Quantify how industrial nitrogen fixation and fossil fuel combustion have altered the natural nitrogen and carbon cycles, and connect these changes to eutrophication and ocean chemistry previews.
Humans have roughly doubled the amount of nitrogen actively cycling through the environment, mainly through the industrial Haber-Bosch process, which manufactures synthetic fertilizer and is credited with helping sustain roughly half of current world food production -- a genuinely enormous humanitarian benefit that comes with a real environmental cost when excess fertilizer runs off farmland uncaptured. Fossil fuel combustion and large-scale deforestation have driven atmospheric CO2 from about 280 parts per million (ppm) before industrialization to more than 420 ppm today, a direct, continuously measured record (the "Keeling Curve," measured atop Mauna Loa in Hawaii since 1958) rather than an estimate or a model.
These two changes ripple outward into problems this course returns to in detail: excess nitrogen and phosphorus running off farmland into waterways feeds algae blooms that can deplete oxygen and kill fish (eutrophication, covered in Unit 7); and the added atmospheric CO2 both traps extra heat (the enhanced greenhouse effect, covered in Unit 9) and dissolves into the ocean, forming carbonic acid and slowly lowering ocean pH (ocean acidification, covered in Unit 7). This week is a bridge: the same two cycles you just learned, now measurably altered by human activity.
Mastery looks like: They can calculate a percent change in atmospheric CO2 from given values and explain how excess nitrogen/phosphorus and excess CO2 each set up a specific downstream environmental problem covered later in the course.
Common stumbles: Treating 50% as a small or unimportant change; confusing the yearly zig-zag in the Keeling Curve with evidence against the longer-term rising trend.