In 1985, with the Darfur region of Sudan deep in drought, a doctoral candidate named Alex de Waal met with a bedridden and nearly blind Arab sheikh named Hilal Abdalla.

The elderly nomad and his tribesmen had pitched their camp across an unforgiving wasteland of rock and sand. Broad black tents rose like sails against the rough horizon. Thorn trees broke ground at lonesome intervals, sparse grazing for the tribe’s camels. The student was long-limbed and gangly, bent forward with the eagerness of youth. The sheikh — tall, stately, stooped by age — asked him in.

"His tent was hung with the paraphernalia of a lifetime’s nomadism — water jars, saddles, spears, swords, leather bags, and an old rifle", De Waal recalled years later. "He invited me to sit opposite him on a fine Persian rug, summoned his retainer to serve sweet tea on a silver platter, and told me the world was coming to an end."

They dined on goat and rice and ate with their hands. De Waal was studying indigenous reactions to the dryness that gripped the region. The elderly nomad described things he had never seen before. Sand blew over fertile lands. The rare rain washed away alluvial soil. Farmers who had once hosted his tribe and his camels were now blocking their migration; the land could no longer support both herders and planters. Many of the sheikh’s tribesmen had lost their stock and scratched at millet farming, relegated to sandy soil between plots of fertile land.

With his stick, the nomad sketched a grid in the sand, a chessboard de Waal understood to be the "moral geography" of the region. The farmers tended to their crops in the black squares, and the sheikh’s people stuck to the white, cutting without conflict like chessboard bishops through the fields. The drought had changed all that. The God-given order was broken, the sheikh said, and he feared the future.

"The way the world was set up since time immemorial was being disturbed", recalled de Waal, now a program director at the Social Science Research Council. "And it was bewildering, depressing, and the consequences were terrible."

Nearly 20 years later, when a new scourge swept across Darfur, de Waal would remember the meeting. Janjaweed fighters in military uniforms, mounted on camels and horses, laid waste to the region. In a campaign of ethnic cleansing targeting the region’s blacks, the armed militiamen raped women, burned houses, and tortured and killed men of fighting age.

Through whole swathes of Darfur, they left only smoke curling into the sky. At their head was a six-foot-four Arab with an athletic build and a commanding presence. In a conflict the United States would call genocide, he topped the State Department’s list of suspected war criminals. De Waal recognised him. His name was Musa Hilal, and he was the sheikh’s son.

On the path from worried elder to militant son lie the roots of a conflict that has forced two million mostly black Africans from their homes and killed between 200,000 and 450,000 people. The fighting in Darfur is usually described as racially motivated, pitting mounted Arabs against black rebels and civilians. But the distinction between "Arab"and "black African" in Darfur is defined more by lifestyle than by any physical difference: Arabs are generally herders, Africans typically farmers. The two groups are not racially distinct. Both are predominantly Muslim.

The fault lines have their origins in another distinction, between settled farmers and nomadic herders fighting over failing lands. The aggression of the warlord Musa Hilal — forged in a time of desertification, drought, and famine — can be traced to the fears of his father and to how climate change shattered a way of life.

Until the rains began to fail, the sheikh’s people had lived amicably with the settled farmers. The nomads were welcome passers-through, grazing their camels on the rocky hillsides that separated the fertile plots. The farmers would share their wells, and the herders would feed their stock on the leavings from the harvest. But with the drought, the nomads ranged farther for their food, and the farmers began to fence off their land — even fallow land — for fear it would be ruined by passing herds. Sometimes they’d burn the grass upon which the animals fed. A few tribes drifted elsewhere or took up farming, but the camel-herding Arabs stuck to their fraying livelihoods — nomadic herding was central to their cultural identity.

The name Darfur means "Land of the Fur", called so for the largest single tribe of farmers in the region. But the vast region holds the homelands — the dars — of many tribes. In the late 1980s, landless and increasingly desperate Arabs banded together to wrest their own dar from the black farmers, publishing in 1987 a manifesto of racial superiority.

It began with complaints of underrepresentation in the government and concluded with a threat to take matters into their own hands: "We fear that if this neglect of the participation of the Arab race continues, things will break loose from the hands of the wise men to those of the ignorant, leading to matters of grave consequences."

Clashes had broken out between the Fur and camel-herding Arabs, and in the two years before an uneasy peace was signed in 1989, 3000 people, mostly Fur, were killed, and hundreds of villages and nomadic camps were burnt. More fighting in the 1990s entrenched the divisions between Arabs and non-Arabs, pitting the pastoralists against the Fur, Zaghawa, and Massaleit, the three tribes that would later form the bulk of the rebellion against the central government. In these disputes, Khartoum often supported the Arabs politically. Sometimes — in an attempt to create a bulwark against revolutionaries from southern Sudan — the government provided arms.

When a rebellion began in Darfur in 2003, it was at first a reaction against Khartoum’s neglect and political marginalisation of the region. But while the rebels initially sought a pan-ethnic front against a distant, uncaring regime, the schism between those who opposed the government and those who supported it soon broke largely on ethnic lines. The camel-herding Arabs became Khartoum’s staunchest stalwarts.

Nomadic Arab militia launched a brutal campaign to push the black farmers from Darfur. They wore military uniforms, sometimes drove military vehicles, and coordinated their attacks with Sudanese aerial bombing. Even so, the conflict was rooted more in land envy than in ethnic hatred.

"Some of the Arab pastoral tribes, particularly the camel herders, did not have their own dar, so were always at the mercy of other tribes for land", said David Mozersky, the International Crisis Group’s project director for the Horn of Africa. "This was fine for hundreds of years, since the system provided land for these groups as they moved."

"But as desertification worsened and as fertile land decreased, some of these pastoralists sought to have their own land, which wasn’t really an option in Darfur. This was one of the main factors that Khartoum used to manipulate and mobilise these Arab tribes to join the Janjaweed and fight on their side. Interestingly, most of the Arab tribes who have their own land rights did not join the government’s fight."

This is an edited extract from Forecast: The Consequences of Climate Change from the Amazon to the Arctic (2009: Scribe) by Stephan Faris.