If we trace the latitude of Mount Everest, which coincides with the 28th parallel north, and follow that line around the world, it leads us westward through Nepal and North India into the deserts of Rajasthan and Sindh, beyond the Persian Gulf to the Arabian Peninsula. After dividing the Red Sea, the line passes over Egypt and the Sahara, all the way across North Africa to Morocco, keeping well below the southern coast of the Mediterranean.

In the Atlantic Ocean the 28th parallel north grazes the Canary Islands before making landfall in Florida and continuing on through the badlands of Texas, into Mexico’s Baja Peninsula. From there it crosses the Pacific, just north of Hawaii, and carries on to the Yellow Sea, after which it penetrates southern China and stretches into Burma, before completing its global circuit through Arunachal Pradesh, Bhutan, Sikkim and finally, the north-eastern corner of Nepal.

The Himalaya are sometimes referred to as the “Third Pole” because they represent the largest accumulation of ice, after Antarctica and the Arctic. However, as our mental circumnavigation of the globe illustrates, one of the coldest places on earth shares the same latitude as many of the warmest, driest spots on our planet. Compared to Saudi Arabia, Libya or southern Texas, the Himalaya are considerably wetter and retain frozen reservoirs that irrigate South and Southeast Asia.

For this reason they are also called the “water towers of Asia”. One of the reasons they are especially vulnerable to climate change is because of their latitude. If Himalayan glaciers dry up and disappear, as some scientists suggest they might, this entire region would then begin to look like the sandy wastes of the Sahara. While the North and South Poles are experiencing rising temperatures, the Himalaya are subject to even greater warming because of their proximity to the equator.

Throughout their existence, the Himalaya have endured the effects of climate change.

Palaeobotanical studies of floral variations over thousands of years demonstrate that meteorological trends have shifted back and forth from centuries of moist and temperate weather to extended periods of arid cold. Vegetation profiles based on pollen data from soil samples, as well as fossil evidence, show that during different eras the dominant tree species alternated from pines and other conifers to broad-leaved oaks and alders. Expansion and retreat of forests and grasslands have mirrored climate shifts throughout the region.

To appreciate the changeable nature of Himalayan weather and its contrasts, we can compare the climate on high passes at varying times of year and in different regions of the mountains. At the eastern end of the Himalaya, in Sikkim, lies Goecha La (4,940 metres) at the foot of Kanchenjunga. Setting off from the roadhead at Yuksam in May, we climb 3,000 metres in three days, ascending from humid, semi-tropical conditions into dense bands of cloud forest, enveloped in mist and drizzle. Our final camp lies above the timberline, where 10 centimetres of snow falls on our tents that night.

Later in the year, in Central Nepal, at the beginning of December, we cross Thorung La (5,416 metres) between the Marsyangdi and Kali Gandaki watersheds.Though the sky is cloudless, most streams and rivulets are frozen and the glare off the snow is blinding. Night temperatures drop to fifteen degrees below zero.

A thousand kilometres to the west, in Ladakh, during August, traversing Shingo La (5,091 metres) the high-altitude desert of the Zanskar Valley is so hot and dry that we become dehydrated and the sun scorches our skin.

If such dramatic extremes exist in the Himalaya, then why should we be concerned about an average increase of a degree or two over the past hundred years?

Perhaps because, unlike previous climate change, the current shifts are a direct result of human consumption and waste. Seemingly insignificant variations in temperature have dire consequences – glacial lakes bursting fragile ice dams, mudslides that bury villages, unpredictable monsoons, disappearing species and disrupted migration patterns.

In addition to annual weather patterns, innumerable microclimates exist in the Himalaya caused by altitude and terrain. Some slopes and valleys lie in rain shadows while other areas attract excessive precipitation. In the foothills of the Central Himalaya, where I live, the northern face of a ridge is generally moist and densely forested while the southern exposure is drier and covered in grass. At higher elevations this pattern is reversed and the northern slopes receive less precipitation.

Anyone who has followed the course of a river like the Ganga knows that its waters pass through distinctly different belts of forest as it twists its way between the ranges, from silver birch (Betula utilis) and deodar (Cedrus deodara) at upper altitudes to Bauhinia creepers and cactus-like Euphorbia royleana lower down. Vegetation and wildlife vary according to temperature and humidity. Human habitation too seeks out the most advantageous climate for growing crops and accessing pastures.

Weather is both cause and consequence in the Himalaya.

