The recent Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) summarizes current knowledge on observed and projected effects of actual and projected warming on agriculture. Observed effects of temperature increases since the 1970s include management responses in agriculture and forestry in the Northern Hemisphere higher latitudes, such as earlier spring planting of crops, and alterations in disturbance regimes of forests due to fires and pests. Future climate change effects on food, fiber and forest products include slight increases in crop productivity at mid-to-high latitudes for local mean temperature increases of 1-3°C depending on the crop, and then decreases beyond that in some regions. At lower latitudes, especially seasonally dry and tropical regions, crop productivity is projected to diminish for even small local temperature increases (1-2°C), thus increasing the risk of hunger. Globally, the potential for food production is projected to increase with increases in local average temperatures for a range of 1-3°C, but above this it is projected to decrease. In North America, moderate climate change in the early decades of the century is projected to increase aggregate yields of rain-fed agriculture by 5-20%, but with considerable variability among regions. Adaptations such as altered cultivars and planting times will allow low-latitude and mid-to-high latitude cereal yields to be maintained at or above baseline yields for modest warming. However, major challenges are projected for crops that are near the warm end of their suitable range or that depend on highly utilized water resources. An important part of the IPCC process is the inclusion of likelihood statements regarding these results or outcomes.