— Rare minerals. Food and water. Arable soil. Air-cleansing forests.

In the intellectual heart of the American military and policy-making world, these are emerging not just as environmental issues, but as the potential stuff of conflict in the 21st century.

In some ways, the role of resources in shaping conflict is nothing new. Much as the Spanish conquistadors sought gold, Saddam Hussein fought for Kuwait ’s oil. And downstream lands have long worried that neighbors will limit water flowing in the Nile, Euphrates and Jordan .

Now a new field of systematic study is opening within research centers, the Pentagon and intelligence institutions. It assumes that the 21st century will be shaped not just by competitive economic growth, but also by potentially disruptive scarcities — depletion of minerals; desertification of land; pollution or overuse of water; weather changes that kill fish and farms.

National security experts have begun to label such factors threats to “natural security” and to study them, often alongside environmental or advocacy groups. A basic question frames their thinking: What are the new relationships among resources, diplomacy, crisis and conflict?

One hint of the complexity can be seen along the Afghanistan - Pakistan border. A war against terrorists seems, at first, to have little to do with resources. But look again: This fall, outraged Pakistani gangs at the border were torching fuel trucks that carry supplies vital to allied troops. And at the end of that supply line, American planning was a small step ahead — a few Marines were trying out new-tech solar collectors, to cut their dependence on diesel fuel.

Strategists also knew that there were other stakes in the outcome. Afghanistan has been seen for decades as a potential route for pipelines carrying Central Asian gas — but only if there is peace. And underground lie huge deposits of riches like lithium , which is crucial to batteries for electronic devices. China in particular has a large interest in those.

The National Intelligence Council has a major effort under way to analyze threats from water and food shortages related to climate change and other environmental causes, one senior American intelligence official said.

National security officials are careful to say that resources are only one factor in the development of conflicts. How politics and diplomacy shape relationships is, in the end, decisive. “But these can be a trigger of instability,” said one senior American intelligence official, whose job forbids him to speak for attribution. “They cause the magnification of other issues.”

Last summer, for example, flooding that uprooted millions of Pakistanis stirred fear in Washington that militant movements might prove more adept than Pakistan’s weak government at delivering aid. Last month, the advocacy group Refugees International reported that the Pakistani Army did respond effectively, but that civilian authorities struggled and American and international agencies should be better prepared for climate-related calamities.

Senior military and intelligence officials who focus on “natural security” issues say the Obama administration has shown a greater understanding of the subtleties than its predecessor, in part because it does not challenge basic assumptions about the harmful impact of climate change.

“But the issues are so complex, the number of actors and the uncertainties are high,” the senior American intelligence officer said. “So the analysis is still speculative. It makes planning much harder and preventive action much harder.”

The natural security risks take many forms. Here are five scenarios, current or anticipated.