The Carbon and Water Cycles

High on the side of a Hawaiian volcano, scientists have measured the carbon dioxide in the air since 1958. Each year it rises through winter and spring and falls through summer, while the whole record slowly climbs.

Closer to home, a big oak in a Virginia yard pulls water out of the soil on a summer day. That water leaves through its leaves as an invisible gas. Where do the carbon and the water go next? And why would air over a Pacific mountain change with the seasons?

Explore: watch the air fill and empty

In this simulation, carbon moves between four boxes: the air, plants and animals, the soil, and the ocean. Counters stand for carbon. You can let the months run, turn fossil fuel burning up or down, and cut or plant a forest. A line graph shows the carbon dioxide in the air. A side panel shows water moving from the soil, through a tree and into the air.

Run the months +1 month Run 10 years Reset

Fossil fuel burning: None

Forest Cut trees down Normal forest Plant more trees

1. 1 Photosynthesis: carbon goes from the air into plants.
2. 2 Respiration: carbon goes from plants and animals to the air.
3. 3 Decomposition: dead things and wastes go to the soil, and decomposers return carbon to the air.
4. 4 Combustion: burning fuel puts old carbon from underground into the air.
5. 5 Ocean: it takes in some carbon from the air and gives some back.

Press “Run the months”. Counters stand for carbon moving between four boxes.

Carbon in the air box over the years (relative units, not real amounts)

In this sketch, over the last 12 months the air box was fullest at the

Run a full year to see

In this sketch, over the last 12 months the air box was emptiest at the

Run a full year to see

In this sketch, this year’s average air carbon, compared with last year’s, is

Run two years to see

Water side panel: soil, trees and air

In this sketch, leaves are off. Very little water vapor leaves the trees.

In this sketch, one forest that drops its leaves in fall stands for the land plants of the Northern Hemisphere, where the lesson says most land plants are. One counter is one made-up chunk of carbon. Box sizes, the monthly rates, how much water vapor trees release in each month, the April-to-October window when leaves are out, the ocean’s share, the three burning levels, cutting to about 60 percent of the trees and planting up to about 140 percent are all made up. The yearly swing is drawn much bigger than the real one: the lesson’s data swing about 6 ppm out of more than 420 ppm. Carbon in rocks, and water shortage changing how much carbon plants take in, are left out.

Rule to keep: photosynthesis takes carbon out of the air, and respiration, decomposition and combustion put it back, so the air box empties in the northern summer, fills in winter, and climbs when burning adds old carbon faster than plants and the ocean take it up.

Try this.

1. Predict in which months the air box will hold the least carbon. Then let a full year run and check.
2. Predict what the line graph will look like over ten years if you turn burning up. Then test it.
3. Cut down the forest. Predict how the summer dip will change. Then watch the water panel too.

Explain: carbon moves between air, life, soil and ocean

Carbon is in the sugar, fat and protein of every living thing. Carbon atoms join in chains and rings that form the frame of these molecules.

The carbon cycle is the movement of carbon among the air, living things, the ocean and the soil. Carbon is also stored in rocks and in fossil fuels underground. Four key processes move carbon around the cycle.

• Photosynthesis is the process in which plants use radiant energy to make sugar (glucose) from carbon dioxide and water. It takes carbon out of the air.
• Respiration is the process in which cells break down sugar and release carbon dioxide and water. Plants, animals and decomposers all do it, day and night.
• Decomposition is the breakdown of dead organisms and wastes by decomposers such as bacteria and fungi. It returns carbon to the air and the soil.
• Combustion is burning. Burning coal, oil or natural gas releases carbon dioxide quickly.

carbon dioxide + water + radiant energy → sugar (glucose) + oxygen

Respiration runs the other way and releases energy from the sugar. You met both in Partners in Energy.

Coal, oil and natural gas formed from living things that died long ago. Their carbon stayed underground for millions of years. Burning them adds that old carbon to today’s air. The ocean also takes in some carbon dioxide at its surface and gives some back.

This cycle keeps carbon available to living things. When people add carbon to the air faster than plants and the ocean take it up, the amount in the air rises. Scientists link that rise to climate change, which can affect the health of ecosystems and which you will study in Big Changes (a later lesson).

