Johann Hari: This summer's drought represents just the beginning of a thirsty century

Western Europe is suffering its worst drying-up since records began. The brittle, cracked reservoirs of Sussex are only the tip: the Spanish Meteorological Institute has just declared a seven-month long "severe drought", France is now patrolled by "water police" forcefully preventing farmers from irrigating their crops, and the country's environment minister declared last month: "The ground under our country is dying of thirst." According to the world's climatologists, this drought is only a harbinger. For much of humanity, this is what the 21st century will look like.

It is impossible to blame any one individual weather event on global warming. But almost all the patterns the climatologists have predicted - more frequent and intense droughts, a rise in global temperatures, and the rapid melting of glaciers, the Arctic ice cap and the Antarctic ice sheet - are coming to pass. How long will we pretend to ourselves it's all a coincidence? Climatologists have long warned us that one of the biggest consequences of global warming will be a transformation in the world's water supplies. Enough denial: it's time to prepare for a thirsty century.

The Nobel Prize-winning scientist Paul Crutzen has provided the clearest intellectual framework for these events. In the 10,000 years since the last glaciation, humanity has been living in a period known as the Holocene. This is when the world's weather settled down, and so did humanity. We began to live in villages and towns, and developed agriculture, writing, and all the other tools that make the lives we know possible. Over millennia, humans have evolved to live in the conditions provided by the Holocene. It is all we know.

There's only one problem. The Holocene is coming to an end. It began to die in the 1780s, when James Watt invented the steam engine and inadvertently changed the course of history. Since then, one creature - man - has become so numerous and so powerful that we are now altering the planet on a geological scale. We have mined and burned so many fossil fuels that we are changing the physics and chemistry of the planet we live on. Man has inaugurated a new climatological age - Crutzen calls it the Anthropocene - and the people of the 21st century are living through the transition. It's a massive, unwitting experiment - and we have nowhere else to go if we do not like the results.

Only now are we beginning to see what this new geological age looks like. Without drastic preventative action now, we are going to hit the highest temperatures since our species evolved in my lifetime - and one of the biggest differences is going to be the location and dependability of the world's water supplies. Last year, the United Nations - after surveying the world's chief scientists - issued a blunt warning. Over the next two decades, 30 per cent of the world's freshwater supply is going to become unusable in a world where 1.2 billion people already do not have access to fresh water.

There are three reasons why. Rainfall is not going to fall in the same place, or at the same times, as before. For example, in sub-Saharan Africa, in the age of the Holocene, people depended on the short rains in March and the long rains in the summer to collect water and grow their crops. But as we move into the Anthropocene, the short rains have disappeared entirely, and the long rains have become erratic. The result is endemic thirst and hunger.

It gets worse. Much of the world's supplies of freshwater are stored in glaciers. For example, the major rivers in India and China only flow because glaciers in the Himalayas catch snow in the winter and it melts off in the spring, releasing the run-off into the Ganges, the Yangtse and many more huge rivers. But as the world warms, these glaciers are disappearing.

In the short-term, this causes flooding as far more water crashes down than normal, but within a few decades, it will cause them to dry up altogether. How will the hundreds of millions of people dependent on this water - for growing their food, as well as for drinking and sanitation - survive?

In South America, the problem is just as bad. Entire cities like Santiago and Lima are dependent on water from glaciers, and are left looking out on nothing but desert without it. We have built human settlements in the places convenient for water supply in the Holocene era - but in the Anthropocene era, freshwater will be in different places where it exists at all.

(I have a mental image of the climate change deniers - from George Bush to Melanie Phillips to John Howard - having a picnic in 2050 on the dry, glacier-free peaks of the Himalayas while people die downstream, and still demanding to know what all the fuss was about).

Finally, as sea levels rise because of the melting of the world's stocks of ice, they will contaminate much of the world's already-existing sources of freshwater. Try pouring a few teaspoons of seawater into a bottle of Evian and see if you can bear to drink it.

The defiantly optimistic part of my mind responded to all this by insisting: even if global warming is allowed to proceed, couldn't technology eventually sort this out? When this comes to pass, can't we build desalination plants and use the 98 per cent of the world's water that is currently undrinkable?

But Stephen Tyndall, director of Greenpeace, explained: "There's two problems with that. Desalination is very energy-intensive, and burns up a lot of greenhouse gases. So while it treats the immediate symptoms of global warming, it actually makes the problem worse in the long term. It's like giving an alcoholic another bottle of vodka. And desalination is incredibly expensive. The rich world might be able to afford it. But as for the poor - we could already provide freshwater to a billion people who don't have it, and we don't. So the idea that we are going to help out all the people affected by global warming by providing desalination is, unfortunately, just not borne out by history."

So the only safe, sane, option is to stabilise the world's climate now by drastically reducing greenhouse gas emissions - but it isn't happening. The G8 summit in Gleneagles ended with fossil fuel business-as-usual. It looks like we are plunging deeper into the Anthropocene, ready or not.

In this thirsty era, global politics is going to be transformed in ways we can only glimpse today. Fortune magazine - the bible of the world's global corporate elites - calls water "the oil of the 21st century" and "the precious commodity that will determine the wealth of nations". The well-known environmentalist pressure group, the CIA, says that by 2015, access to drinking water could be the major source of international conflict around the world.

So enjoy Europe's warm, dry summer. It is coming at a terrible price.

j.hari@independent.co.uk