Colonial migration to the hills is the reason my home town exists because the first British residents came here to escape searing temperatures, oppressive humidity and virulent fevers on the plains. In May and June, when the heat in Delhi climbs to 45° Celsius, our thermometers seldom rise above 30°. Landour, Mussoorie, Murree, Dalhousie, Shimla, Nainital and Darjeeling continue to be summer resorts, where tourists migrate each year because of intense heat at lower elevations.

Climate affects everything in the Himalaya, from biology and glaciology to mountaineering and philosophy. On the highest peaks, almost as many climbing deaths are linked to bad weather and storms as they are to altitude sickness, avalanches or fatal falls put together. Most Himalayan agriculture depends entirely on seasonal rainfall from rice paddies in the valleys to buckwheat above the treeline.

The composition of Himalayan forests serves as a natural hydrometer, signalling levels of humidity in the air and moisture in the soil. Epiphytic species, such as ferns or orchids that take root in the saturated branches of a cloud forest, rely on mist as much as rain.

Governments have fallen as a result of weak monsoons and wars have begun and ended as temperatures climbed and dropped. Artists and poets have been inspired by the monsoon. Pahari miniature paintings illustrate approaching storms as metaphors of desire – a young woman standing on a balcony, ardently awaiting the arrival of her beloved as thunderheads darken and swell.

Religious faiths teach lessons based on climate from the stoic asceticism of monastic retreats in barren highlands to the voluptuous divinity of mother goddesses, whose fecund blessings enrich the soil. Indra, the god of storms, hurls thunderbolts from heaven and Lord Shiva catches the monsoon torrents in his matted dreadlocks. White clouds symbolise purity and transience for Buddhist teachers just as rainbows serve as archways to paradise.

Migration is governed by the seasons.

Rosefinches and black-necked cranes fly south or north as winds change direction and temperatures rise or fall. Nomadic shepherds time their departures and arrivals according to weather patterns, reaching high meadows with their herds soon after the monsoon brings forth an abundance of grass and herbaceous plants.

Fish, ferns and fungi flourish in this wet season while erosion shapes the ridges, carving ravines and uprooting trees, while washing away the detritus of glaciers. Even the deities and their devotees move up and down the valleys from winter sanctuaries below the snow line to summer temples near the sources of rivers. In Garhwal and Kumaon, biannual pilgrimages transport gods and goddesses from lower to upper elevations in the spring, then back again in the fall.

These ritual journeys are often compared to a bride departing her parental home or returning from her husband’s abode, suggesting the displacement of marriage as well as the reproductive process of conception, gestation and birth. Perhaps it is only natural in a landscape fostering seasonal journeys that human beings would believe in the transmigration of the soul.

Climate change in the Himalaya is everyone’s problem.

The causes often lie far away, sometimes in distant corners of the globe. In 1991, during the First Gulf War, when Kuwait’s oil wells were set on fire, a pall of smoke drifted thousands of kilometres eastward. Flying home to India that year, I remember seeing a shadowy black smear stretching towards the horizon.

When I reached Mussoorie, the oak leaves were covered with an oily film, the residue of fossil fuels set alight by war.This greasy soot also settled on snow peaks and glaciers leaving a stain that not only sullied the face of the Himalaya but accelerated their melting. Seasonal winds carry various kinds of particulate matter from sand and dust to vehicular pollutants.

In his book Life in the Himalaya:An Ecosystem at Risk, botanist and environmental scientist Maharaj K Pandit links the darkening of the surface of glaciers and ice fields to both local and global pollution. “A number of recent studies have shown that the deposition of mineral dust and black carbon has contributed to the darkening of the western Himalayan snow cover, which accelerates the seasonal snowmelt and the regional snow albedo feedback producing more warming and higher glacial ablations.”

Calculating the human cost of Himalayan warming would be a staggering exercise.

These high ranges separate the two largest populations on earth, with China (1.418 billion) to the north and India (1.363 billion) to the south. While the upper regions are sparsely populated, particularly above 2,500 metres, they provide a vital source of fresh water for those who live on either side.

The roof of Asia is sharply pitched, collecting snow and draining moisture into the valleys. Both on a local and global scale the diminishing ice cap could quickly deplete some of the most important river systems upon which billions of people rely – not just the Brahmaputra, Ganga and Indus but also the Salween and Irrawaddy in Myanmar, the Yellow River and the Mekong in China and Southeast Asia. If these giant waterways, all of which have tributaries in the Himalaya, were to diminish or disappear, the effects downstream would be devastating and irreversible.

Wild Himalaya

Excerpted with permission from Wild Himalaya: A Natural History Of The Greatest Mountain Range On Earth, Stephen Alter, Aleph Book Company.