Agricultural crops are naturally more reflective than local vegetation and soils. Even different crops and different varieties of the same crops vary considerably in 'albedo' - their solar reflectivity. It is the arrangement of leaves at the top of the plant and the surface texture of the leaves that affects a plant's solar reflectivity. For example, waxy leaves, as found in some barley varieties, or hairy leaves, such as some sorghum varieties, reflect back greater amounts of sunlight than leaves without these characteristics.
In major agricultural areas, crop varieties could be chosen to reflect the maximum amount of heat from the sun. This practice, termed 'bio-geoengineering', could be implemented in the short-term and cost less than other geoengineering alternatives. Farmers are already growing crops in these areas and it would be relatively cheap and easy to switch to suitable crop varieties with greater albedo properties. In the medium term, selective breeding and/or genetic modifications offer the potential to develop crops with more reflective properties.
The researchers investigated the impact of growing crops such as rice, wheat, soybeans, sugarcane and millet, with an increased albedo of 20 per cent, in areas already under cultivation. The study suggests the impact would be regional and seasonal rather than global. The benefits would be felt across a 30 degree-wide latitudinal band, affecting central North America and mid-latitude Europe and Asia.
A further benefit of bio-geoengineering is that crops lose less moisture in the cooler temperatures. Together with projected changes in rainfall patterns, this could increase the moisture content of soils, especially in southern-central United States. Crop yields are unlikely to be reduced and should not affect the supply of food. There is, however, a negative consequence: a knock-on effect of altering the climate in this region is that water availability may be reduced for some other regions of the world and Australia and parts of the subtropics could experience greater droughts. This uneven effect on different regions of the world will be a challenge for decision-makers.
The researchers suggest bio-geoengineering could help reduce the impact of hotter temperatures on health and agriculture in the industrialised northern hemisphere as a temporary measure, but it is not a long-term alternative to reducing global CO2 emissions. In addition, new varieties produced to have greater albedo would need to be tested to ensure they do not have other negative environmental impacts, such as increased water use. When choosing the most appropriate crop variety, input needs, such as fertilisers and pesticides, will also need to be considered.