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Difficulty: HardClimate Change, Global Warming and Ozone Layer Depletion

Methane (CH4CH_4) plays a critical role in radiative forcing and atmospheric chemistry. Which of the following statements accurately describes the primary atmospheric sink of methane and its indirect contribution to global warming?

  1. It is primarily removed by reaction with hydroxyl radicals (OHOH) in the troposphere, indirectly enhancing warming through the production of stratospheric water vapor and tropospheric ozone.Answer
  2. B
    It is primarily absorbed by deep oceans via solubility carbon pumps, directly causing stratospheric ozone depletion through catalytic bromine release.
  3. C
    It is primarily removed by direct UV photolysis in the mesosphere, leading to net cooling of the lower troposphere through cloud condensation enhancement.
  4. D
    It is primarily converted into nitrous oxide (N2ON_2O) by soil microbial action, reducing overall radiative forcing while increasing thermal light scattering.

Answer

Methane is primarily removed by reaction with hydroxyl radicals (OHOH) in the troposphere, indirectly enhancing warming through the production of stratospheric water vapor and tropospheric ozone.
The dominant sink for atmospheric methane (CH4CH_4) is oxidation by hydroxyl radicals (OHOH) in the troposphere. This process accounts for roughly 90% of methane removal. Additionally, the oxidation of methane leads to indirect global warming by contributing to the formation of tropospheric ozone (O3O_3) and increasing stratospheric water vapor (H2OH_2O), both of which trap outbound terrestrial infrared radiation.

Step-by-Step Solution

1
Identify the primary atmospheric sink of methane (CH4CH_4).
Approximately 90% of atmospheric methane is removed in the troposphere via chemical oxidation by hydroxyl radicals (OHOH).
Hydroxyl radicals act as the primary chemical scavenger in the troposphere for methane and other trace volatile organic compounds.
2
Analyze the indirect climate impacts of tropospheric methane oxidation.
The oxidation sequence produces formaldehyde, carbon monoxide, tropospheric ozone (O3O_3), and brings water vapor (H2OH_2O) into the dry stratosphere.
Both tropospheric ozone and stratospheric water vapor are powerful greenhouse gases that increase positive radiative forcing beyond methane's direct absorption bandwidth.

Key Concept

Atmospheric Methane Sinks and Indirect Radiative Forcing
Estimated Time:1m 30s
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Topics

Climate Change, Global Warming and Ozone Layer Depletion