Science · Environmental Science
Chapter 1: Environmental Systems and Science
Positive feedback is the destabilizing one.
In 2020, with much of the world in lockdown, fossil-fuel carbon dioxide emissions fell by about 5%, the largest one-year drop on record. At Mauna Loa in Hawaii, carbon dioxide in the air still rose, from about 412 to about 414 parts per million.
Emissions fell, so why did the amount in the air keep rising?
Make your guess to turn the page.
Still to come: 9 pages · Lab · Words to Know · What We Learned · 4 questions · Explain It in Writing · Open Questions
The idea this lesson is working on — Students picture steady state as stillness, and they expect a fall in emissions to mean a fall in the amount in the air. The book uses one equation for both: the stock changes by inflow minus outflow, so it rises while emissions fall as long as they stay above what the ocean and land take up, and it holds steady only while two busy flows match. Ask: what goes in, what comes out, and which is bigger? The idea to reach is that a stock follows the difference between its flows, not either flow alone.
The figures and where they come from — Mauna Loa yearly means are from NOAA’s Global Monitoring Laboratory, rounded. The budget of about 11 billion tons of carbon in and about 6 taken up by ocean and land is the rounded recent average of the Global Carbon Project; 1 ppm of carbon dioxide is about 2.12 billion tons of carbon. The 2020 drop in fossil emissions is usually given as 5 to 6%. The lab’s loop multiplies the departure by 1.55 or 0.45 each round; real feedbacks are not that tidy.
Systems respond with a delay — Effects can appear long after the cause and continue long after it stops. Lags make problems easy to underestimate while they are building and hard to reverse once they are visible, and they are a common cause of boom and bust in managed fisheries.