Global agriculture will face many challenges over the coming decades. Degrading soils and water resources will place enormous strains on achieving food security for growing populations. These conditions may be worsened by climate change. While a global warming of less than 2.5°C could have no significant effect on overall food production, a warming of more than 2.5°C could reduce global food supplies and contribute to higher food prices.

Some agricultural regions will be threatened by climate change, while others may benefit. The impact on crop yields and productivity will vary considerably. Added heat stress, shifting monsoons, and drier soils may reduce yields by as much as a third in the tropics and subtropics, where crops are already near their maximum heat tolerance. Mid-continental areas such as the US grain belt, vast sections of mid-latitude Asia, sub-Saharan Africa and parts of Australia are all expected to experience drier and hotter conditions. Meanwhile, longer growing seasons and increased rains may boost yields in many temperate regions; records show that the season has already lengthened in the UK, Scandinavia, Europe and North America.

Higher temperatures will influence production patterns. Plant growth and health may benefit from fewer freezes and chills, but some crops may be damaged by higher temperatures, particularly if combined with water shortages. Certain weeds may expand their range into higher-latitude habitats. There is also some evidence that the poleward expansion of insects and plant diseases will add to the risk of crop losses.

Soil moisture will be affected by changing precipitation patterns. Based on a global warming of 1.4 – 5.8^oC over the next 100 years, climate models project that both evaporation and precipitation will increase, as will the frequency of intense rainfalls. While some regions may become wetter, in others the net effect of an intensified hydrological cycle will be a loss of soil moisture and increased erosion. Some regions that are already drought-prone may suffer longer and more severe dry spells. The models also project seasonal shifts in precipitation patterns: soil moisture will decline in some mid-latitude continental regions during the summer, while rain and snow will probably increase at high latitudes during the winter.

More carbon dioxide in the atmosphere could boost productivity. In principle, higher levels of CO₂ should stimulate photosynthesis in certain plants. This is particularly true for so-called C3 plants because increased carbon dioxide tends to suppress their photo-respiration. C3 plants make up the majority of species globally, especially in cooler and wetter habitats, and include most crop species, such as wheat, rice, barley, cassava and potato. Experiments based on a 50% increase of current CO₂ concentrations have confirmed that "CO₂ fertilization" can increase mean yields of C3 crops by 15% under optimal conditions. C4 plants would also use water more efficiently, but the effects on yields would be smaller in the absence of water shortages. C4 plants include such tropical crops as maize, sugar cane, sorghum and millet, which are important for the food security of many developing countries, as well as pasture and forage grasses. These positive effects could be reduced, however, by accompanying changes in temperature, precipitation, pests, and the availability of nutrients.

The productivity of rangelands and pastures would also be affected. For example, livestock would become costlier if agricultural disruption leads to higher grain prices. In general, it seems that intensively managed livestock systems will more easily adapt to climate change than will crop systems. This may not be the case for pastoral systems, however, where communities tend to adopt new methods and technologies more slowly and where livestock depend more fully on the productivity and quality of the rangelands, which may become degraded.

The global yield from marine fisheries should remain unchanged by global warming. The principal effects will be felt at the national and local levels as the mix of species changes and people respond by relocating fisheries. These possible local effects could threaten the food security of countries that are highly dependent on fish. In general, some of the positive effects of climate change could include longer growing seasons, lower natural winter mortality, and faster growth rates at higher latitudes. The negative ones could include upsets in established reproductive patterns, migration routes, and ecosystem relationships.

Food security risks are primarily local and national. Studies suggest that global agricultural production could be maintained relative to the expected baseline levels over the next 100 years with moderate climate change (below a 2°C warming). However, regional effects would vary widely, and some countries may experience reduced output even if they take measures to adapt. This conclusion takes into account the beneficial effects of CO₂ fertilization but not other possible effects of climate change, including changes in agricultural pests and soils.

The most vulnerable people are the landless, poor, and isolated. Poor terms of trade, weak infrastructure, lack of access to technology and information, and armed conflict will make it more difficult for these people to cope with the agricultural consequences of climate change. Many of the world's poorest areas, dependent on isolated agricultural systems in semi-arid and arid regions, face the greatest risk. Many of these at-risk populations live in sub-Saharan Africa; South, East and Southeast Asia; tropical areas of Latin America; and some Pacific island nations.

Effective policies can help to improve food security. The negative effects of climate change can be limited by changes in crops and crop varieties, improved water-management and irrigation systems, adapted planting schedules and tillage practices, and better watershed management and land-use planning. In addition to addressing the physiological response of plants and animals, policies can seek to improve how production and distribution systems cope with fluctuations in yields.