The phrase "net sink" means something that absorbs or takes in more than it gives out. It acts as a collector or storage for positive results or values, rather than releasing or losing them. Full definition
What this amounts to is an additional divergence from prevailing assumptions that the biosphere will continue to provide a strong net sink of carbon throughout the century. (critical-angle.net)
Illegal and unsustainable logging has been a critical factor in turning tropical forests from net sinks to net sources of carbon emissions since 2003. (eia-global.org)
In northern Eurasia, the annual net sink rate increased from the 1960s to the 2000s, but since then, the rate at which carbon is sequestered by the region has leveled and even showed signs of weakening, said Michael Rawlins, an assistant professor in the University of Massachusetts» Department of Geosciences. (scientificamerican.com)