Environmental economics is a sub-field of economics concerned with environmental issues. It has become a widely studied topic due to growing environmental concerns in the twenty-first century. Quoting from the National Bureau of Economic Research Environmental Economics program: ... Environmental Economics ... undertakes theoretical or empirical studies of the economic effects of national or local environmental policies around the world ... . Particular issues include the costs and benefits of alternative environmental policies to deal with air pollution, water quality, toxic substances, solid waste, and global warming. Environmental economics is distinguished from ecological economics in that ecological economics emphasizes the economy as a subsystem of the ecosystem with its focus upon preserving natural capital. One survey of German economists found that ecological and environmental economics are different schools of economic thought, with ecological economists emphasizing "strong" sustainability and rejecting the proposition that natural capital can be substituted by human-made capital.