The graph shows global average yields of some major agricultural crops from 1961 to 2007. The source of the data is the Food and Agriculture Organisation (FAO) of the United Nations. The remarkable improvements in the yields of most major crops since the 1940s is known as the Green Revolution. Per capita calorific food production has increased, as production has outpaced population growth during this period.
How has this doubling or tripling of agricultural yields been achieved? A major driver of high yield is the breeding and gradual adoption of new plant varieties with increased harvest index. The harvest index is the fraction by weight of the total above-ground biomass which contributes useful product (the grain from a rice plant, for example.)
Intensive farming methods, such as use of fertilizers, irrigation and pest control, are often required to make the new cultivars viable.
The benign outlook promised by the above graph seems to be contradicted by recent reports, such as United Nations Environment Programme GEO4 report. Firstly while most crops and regions continue to show improved yields, there are physical limits on possible future gains in harvest fraction through plant breeding. Secondly, environmental constraints (and climate change) will increasingly impact agriculture in many parts of the world. According to one study, more than 30% of terrestrial photosynthesis has already been appropriated by humans. Nearly 50% of the available rainwater resources are exploited by agriculture for crop irrigation.
The view that resource constraints (on crop yields, land area, water etc) are about to impact agriculture is a reversal of prevailing wisdom. It has been referred to as Green Revolution Fatigue. Is there evidence in the FAO data that the rate of yield improvement is slowing? The graph below shows that wheat yield growth is slowing and may be saturating.