The Paris Agreement calls for emissions reductions to limit climate change, but how will the carbon cycle change if it is successful? The land and oceans currently absorb roughly half of anthropogenic emissions, but this fraction will decline in the future. The amount of carbon that can be released before climate is mitigated depends on the amount of carbon the ocean and terrestrial ecosystems can absorb. Policy is based on model projections, but observations and theory suggest that climate effects emerging in today's climate will increase and carbon cycle tipping points may be crossed. Warming temperatures, drought, and a slowing growth rate of CO2 itself will reduce land and ocean sinks and create new sources, making carbon sequestration in forests, soils, and other land and aquatic vegetation more difficult. Observations, data-assimilative models, and prediction systems are needed for managing ongoing long-term changes to land and ocean systems after achieving net-zero emissions. International agreements call for stabilizing climate at 1.5 degrees above preindustrial, while the world is already seeing damaging extremes below that. If climate is stabilized near the 1.5 degrees target, the driving force for most sinks will slow, while feedbacks from the warmer climate will continue to cause sources. Once emissions are reduced to net zero, carbon cycle-climate feedbacks will require observations to support ongoing active management to maintain storage.

World soils and terrestrial ecosystems have been a source of atmospheric abundance of CO2 ever since settled agriculture began about 10-13 millennia ago. The amount of CO2-C emitted into the atmosphere is estimated at 136 +/- 55 Pg from terrestrial ecosystems, of which emission from world soils is estimated at 78 +/- 12 Pg. Conversion of natural to agricultural ecosystems decreases soil organic carbon (SOC) pool by 30-50% over 50-100 years in temperate regions, and 50-75% over 20-50 years in tropical climates. The projected global warming, with estimated increase in mean annual temperature of 4-6 degrees C by 2100, may have a profound impact on the total soil C pool and its dynamics. The SOC pool may increase due to increase in biomass production and accretion into the soil due to the so-called CO2 fertilization effect, which may also enhance production of the root biomass. Increase in weathering of silicates due to increase in temperature, and that of the formation of secondary carbonates due to increase in partial pressure of CO2 in soil air may also increase the total C pool. In contrast, however, SOC pool may decrease because of: (i) increase in rate of respiration and mineralization, (ii) increase in losses by soil erosion, and (iii) decrease in protective effects of stable aggregates which encapsulate organic matter. Furthermore, the relative increase in temperature projected to be more in arctic and boreal regions, will render Cryosols under permafrost from a net sink to a net source of CO2 if and when permafrost thaws. Thus, SOC pool of world soils may decrease with increase in mean global temperature. In contrast, the biotic pool may increase primarily because of the CO2 fertilization effect. The magnitude of CO2 fertilization effect may be constrained by lack of essential nutrients (e.g., N, P) and water. The potential of SOC sequestration in agricultural soils of Europe is 70-190 Tg C yr(-1). This potential is realizable through adoption of recommended land use and management, and restoration of degraded soils and ecosystems including wetlands.