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RAS question

In the carbon cycle, the largest reservoir of carbon is:

Correct answer: (D) Oceans.

In the carbon cycle, the oceans are the largest active carbon reservoir, storing about 38,000 GtC as dissolved carbon dioxide and carbonates.

  1. (A)

    Atmosphere

  2. (B)

    Forests

  3. (C)

    Fossil fuels

  4. (D)

    Oceans

Explanation

The answer is oceans because, among the active reservoirs named in the options, they store far more carbon than the atmosphere, forests or fossil fuels. The given figures put ocean carbon at about 38,000 GtC, mainly as dissolved carbon dioxide and carbonates. NOAA's carbon-cycle explanation supports the same idea: at human-relevant timescales, the atmosphere exchanges carbon with the terrestrial biosphere, oceans and fossil fuels, and the oceans store very large amounts of carbon because of their vast surface area and the solubility of carbon dioxide in water. The lithosphere contains more carbon overall, but it is not the relevant active reservoir here because its cycling is very slow.

Why the other options are wrong

  • (A) The atmosphere holds about 800 GtC, which is far below the ocean reservoir and is also described by NOAA as exchanging carbon with the oceans rather than exceeding them.
  • (B) Forests belong to the terrestrial biosphere, whose total carbon stock is about 2,000 GtC, much smaller than the ocean stock.
  • (C) Fossil fuels hold about 10,000 GtC, so they are significant but still well below the roughly 38,000 GtC stored in the oceans.

Concept

This tests the carbon-cycle distinction between active reservoirs and very slow geological stores. It recurs in RAS environment questions because climate-change basics often hinge on where carbon is stored, exchanged and released.

Source

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