We need not become earth scientists, but we do need to be conversant with the earth’s bio-physical systems that are transformed by human behavior.

Scientists divide the earth into four broad "spheres":

Lithosphere: the solid (rock) earth

Hydrosphere: water in various forms (ice, liquid and vapor) (1.5 million trillion m/t)

Atmosphere: air and weather (5.1 t m/t)

Biosphere: living matter (species) and chemical constituents (8 t m/t)

The biospshere is typically subdivided into "ecosystems" variously defined (forests, grasslands, etc.) to direct attention to an interacting web of biotic and abiotic elements in which life forms (species) are embedded.

Our approach to assessing the human role in environment will take up several "cycles" and subsystems of the earth’s spheres:

Solar radiation is source of most energy that does work in the earth systems (though geothermal energy caused by radioactive decay causes longer term work like some geological processes).

The basic paths are : solar radiation input; some reflected/scattered by the atmosphere, some reflected by the earth’s surface; absorption at the surface (land and water) to become heat, and the use of that energy to do work: like create atmospheric circulation, the hydrological cycle, and to power ecosystems (thru the bio-geochemical cycles). (see fig. 5.6). This comes up again in our look at global warming.

Cycles of matter in Ecosystem (pleas note that I will refer to these as the bio-geochemical cycles---the ones we examine can also be called nutrient cycles). We will examine only two of these offered in the text: the carbon and nitrogen cycles.

Carbon Cycle (See Fig. 5.9):

We’ll need to know about the carbon cycle in future chapters, and it is one of the most obvious and controversial points in the whole earth system in terms of human transformations (e.g., global warming, etc.).

We’ll start with where the carbon rests in the system. There are two large storages of carbon in the earth system:

The key fluxes (movement of carbon) between these two large storages are:

From air to biosphere and then to lithosphere (203 billion tons / yr):

Other major source is burning of carbon, either of biomass or fossil fuels. Some of this is "natural", but a net and continuing increase ion this flux is a result of human activity, direct and indirect.

We’ll continue looking at bio-geochemical cycles next week.