CoachA biogeochemical cycle is the movement of an element between reservoirs (the atmosphere, the ocean, soils, living biomass, rocks) by fluxes measured in mass per year. A reservoir of mass with total outflow has a residence time , the average time an atom spends in it; at steady state inflow equals outflow and is constant. The cycles are coupled through the stoichiometry of life — the Redfield ratio of marine plankton, by atoms — so that the supply of the scarcest element sets the rate at which all the others are taken up.
Examples
Example 25.6 (What the numbers say)
Emissions of a year with an airborne fraction of leave , or , in the air each year — the observed slope. Cumulative emissions since 1850 are about , of which is in the air (140 ppm), the rest in ocean and land. To hold the atmosphere at any level, net emissions must fall to what the slow sinks — deep-ocean mixing and weathering, together under a year — can take: net zero is not a slogan but the box model’s steady-state condition with a very long . For nitrogen, the doubling of the fixed-nitrogen flux has doubled the nitrate in rivers, produced dead zones at the mouths of the Mississippi and the Baltic, and raised the atmosphere’s , a greenhouse gas with a residence time of a century, by a fifth. Chapter 27 follows the consequences for living things.