carbon sinks may be reaching their limit as forests mature and climate conditions change. And some may literally go up in smoke if wildfires become more frequent, as some climate simulations predict. Planting forests and adopting carbon-conserving practices such as no-till agriculture may increase carbon sinks somewhat, but this would not come close to compensating for carbon emissions, which continue to accelerate.See also Fire as the dominant driver of central Canadian boreal forest carbon balance by Ben Bond-Lamberty et al (Nature 450). The arctic tundra and boreal forest are the first and second largest carbon stores among terrestrial biomes.
[P.S.: I should have been paying closer attention: Andy Revkin blogged SOCCR yesterday here - but he doesn't pick up the key issue of carbon cycle feedback.]