### Changes in agriculture

#### Indicators:

Current crop-growing area with change in potential yield (graph) Potential crop area Current crop-growing area with change in potential yield (map)

The indicators in this view illustrate a number of results on crop productivity (yield or production per unit area) of the Land-Cover Model (LCM), including changes in mean productivity, and changes in yields in current and potential crop-growing areas. Changes in crop productivity represent the effects of climate only. The effects of management and technological development have therefore not been accounted for.

Climate influences potential crop yields under local climate and soil conditions. The potential yield differs from the theoretical maximum yield, the latter representing the global maximum yield under crop-specific optimal climate and soil conditions, so that each crop is represented by one global value of the theoretical maximum yield.

| #### Current crop-growing area with change in potential yield
| unit: km2 (square kilometer, which match 100 ha) dimension: region, aggregated food crop, change in yield

The indicator presents the areas within the 1990 crop growing areas where potential yields for the seven aggregated food crops decrease (more than 10%) or increase (more than 10%) compared to the 1990 level or remain stable.

In IMAGE 2.4, potential yield is modelled as a function of climate and CO2 concentration (caused by CO2-fertilization and enhanced water-use efficiency):

- Changes in local climate (i.e. precipitation and temperature) can both shorten and lengthen the growing season of crops (see indicators), and thus potential yield.
- For calculating the change in potential yield caused by increased CO2, we use a method similar to that for natural vegetation. The yield-response to CO2-fertilization is stronger for C3-crops (such as cereals) than for C4-crops (maize and millet). With increasing CO2, all plants use water more efficiently. Hence, reduced moisture availability can be offset by enhanced water-use efficiency. In earlier versions of IMAGE, the impacts of CO2 on crop yields were not considered.

The model results show that impacts on potential yield can be both positive and negative depending on local climate changes. The results are in general agreement with the IPCC, which concluded that changes to total global output of agriculture are estimated to be small, production in high-latitude countries is estimated to increase and production in low-latitude counties is estimated to decrease. It should be noted that the regional IMAGE 2.4 results were found to be strongly dependent on the GCM pattern used. Furthermore, sensitivity runs indicate that the results are dominated by the impacts of increased CO2 concentration. Potential yields in most regions are negatively affected if the CO2-effect is ignored (results not shown).

There are several uncertainties in assessing the impacts on agricultural yields:

- This indicator represents the potential yield and not the actual yield. For the latter, several other factors, including adaptation, play an important role.
- Results strongly depend on the GCM pattern used for scaling.
- Fertilization impacts and improvement of water use efficiency caused by increased CO2 have been measured mostly under laboratory conditions. The impacts under field conditions are less certain. The impacts of CO2-fertilization and enhanced water efficiency in annual crops is, however, less controversial than in natural ecosystems where other factors usually are limiting and the effects are less pronounced.

In regions where decrease in potential agricultural yield occurs, additional investments in agriculture will be required. Because the strongest negative impacts tend to be in the least developed countries with a relatively high share of agriculture in GDP. We focus on those areas with decreasing yields. The quantitative interpretation of the indicator should be done with care, in view of the uncertainties and the factors not taken into account in IMAGE 2.4.

| #### Potential crop area
| unit: km2 (square kilometer which match 100 ha) dimension: region, crop, latitudinal belt

Potential crop areas are defined as those areas where the potential yield of the crop considered (i.e. for pasture and fodder species, 7 aggregated food crops and 4 biofuel crops) exceeds 20% of the theoretical maximum yield. Areas with crop yields below 20% of the theoretical maximum are considered to be marginal land (for the crop considered). Note that many areas are counted more than once, since in those areas the potential yield of several crops exceeds 20% of the theoretical maximum yield. Hence, the sum of the potential crop areas can be larger than the total land cover area.

#### Current crop-growing area with change in potential yield (map) |  |
unit:none dimension: crop |

This map indicates the areas where potential yields in current crop-growing areas have been altered by climate change. Here, it is indicated if yields have stabilized, decreased or increased since 1970.