Carbon Cycle
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40 free Carbon Cycle trivia questions with answers — science & nature quiz, new questions added Oct 2026.
Every carbon atom in your body has been somewhere else first: in the air, in a leaf, in the sea, perhaps shut inside a rock for millions of years. The carbon cycle is the story of that journey. It links the smallest green cell to whole mountain ranges, and it explains why burning fossil fuels changes the climate of the whole planet. This quiz follows carbon through each stop on the route. Expect questions on how plants and animals trade carbon with the air, where the planet keeps its biggest stores, how oceans and rocks lock carbon away over long ages, and what scientists have measured as human activity tips the balance. A few questions look at the people and experiments behind the science. It is written for middle school and high school students revising the topic, for teachers who want a ready-made multiple choice set, and for anyone curious about how Earth recycles the element life is built on. The early questions are the gentlest, and every answer comes with a short explanation.
30 of 40 questions with answers and explanations. Play the quiz
Q 01Where was carbon, the fourth most abundant element in the Universe, first forged before any of it reached Earth?
Inside stars
Inside stars, helium nuclei fuse into carbon through the triple-alpha process, which runs only in giant and supergiant stars. NASA calls carbon the backbone of life on Earth.
Q 02Which British Isles landmark is built from the skeletons of tiny algae that locked carbon away in an ancient sea?
White Cliffs of Dover
The White Cliffs of Dover are chalk, laid down as white mud on a Late Cretaceous sea floor at about half a millimetre a year. They now stand up to 110 metres tall.
Q 03Which planet's carbon dioxide atmosphere traps so much heat that the surface is hot enough to melt lead?
Venus
Venus has an atmosphere that is 96.5% carbon dioxide, which holds its surface near 467 °C, hotter even than Mercury. On Earth the carbon cycle keeps most carbon out of the air.
Q 04Which fuel, cut from bogs and dried in rural Ireland, holds more carbon worldwide than all forests combined?
Peat
Peat stores up to 550 billion tonnes of carbon while covering just 3% of the land surface. It builds up where waterlogged ground stops dead plants from decaying fully.
Q 05Which thawing Arctic layer holds about twice as much carbon as the entire atmosphere, locked in plant remains?
Permafrost
Permafrost in the far north holds an estimated 1,400 to 1,650 billion tonnes of carbon, built up over thousands of years. As it thaws, microbes turn part of that store into greenhouse gases.
Q 06Which gemstone is pure carbon, crystallised under enormous pressure 150 to 250 km down in Earth's mantle?
Diamond
Diamond finds at Juina in Brazil showed that the carbon cycle reaches the lower mantle, because the stones held traces of melted ocean crust. A few come from 800 km down.
Q 07Which fossil fuel formed from swamp forests that covered the tropics about 300 million years ago?
Coal
Coal gave the Carboniferous period its name, which means 'coal-bearing'. About 90% of all coal beds were laid down in that period and the Permian that followed.
Q 08Which gas, given off by cattle, rice paddies and swamps, traps far more heat per molecule than carbon dioxide?
Methane
Methane has around 80 times the warming power of carbon dioxide over 20 years, yet it lasts less than 12 years in the atmosphere. It is also the main ingredient of natural gas.
Q 09Unlike plants, which take in carbon dioxide, animals get nearly all the carbon in their bodies from what source?
The food they eat
The food they eat carries carbon that a plant or alga first captured from the air. Organisms that must get their carbon by consuming others are called heterotrophs.
Q 10Which ocean drifters capture about as much carbon dioxide as all land plants, despite being 1% of plant biomass?
Phytoplankton
Phytoplankton account for about half of all the carbon captured by living things each year. Warmer seas could cut their numbers, because they grow best in cool, nutrient-rich water.
Q 11Which process, powered by sunlight, removes more carbon dioxide from Earth's atmosphere than any other?
Photosynthesis
Photosynthesis drives the fast carbon cycle: plants and algae use the Sun's energy to combine carbon dioxide with water into sugars. That cycle shifts between 1,000 and 100,000 million tonnes of carbon every year.
Q 12Which process in the cells of animals and plants breaks down sugar for energy and returns CO2 to the air?
Respiration
Respiration runs in plants as well as animals, so a forest gives back part of the carbon it takes in. Chemically it is the sugar-building reaction of green leaves in reverse.
Q 13Which group of organisms, including fungi and bacteria, returns the carbon in dead leaves and animals to the air?
Q 21What do scientists call a forest, ocean or other store that takes in more carbon than it gives back?
A sink
A sink on land and another in the ocean each take up about a quarter of the carbon that human activity emits every year. A store that gives out more than it takes in is a source.
