Jose Maria Figueres Olsen Former President Republic of Costa Rica The heated debate about global climate change continues. Some say it is the gravest calamity our species has ever encountered. Others deny its existence altogether. As with most caseS of human decision making, the truth is most likely somewhere in the middle. The challenge of this particular set of decisions is the overwhelming sense of uncertainty. Science cannot fully attribute the climatic catastrophes occurring before our eyes to increasing levels of greenhouse gas concentrations. Neither can Science prove that extreme events and warming trends are unrelated to human behavior. Economic models, sophisticated as they are, cannot agree on the costs of reducing carbon dioxide (C~) emissions in industrialized countries. International negotiations are thus mired in the morass of scientific and economic uncertainty. The are only two elements of certainty in the whole debate. The frrst is the need for precaution. The potential impacts are such, that the risk of inaction is unaffordable to the human race. Under the current state of knowledge, mankind must take cautious but unequivocal steps to reverse current patterns.