Climate change is an abbreviated way of representing the effects that human-caused release of greenhouse gasses (GHG) are having on the global climate. The increased concentration of these gasses in the atmosphere is causing the average global temperature to rise and leading to a wide variety of follow-on effects, including rising sea levels, changes in rainfall amount and distribution, increase in the frequency and severity of extreme weather events, and others. These effects, in turn, generate impacts on a variety of human situations and activities, including, importantly, the practice of agriculture.
Long-lived greenhouse gasses emitted by human activity are causing the temperature of the planet to rise. The four most prominent of these gasses are carbon dioxide (63%), methane (18%), nitrous oxide (6%), and various industrial halocarbons (13%). The rising global temperature (about 0.8⁰C in the past century) increases the moisture holding capacity of the atmosphere and the rate of evaporation from open water surfaces, as well as the energy content of the ocean-atmospheric system, which, in turn, drives a multitude of other changes in global climate.
Agriculture, both crop agriculture and animal husbandry, depend critically on the prevailing climate in its particular locality. As climates change, agricultural systems that depend on those climates are profoundly affected. Those effects pass on to both producers who depend on agriculture for their livelihoods, and to non-agricultural populations which consume the surplus food produced.
Agriculture’s contributions to greenhouse gas emissions
About two-thirds of the enhanced levels of CO2 in the atmosphere results from the burning of fossil fuel, a consequence of the industrial revolution whose climate-altering effects were predicted by Swedish chemist and Nobel laureate Svante Arrhenius in 1896.
However the practice of agriculture contributes to global warming as well as being affected by it. Land use changes such as logging and land clearing for agriculture are responsible for about one-third of the enhanced level of atmospheric CO2. Land use practices release carbon through the burning or decay of above-ground biomass and the consumption and oxidation of below-ground organic matter by microorganisms. Globally, about half of all soil carbon in managed ecosystems has been lost to the atmosphere over the past two centuries as a result of land clearing and tillage (McCarl, et al., 2007).
Agriculture is responsible for about half of the methane in the atmosphere. Ruminant digestion (32%) and rice cultivation (19%) are the primary sources. The remainder results from various natural and anthropogenic sources (Chen and Prinn, 2006).
About 40% of nitrous oxide emissions are anthropogenic, and the primary driver for the industrial era increase in N2O levels is enhanced microbial production in expanding and more heavily-fertilized agricultural lands (Denman et al., 2007).
Agriculture has the potential to help mitigate the rising concentration of GHG in the atmosphere. Its contribution can come in several ways. One is by employing improved management practices for forests and dryland agricultural areas. Such practices are discussed in the GiZ publications Running dry?, Central Asia: Acting locally – cooperating regionally, and Climate change and agriculture.
Another way agriculture can help reduce net GHG emissions is by making intensive irrigated agriculture more productive and hence reducing the need to clear fragile drylands to expand more extensive rainfed agriculture.
Production of biofuels is another possible way to reduce the net release of GHGs by substituting the solar energy used in photosynthesis for the burning of fossil fuels, a practice which releases long-sequestered carbon. Such measures must be approached very cautiously, however, because of the high CO2 footprint of intensively-cultivated crops such as maize which are sometimes fermented to produce liquid biofuels. Net reductions of CO2 emission from maize to ethanol conversions can be quite small, on the order of 10 to 20%, and such conversions have the potential to drive up foodgrain prices, negatively affecting poor urban consumers.
Impact pathways to agriculture
Even if aggressive actions are taken worldwide to stem the release of GHG, global warming will continue for at least a century as a result of forces already set in motion. Consequently, agriculture will almost certainly have to adapt, to some degree, to changes in temperature and precipitation under both irrigated and rainfed conditions. See the GiZ publication Water and Adaptation to Climate Change for a more extensive discussion of impact pathways.
The most direct impact of GCC on agricultural water supply is its alteration of precipitation patterns. Changes will occur in average annual amount, temporal distribution, and intensity and duration of individual events. The effects will be felt directly, in the case of rainfed agriculture, and indirectly, through their effect on watershed hydrology and runoff, in the case of irrigated agriculture. Higher global temperatures will evaporate more water and increase the moisture-holding capacity of the atmosphere. Annual precipitation will increase in some areas and decrease in others. In general, areas which are already humid will receive more rainfall and already dry areas will receive less. Regional climate models can provide more detailed predictions, but their resolution is still relatively coarse. Worldwide, the expectation is for more intense precipitation events and longer dry periods between events.
Impacts on stream hydrology will likewise be mixed. Watershed responses to reduced rainfall and higher temperatures are typically amplified, so, for example, a 20% reduction in rainfall might yield a 50% reduction in runoff. Streamflow will be flashier in response to higher intensity storms, and floods will be more frequent and more severe.
One critical impact on hydrology is temperature-related, as rising temperatures push mountain snowlines higher and cause more precipitation to fall as rain rather than snow at higher elevations. This effect is already shifting the peaks of runoff hydrographs in snow-fed rivers such as the Columbia in the Western United States to occur earlier in the year and reducing summertime flows, when demand for irrigation is the greatest. This loss of natural storage has powerful negative implications for the extensive irrigated areas which depend on snow and glacier-fed rivers for their water supply, such as the vast Indo-Gangetic Plain and large areas of North China. At the same time, evaporation losses from artificial reservoirs will increase, reducing their useful supply, while more intense rainfall events may increase reservoir sedimentation rates.
Demand for agricultural water is also affected by climate change. Rising temperatures increase evapotranspiration by plants of all types, including agricultural crops, depleting soil moisture more rapidly and increasing irrigation demand. At the same time, the increased level of CO2in the atmosphere, a raw material for photosynthesis, will increase the rate of biomass formation, to a point. However this effect tends to level off as other factors such as nutrients and water become limiting (Chandler, 2007).
A Framework for Understanding Climate Change Adaptation.
Chandler, David (n.d.): Climate myths: higher CO2 levels will boost plant growth and food production. http://environment.newscientist.com/channel/earth/climate-change/dn11655 (accessed May 2007)
Chen, Y-H.; Prinn, R.G (2006): Estimation of atmospheric methane emission between 1996-2001 using a 3-D global chemical transport model. J. Geophys. Res., 111, D10307.
Denman, K.L., Brasseur, G., Chidthaisong, A., et al. (2007): Couplings between changes in the climate system and biogeochemistry. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change; Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller, H.L., Eds.; Cambridge University Press, Cambridge, UK 2007.
GTZ (2010): Central Asia: acting locally – cooperating regionally. GTZ, Eschborn.
McCarl, Bruce; Metting, F. B.; Rice, Charles (2007): Soil carbon sequestration. Climatic Change, 80, 1-3.
Rosegrant, M, et al. (2008): Climate change and agriculture; threats and opportunities. GTZ, Eschborn.
Sorensen, L., et al. (2009): Running dry? Climate change in drylands and how to cope with it. GTZ, Eschborn.
Svendsen, M, and N. Künkel (2008): Water and adaptation to climate change: consequences for developing countries. GTZ, Eschborn.