Rural land use

Agriculture is one of the major sources of toxic pollutants, largely through the runoff, or air-borne transport of pesticides, herbicides and fungicides. There are probably thousands of such compounds in existence, and these include a number of POPs (q.v), such as aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, mirex, toxaphene, lindane (hexachlorocyclohexane, HCH), and hexachlorobenzene. While growing numbers of national and international measures exist to curb the uses of the more dangerous or persistent toxins, the development of restrictions in many places cannot keep pace with the development of new compounds.

Agriculture is also the source of considerable amounts of nutrients through the application of fertilisers. On average, more than 90 kilogrammes of fertiliser are used per hectare of cropland per year, although the total amount varies considerably. In fact use has declined in much of the developed world since the 1980s, but has increased massively, particularly in Asia. Large amounts of these fertilisers are lost from agricultural systems and enter the rivers or into groundwater, and large amounts then enter coastal waters where they create problems of eutrophication such as algal blooms and reduced oxygen levels. Nitrogen fertilisers may also contribute to the increase of nitrogen-compounds in the atmosphere, possibly fuelling eutrophication episodes such as algal blooms even away from coastlines.

Pastoralism, or the rearing of livestock, can have some of the same impacts as crop-based agriculture, particularly in areas where pastures are treated with pesticides or fertilisers, or in areas where livestock are fed with crops. Aside from these problems, livestock in more natural areas have a significant impact on natural vegetation and, where overgrazing is a problem, it can also lead to high levels of erosion, with the carriage of sediments into coastal waters, or even as dust to more remote regions. After CO2, methane is one of the most important greenhouse gases and is rapidly increasing in the atmosphere. Two major sources for this increase are from rice paddies and from livestock, both of which are considered to be having a discernible effect on radiative forcing.