This may not be the most tactful time to bring it up, with much of Pakistan underwater and many millions homeless, but Pakistan’s real problem is not too much water. It is too little water—and one day it could cause a war.

The disastrous floods (to which the response of both Pakistan’s government and the international community has been far too slow) are due to this year’s monsoon being much stronger than usual. But that is just bad weather, in the end: every 50 or 100 years, you can expect the weather to do something really extreme. It comes in various forms—blizzards, floods, hurricanes—but it happens everywhere.

The long-term threat to Pakistan’s well-being is that the country is gradually drying out. The Indus river system is the main year-round source of water for both Pakistan and northwestern India, but the glaciers up on the Tibetan Plateau that feed the system’s various tributaries are melting.

While they are melting, of course, the amount of water in the system will not fall steeply, but according to the Chinese Academy of Sciences, some of the glaciers will be gone in as little as 20 years. Then the river levels will drop permanently, and the real problems will begin.

When India and Pakistan got their independence from Britain in 1947, there was plenty of water in the Indus system for everyone. In fact, almost half of the water was flowing into the Arabian Sea unused. But the population has grown fast over the years, especially on the Pakistani side of the border—from 34 million in 1947 to 175 million now—and the amount of water in the rivers has not.

The per capita supply of water in Pakistan has fallen from over 5,000 cubic metres annually in 1947 to only about 1,000 cubic metres today, a level defined by the United Nations as “high stress”. Ninety-six percent of that goes to irrigation, and the Indus no longer reaches the sea in most years. That’s what has already happened, even before the melting of the glaciers has gone very far.

Fifteen or 20 years from now, the water shortage (and therefore also food scarcities) will be a permanent political obsession in Pakistan. Even now, Pakistani politicians tend to blame India for their country’s water shortage (and vice versa, of course). It will get worse when the shortage grows acute.

What turns a problem into a potential conflict is the fact that five of the six tributaries that make up the Indus system cross Indian-controlled Kashmir on their way to Pakistan. There is a treaty, dating from 1960, that divides the water between the two countries, with India getting the water from the eastern three rivers and Pakistan owning the flow from the western three. But the treaty contains a time bomb.

India’s three rivers contain only about one-fifth of the system’s total flow. To boost India’s share up to around 30 percent, therefore, the World Bank arbitrators proposed that the treaty also let India extract a certain amount of water from two of Pakistan’s rivers before they leave Indian territory. The proposal was reluctantly accepted by Pakistan.

The amount is not small—it is, in fact, enough water to irrigate 320,000 hectares—and it is a fixed amount, regardless of how much water there actually is in the river. Now roll the tape forward 20 years: the glacial meltwater is coming to an end, and the total flow of the Indus system is down by half. But almost all of the loss is in Pakistan’s three rivers, since the smaller Indian three do not depend heavily on glaciers.

So India is still getting as much water as ever from the eastern three rivers, and it is still taking its full treaty allocation of water from two of Pakistan’s rivers, although they depend on glacial meltwater and now have far less water in them. As a result, India’s total share of the Indus waters rises sharply (and quite legally) just as Pakistanis start to starve.

In these circumstances, would an Indian government voluntarily take less water than the treaty allows? Get real. India will be having difficulties with its food supply too, though it will not be in such grave danger as Pakistan. Any Indian government that “gave India’s water away” would promptly be driven from power—by parliament if it was the usual fractious coalition, or by voters at the next election if it were an unusually disciplined single party.

On the other hand, no Pakistani government, civilian or military, could just sit by as land that has been irrigated for a century goes back to desert and food rationing is imposed nationwide. Especially not if India’s fields just across the border were still green. That is the nightmare confrontation that lies down the road for these two nuclear powers.

Meanwhile, the homes of millions of Pakistanis are underwater. In terms of human suffering, it is 20 times worse than Hurricane Katrina was in the United States five years ago, and it needs a proportionate response now. But the future holds something much worse for Pakistan and for India, unless they start revising this 50-year-old treaty now, before the crisis arrives.