This dissertation is an ethnographic examination of an environmental policy known as REDD+ (Reducing Emissions from Deforestation and Forest Degradation-the "+" signifies improved carbon stocks). REDD+ was designed to lessen climate change by reducing deforestation in Amazonia-a goal that would be achieved by paying landholders to keep their forests standing. This proposal has been highly influential at United Nations (UN) environmental forums. I study REDD+ as a scheme that, in the region of Amazonia in which I worked, reflects peasant and scientific understandings of tropical forests. REDD+ proposals engage forests as humanized spaces long transformed by global capitalist dynamics and that will be further transformed by purportedly unavoidable socio-environmental crises. I claim that this policy marks a profound shift in the ecological imagination. Environmental debates at the UN have often portrayed Amazonia as "pristine Nature"-a non-human realm that experts believed could be made into an inside in which they could further contemporary modes of human living. In contrast, I argue that REDD+ schemes engage Amazonia as a human-shaped space of intensifying environmental crises that experts cannot bring under control. In chapter one I explore the links between REDD+ proposals and midtwentieth-century development projects in Brazil, focusing on how developmental planners and REDD+ proponents both assumed that Amazonia's transformations by the forces of industrial capitalism were unavoidable. In chapter two I examine descriptions of REDD+ offered by Amazonian peasants. They saw this policy as yet another instance of economic forces that challenges poor peasants to establish particularly hostile relations with humans and non-humans alike. I examine the scientific practice of pro-REDD+ scientists in chapter three. I show that Amazonbased environmental scientists investigate Amazonia as a shifting socio-natural situation that will continue unraveling in the foreseeable future. Chapter four studies REDD+ contributions to UN negotiations that climate diplomats themselves see as insufficient to avoid potentially catastrophic climate change. My multi-sited approach advances the understanding of contemporary environmental politics by examining REDD+ as radical philosophical-anthropological proposition: that humans should learn to endure the worlds they have made into precarious spaces.