Explain: water moves, and plants help move it

You learned the water cycle in grade 6. Here is a review, plus the part living things play.

• Evaporation is liquid water changing into water vapor, a gas. Heat from the Sun drives it.
• Condensation is water vapor cooling into tiny droplets that form clouds.
• Precipitation is water falling from clouds as rain, snow, sleet or hail.
• Infiltration is water soaking into the ground.
• Runoff is water flowing over the land into streams and rivers. Gravity pulls it downhill.
• Transpiration is the release of water vapor from plant leaves. Roots take water from the soil. It moves up the stem and exits through tiny openings in the leaves.

The water cycle makes fresh water available to every living thing. It also shapes the weather, because water vapor forms the clouds that bring rain. Moving water carries things too. Runoff from a Virginia field can carry soil, nutrients and carbon into a stream that flows to the Chesapeake Bay.

Explain: how the cycles connect

The two cycles meet inside living things.

Process	What happens to carbon	What happens to water
Photosynthesis	Taken out of the air	Taken from the soil
Respiration	Released to the air	Released
Transpiration	No change	Released to the air
Runoff	Carried to streams	Flows to streams

A dry summer means less water for plants. They close the tiny openings in their leaves to save water, so they take in less carbon dioxide. Nitrogen joins in too, as you will see in Nitrogen for Life.

Now think about Hawaii. Most of Earth’s land, and most land plants, are in the Northern Hemisphere. Each northern spring, leaves grow and photosynthesis speeds up. Each fall, leaves drop, but respiration and decomposition keep going. Air at Mauna Loa mixes with air from across the north.

One thing does not cycle. Matter is used again and again, but energy comes in as sunlight and leaves as heat. You saw that in Food Webs and Energy.

Elaborate: a year of air at Mauna Loa

Here is real data. Each value is the average carbon dioxide in clean, dry air for one month of 2025. The unit ppm means parts per million: 400 ppm is 400 carbon dioxide molecules in every million molecules of air.

Month (2025)	Carbon dioxide (ppm)	Month (2025)	Carbon dioxide (ppm)
January	426.65	July	427.87
February	427.09	August	425.48
March	428.15	September	424.37
April	429.64	October	424.87
May	430.51	November	426.46
June	429.61	December	427.49

Source: NOAA Global Monitoring Laboratory and Scripps Institution of Oceanography, monthly averages at Mauna Loa Observatory, Hawaii, about 3,400 meters up (file co2_mm_mlo.txt, created October 9, 2026). The same file gives 317.51 ppm for May 1958.

1. Which month had the highest value, and which had the lowest? How many ppm apart are they?
2. Describe the trend from May to September, then from September to December.
3. Compare May 2025 with May 1958. Why compare the same month, not May with September?

Design problem

Does light change how much water a plant releases through its leaves? Plan an investigation with potted plants.

One way to do it

• Question: Do plants in bright light lose more water by transpiration than plants in shade?
• Independent variable: light, either under a bright lamp or on a shaded shelf, with both places kept at the same temperature.
• Dependent variable: the loss in mass of each pot, in grams, after 48 hours.
• Constants: same kind and size of plant, same pot and soil, same watering, same room temperature.
• Control: a pot of wet soil with no plant in each place. It shows any water lost some other way.
• Setup: wrap the soil surface in plastic so water can leave only through the plant.
• Repeated trials: three plants in each place. Find the mean loss.
• Safety: use a common, nontoxic houseplant, wash your hands after handling soil, and wipe up spills.

If the plants in light lose more mass than those in shade, the data support the idea that light speeds up transpiration.

Evaluate: check your thinking

The air at Mauna Loa goes up and down because northern plants take in more carbon dioxide than is released in summer, and less in winter. Year after year, the record also climbs, mostly because burning fossil fuels adds old carbon. The oak’s water is now in the air and may form a cloud. Some of the carbon in your body can someday return to the air.

Science practices you used: reading a data table and finding a pattern, doing a subtraction with data, using a model and naming its limits, making a claim with evidence and reasoning, and planning a fair test with a control and repeated trials.