seasonal forecasts to provide relevant weather advisories to farmers,” said Mrutyunjay Mohapatra, Director General of IMD and a co-author of the study.