Ecoscience: Population, Resources, Environment

By John P. Holdren; Paul R. Ehrlich et al. | Go to book overview

CHAPTER 2
The Physical World

All the rivers run into the sea; yet the sea is not full.

--Ecclesiastes

The biosphere is that part of Earth where life exists. In vertical dimension it extends from the deepest trenches in the ocean floor, more than 11,000 meters (36,000 feet)1 below sea level, to at least 10,000 meters (m) above sea level, where spores (reproductive cells) of bacteria and fungi can be found floating free in the atmosphere. By far most living things -- most of which depend directly or indirectly on the capture of solar energy by photosynthesis in plants and certain bacteria -- exist in the narrower region extending from the limit of penetration of sunlight in the clearest oceans, less than 200 meters from the surface, to the highest value of the permanent snow line in tropical and subtropical mountain ranges -- about 6000 meters, or 20,000 feet. ( Everest, the highest mountain, rises almost 8900 meters above sea level.) By any definition, the biosphere is as a mere film in thickness compared to the size of the ball of rock on which it sits -- about like the skin of an apple, in fact. The radius of Earth is about 6370 kilometers (km), or 4000 miles (mi).

Of course, conditions within the thin envelope of the biosphere are influenced by physical processes taking place far outside it: by the energy emitted by the sun, 150

____________________
1
Throughout this book physical dimensions are given in metric units, sometimes accompanied by the English equivalent to ease the transition for readers not completely accustomed to the metric system. For more precise conversion factors, see the tables inside the covers of the book.

-11-

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