05/22/2009 05:12 am ET Updated May 25, 2011

Use Energy, Get Rich And Save The Planet

The old wealth-is-bad IPAT theory may have made intuitive sense, but it didn't jibe with the data that has been analyzed since that first Earth Day. By the 1990s, researchers realized that graphs of environmental impact didn't produce a simple upward-sloping line as countries got richer. The line more often rose, flattened out and then reversed so that it sloped downward, forming the shape of a dome or an inverted U -- what's called a Kuznets curve. (See for an example.)

In dozens of studies, researchers identified Kuznets curves for a variety of environmental problems. There are exceptions to the trend, especially in countries with inept governments and poor systems of property rights, but in general, richer is eventually greener. As incomes go up, people often focus first on cleaning up their drinking water, and then later on air pollutants like sulfur dioxide.