Q 22Which sedimentary rock, built largely from sea creatures' shells, holds about 80% of the carbon in Earth's rocks?
Limestone
Limestone can keep its carbon locked away for hundreds of millions of years. Heat and pressure deep underground turn it into marble, which NASA calls its metamorphic cousin.
Q 23Which landforms, the slow carbon cycle's main route back to the air, emit over 100 times less CO2 than fossil fuels?
Volcanoes
Decomposers
Decomposers such as fungi feed by releasing enzymes onto dead matter and absorbing what dissolves, a habit called external digestion. Fungi lead the work on leaf litter in many ecosystems.
Q 14Which chemical reaction do a campfire, a car engine and a forest fire share, turning stored carbon into CO2?
Combustion
Combustion of fossil fuels is the main reason carbon dioxide in the atmosphere is rising. It releases in moments carbon that took millions of years to accumulate underground.
Q 15Which gas do plants release as a waste product when they build carbon dioxide and water into sugar?
Oxygen
Oxygen given off by plants comes from the water they split, not from the carbon dioxide they absorb. Robert Hill's experiments on isolated chloroplasts in 1937 and 1939 pointed to water as the source.
Q 16Which simple sugar, with six carbon atoms in every molecule, do green plants build from carbon dioxide and water?
Glucose
Glucose is the most abundant simple sugar, and plants chain it into cellulose for their cell walls. That makes cellulose the most abundant carbohydrate in the world.
Q 17Through which tiny pores, found mostly on the undersides of leaves, does carbon dioxide enter a plant?
Stomata
Stomata sit only on the lower leaf surface in most tree species. Each pore is bordered by a pair of guard cells that control how wide it opens.
Q 18What name do ecologists give plants and algae, the organisms that first capture carbon at the base of an ecosystem?
Producers
Producers are also called autotrophs, because they need no living source of carbon or energy. They form the lowest trophic level of an ecosystem.
Q 19Which of Earth's spheres holds most of its carbon, some of it locked away for hundreds of millions of years?
The lithosphere
The lithosphere, Earth's rocky outer shell, holds more carbon than the oceans, air and living things put together. Some of that store is the buried remains of ancient life.
Q 20Which land store holds about 1,500 gigatons of carbon, roughly three times as much as all plants above ground?
Soil
Soil can hold its carbon for thousands of years before erosion washes it away or microbes breathe it back into the air. Warmer ground speeds up that release.
Volcanoes emit between 130 and 380 million tonnes of carbon dioxide a year. Their fresh lava then gives the slow carbon cycle new rock to work on.
Q 24Which change to seawater, nicknamed the evil twin of global warming, is caused by the ocean absorbing CO2?
Acidification
Acidification has lowered the pH of the ocean surface by 0.1 since 1750, a 30 percent change. Corals, crabs and snails then struggle to build their shells and skeletons.
Q 25Carbon dioxide warms the planet by absorbing which kind of radiation given off by Earth's land and oceans?
Infrared
Infrared energy soaked up by greenhouse gases is sent back out in all directions, and some of it returns to heat the surface. Without that effect Earth would be a frozen -18 °C.
Q 26Extra CO2 speeds plant growth only until plants run short of water or which nutrient, a staple of fertiliser?
Nitrogen
Nitrogen sets a ceiling on carbon fertilisation, the extra growth plants manage in air richer in carbon dioxide. The nitrogen cycle is another of Earth's major biogeochemical cycles.
Q 27From which natural archive do scientists read 800,000 years of past CO2 levels, sealed in tiny air bubbles?
Ice cores
Ice cores from Antarctica show that carbon dioxide and temperature rose and fell together over the past 800,000 years. The deepest drill holes reach more than two miles down.
Q 28Since which era, when people first burned fossil fuels on a large scale, has CO2 in the atmosphere kept climbing?
The Industrial Revolution
The Industrial Revolution began moving carbon out of the slow cycle and into the fast one. Left alone, NASA notes, that carbon would have leaked out over millions of years.
Q 29Which floating fern may have cooled Earth 49 million years ago by blooming across the Arctic Ocean and burying carbon?
Azolla
Azolla settled on the stagnant sea floor for about 800,000 years, and the carbon it buried may have helped end a hothouse era when palm trees grew at the poles.
Q 30On which Hawaiian mountain has an observatory tracked the rise of carbon dioxide in the air since 1958?
Mauna Loa
Mauna Loa was chosen for its remote location far from continents and its lack of vegetation. Its record is the foundational study of greenhouse gases in the atmosphere.