It's well known that the oceans and land-based ecosystems have historically absorbed about 50% of human emissions of CO2, with the other half remaining in the atmosphere.  But scientists have worried that this natural uptake of CO2 might not last forever.  This new paper suggests that as the world warms, land-based CO2 sinks might already be close to tipping, which would substantially accelerate the accumulation of CO2 in the atmosphere.