The first sign that we are entering a dead zone is the carcass of a camel, gathering flies and red dust. Since camels can go for three weeks without water, according to local farmers, the heap of fur, hair and bleached bones is an ominous sight. We enter a mud-walled, straw-roofed village. Instead of offering the usual smiles and waves, the children duck away. The reason for the villagers' fear becomes evident a few minutes later: nine turbaned men on horseback, members of the Arab militia known as the Janjaweed, appear with rifles over their shoulders. We are gone before they can react, but their presence on the road in broad daylight provides a hint of their sense of invulnerability.

Two more hours across scorched mountains and rocky desert, and we are in Iriba, the logistics base in northeast Chad for six camps of refugees from Darfur. Aid workers there tell me that as horrific as the suffering in Darfur is today, it is almost surely going to get worse. "The water is going. The firewood is gone. The land has lost its ability to regenerate," says Palouma Ponlibae, an agriculture and natural-resources officer for the relief agency CARE. "The refugees are going to have to move. There's going to be nothing here to sustain life."

Darfur, a barren, mountainous land just below the Sahara in western Sudan, is the world's worst man-made disaster. In four years, according to the U.N., fighting has killed more than 200,000 people and made refugees of 2.5 million more. The conflict is typically characterized as genocide, waged by the Arab Janjaweed and their backers in the Sudanese government, against Darfur's black Africans. But what is often overlooked is that the roots of the conflict may have more to do with ecology than ethnicity. To live on the poor and arid soil of the Sahel--just south of the Sahara--is to be mired in an eternal fight for water, food and shelter. The few pockets of good land have been the focus of intermittent conflict for decades between nomads (who tend to be Arabs) and settled farmers (who are both Arab and African). That competition is intensifying. The Sahara is advancing steadily south, smothering soil with sand. Rainfall has been declining in the region for the past half-century, according to the National Center for Atmospheric Research. In Darfur there are too many people in a hot, poor, shrinking land, and it's not hard to start a fight in a place like that.

The devastation of Darfur highlights the potentially catastrophic effects of climate change on societies across Africa. The U.N. estimates that the lives of as many as 90 million Africans--most of them in and around the Sahara--could be "at risk" on account of global warming. Many of Africa's armed conflicts can be explained as tinderboxes of climate change lit by the spark of ancient rivalry. In Somalia, nearly two decades of anarchy have been exacerbated by eight years of drought. In Zimbabwe, relief agencies say President Robert Mugabe's disastrous rule is being overtaken by an even greater catastrophe, a three-month drought that wiped out the maize crop, fueling tensions between government-allied haves and opposition have-nots. Apart from drought, other environmental challenges can prove deadly. A growing number of experts believe the 1994 genocide in Rwanda is best understood as a contest between too many people on too little land.

Environmental skeptics, including the Bush Administration, dispute the more dire predictions about climate change. But others in the developed world are beginning to sound alarms about the weather's role in warmaking. On April 16, 11 former U.S. admirals and generals published a report for the think tank CNA Corporation that described climate change as a "threat multiplier" in volatile parts of the world. The next day, British Foreign Secretary Margaret Beckett hosted the first-ever debate on climate change and armed conflict at the U.N. Security Council. "What makes wars start?" asked Beckett. "Fights over water. Changing patterns of rainfall. Fights over food production, land use. There are few greater potential threats to our economies too ... but also to peace and security itself."

So Darfur is a test case--not just of the world's commitment to stop genocide but also of its ability to prevent future African resource wars. Already, the fighting in western Sudan has spilled into Chad and the Central African Republic. At the Guereda refugee camp in Chad, near the Sudanese border, staff members from the International Medical Corps increasingly find themselves mediating conflicts between refugees and local farmers, who complain that the influx of refugees has ruined their land. The refugee camps house concentrated populations that are too big for the land to support, and water and firewood are all but exhausted. "Resources are simply insufficient to meet the overwhelming needs," warns Serge Malé, U.N. High Commissioner for Refugees representative in Chad.

