Land is one fourth of Earth's surface and it holds three times as much carbon as the atmosphere does. About 1,600 billion tons of this carbon is in the soil as organic matter and some 540-610 billion tons is in living vegetation. Although the volume of carbon on Earth's surface and in the atmosphere pales in comparison to the many trillions of tons stored deep under the surface as sediments, sedimentary rocks, and fossil fuels, surface carbon is crucial to climate change and life due to its inherent mobility.3
Surface carbon moves from the atmosphere to the land and back, and in this process it drives the engine of life on the planet. Plants use carbon dioxide (CO2) from the atmosphere to grow and produce food and resources that sustain the rest of the biota. When these organisms breathe, grow, die, and eventually decompose, carbon is released to the atmosphere and the soil. Carbon from this past life provides the fuel for new life. Indeed, life depends on this harmonized movement of carbon from one sink to another. Large-scale disruption or changes on land drastically alter the harmonious movement of carbon.
Land use changes and fossil fuel burning are the two major sources of the increased
CO2 in the atmosphere that is changing the global climate. (See Box 3-1.) Burning fossil fuel releases carbon that has been buried for millions of years, while deforestation, intensive tillage, and overgrazing release carbon from living or recently living plants and soil organic matter. Some land use changes affect climate by altering regional precipitation patterns, as is occurring now in the Amazon and Volta basins. Overall, land use and land use changes account for around 31 percent of total human-induced greenhouse gas emissions into the atmosphere. Yet other types of land use can play the opposite role. Growing plants can remove huge amounts of carbon from the atmosphere and store it in vegetation and soils in ways that not only stabilize the climate but also benefit food and fiber production and the environment. So it is imperative that any climate change mitigation strategy address this sector.4
Extensive action to influence land use is also going to be essential to sustain food and forest production in the face of climate change. Agricultural systems have developed during a time of relatively predictable local weather patterns. The choice of crops and varieties, the timing of input application, vulnerability to pests and diseases, the timing of management practices—all these are closely linked to temperature and rainfall. With climate changing, production conditions will change—and quite radically in some places—which will lead to major shifts in farming systems.
Climate scenarios for 2020 predict that in Mexico, for example, 300,000 hectares will become unsuitable for maize production, leading to estimated yearly losses of $140 million and immense socioeconomic disruption. And in North America, the areas with the optimum temperature for producing syrup from maple trees are shifting northward, leaving farmers in the state of Vermont at risk of
Farming and Land Use to Cool the Planet
Carbon dioxide (77 percent), nitrous oxide (8 percent), and methane (14 percent) are the three main greenhouse gases that trap infrared radiation and contribute to climate change. Land use changes contribute to the release of all three of these greenhouse gases. (See Table.) Of the total annual human-induced GHG emissions in 2004 of 49 billion tons of carbon-dioxide equivalent, roughly 3 1 percent—15 billion tons—was from land use. By comparison, fossil fuel burning accounts for 27.7 billion tons of CO2-equivalent emissions annually.
Deforestation and devegetation release carbon in two ways. First the decay of the plant matter itself releases carbon dioxide. Second, soil exposed to the elements is more prone to ero sion. Subsequent land uses like agriculture and grazing exacerbate soil erosion and exposure. The atmosphere oxidizes the soil carbon, releasing more carbon dioxide into the atmosphere. Application of nitrogenous fertilizers leads to soils releasing nitrous oxide. Methane is released from the rumens of livestock like cattle,goats, and sheep when they eat and from manure and water-logged rice plantations.
Naturally occurring forest and grass fires also contribute significantly to GHG emissions. In the El Niño year of 1997-98, fires accounted for 2.1 billion tons of carbon emissions . Due to the unpredictability of these events, annual emissions from this source vary from year to year.
Greenhouse Gas Emitted
(million tons CO2 equivalent) 6,500
Soil fertilization (inorganic fertilizers and applied manure) 2,100
Gases from food digestion in cattle (enteric fermentation in rumens) 1,800
Biomass burning 700
Paddy (flooded) rice production (anaerobic decomposition) 600
Livestock manure 400
Other (e.g., delivery of irrigation water) 900
Nitrous oxide* Methane*
Methane, nitrous oxide* Methane*
Methane, nitrous oxide* Carbon dioxide, nitrous oxide*
Deforestation (including peat) For agriculture or livestock
* The greenhouse gas impact of 1 unit of nitrous oxide is equivalent to 298 units of carbon dioxide; 1 unit of methane is equivalent to 25 units of carbon dioxide. Source: See endnote 4.
losing not only their signature product but generations of culture and knowledge.5
The Gangotri glacier in the Himalayas, which provides up to 70 percent of the water in the Ganges River, is retreating 35 meters yearly. Once it disappears, the Ganges will become a seasonal river, depriving 40 percent of India's irrigated cropland and some 400 million people of water. The frequency, intensity, and duration of rainfall are also likely
Farming and Land Use to Cool the Planet to change, increasing production risks, especially in semiarid and arid rainfed production areas. Monsoons will be heavier, more variable, and with greater risk of flooding. An increased incidence of drought threatens nearly 2 billion people who rely on livestock grazing for part of their livelihoods, particularly the 200 million who are completely dependent on pastoral systems. The incidence and intensity of natural fires is predicted to increase.6
The poorest farmers who have little insurance against these calamities often live and farm in areas prone to natural disasters. More-frequent extreme events will create both a humanitarian and a food crisis.
On the other hand, climatic conditions may improve in some places. In the highlands of East Africa, for example, rains may become more reliable and growing seasons for some crops may expand. The growing season in northern latitudes in Canada and Russia will extend as temperatures rise. Even in these situations, however, there will be high costs for adapting to new conditions, including finding crop varieties and management that are adapted to new climate regimes at this latitude. The impacts on pest and disease regimes are largely unknown and could offset any benefits. For instance, the Eastern spruce budworm is a serious pest defoliating North American forests. Changing climate is shifting the geographic range of the warblers that feed on the budworms, increasing the odds for budworm outbreak.7
Many of the key strategies described in this chapter for agricultural, forest, and other land use systems to mitigate climate change— that is, to reduce GHG emissions or increase the storage of carbon in production and natural systems—also will help rural communities adapt to that change. Mobilizing action for adaptation in these directions rather than relying only on other types of interventions, such as seed varieties or shifts in market sup ply chains, could have significant success in slowing climate change.
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