Rainforests & Global Climate
Understand how rainforests affect climate. Growing trees use photosynthesis to absorb carbon dioxide, release oxygen and store carbon in wood. A forest is a carbon sink when it takes in more carbon than it releases. Burning or decay can return stored carbon as CO2, while clearing removes growing trees. Transpiration supplies moisture for rainfall, so losing trees can also change regional rain patterns.
What a learner can do afterwards
- Explain that growing trees absorb CO2 and store carbon, and that a forest is a carbon sink when it takes in more carbon than it releases over the same period
- Describe how burning or decay can release stored carbon as CO2, adding to warming, and how clearing also removes trees that would absorb CO2
- Explain that transpiration supplies moisture for rainfall, and that losing trees can change rain patterns without being the only possible cause
1 · Read
Growing trees use sunlight, water and carbon dioxide (CO2) to make food. This is photosynthesis, which also releases oxygen. Some carbon becomes wood in trunks, branches and roots. Plants, animals and decaying material also return CO2 to the air. Over the same period, a forest is a carbon sink if it takes in more carbon than it releases. A forest can hold much carbon without being a sink.
Clearing can affect CO2 even when no wood burns. Suppose growing trees are removed and no new trees replace them. The removed trees no longer take in CO2 through photosynthesis. Keeping their wood as furniture can store some carbon, but it does not keep the trees' CO2 uptake going. This loss of uptake is separate from carbon released by burning or decay.
Burning wood combines its carbon with oxygen from the air to form CO2. Some carbon can remain in charcoal or unburned wood. Cutting does not release all carbon at once: furniture can store it for years. Decay returns carbon to the air, usually more slowly than burning. CO2 absorbs and sends out some of Earth's outgoing heat. Adding CO2 slows heat loss to space and increases warming, separate from the fire's heat.
Burning can return stored carbon quickly. Decay can return it over time.
In the rainforest water cycle, roots take in water and leaves release water vapor. This release is transpiration. When moist air cools, water vapor can form tiny cloud droplets. Droplets can join together and fall as rain. With fewer trees, less water vapor enters the air through transpiration. Rainfall may decrease nearby or in places reached by that moist air.
Trees supply some of the moisture for rain. They do not control all rainfall.
Winds and other weather changes also affect rain. Less rain after forest clearing fits what we know about transpiration, but it does not prove clearing was the only cause. Compare rainfall over several years and check other changes before reaching that conclusion.
Growing trees store carbon and release water vapor. Burning or decay can return carbon as CO2. Losing trees can add to warming and change rainfall.
This lesson, practice and flashcards were written for this dataset. The forest carbon-balance figures are made-up practice values.
The readings and flashcards were written for this dataset. Fact references included CLEAN resource records, Khan Academy's carbon-cycle article and OpenStax Biology 2e. The linked video, What if there were 1 trillion more trees? - Jean-François Bastin, is a TED-Ed lesson. Its licensing terms have not been verified.
More to read
A carbon store and a carbon sink
A forest can contain a large store of carbon in its trees. That tells us how much carbon is held there. It does not tell us whether the store is growing or shrinking.
Carbon enters as plants take in carbon dioxide. Plants and animals return some carbon dioxide to the air as they use food. Decay returns carbon too.
A carbon sink takes in more carbon than it releases over the same period. A forest with a large store could still lose carbon during a year of heavy burning. To describe its carbon balance, compare what entered and what left during that year.
What age cannot tell us about a tree
Knowing which tree is older does not settle which one holds more carbon. Different kinds of trees grow to different sizes. Even trees of the same age can have different amounts of wood.
Wood contains carbon. For trees of the same kind, a greater mass of dry wood usually means more carbon stored in the wood. Dry mass leaves out the water, so a wetter piece does not appear to contain more carbon just because it holds more water.
Researchers use information about a tree's kind and size to estimate its carbon store. Age alone leaves out information they need.
Carbon in a wooden table
Imagine a tree is cut and some of its wood becomes a table. The table still contains carbon that the tree took from the air while growing. Cutting did not turn all that carbon into gas.
What happens next matters. A table kept for years can keep carbon stored for years. If wood burns, carbon combines with oxygen from the air and forms carbon dioxide. Some carbon can remain in charcoal or unburned wood. If wood rots, decay can return carbon to the air over time.
Clearing also removes growing trees. Those trees can no longer take in carbon dioxide through photosynthesis.
Why extra carbon dioxide adds warming
Earth's surface warms in sunlight and sends energy outward as invisible infrared radiation. Carbon dioxide absorbs some of this energy and sends energy out in all directions. Some travels back toward the surface. This slows heat loss to space.
The natural greenhouse effect keeps Earth's surface warmer. Adding more carbon dioxide increases greenhouse warming. Burning forest wood can add carbon dioxide that contains carbon once stored in trees.
Keeping forests standing helps protect their carbon stores. It does not cancel every other source of carbon dioxide. Reducing emissions from coal, oil and gas is needed too.
Moisture from the forest
Water taken in by roots can travel through a tree and pass from its leaves into the air as water vapor. This is transpiration. The vapor is a gas. When moist air cools, some vapor can become tiny liquid droplets in clouds. Droplets can join and fall as rain.
Air moves, so moisture released over a forest can help supply rain elsewhere. Removing many trees can reduce the moisture supplied by transpiration and change rainfall patterns.
Other changes matter too, including winds. If farms report less rain after clearing, the tree loss is a possible cause. The report alone does not show that no other cause contributed.
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