A more substantive precautionary principle of international law is evolving as new treaties articulate new measures of precaution in different contexts. Although there is considerable controversy over how to articulate or define a precautionary "principle" of law, the goal is to ensure that the mere lack of scientific knowledge about risks cannot justify a failure to take appropriate precautions. Where we have sufficient evidence of risk, we often take precautions, despite a lack of certainty about those risks.

In a recent article reviewing the U.S. Supreme Court's environmental decisions over the last 30 years (1969-1999), Professor Richard Lazarus argues that "the Justices have never fully appreciated environmental law as a distinct area of law."¹

A recent headline on the front page of the Wall Street Journal hailed the opening of the nation's first "butterfly bank." The "deposits" in this unusual bank are conservation credits earned by preserving an important area of habitat for the Quino checkerspot butterfly, an endangered species restricted to California. The bank's intended customers are other landowners who hope to develop other sites where the butterfly occurs. In order to do so, they can buy credits from the private entrepreneur who established the butterfly bank.

The focus of much dialogue and debate in the public eye over climate change and greenhouse gas emissions (GHGs) tends to focus on industrial emissions of pollution for manufacturing or the production of electricity. Emissions from transportation sources (like trains, planes, and automobiles) and from the heating, cooling, and lighting of buildings themselves are less readily visible, yet each constitutes roughly a third of America's total greenhouse gas emissions.

Editors' Summary