At another camp, Touloum, home to 22,400, women's welfare officer Mariam Bakhet Ahmed tells me that this year, local villagers have raped 50 refugee women who ventured out for firewood. Touloum camp chief Haroon Ibra Diar describes how, when his people fled to Chad from the village of Abugamra, Sudan, in April 2004, the Janjaweed were employing macabre energy-saving measures. "They beheaded people and used their heads for firewood," he says. When I ask him what the future holds, he says, "We are farmers. But how can we farm here? There's not even enough water to drink. It's a land of death. That's all that it offers us."

The shifting dynamics of the fighting in Darfur illustrate why the prism through which the war is commonly explained--ethnic animosity between Arabs and blacks--may be less applicable than other factors, including the environment. Because of Darfur's harsh, dry terrain, the region's Arab herders and its non-Arab farmers have had to work together in the past: the farmers allowed the herders' livestock on their land in exchange for goods such as milk and meat. As resources become more scarce, that history of cooperation may help persuade some local Arabs and non-Arabs to join forces against the central government. Commanders of the non-Arab rebels told me some Janjaweed commanders have defected, in part out of fear that they will be abandoned by their backers in Khartoum and face arrest for war crimes.

But those are still just small indications of change amid the continuing carnage in Darfur. A U.S.-brokered peace deal last year between rebel groups and the Sudanese government was not worth the paper it was written on. The U.N. has resolved to send in peacekeepers but has been stymied by Sudan's refusal to accept them. After a U.N. panel revealed that contrary to Sudan's denials, government planes have been transporting arms and military equipment to Darfur, Khartoum said it would accept the deployment of 3,000 U.N. troops. They would complement the 7,000 African Union peacekeepers already on the ground, but even that falls far short of what's needed to police a conflict involving hundreds of thousands of fighters and 2.5 million refugees. So far the Bush Administration has been unable to persuade other Security Council members, particularly China, to support more robust measures.

The pillage of Darfur won't end until the world's powers pressure all sides to agree to a truce and allow for the deployment of a larger peacekeeping force. But that's just a start toward fixing Darfur's problems--and preventing similar conflicts from erupting elsewhere. In the longer term, Darfur and the rest of sub-Saharan Africa need sensible land-use policies and careful water management. And as climate change shrinks the availability of arable land and natural resources, Africa will need the developed world to do its part to curb the carbon emissions that contribute to global warming.

For Africa, the cost of inaction could be devastating. Philip E. Clapp, president of the Washington-based National Environmental Trust, warns that Darfur may be "an advance warning" of climate-related apocalypses to come. Take rising sea levels: five of Africa's 10 largest cities are coastal, and 40% of Asia's population of 3.9 billion--1.5 billion people--live within 62 miles of the sea. "Darfur is small by comparison with what is projected," says Clapp. "It may be our last warning before the consequences of climate change become so enormous that they are beyond the capacity of industrialized nations to deal with."

For the thousands of victims of Darfur, hope is nearly extinguished. In Touloum, I meet a filthy young man in rags, distractedly unpicking the threads of a knitted woolen cap. Diar, the camp chief, introduces him as Abdoolcarim Abdur, or "Adam." Diar says Adam, 22, saw his entire family cut down in front of him in 2004, and--as has happened to 40% of Darfur's survivors, according to Médecins Sans Frontières--something snapped. Adam became alternately petrified and violent, convinced that another Janjaweed onslaught was imminent. Afraid of his outbursts, his fellow refugees carried him to Chad tied to a door.

Adam tells me he gets terrible headaches from the Janjaweed horsemen in his head. They gallop around and around in his skull. He asks if I can give him a lift back into Sudan. "It's the time of mangoes and guavas," he says. I shake my head, and he wanders off. Watching him leave, Diar tells me Adam is obsessed with going home. Sometimes, when the headaches are bad, he disappears for days. He runs out into the the desert and back toward the war.

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