EFFECTS OF CHANGE IN CLIMATE ON AGRICULTURE
Agriculture feeds and clothes the world. Although the long-term effects of climate change are still largely unknown, scientists can observe short-term effects of climate change on crops and animals. In addition, scientists can prognosticate about the changes that are likely to occur in agriculture if global climate change causes changes in temperatures and rainfall. The National Climatic Data Centre has published a FAQ on global warming that might be useful to read.
Data have shown that levels of atmospheric CO2 are increasing. Research is being conducted to determine what types of plant responses can be expected from these changes (see section on CO2 increase below). Others worry that climate change is going to permanently alter weather patterns, temperatures, and rainfall. NOAA data show that for much of the Southeast, annual average rainfall has been relatively constant or slowly increasing; air temperatures are slightly lower than 100 years ago. However, the frequency of rainfall events greater than 2 inches is increasing, leading to longer dry periods between rain events. Crop yields are likely affected by these changes to some extent already, but it is not clear if future changes will be catastrophic or not. Plants are surprisingly resilient, and can withstand a variety of conditions while still being productive. In addition, other factors such as location, soil fertility, crop varieties, and management practices will all affect future yields. Below we list some of the effects we could expect for agriculture due to various aspects of climate change.
Many scientists project that the average temperatures throughout the US will rise in the next few decades. Much of this warming could occur at night, but the models are not clear on this. If temperatures increase, cooler areas of the country might be more habitable for some of the main food crops grown in the US – thus, expanding the areas in which certain crops could be grown or moving their ranges north. For example, less frequent freezes could allow citrus to move north from its current range in Florida to other areas of the Southeast. In areas where crops are being grown in their warmest productive temperature ranges already, heat stress or increased disease could reduce yields. However, research on new crop varieties and technological advances could improve yields in spite of reductions due to temperature increases. A report from the IPCC (Intergovernmental Panel on Climate Change) is optimistic that general crop yields for the next century could increase in a range from 5-20% during the first few decades of the 21st century, and they expect the crop yield to remain somewhat steady (but positive) through the rest of the century. If climate change reduces the global amount of arable land, however, total yields could still decrease.
Extreme Weather Events
Some scientists believe that climate change will lead to more extreme weather events. Extreme weather events include heat waves, droughts, strong winds, and heavy rains. Climate models do not do a good job of predicting how extreme weather events might change under global warming. For example, models do not agree on whether the number of hurricanes in a warmer world would be more or less than current values, but scientists generally feel that the strength of the largest hurricanes will increase. The length of the hurricane season could also increase. Observational changes in the number of tornadoes per year we see now may be due to increases in the number of people watching the skies and the growth of urban areas rather than any strict climate changes. It is not clear if observed changes in extreme weather events we see now are part of long natural cycles, or if they are in response to climate change. Nonetheless, all of these events can be detrimental to crop growth.
Droughts are damaging because of the long-term lack of water available to the plants. Droughts have been responsible for some of the more serious famines in the world, although sociological factors are also important. Heat waves can cause extreme heat stress in crops, which can limit yields if they occur during certain times of the plants’ life-cycle (pollination, pod or fruit set). Also, heat waves can result in wilted plants (due to elevated transpiration rates) which can cause yield loss if not counteracted by irrigation. Strong winds can cause leaf and limb damage, as well as “sandblasting” of the soil against the foliage. Heavy rains that often result in flooding can also be detrimental to crops and to soil structure. Most plants cannot survive in prolonged waterlogged conditions because the roots need to breathe. In addition, flooding can erode topsoil from prime growing areas, resulting in irreversible habitat damage. Heavy winds combined with rain (from events such as hurricanes and tornadoes) can down large trees, and damage houses, barns and other structures involved in production agriculture.
Carbon Dioxide Increase
Scientists are in agreement that the levels of atmospheric CO2 (carbon dioxide) have increased in recent years. Prior to the Industrial Revolution, they were measured at 280 parts per million by volume (ppmv); currently the levels are around 380 ppmv. These levels have been steadily increasing by 1.9 ppm yearly since the year 2000, largely as a result of fossil fuel burning. Carbon dioxide is critical to photosynthesis (and thus plant growth). Scientists agree that even small increases in carbon dioxide result in more plant growth. It is likely that higher levels of carbon dioxide will result in higher harvestable crop yields. However, this depends critically on the availability of sufficient water and nutrients necessary for plant growth. Some scientists believe that one drawback to this increased productivity will be crops with lower nutrient and protein levels. If true, this could have a significant, widespread impact on long-term human health if additional fertilizers were not incorporated into crop production.
Weeds, Pests and Disease
While crops are expected to respond to increased CO2 with strong vegetative growth, other plants are also thought to respond in a similar fashion. Weeds have become more prolific and are expected to invade new habitats as global warming increases. For example, researchers at Duke University found that poison ivy is actually becoming more toxic as levels of atmospheric carbon dioxide increase. Studies have also shown that herbicides become less effective in a higher carbon dioxide environment, meaning that higher rates of herbicides will be necessary to achieve the same levels of control. Insect pests, some of which carry plant diseases, could become more prolific and widespread as temperatures increase. If pests live longer and reproduce more each year, it is possible that they could spread crop diseases into new production areas. It is also possible that increases in temperature, moisture and carbon dioxide could result in higher populations of destructive pests.
Irrigation and Rainfall
Changes in climate may also impact the water availability and water needs for agriculture. If temperature increases and more sporadic rainfall events result from global warming, it is possible that irrigation needs could increase in the future. For example, rainfall in parts of the south eastern US states has increased about 10% over the past century. However, part of this increase may be due to changes in the frequency of tropical storms. Tropical storms usually result in rainfall events greater than 2 inches in a day which occur at irregular intervals; these are less useful in an agricultural sense than are rainfall events that occur more frequently, even with lower accumulations. Plants growing in a high carbon dioxide environment may have lower water needs. In addition, widespread increased humidity will slow transpiration, further reducing the need for water. However, these benefits will probably be overshadowed by the lack of available water due to increased droughts and heat waves. The crops will transpire more heavily than when under “normal” growing conditions, and would likely need more water to adjust to these climactic changes. In anticipation of these changes, plant breeders are currently working to develop new varieties of crops that are considered to be drought tolerant, and more adaptable to varying levels of temperature and moisture.
While crops could be impacted by climate change, it is likely that farm animals would be even more susceptible to changes in the climate.
It is expected that increased air temperatures will cause more stress on livestock. Both humans and livestock are warm-blooded animals, so both are affected by increased heat and humidity. During stifling heat, livestock reproduction declines as well as their appetite. Decreased appetite will lengthen the time needed for the livestock to reach their target weight (most animals only eat about half of normal quantities when they are heat-stressed). Stress can also increase the incidence of sickness, decrease rates of reproduction, and increase fighting among animals in confinement. In some areas, night-time temperatures are even more above average than daytime temperatures during heat-waves, which has resulted in increased mortality rates. Despite the warmer winter temperatures, global warming could have a negative overall impact upon livestock.
As indicated above, increased carbon dioxide may result in feed and forage that is less nutritious even if there is more of it. It is likely that growers would be forced to use feed additives in order to see the expected growth gains in livestock, and to avoid illnesses. This increased cost to the grower would result in increased food costs to the consumer. Availability could also decrease if there is not enough water and nutrients in stressed soils to keep up with plant growth.
Insect parasites and diseases could also become more prolific as global warming progresses. New diseases may also emerge in the Southeast that were once considered to inhabit only tropical areas. It is expected that in cases of increased heat stress and humidity, most livestock will not be able to fight these diseases without the use of costly medicines.