Environment & climate · Long read · 13 min read

A Changing Sky: Climate and the Fragile Hills of Munnar

Ice-age grasslands, a flood that rewrote the map, winters that come and go, and a mountain goat with nowhere higher to climb. What the past tells us about Munnar's climate, and what a warming world is doing to it now.

By GK Global, Adimali, Munnar · Trails & Tales by GK

On a clear January morning at Chenduvarai or Sevenmallay, the grass between the tea bushes can be white with frost, and the thermometer can read below zero. A few months later the same slopes can be lashed by rain heavy enough to bring a hillside down. Both are part of Munnar's climate, and both are changing.

Climate change is often spoken of as something new. In the high ranges it is not: these mountains have lived through ice ages, wet and dry millennia and catastrophic floods long before anyone measured them. What is new is the speed of today's warming and the fact that it is arriving in a landscape already reshaped by 150 years of plantations, roads and towns. This story follows Munnar's climate from the deep past to the present, using the scientific record, and tries to be careful about what is known, what is likely and what is still uncertain.

Islands in the sky

Munnar sits on one of the sky islands of the southern Western Ghats: high plateaus, mostly above about 1,500 m, separated from each other by warm, dry valleys. On top of them lies a pattern found almost nowhere else in the tropics, rolling montane grasslands broken by dark, compact patches of evergreen forest called sholas, which fill the folds and hollows of the hills. Eravikulam National Park, above Munnar, protects one of the finest surviving examples.

Sholas and grasslands do different jobs. The forests hold water like sponges: studies have found shola soils keep about twice as much moisture as grassland soils through the dry season. The grasslands catch rain and mist and release it slowly into the streams. Together they feed the rivers that meet at Munnar, and further down, the Periyar and the reservoirs that power much of Kerala. When people call these hills the water towers of Kerala, this is what they mean.

Being islands, they are also traps. Plants and animals adapted to the cool tops cannot easily cross the hot valleys between them. A study of a small high-altitude butterfly, the red-disc bush brown, found that the roughly 50 km of hotter, drier country between the Munnar and Kodaikanal plateaus is a real barrier to it. That isolation explains why the sky islands are so rich in species found nowhere else, and why a warming climate is such a serious threat to them.

The Ice Age on the hilltops

How old is this landscape? The best evidence comes from peat bogs on the high plateaus, which have been building up layer by layer for tens of thousands of years. Scientists, among them the ecologist Raman Sukumar and his colleagues, have read the history of the vegetation in these layers through two clues: pollen grains, and the ratio of carbon isotopes in the peat, which differs between most tropical grasses (C4 plants) and trees (C3 plants).

The story they tell runs roughly like this. The peat record reaches back about 40,000 years, and grasslands were already present then, long before people settled the hills. Around the coldest part of the last Ice Age, about 20,000 to 16,000 years ago, the climate was drier and the plateaus were dominated by grass, with forests confined to sheltered, humid valleys and bogs. As the Ice Age ended and the monsoon strengthened, a wetter phase about 12,000 years ago allowed the shola forests to spread. Then, around 6,000 years ago, the monsoon weakened again and the grasslands regained ground, giving roughly the balance we see today.

The colder past also left living relics. Several plants and animals of the high ranges have their closest relatives in the Himalayas or other temperate mountains, and biologists think they arrived when India's climate was much colder and survived on the cool hilltops when the lowlands warmed. Today they are stranded at the top. There is, quite literally, nowhere higher for them to go.

The lesson of the deep past cuts two ways. It shows that the grasslands are ancient and natural, not damaged forest, and it shows that the balance between grass and forest is sensitive to climate. Small shifts in temperature and rainfall have redrawn this map before.

Frost: the gardener of the grasslands

Why do the sholas stay in the hollows instead of spreading over the hills? For a long time foresters blamed fire and grazing. Recent experiments in the Nilgiris point to a different answer: frost. On clear winter nights, cold air drains into the open grasslands and the temperature at ground level can fall below freezing, while the air inside a shola stays a few degrees warmer.

In a study published in the Journal of Ecology in 2019, researchers Ashish Joshi, Jayashree Ratnam and Mahesh Sankaran planted seedlings of five native shola trees in the grassland, some exposed to the winter cold and some protected from it. Roughly 40% of the native seedlings survived in the protected plots, but only about 10% where frost could reach them. Fire, by contrast, played a negligible role. The frost, in other words, has been tending this landscape for thousands of years, keeping the forest in the valleys and the grass on the slopes.

