Lesson 5 of 6 · Carbon & other material cycles
Count the stores and the transfers
8 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish a material stock from its flows and explain how changed transfer rates connect air, water, rock, and life.
Follow a linked name to explore its archive card. “Back to your lesson” brings you to the same part of the reading.
Words with a dotted underline open a short definition right here in the lesson.
A reservoir is not a rate
Carbon is present in living organisms, soils, the atmosphere, oceans, and rocks. A is a store of material; its stock is the amount present at a chosen time. A describes a transfer rate, such as a mass of carbon per year. Comparing a stock in tonnes with a flux in tonnes per year as though they were the same quantity hides an essential difference.
A reservoir’s size changes when its total incoming and outgoing transfers differ. A large store can have a small annual change even while large amounts enter and leave. Conversely, a relatively small store can change rapidly if its flows change. Always state the boundary and interval: a forest’s growing trunks, the whole forest , and the global land are different systems with different accounts.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Follow a carbon atom through different clocks
Photosynthesis transfers carbon from carbon dioxide into organic matter. Respiration, decomposition, and burning can return carbon to the atmosphere. Carbon dioxide also exchanges between the atmosphere and ocean, where it participates in several dissolved chemical forms. Some carbon moves through organisms quickly; some remains in soils, deep water, sediments, or rocks much longer. A cycle does not imply that every atom takes the same route or returns on schedule.
Geological processes connect these faster exchanges to longer storage and release. Burning fossil fuels transfers carbon from long-stored deposits into the active atmosphere–ocean–land system. Human activities have increased atmospheric carbon dioxide because additional releases have exceeded the additional uptake. Natural exchanges still occur. Their existence does not cancel the effect of a changed balance, just as money entering and leaving an account can coexist with a growing debt.
Work through a decision
Check an invented carbon budget
Imagine a clearly bounded land system containing 1,000 units of carbon at the beginning of a year. During that year, photosynthesis brings in 80 units, while respiration, decomposition, and exports together remove 75. With all relevant transfers included, the stock ends at 1,005 units. The net gain is five units; the incoming flow is 80 units per year. They describe different aspects of the same account.
If the following year brings in 70 and loses 75, the stock falls by five even though photosynthesis continues. The exercise uses invented equal-sized units and fixed annual totals, not a forecast of a real forest. Before assigning climate benefits to a real project, examine its full boundary, measurement uncertainty, time period, and how durable the stored carbon is. Growth alone does not report every transfer.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Other elements connect the same landscape
Nitrogen and phosphorus also move among organisms, water, soil, and rock, but their chemical pathways differ from carbon’s. Nutrients enable growth; their location and available chemical form matter. Runoff can carry material from land into a lake or coastal water. Too much nutrient delivery can contribute to excessive growth and later oxygen depletion, linking a land-management decision with an aquatic response downstream.
A river photograph shows routes through a landscape, but not the mass or chemical forms of everything moving in the water. Testing a material budget needs concentration measurements together with water flow and time. Explain why a large reservoir alone does not identify a large yearly source. Then trace one transfer across two Earth systems and name the observations needed to measure its rate rather than simply assume it.
An orbital photograph of the Nura River mouth at Lake Tengiz, Kazakhstan, preserved from the NASA image held in the archive. The delivery image is resized and compressed without adding a chemical color scale. The branching landscape shows possible transport routes; these colors are not measurements of carbon or nutrient concentration.
Look closely
Trace a channel toward the lake. What two kinds of measurement would help estimate how much dissolved material moves along that route during a month?
See what to notice
Combine material concentration with the volume of water flowing through over time, using compatible units and representative sampling. Channel appearance alone does not supply either total. Both concentration and flow can vary during the month, so a useful estimate must account for that variation.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.