"Almost invariably, we discuss Darfur in a convenient military and political shorthand -- an ethnic conflict pitting Arab militias against black rebels and farmers," Ban wrote in an editorial that appeared Saturday in The Washington Post. "Look to its roots, though, and you discover a more complex dynamic."

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Rainfall in Sudan began declining two decades ago, a phenomenon due "to some degree, from man-made global warming," said Ban, who has made both Darfur and climate change priorities.

Settled farmers and Arab nomadic herders had gotten along until the drought, he wrote, but as conditions worsened, water and food shortages disrupted the peace and "evolved into the full-fledged tragedy we witness today."

Ban said similar ecological problems are behind conflicts in other countries, including Somalia and Ivory Coast.

The Darfur conflict began in 2003 when local rebels took up arms against the Sudanese government, accusing it of decades of neglect. Sudanese leaders are accused of unleashing the pro-government Arab militia, the janjaweed, to fight them - a charge they deny.

After months of U.N. and Western pressure, Sudan agreed in the past week to allow a joint U.N.-African Union force of up to 19,000 peacekeepers to replace the 7,000-member AU mission now in Darfur.

Ban called the agreement "significant progress" after "four years of diplomatic inertia." But he warned that a long-term solution was needed for "the essential dilemma" -- the scarcity of good land.

"Any peace in Darfur must be built on solutions that go to the root causes of the conflict," he said.

The U.N. chief called for sustained economic development, possibly involving new irrigation and water storage techniques and efforts to improve health, education and roads.

"The international community needs to help organize these efforts, teaming with the Sudanese government as well as the international aid agencies and nongovernmental organizations working so heroically on the ground," Ban said.