In En-ROADS, higher temperature reduces crop yield, increasing the demand for cropland. This need for more cropland increases deforestation, thus both increasing CO2 emissions and reducing CO2 removals.
What is crop yield?
Crop yield is measured as the amount of food and animal fodder that is harvested per hectare of land per year. To increase the amount of food grown, you can increase the amount of land, increase the yield, or both. Crop yields have increased substantially in the 20th century due to factors such as breeding and technological advances, and evidence suggests that crop yields can grow further. In En-ROADS, by 2100 potential crop yield grows to 2.5 times its 2020 value, which can be seen in the “Crop Yield” graph under Graphs > Land, Forestry, and Food. Users can adjust this growth assumption under Simulation > Assumptions > Agriculture > Potential crop yield.
Effect of temperature on crop yield
The crop yield is affected by a feedback from temperature. The default assumption in En-ROADS is that the rise in global temperature reduces overall crop yield by 4% for each degree increase from the temperature in 2020. This is derived from research by Zhao et al., 2017. Users can adjust this assumption under Simulation > Assumptions > Agriculture > Effect of temperature on crop yield.
Note that the Assumptions in En-ROADS only affect the Current Scenario. For example, if you change the “Effect of temperature on crop yield” assumption to zero, the Baseline Scenario will still include the effect of temperature on crop yield, whereas the Current Scenario will not.
Repercussions of this feedback
The effect of temperature on crop yield is referred to as a reinforcing feedback cycle: higher temperatures reduce crop yield, which increases the demand for land for farming. This leads to deforestation, which releases stored CO2 from forests and soil into the atmosphere and reduces the amount of CO2 that forests remove from the atmosphere. More CO2 increases temperature, perpetuating the cycle:
The effect of temperature on crop yield and hence on deforestation is evident in a scenario in which the effect of temperature on crop yield is turned off in the Assumptions. See the scenario here. Crop yield in this scenario is higher than in the Baseline Scenario (as the Baseline Scenario includes the feedback from temperature to crop yield), and thus less land is needed for farming and deforestation is lower than in the Baseline Scenario:
Actions in En-ROADS that reduce temperature will reduce emissions from land use, shown in the “CO2 Net Emissions from Land Use without Bioenergy” graph, and will slightly increase crop yield relative to the Baseline Scenario. See an example scenario in En-ROADS here. The “Decrease in Crop Yield from Temperature” graph under Graphs > Impacts displays the impact of global temperature increases on crop yields of maize, wheat, rice, and soybean.
Crop yield in En-ROADS is also driven by food needs. If there is less food from animals or less wasted food, the need for food production is lower and consequently the need for crop yield expansion is lower.