Out of total Oxygen present in the earth’s atmosphere, the estimated production of oxygen by Amazon forest through photosynthesis is

Correct — option (4), 20 percent.

20 percent, the figure in RPSC's key, is a widely repeated one. FactCheck.org reports that news outlets and politicians cited it during the Amazon fires of August 2019, and that one scientist had heard it for at least a decade.

French President Emmanuel Macron's tweet of 22 August 2019 called the Amazon "the lungs which produces 20% of our planet's oxygen".

Of the four options, 20 percent is that figure, and it is RPSC's key.

Scientists have challenged it. FactCheck.org (3 September 2019) reports estimates closer to 6 to 9 percent of the world's oxygen: around 6 percent or less from environmental scientist Jonathan Foley, and 9 percent from Oxford ecologist Yadvinder Malhi.

They add a second correction. The Amazon's plants respire and its microorganisms decompose leaf litter and dead wood, using up nearly all the oxygen the forest makes. Malhi calls its net contribution to the world's oxygen "effectively zero".

The idea to remember: 20 percent is the popular figure for the Amazon's oxygen; the 2019 scientific estimates are about 6 to 9 percent, with a net contribution near zero.

Why the others are wrong

• (1)
  40 percent — 40 percent is twice the popular 20 percent claim and several times the 6 to 9 percent that scientists estimated in 2019.

  The arithmetic behind the estimates rules it out: tropical forests do about a quarter to a little more than a third of land photosynthesis, and the Amazon alone does about 12 to 16 percent of all land photosynthesis, according to FactCheck.org.

• (2)
  50 percent — About half is the share FactCheck.org gives not to the Amazon but to the ocean, which accounts for about half of all photosynthesis.

  That leaves land plants with roughly the other half, and the Amazon is only one part of land vegetation. So the Amazon, one part of that land half, cannot account for 50 percent on these estimates.

• (3)
  70 percent — 70 percent is more than all land photosynthesis put together, since the ocean accounts for about half of all photosynthesis by FactCheck.org's account.

  The atmosphere's oxygen, around 21 percent of the air, was built up over millions of years, largely by phytoplankton in the sea, not by any present-day forest.

Plants release oxygen in photosynthesis and use oxygen in respiration. Microorganisms that break down dead leaves and wood use oxygen too. A forest's net effect on the air is what it produces minus what its plants and microbes consume.

For the Amazon, FactCheck.org reports that plants use about half or more of the oxygen they produce, and other creatures, mostly microorganisms, consume the remaining half or so, leaving a net contribution near zero.

The oxygen in the atmosphere came from photosynthetic organisms that died long ago and were buried before they could rot, a very small yearly surplus that added up over millions of years.

Key facts

• Emmanuel Macron's tweet of 22 August 2019 called the Amazon "the lungs which produces 20% of our planet's oxygen".
• FactCheck.org (3 September 2019): scientists estimate the Amazon's share of the world's oxygen at closer to 6 to 9 percent.
• Estimates quoted in 2019: Jonathan Foley, around 6 percent or less; Yadvinder Malhi, 9 percent.
• The Amazon does about 12 to 16 percent of land photosynthesis; the ocean accounts for about half of all photosynthesis (FactCheck.org).
• The Amazon consumes nearly all the oxygen it produces; Malhi describes its net contribution as "effectively zero".

Common traps

• A share of land photosynthesis is not a share of the world's oxygen. The Amazon's 12 to 16 percent of land photosynthesis becomes about 6 to 9 percent once ocean photosynthesis is counted.
• Gross production is not net contribution. The Amazon's plants and microbes use nearly all the oxygen the forest produces.
• "Lungs of the planet" is a label for the forest's importance, not a measurement of its oxygen output.

UnlockIAS will link similar questions from RAS Pre 2013 here once that paper is published on this site.

• practice — not a real PYQ

  According to FactCheck.org (September 2019), about what share of all photosynthesis on Earth does the ocean account for?

  • (a)About one-tenth
  • (b)About one-quarter
  • (c)About half
  • (d)About nine-tenths
  Answer(3) — FactCheck.org states that the ocean accounts for about half of all photosynthesis. Options (1) and (2) are too small, and option (4) would leave land plants, including all tropical forests, with only a tenth.
• practice — not a real PYQ

  Consider the following statements about the Amazon rainforest and oxygen : A. Its plants use part of the oxygen they produce in respiration. B. Microorganisms that decompose leaf litter and dead wood in the forest consume oxygen. C. The oxygen of the present atmosphere was built up largely by phytoplankton in the sea over millions of years. Which of the statements given above are correct ?

  • (a)A and B only
  • (b)B and C only
  • (c)A and C only
  • (d)A, B and C
  Answer(4) — FactCheck.org reports all three: Amazon plants respire much of the oxygen they make (A); other creatures, mostly microorganisms decomposing debris, consume the remaining half or so (B); atmospheric oxygen was largely built up by phytoplankton over millions of years (C). Options (1), (2) and (3) each omit a correct statement.