Our joint statement suggests three main avenues of effort for the scientific and technological community in supporting human needs, maintaining the environment, and moving toward sustainable human consumption patterns. I want to speak on the second of these-the need to actively generate new knowledge. To do so, we need to change science itself, to go beyond what we already know and expand the world's capacity system for discovering new things.

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A bricf rcvicw is given of the concepts age distribution, transit time distribution, turn-over time, average age and average transit time (residcnce time) and their relations. The characteristics of natural reservoirs are discusscd in terms of these concepts, and a classification is proposed based on whether the average age is larger, equal t o or smaller than tho averagc transit time. Somc examples illustrate the diffcrcnces betwcen thcse various cases.

A bricf rcvicw is given of the concepts age distribution, transit time distribution, turn-over time, average age and average transit time (residcnce time) and their relations. The characteristics of natural reservoirs are discusscd in terms of these concepts, and a classification is proposed based on whether the average age is larger, equal t o or smaller than tho averagc transit time. Somc examples illustrate the diffcrcnces betwcen thcse various cases.