PARAUAPEBAS, Brazil — The Amazon has been viewed for ages as a vast quilt of rain forest interspersed by remote river outposts. But the surging population growth of cities in the jungle is turning that rural vision on its head and alarming scientists, as an array of new industrial projects transforms the Amazon into Brazil’s fastest-growing region.

The torrid expansion of rain forest cities is visible in places like Parauapebas, which has changed in a generation from an obscure frontier settlement with gold miners and gunfights to a sprawling urban area with an air-conditioned shopping mall, gated communities and a dealership selling Chevy pickup trucks.

Scientists are studying such developments and focusing on the demands on the resources of the Amazon, the world’s largest remaining area of tropical forest. Though Brazilian officials have historically viewed the colonization of the Amazon as a matter of national security — military rulers built roads to the forest under the slogan “Occupy it to avoid surrendering it” — deforestation in the region already ranks among the largest contributors to global greenhouse-gas emissions.

Brazil has shifted away from colonization, but policies that regularize land claims by squatters still lure migrants to the Amazon. And while the country has recently made progress in curbing deforestation, largely by enforcing logging laws and carving out protected forest areas, biologists and other climate researchers fear that the sharp increase in migration to cities in the Amazon, which now has a population approaching 25 million, could erode those gains.