The same experiment held a warning. Seedlings of the Australian wattle (Acacia), an introduced tree that has spread across the southern hills, survived far better: about 90% in protected plots and nearly 40% even with frost. Warmer winters with fewer frosts could let trees advance into the grasslands, but the trees most ready to take that chance are not native sholas but invasive wattle. Either way, the authors concluded, the ancient grasslands could be lost.

A landscape remade: the colonial century

Long before global warming was measured, the high ranges' ecosystems were transformed by people. When the British arrived, many officials regarded the grasslands as degraded forest, the result of fires lit by hill communities and herders. From the 1850s, foresters in the Nilgiris set out to "improve" them by planting fast-growing exotic trees: wattle and blackwood from Australia from 1856, and later eucalyptus and pine. Similar plantings spread across the southern hills, including the high ranges, where eucalyptus was grown for firewood for the tea factories.

In the Kannan Devan Hills themselves, the greatest change was tea. From the 1880s, large areas of shola and grassland were cleared for plantations, which now cover the lower and middle slopes around Munnar, while the highest grasslands of Eravikulam were kept as a game preserve and, from 1975, protected for the Nilgiri tahr. (We tell that story in Three Rivers and a Tea Empire and The Strangers Who Shaped Munnar.)

The cost of these changes is clearest where they have been measured. On the Nilgiri plateau, studies estimate that about 83% of the grasslands and around half of the sholas have been lost since the mid-1800s, mainly to plantations, farms and invasive trees; in the Palani hills, about two-thirds of the grasslands disappeared in around 40 years. Comparable studies for the high ranges are fewer, but the pattern is familiar. The result is a landscape with less of its natural sponge, more exposed slopes, and invasive plants waiting to exploit any change in the climate.

The memory of water: 1924

Extreme rain is not new to Munnar. In July 1924, the year 1099 of the Malayalam calendar, the monsoon brought about three weeks of exceptional rain. Kerala received about 3,368 mm in the June–September season that year, some 64% above normal, and figures of about 485 mm are often quoted for Munnar during the flood. Hillsides collapsed across the high ranges. At Karinthiri Malai a whole mountainside slid away and with it the old road from the plains; in Munnar the Kundala Valley Railway was destroyed and never rebuilt, and the town itself had to be moved, leaving the original site as Old Munnar. (Read the full story in our piece on Karinthiri Malai.)

The Great Flood of '99 is a vital reminder: these hills have always been capable of catastrophe. The question today is not whether such events can happen, but whether a warming climate is making them more frequent and more intense.

A warming century

The broad picture for India is now clear. The Ministry of Earth Sciences' first national climate assessment, published in 2020, found that India's average temperature rose by about 0.7°C between 1901 and 2018, and that the frequency of very heavy rain days, with more than 150 mm in a day, rose by about 75% between 1950 and 2015. Severe cyclonic storms over the Arabian Sea also became more frequent after the monsoon between 1998 and 2018.

The Arabian Sea matters directly to Munnar, because it is the source of the monsoon that falls on these hills. Researchers at the Cochin University of Science and Technology, led by atmospheric scientist S. Abhilash, have reported that the Arabian Sea has warmed about one and a half times faster than other seas over three decades, with its highest surface temperatures rising from about 29°C to above 30°C. They link this to a new kind of rain over Kerala: tall, deep cumulonimbus clouds, once rare on this coast, that can dump a cloudburst of rain on a small area in a few hours, in and out of the monsoon season.

For the Western Ghats, this means the danger is not only how much rain falls in a season, but how much falls in a day or an hour on slopes already soaked.

2018 and 2020: the floods return

In August 2018 Kerala suffered its worst floods since 1924. By 21 August the state had received 53% more rain than normal for the monsoon season, and a study by Vimal Mishra and colleagues at IIT Gandhinagar estimated that the rain over the catchments of Idukki, Kakki and the Periyar over one to fifteen days was rarer than a once-in-500-years event. With reservoirs already nearly full, water had to be released rapidly downstream. In the high ranges, landslides cut roads and isolated Munnar for days.

