Global Agro-Ecological Zones (GAEZ)

Global Agro-Ecological Zones (GAEZ)

Measuring the potential of farmland

FAO’s GAEZ system produces an outlook for world food production – and reveals limits to growth in several regions

Over the next three decades world food supply will grow faster than population, although undernourishment will remain widespread. That was the main conclusion of Agriculture: Towards 2015/2030, an FAO report released in July 2000.

Underpinning the report’s widely quoted forecasts were months of serious number-crunching by a joint project of AG’s Land and Water Development Division (AGL) and the International Institute for Applied Systems Analysis (IIASA). Using a new land resources evaluation tool called the Global Agro-Ecological Zones (GAEZ) system, the project was able to a combine massive climatic, soil and terrain data sets covering most of the Earth’s land surface. The result: crop suitability and land productivity assessments for the entire globe.

About one-quarter of the global land surface is “sufficiently suitable” for crop cultivation, but only 3.5% is problem-free

What is the GAEZ system? With IIASA, AGL has developed over the past 25 years an Agro-Ecological Zones (AEZ) methodology, which provides a standardized framework for characterizing climate, soil and terrain conditions relevant to agricultural production. AEZ has been used in several countries – including Bangladesh and Canada – to evaluate crop production potentials. What’s now made the AEZ approach global is the information revolution – the availability of digital global databases of climatic parameters, topography, soil and terrain, land cover and population distribution. These data have enabled us not only to revise and improve AEZ calculation procedures, but extend its crop suitability and land productivity assessments to temperate and boreal environments. We can now produce global assessments of agricultural potential.

In simple terms, what is the GAEZ methodology? The AEZ framework contains three basic elements. The first is what we call LUTs – Land Utilization Types – which are selected agricultural production systems with defined input and management relationships, and crop-specific environmental requirements and adaptability characteristics. The second is geo-referenced climate, soil and terrain data which are combined into a land resources database. The vital third element is the procedure for calculating potential yields by matching crop/LUT environmental requirements with the environmental characteristics captured in the database.

What went into the GAEZ land resources database? We used the FAO/UNESCO Digital Soil Map of the World (DSMW) for building a land surface database with more than 9.2 million grid-cells, plus soil association and attribute tables, a slope distribution database, and a layer providing distributions in terms of eleven aggregate land-cover classes.

For climate, we used a recent global climatic data set compiled by the University of East Anglia‘s Climate Research Unit. This database contains climate averages for the period 1961-90, as well as year-by-year data of the period 1901-1996. These are used to characterize each half-degree grid-cell in terms of thermal climates, temperature profiles, accumulated temperature sums, length of growing periods and moisture deficits. Terrain slopes were derived from the Global 30 Arc Second Elevation Database developed at the USGS Eros Data Center. At IIASA, rules based on altitude differences of neighbouring grid-cells were applied to compile a terrain-slope distribution database with seven average slope range classes.

How did you assess crop productivity? For rain-fed land, we used a water-balance model to quantify the start and duration of the period when sufficient water is available to sustain crop growth. Soil moisture conditions together with other climate characteristics – such as radiation and temperature – were used in a simplified crop growth model to calculate potential biomass production and yield. For the assessment of irrigated land productivity the duration of the period with temperatures conducive for crop growth was used for matching the crop cycle length and for the calculation of biomass production and yield.

The calculated potential yields are then combined in a semi-quantitative manner with a number of reduction factors directly or indirectly related to climate (like pest and diseases), and with soil and terrain conditions. The reduction factors, which are successively applied to the potential yields, vary with crop type, climate, soil and terrain conditions, and assumptions at the level of inputs and management. For determining irrigated land productivity potentials, we assumed that water resources of good quality are available, and that irrigation infrastructure is in place. In other words, the procedures identify areas where climate, soils and physiography permit irrigated crop cultivation, but do not assess availability of sufficient water supply. But the Global AEZ could easily be linked to watershed data to define limits to water availability.

(Climate change. The GAEZ study estimated the potential effects of global climate change on rainfed agriculture. A temperature increase of 3°C, paired with a 10% rainfall increase, would lead globally to about 4% more cultivable rain-fed land. The beneficiaries, however, would not be evenly distributed – while the increase in cultivable land in developed countries might exceed 25%, in developing regions there would be an 11% drop.)

What were the main findings of the GAEZ study? Considering current climate and the main crop types modelled in Global AEZ – and optimizing across low, intermediate and high input levels – we conclude that a little more than one-quarter of the global land surface can be regarded as “sufficiently suitable” for crop cultivation. For the developed countries this amounts to about 20% and for developing countries to about 30% of their respective land surfaces. This gross estimate of land with cultivation potential is twice the area that was actually in cultivation in 1994-96, according to FAO statistics. From this, we concluded that the Earth’s land and climate resources are adequate to meet the food needs of for a world population of 8.9 thousand million, as projected for the year 2050.

However, we don’t expect the area of cultivated land – at global scale – to increase very much. Most of the increase in future food production will come through improvements in input use and technology, especially in developing regions where the gap between actual and potential yields is still very wide. In fact, a major expansion of cultivated land would be undesirable for environmental reasons, because of important implications for biodiversity and global biogeochemical cycles.

Globally, at least, the outlook is optimistic… Globally, yes. But there are several regions where rain-fed cultivation potential has already been exhausted, notably in parts of Asia. On the basis of currently available data, the GAEZ approach estimates that 10.5 thousand million ha – which is more than three-quarters of the global land surface – suffer rather severe constraints for rain-fed cultivation. Assuming availability of water resources, only about 1.8% of arid and hyper-arid zones was assessed as prime land for cereals under irrigation. Overall, the analysis concludes that only 3.5% of the land surface can be regarded to be entirely free of constraining factors. Only for some parts in Europe did the share of essentially constraint-free conditions reach 20% and more.

What plans are there for GAEZ? There is now rich experience with the application of AEZ at national, regional and global levels, and we see it as a source of comprehensive information for decision-makers, especially national and international organizations dealing with agriculture, land and water, food security, climate variability and climate change. Apart from general refinement of the basic methodology and data, we want to add water resources data in the GAEZ database, and use IIASA’s Climate and Human Activities-sensitive Runoff Model (CHARM) to enhance the assessment of irrigation production potentials at watershed level. We are also planning specific AEZ studies on the effect of climatic variability on food security in the Horn of Africa, Southern Africa, Bangladesh and China.