IMD's 70-Year Data: India's Climate Zones Shift

Environment and Climate - Internal (Mains: General Studies-III) | News context: 23 October 2026 | Primary study: Mausam journal 77(4), October 2026 | Published: 24 October 2026

Why in the News

Year-to-year swings in rainfall are common in meteorology, but when a region's very 'climate class' changes, it signals long-term change. A new study by researchers of the India Meteorological Department (IMD) shows exactly this: based on data from 1951 to 2020, a large part of northwest India has shifted from a 'temperate' to an 'arid steppe-hot' climate zone, and northeast India has changed from 'tropical savanna' towards 'temperate'. In all, the climate class of about 7 per cent of India's area has changed - under the combined pressure of rising temperature and falling rainfall. The study is published in IMD's peer-reviewed journal 'Mausam' (Mausam 77(4)); news coverage came on 23 October 2026. The news called it '70 years of change' - the study period 1951-2020 is about 69 years.

How Was the Study Done?

The team of Ananya Karmakar (IMD, Pune), Rajiv Chattopadhyay (Indian Institute of Tropical Meteorology), Pulak Guhathakurta, K.S. Hosalikar and Director General Mrutyunjay Mohapatra used the six categories of the Koppen-Geiger climate classification. Measured rainfall and maximum-minimum temperature data for 1951-2020 from the country's meteorological subdivisions were taken, and for each decade a 30-year moving climatological mean was computed to redraw the map of climate zones. In the comparison, the largest changes appeared between the two periods 1961-1990 and 1991-2020 - the arid zone, once confined to western Uttar Pradesh, has now spread to the state's eastern parts; as per news reports, the arid zone grew from about one-fourth to one-third of the total geographical area.

What Cause Was Given?

The study links this change to the negative phase of the 'Victoria Mode' (VM) - a climate oscillation of the Pacific Ocean - which reduced moisture transport and increased divergence over northern India and the Indo-Gangetic plain. The result: a rising trend in temperature and a falling trend in rainfall together shifted the climate zones. The researchers warn that this change can adversely affect future agriculture in northern India's agro-climatic zone, and this information will also enrich IMD's forecasting system.

What Is the Koppen-Geiger Classification, and Why Does a Climate Zone Shift Matter?

The Koppen-Geiger classification is the world's most widely used climate mapping - it assigns a category to a region (such as arid, temperate, tropical savanna) from the monthly patterns of temperature and rainfall. A single drought year does not change a zone; decades-long averages do. So a climate zone shifting is a deeper signal than seasonal fluctuation. It has practical meanings: the planning of agro-climatic zones, the sowing-harvesting calendar, crop choice, irrigation needs and water-resource planning - all rest on long-term climate norms. If the baseline itself is changing, planning documents too need a new basis.

What Is the Victoria Mode?

El Nino-La Nina and the Indian Ocean Dipole (IOD) are commonly discussed for the Indian monsoon, but scientists are now also linking oscillations like the 'Victoria Mode' (VM) of the North Pacific. The VM is an oscillation of North Pacific sea-surface temperature patterns, which sends atmospheric waves as far as Asia through the Hadley circulation and jet streams. According to the study, in its negative phase the moisture flow reaching the Indo-Gangetic plain weakens and the divergence of air increases - that is, moisture does not settle there. This mechanism explanation is the study's analysis; proving a teleconnection is complex, so read it as a 'contributing factor', not as the sole cause.

One Study Found More Rain, Another Drought - a Contradiction?

There is an important subtlety here. A 2024 study published in Geophysical Research Letters (Joseph et al.) found that between 1979 and 2022, monsoon-season rainfall in northwest India rose by about 40 per cent compared to the 1980s - from the expansion of the Hadley cell and warming of the Indian Ocean. At a surface glance this seems to clash with the new study, but the two measure different things: the GRL study looks only at monsoon-season (June-September) rainfall quantity, while the Mausam study determines the climate class from the combination of annual rainfall and temperature. Despite rising monsoon rain, more heat and shortfalls in non-monsoon months can push the annual 'aridity index' towards the arid zone. Both findings can be correct on their different criteria - so do not treat any one headline as complete proof of a 'climate trend'. Likewise, the finding of rising rainfall in peninsular India is consistent with both.

The Region-Wise Picture

Putting together the regional information from the reports, three patterns emerge. Northwest India: eastward expansion of the arid-hot zone - the arid belt once limited to western Uttar Pradesh has now entered eastern Uttar Pradesh; this is a farm-dense region, so the concern is direct. Northeast India: a class change from tropical savanna towards temperate - its balance has shifted in a different direction. Peninsular India: towards wetness from an increase in overall rainfall - this trend matches other recent studies. All three patterns run in opposite directions within one country - this is the policy challenge of 'one nation, many climate fates': region-specific adaptation plans instead of a national average.

Limits and Cautions

Several things are to be kept measured. First, climate-zone classification is observation-based; the causal attribution (the Victoria Mode's role) is the study's analytical association - it is a plausible explanation, not final causal proof. Second, the news spoke of a '1951-1980 versus 1991-2020' comparison, while the journal abstract places the major changes between '1961-1990 versus 1991-2020' - the two are different; we have followed the journal's account. Third, the figure 'arid zone from one-fourth to one-third' is from news reports; the journal abstract has 'about 7 per cent of the area changing class' - the two are different measures (area expansion versus class change). Fourth, a subdivision-level map can hide district-level experience - variation within one subdivision is possible. Fifth, the adverse effect on agriculture is a projection for the future, not a measured result.

Three Questions from a Policy View

After this study, three questions face policymakers. First, will the boundaries of the national agro-climatic zones (currently 15 zones) be reviewed in line with the new climate map? Second, will crop insurance and farm advisory take long-term zone change into account, or only the year's weather? Third, who will independently assess irrigation-groundwater pressure and the sustainability of cropping patterns (such as the paddy-wheat cycle) in the eastward expansion of the arid zone? These questions lie outside the study's scope, but their answers will decide the policy value of this science.

What to Watch Next

Next steps: climate-zone mapping from the subdivision down to the district level, an official review of the agro-climatic zones, and inclusion of the new maps in ICAR's agro-advisory systems. Alongside, monitoring of remote oscillations like the Victoria Mode in Indian monsoon forecasting, and clearly separate reporting of monsoon versus annual criteria.

Sequence of Dates

Study period 1951-2020; the paper is in Mausam 77(4) (issue October 2026, online record late September 2026); news coverage 23 October 2026. Keep the paper date and the news date separate.

Reference Points for the Reader

Remember three things. First, a 'climate zone shift' is a change in averages, not one year's weather - Koppen-Geiger, monsoon indices and annual rainfall trends are three different measures. Second, remote oceanic oscillations (El Nino, IOD, Victoria Mode) affect the Indian monsoon, but their contribution is a study-specific analysis. Third, the core message for policy: long-term reassessment of the planning base (agricultural zones, water allocation) is now necessary, not optional.

Mains Practice Question

What effects can the long-term shifting of India's climate zones have on agriculture, water resources and disaster management? Clarifying the difference between the criteria of monsoon rainfall and annual climate classification, give suggestions for adaptation policy. (250 words, 15 marks)

Climate Zones Themselves Have Shifted: IMD's 70-Year Data Shows a Drier, Hotter North India and a Wetter Peninsula