Human activities in the land sector produce the second highest level of greenhouse gas emissions globally and the highest in some countries, but the land is also a major sink for anthropogenic emissions. This land carbon sink is at risk because plant growth, survival and distributions are dependent on climate conditions and particularly extreme events: higher temperatures, reduction in water availability in some areas and flooding in others, droughts and wildfires. Agricultural and forest productivity are affected by the combined pressures of climate change, demand for land resources for food production, deforestation and degradation of natural systems, and population pressure. The land sector provides numerous opportunities for mitigation, many of which are cost-effective and can be implemented quickly. However, the land sink has limits because the uptake of carbon represents replacing the stock that was previously depleted by human activities, and productivity is limited by nutrient and water availability. Potential mitigation activities should be assessed in terms of the magnitude, timeframe and cost of the changes in carbon stocks.