Only about 45% of the total CO2 emitted from fossil fuel burning and land use change stayed in the atmosphere on average during the past few decades. The remaining CO2 was taken up by the carbon reservoirs (the ‘CO2 sinks’) in the ocean and on land. The sinks are sensitive to climate and elevated CO2 levels. Their efficiency in removing CO2 emissions from the atmosphere is expected to decrease in the future under increasing atmospheric CO2 because of their response to elevated CO2 levels, warming and other climate changes. Recent evidence from observations and models suggests that the efficiency of the sinks could have already decreased in the past few decades, but the uncertainties are very large. There is an urgent need for reducing these uncertainties by better monitoring the CO2 emissions and sinks, and by improving our understanding of the sinks dynamics.