Karachi, 2050: The sprawling megacity lies crumbling, desiccated by another deadly heatwave, its millions of inhabitants suffering life-threatening water shortages and unable to buy bread that has become too expensive to eat.

It sounds like the stuff of dystopian fiction but it could be the reality Pakistan is facing. With its northern glaciers melting and its population surging -- the country's climate change time bomb is already ticking.

In a nation facing Islamist violence and an unprecedented energy shortage slowing economic growth, the environment is a subject little discussed.

But the warning signs are there, including catastrophic floods which displaced millions, and a deadly heatwave this summer that killed 1,200 people.

Three of the world's most spectacular mountain ranges intersect in Pakistan's north: the Himalayas, the Hindu Khush and the Karakoram, forming the largest reservoir of ice outside the poles.

The mountain glaciers feed into the Indus River and its tributaries to irrigate the rest of the country, winding through the breadbasket of central Punjab and stretching south to finally merge with the Arabian Sea near Karachi.

The future of Pakistan, a Muslim giant whose population the UN predicts will surge past 300 million people by 2050, can be read in part by the melting of glaciers like Passu, at the gateway to China.

From its magnificent rocky slopes, the glacial melt is obvious.

"When we would come here 25 years ago, the glacier reached that rock up there," explains Javed Akhtar, indicating an area some 500 metres (1,600 feet) from the tip of the ice.

Akhtar, his face bronzed by the sun, is a villager who has been employed by a team of glaciologists measuring the impact of climate change.

Temperatures in northern Pakistan have increased by 1.9 degrees Celsius in the past century, authorities say, causing "glof" -- glacial lake outburst floods, where the dams of such lakes abruptly rupture, sending water cascading down the slopes.

Today, thirty glacial lakes are under observation in the north. According to the Intergovernmental Panel on Climate Change (IPCC), such mass loss of water is "projected to accelerate throughout the 21st century, reducing water availability, hydropower potential, and changing seasonality of flows in regions supplied by meltwater from major mountain ranges".

In Pakistan, most of the country is fed by the lush, fertile plains of one such region: Punjab.

- The breadbasket -

Despite its growing population, Pakistan remains self-sufficient in agricultural terms, largely thanks to the rich Punjabi soil.

But in recent years the region has seen unprecedented, deadly floods that wipe out millions of acres of prime farmland.

The disasters are caused by monsoon rains, but are a bellwether for the havoc that melting glaciers could cause, with any variation in water levels threatening farmers' crops.

"When there is too much water it's not good for rice, and when there is not enough, that's also bad. And it's the same for wheat," says farmer Mohsin Ameen Chattha during a walk through his family land just outside the Punjabi capital of Lahore.

Surplus monsoon water is mostly stored in Pakistan's two large reservoirs, the Tarbela and the Mangla dams -- but, warns Ghulam Rasul, director general of Pakistan's meteorological department, the supply would hardly last 30 days.

"That is not sufficient," he says.

Throughout the rest of the year, farmers rely on the rivers, primarily the glacier-fed Indus, to irrigate their land.

For now, the production of rice and wheat is still rising.

But if the glaciers were to one day disappear, "we would be totally dependent on the monsoon. And already it varies," says Rasul.

"All this has an impact on food security" for the country, he added.

If its daily wheat production should no longer suffice, Pakistan would have to begin importing the grain -- driving the price of bread up.

- Karachi: The perfect storm? -

Like the Indus, the ominous warning signs all culminate around Karachi.

The city draws almost all of its water from the river and already fails to meet even half of the four billion litres a day its inhabitants require, in part because of its inadequate pump network and .

By 2050 the IPCC predicts a decrease in the freshwater supply of South Asia, particularly in large river basins such as the Indus.

That means Karachi will somehow have to manage its growing population with even less water -- a population with a significant poverty rate that will also struggle should food prices rise.

"In the long term, it is a huge challenge," says Syed Mashkoor ul-Hasnain of the Karachi Water Company.

To make matters worse, the meteorologist Rasul predicted changes in atmospheric pressure over the Arabian Sea that could reduce the breezes that currently temper the sweltering heat of the port.

In June an unprecedented heatwave took 1,200 lives, mostly in poor neighbourhoods of Karachi -- heat traps with their massive concrete buildings, lack of shade, and the absence of aqueducts.

Could it have been a taste of the future? Back on the Passu Glacier, the research assistant Javed Akhtar is unequivocal.

"A calamity is coming," he warns.