Human activities in the recent past have caused the increased concentration of carbon dioxide in the atmosphere, but a lot of it does not remain in the lower atmosphere because of 1) Its escape into the outer stratosphere. 2) The photosynthesis by phytoplankton in the oceans. 3) The trapping of air in the polar ice caps Which of the statements given above is/are correct?

Only statement 2 is correct. Phytoplankton in the surface ocean fix dissolved CO2 through photosynthesis, and their remains sink to deep water, which is the biological pump. Together with physical dissolution of CO2 into seawater, this makes the oceans one of the largest active carbon sinks, taking up roughly a quarter of human CO2 emissions each year.

• Statement 1 is wrong. CO2 is a long-lived gas that mixes through the troposphere and lower stratosphere. It does not escape in any meaningful amount into the outer stratosphere or to space, because Earth's gravity holds it.
• Statement 3 is wrong. Air trapped in polar ice is an archive, not a sink. Ice cores are how we reconstruct past CO2 levels, and the trapped volume is negligible compared with ocean and land sinks.

Concept: carbon sinks and the carbon cycle. For Mains GS-III (Environment and Climate Change), link this to ocean acidification, which is the price of the same ocean uptake, and to the limits of natural sinks in meeting emission targets.

Trap: Aspirants pick A because 'escape into the stratosphere' sounds like a natural removal process, or pick C because ice cores are the famous CO2 record. Neither is a sink.