The future trajectory of the carbon sink influences how forests should be managed for climate-change mitigation. If the world’s forests remain net absorbers, conservation would be appealing. Preserved mature forests would absorb almost as much carbon as younger ones. Because decomposing harvest residues and roots add immediately to the CO2 emissions, and it takes decades for increased use of wood products to compensate, avoiding harvest could generate extra climate benefits, at least in the short run. Conversely, if mature forests become carbon sources, increased harvesting may be the best itigation option. Harvesting would reduce losses from decomposition while promoting wood as a fossil-fuel substitute. Today’s forest management is more of a gamble than a scientific debate. By following ‘no-regret’ strategies, we can buy time while we learn more. The future of the world’s forest should not depend on tossing a coin.