Environmental economics is a sub-field of economics concerned with environmental issues. It has become a widely studied subject due to growing environmental concerns in the twenty-first century. 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 human-made ("physical") capital can substitute for natural capital. History The modern field of environmental economics has been traced to the 1960s. with significant contribution from Post-Keynesian economist Paul Davidson (economist) who had just completed a management position with the Continental Oil Company. Topics and concepts Market failure Central to environmental economics is the concept of market failure. Market failure means that markets fail to allocate resources efficiently. As stated by Hanley, Shogren, and White (2007): "A market failure occurs when the market does not allocate scarce resources to generate the greatest social welfare. A wedge exists between what a private person does given market prices and what society might want him or her to do to protect the environment. Such a wedge implies wastefulness or economic inefficiency; resources can be reallocated to make at least one person better off without making anyone else worse off." This results in a inefficient market that needs to be corrected through avenues such as government intervention. Common forms of market failure include externalities, non-excludability and non-rivalry. Externality An externality exists when a person makes a choice that affects other people in a way that is not accounted for in the market price. An externality can be positive or negative but is usually associated with negative externalities in environmental economics. For instance, water seepage in residential buildings occurring in upper floors affect the lower floors. Another example concerns how the sale of Amazon timber disregards the amount of carbon dioxide released in the cutting. Or a firm emitting pollution will typically not take into account the costs that its pollution imposes on others. As a result, pollution