CSIR NET Dec 2015 Q121: Meteorology

Although the total quantity of carbon stored in other Earth reservoirs (oceans, rocks/carbonates, biomass, soils) vastly exceeds the carbon present in the atmosphere, it is specifically the carbon present as a gaseous compound (CO2) in the atmosphere that directly participates in the greenhouse effect, absorbing and re-emitting outgoing longwave (infrared) radiation and thereby influencing global radiative forcing and climate. Carbon locked in solid rock (carbonates), dissolved in the deep ocean, or bound in organic matter does not directly interact with radiation in the same way and does not contribute to the greenhouse effect unless and until it is converted to and released as atmospheric CO2 gas. Because it is specifically this atmospheric gaseous carbon pool -- relatively small in mass but radiatively critical -- that governs the greenhouse effect, even a modest annual increase (a few ppm/year) in this particular reservoir is of grave concern, regardless of how large the other, radiatively inactive carbon reservoirs are. This matches option 2.

Option 1 (oceans are not a carbon sink) is factually incorrect, since oceans are in fact a major carbon sink, absorbing a substantial fraction of anthropogenic CO2 emissions, so this cannot be the reason for concern. Option 2 is correct: it is the atmospheric, gaseous form of carbon that is radiatively active and directly drives the greenhouse effect, making its increase disproportionately significant compared to carbon held in other, radiatively inactive reservoirs. Option 3 (no uptake of CO2 by other Earth system components once emitted) is inaccurate, since a substantial fraction of emitted CO2 is indeed taken up by oceans and the terrestrial biosphere over time, even though uptake is incomplete and occurs on varying timescales. Option 4 (CO2 is a reactant in several atmospheric chemical reactions) describes a secondary chemical role of CO2 but does not directly explain why the greenhouse/climatic concern is specifically tied to its atmospheric gaseous abundance relative to other, far larger, non-atmospheric carbon reservoirs.