3.2 Impacts on air and climate

The burning of forest for the cultivation of new land, slash-and-burn, the burning of straw, ploughing, and the draining of wetland all release carbondioxide (CO2) into the atmosphere and may hence contribute towards the greenhouse-effect on a global scale. In principle, it may be wise to avoid farming systems which clearly contribute towards increasing the discharges of CO2. The cultivation of wet-rice may serve as a source of significant emissions of methane (CH4), which is also a greenhouse gas. Work in progress is attempting to solve this problem by developing varieties of rice which are less dependent on being covered by water.

Large irrigation projects will increase the evaporation locally so that the air humidity increases, possibly changing the conditions of life for insects and micro-organisms (e.g. pests and mould on crops). New irrigation methods based on under-ground pipes and drip-irrigation from surface pipes give less evaporation than spraying techniques or traditional irrigation by means of canals. The highest levels of evaporation occur when watering takes place at mid-day when sun intensity is great and winds are strong.

Deforestation and over-grazing change the reflection from the earth's surface, but it is not clear whether such changes have any significant impact on weather and climate on a global or local scale. Available reports differ strongly in their conclusions because increased reflection may trigger other processes which counteract any possible increases of atmospheric temperatures. High temperatures in the top centimetres of soil may influence the germination ability of some species and varieties and hence the composition of the vegetation.
Some researchers claim that they can detect a local increase in precipitation following afforestation. In areas with a lot of fog, trees may catch moisture which can also be utilized by other plants when the fog condensates and the water falls to the ground.