A number of different physical variables impact agriculture. These include temperature, precipitation, humidity, wind speed, and radiation. The absolute levels of these variables and their variability on a daily, monthly, and annual basis, affect crop yields as well as livestock health.
Crops are particularly sensitive to absolute temperature variation even over short time scales, in some cases a few hours (for extreme cold) and days (for warmth). Likewise, extreme events such as floods, and inter-annual variations in rainfall connected with cycles such as ENSO can also impact crops significantly. For example, the major drought in Australia from 1998-2010 led to significantly lower crop yields. Major cold snaps in Florida in 1983 and 1985 killed a third of all citrus trees, with an accompanying loss of $2 billion. At the other end of the spectrum, the North Atlantic Oscillation has caused sunnier summers in Britain, leading to increased wheat yields.
As it turns out, anthropogenic impacts can greatly magnify the effects of climate change on crops, livestock, and fisheries. For example, soil erosion, overgrazing, air pollution, salinization of groundwater, and pests and overuse of pesticides tend to exacerbate the impacts of the changing climate such as droughts and heatwaves. Here, we describe the forecasts and impacts of changes in climate variables, followed by anthropogenic changes.
Model forecasts under SRES A1B and A2 are for 2-4oC warming by 2100 with the most significant increase in high-latitude regions. In addition, the forecasts indicate a much higher likelihood of heat waves in the future. As it turns out, an increase in average temperature can have a positive impact on agriculture, lengthening the growing season in regions with cool spring and fall seasons. Regional and global simulations allow predictions of temperature increase on crop yield. The results (see figure below) show that modest temperature increases produce increased yields for some crops. Warming will also lead to a decrease in the occurrence of severe winter cold stress on crops, causing a pole-ward shift in the feasibility of regions for agricultural activities. This is especially important for high-yield tropical crops such as rice. Warming will have a greater impact in the Northern Hemisphere, where there is more cultivated area in high latitude areas.
However, warming ultimately reaches a limit where yield curves start to decrease for all crops. Globally, this threshold is reached at temperature increases over about 3oC. Crop yields also decline precipitously at temperatures above 30oC; although plants develop faster in warm temperatures, photosynthesis has a temperature optimum in the range of 20° to 25°C, and, above this range, plants have less time to accumulate carbohydrates, fats, and proteins. Because individual plants cannot move, they have developed mechanisms that allow them to tolerate higher temperatures and adapt to changing soil conditions. These mechanisms include the production of proteins that lessen heat-shock, as well as the ability to conduct photosynthesis during periods of heat. Such adaptations will partially determine where a crop can survive the impact of climate change.
Warming will have negative impacts on crop yield in regions where summer heat limits production, and it will lead to more frequent extreme high heat stress on crops. Heat stress varies by plant but includes lack of emergence of new plant material or damage to it, water deficit as a result of high evaporation, damage to reproductive development, and death. In addition, climate change will lead to increased soil evaporation rates, which stress crops and also increase the intensity of droughts.
Heat waves are usually known for their human toll. For example, the Great Chicago heat wave of 1995 led to over 700 deaths as a result of five extremely warm and humid days with a heat index reaching 125 degrees. A prolonged heat wave in Northern Europe in 2003 killed more than 40,000 people and led to a 20 to 36% decrease in the yields of grains and fruits. Heat waves are defined differently in different places, but usually, are defined as a specific number of days over a certain temperature. Prolonged heat waves can also cause significant damage to crops and livestock, with major economic losses. In Russia, an extended heat wave in 2010 caused 50,000 deaths and a loss of 25% of the grain yield at a cost of $15 billion. In the central US, more than 6000 cattle died in the July 2011 heat wave. And the heat wave of 2012 in the same region resulted in the worst corn crop in two decades.