Two years later came the deadliest landslide in the region's recorded history. Late on the night of 6 August 2020, after torrential rain, a hillside above the tea workers' lines at Pettimudi, north of Munnar, gave way. A study in the journal Landslides recorded that the slide began on steep shola slopes, ran about 1.2 km down through the tea estate and buried the homes below. Seventy people died, almost all of them estate workers and their families. The same study recorded rainfall of 616 mm in a day in the area, the highest ever recorded there.

No single disaster can be blamed on climate change alone. Slope, soil, land use, the siting of homes and the management of dams all matter. But the science of attribution is growing: after the 2024 Wayanad landslides elsewhere in Kerala, scientists estimated that climate change had made the triggering rainfall around 10% heavier. The pattern that the national records describe, more frequent extreme downpours over a warmer sea, is exactly the pattern that makes steep, rain-soaked hills like Munnar's more dangerous.

Winters out of step

Munnar's frosts are part of its fame, and they bring visitors every January. They are also becoming harder to predict. In the winter of 2021–22, cloudy skies and unseasonal rain meant the usual January frost largely failed to appear, with the lowest readings around 1°C at Chenduvarai. In other years, frost has arrived late or lingered unusually long: tea industry sources have reported sub-zero readings of −1°C to −2°C at estates such as Chenduvarai, Sevenmallay and Letchmi well into February, with frost wilting tea across some 550 hectares in a single season.

This can sound like a contradiction: how can a warming world bring such cold nights? The answer is that climate change is not just about averages. What tea planters and scientists describe is a more erratic calendar: frost seasons that start late, end late or fail altogether, and a widening gap between hot days and cold nights, which climatologists such as Gopakumar Cholayil warn is a growing risk for tea and other high-range crops. For an ecosystem in which frost decides where forest and grassland grow, an unreliable winter matters as much as a warmer one.

The tahr's shrinking world

No animal shows the stakes more clearly than the Nilgiri tahr, the endangered mountain goat found only in the southern Western Ghats, whose largest population lives in Eravikulam National Park above Munnar. The tahr depends on the high grasslands and the cliffs beside them. It cannot simply move uphill as the climate warms, because on these sky islands it is already near the top.

In 2018 a team including scientists from the Ashoka Trust for Research in Ecology and the Environment (ATREE) published a study in Ecological Engineering modelling where the tahr's climate would remain suitable in future. Under a scenario in which greenhouse gas emissions keep rising, it projected the loss of around 61% of the tahr's suitable habitat as early as the 2030s, rising to about 63% by the 2080s, with some southern ranges becoming entirely unsuitable. The authors warned that the existing network of protected areas may not be enough to conserve the species unless climate change is built into its management. (Read more in our story on the Nilgiri tahr.)

The same pressures face the frogs, lizards, birds, butterflies and plants of the sholas and grasslands, many of them found on a single sky island. The famous Neelakurinji, whose mass flowering every twelve years turns the Munnar hills blue, grows only in these high grasslands; its future depends on theirs.

What can be done

Munnar cannot stop global warming on its own. But much of what makes the hills vulnerable is local, and so are many of the remedies:

• Protect what remains. Every surviving shola and grassland is part of the water tower. Keeping them intact is the cheapest flood defence and water supply the region has.
• Remove invasive trees and restore grasslands. Projects in the Nilgiris and Palanis are clearing wattle and replanting native species; the frost research shows why grasslands need active care.
• Build with the slope, not against it. Landslide mapping, limits on construction on steep and unstable ground, and safe housing for estate workers save lives.
• Prepare for extremes. Better rain gauges in the hills, early warnings and reservoir rules that respond to forecasts can reduce the toll of the next great flood.
• Keep watching. Long-term weather records from the estates and research stations, and monitoring of tahr and grassland, are the only way to know how fast the high ranges are changing.

Visitors can help too: by keeping to paths, carrying out plastic, respecting closures in the monsoon and choosing operators who know the hills. (See our guide Fragile Hills: How to Travel Gently.)

The frost on the grass, the mist in the shola and the river at the confluence have been shaped by climate for forty thousand years. For the first time, the people who live in these hills are shaping the climate back.

Note on sources: much of the detailed ecological research on sky islands has been carried out in the Nilgiris and Palani hills, which share Munnar's ecosystems; where we use those results, we say so. Rainfall figures for historical floods come from different records and should be read as approximate. Climate projections depend on future emissions and on the models used.