Models indicate that precipitation will increase in high latitudes including places such as Northern Europe and Canada, but decrease in most subtropical land regions, including the southern tier of states from Texas to California. Droughts will become longer and more intense in these areas. A decrease in precipitation can reduce soil moisture over the short term and increase soil erosion rates over the long term. Likewise, as we have seen, the intensity of extreme events such as cyclones and hurricanes is likely to increase, also leading to potentially more significant crop damage, and also potentially, soil erosion. Waterlogged soils can cause severe damage to root systems and limit the uptake of nutrients. Flooding can cause permanent loss of many crops. One of the most significant periods of flooding in the US took place in the Midwest in 1993 when the Mississippi and Missouri rivers flooded their banks and submerged huge areas of farmland, a total of nine million acres. The estimated crop losses during this event were $7 billion. In the 2008 Midwest floods, Iowa alone lost $4 billion in damaged crops.
Precipitation may be crucial for determining the impact of climate on crops. Decreases in precipitation and evaporation-precipitation ratios in marginal areas that are currently entirely fed by rain may change ecosystem function to the point where irrigation is required. Estimates suggest that impact of increased evaporation will require increased irrigation requirements of 5-8% globally with higher amounts (15%) in Southeast Asia.
A key variable controlled by a combination of heat stress and rainfall is the 120-day growing period, the minimum duration required for crops such as corn to survive. Climate change has the potential to shrink the 120-day growing period, and this will greatly impact the sustainability of crop production.
Drought can be devastating to agriculture. During the decade-long “Federation” drought in Australia, from 1901-1903, an estimated 52 million sheep perished. In the long Dustbowl of the 1930s, over 75% of topsoil was lost in areas of Kansas, Oklahoma, and Texas, and crops were ruined. In fact, in many areas, agriculture never recovered from the drought, and the economic losses and human suffering are still legendary.
The following videos describe recent droughts in Texas and Australia:
All SRES emission scenarios call for CO2 levels to increase significantly over the course of the 21st century. Even with drastic actions, levels are predicted to reach 550 ppm mid-century before decreasing. Worst-case scenarios have levels continuing to rise beyond 650 ppm at the end of the century. Compared to other climate variables, increasing CO2 generally has a positive impact on crops, leading to increased crop yields (see figure below). CO2 is key to photosynthesis; the gas is what is known as a limiting nutrient for plant growth. Without a certain amount of CO2, plants will fail to grow. CO2 acts as a fertilizer for crops like rice, soybeans, and wheat and enhances growth rates. The impact of increased CO2 is only known via experiments, and they show increases in production from 5-20% at CO2 levels of 550 ppm. The key uncertainty is how realistic experiments are at predicting the real world. There is a consensus among scientists that the real changes in yield might be slightly less than those in the lab.
Pollution from industry will increase tropospheric ozone levels. Ozone levels in the lower atmosphere are determined by both emissions and temperature, thus ground levels are almost certain to increase. Higher levels of ground-level ozone limit the growth of crops.
As with temperature and precipitation, the negative impact of increasing ozone on crops will offset the beneficial impact from elevated CO2 levels. At the same time, the ozone layer is becoming thinner in other areas, leading to increases in UV-B exposure. The future impact of ozone and UV-B on crops is not completely understood, as the increasing CO2 may possibly increase or decrease the effect. Increasing ozone and UV-B exposure can lead to reduced rates of photosynthesis and a number of other measures of crop stress, including sensitivity to drought.
Food production will be affected by a number of other factors, including rising sea level (see Module 10) that will swamp low-lying coastal areas that include some of the most productive areas of the world today. The potential area of crop production is also being reduced by desertification and salinization (increase in harmful salt levels in topsoil as a result of excess evaporation) as well as soil erosion over vast areas of the world. Soil erosion can result directly from climate change, for example, from increased precipitation in major storms. However, it is also a product of over-cultivation of crops and other poor agricultural practices.
The impact of climate change (summarized in the figures below) on North American agriculture varies significantly by region. Projections suggest yield increases of 5-20 percent over the first decades of the century as a result of warming and higher CO2, with generally positive effects for the nation as a whole for much of the century. However, regions of the continent will be much more vulnerable than others. In particular, the Great Plains will likely face declining yields as a result of drought. Crops that are limited by growing season, for example, fruit in the northeastern US, will benefit from improved growing conditions, whereas those crops that are near their climate thresholds, for example, grapes in California (as a result of low rainfall) will likely face lower yields and poorer quality. Drought in California will likely impact the yields of numerous crops.
Links
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