What are the factors that affect crop yield?

Compared with any other industry, agriculture may be able to more clearly highlight the scope and seriousness of the potential impact of climate change on food production, food security, loss of livelihood sources, environmental damage and ecological migration. Carrying out the "Green Earth Revolution" in the fields of crops and agricultural technology will help to reduce the emission of polluting gases, reduce losses and enhance our adaptability to change.

One of the greatest achievements of mankind in the 20th century is to successfully expand the scale of grain production, so that it can meet the growing grain demand due to population growth and income level improvement. The United Nations Food and Agriculture Organization estimates that as these two factors continue to promote the growth of food demand, the world food demand will increase by 50% compared with 1998 in 2030. Climate change will be an important factor to determine whether this demand can be met.

The latest assessment of the Intergovernmental Panel on Climate Change predicts that during the period of 1990-2 100, the global average surface temperature will rise by 1.4 ~ 5.8℃, while the sea level will rise by 9 ~ 88 cm in the same period. The global temperature rose by 0.6℃ in the whole 20th century, which was mainly caused by human activities.

Yes

Rising temperature will have the following effects on crop yield:

* transfer the best planting area of crops;

* Change the rainfall type (rainfall and variability) and the possible total amount of soil water evaporation and transpiration loss;

Reduce winter snowfall and water storage in the form of glaciers;

* Transfer the occurrence of crop diseases and insect pests;

:: Impact on crop yield through carbon dioxide and temperature;

* Due to sea level rise and fragile flood control capacity, the cultivated land area has decreased.

The overall impact of these effects will vary with altitude, soil type, crops and other local factors. These changes and the uncertainty of long-term climate prediction (especially in the regional scope) make our discussion on the impact of climate change on crop yield only preliminary at best. These general conclusions usually only indicate the scope of what may happen in the future.

Generally speaking, agriculture in many temperate regions may benefit from it, which is manifested in the extension of crop growth period, the reduction of livestock wintering cost, the increase of crop yield and the faster growth of forests. But for many tropical areas, the overall situation is negative: the variability of rainfall may become greater, the probability of extreme weather phenomena will increase, and crops may be reduced. Improvements in crops, farming techniques and land and water resources management may be compensated, but it will be extremely difficult to increase food production in these tropical areas.

As the temperature rises, some areas where crops grow well due to certain climatic conditions may move to the poles and higher altitudes. This will lead to the reduction of grain output and export income in some tropical countries. For example, for 26 poor countries in Africa and Central America, coffee ranks first, second or third in their agricultural exports. However, coffee is very sensitive to the change of annual average temperature. In Uganda, a temperature rise of only 2℃ will greatly reduce the land area suitable for growing coffee in northern latitude, and global warming may expand the potential crop planting area northward, so Canada and Russia will gain the largest area in this area. However, the soil types in the new climate zone may not be suitable for the current high-density agricultural operations in these major agricultural producing countries.

Changes in Rainfall Due to global warming, rainfall types in many parts of the world may change. According to the prediction of the Intergovernmental Panel on Climate Change, the global average annual rainfall will increase in 2 1 century, although the rainfall will decrease to some extent in some areas. In areas where rainfall is expected to increase, the annual rainfall may vary greatly. Water shortage during flowering, pollination and grain filling usually reduces the yield of corn, soybean, wheat and sorghum.

The change of rainfall and the increase of soil water evaporation and transpiration loss will further lead to the shortage of water resources in some parts of the world and affect its water quality. Access to water resources is the main factor to ensure food security (Figure 2). Agricultural water consumption accounts for almost 70% of global water consumption, and the proportion in Asia and West Asia is as high as 95%. Therefore, the shortage of water resources will seriously affect agriculture.

Changes in rainfall will also affect soil moisture. The recent analysis of 15 climate models in the world shows that all the climate models give some identical prediction results. Due to global warming, the southwestern United States, Mexico, Central America, the Mediterranean, Australia and southern Africa will increase soil moisture evaporation and make the soil dry all year round. In the Amazon basin and many areas of West Africa, soil drying will occur in June, July and August, while in the Asian monsoon region, soil drying will occur in 65438+February, 65438+ 10 and February. However, these 15 models can only get very consistent conclusions in the non-planting season in the middle and high latitudes of the northern hemisphere. The conclusion of this study is that global warming may lead to insufficient soil moisture, which will lead to the overall reduction of global grain production potential.

Changes in rainfall will also affect the flow of rivers and irrigation water. In areas that rely on snowmelt for agricultural irrigation, such as many areas in South Asia, the reduction of glaciers and snowfall may have serious consequences for water supply in summer.

Climate change will further aggravate the frequency and intensity of droughts in Central Asia, North Africa and South Africa, the Middle East, the Mediterranean region and Australia. The increase of the frequency and intensity of extreme weather phenomena (including drought, storm and flood) will lead to the destruction of crops and land degradation. Drought and flood have become the most common causes of serious food shortages in developing countries.

The trend of climate warming will increase the number, growth rate and geographical distribution of many major crop pests. At the same time, according to the change of rainfall type, climate warming may also stimulate microbial pathogens.

The impact of climate change on crop pests has already appeared in some areas. Here are some examples from around the world:

* In northern New Zealand, the decrease of frost probability has led to an increase in the number of tropical grassland caterpillars, which has caused serious damage to pastures.

* Citrus canker is a highly contagious bacterial disease, and it likes hot and heavy rain. The disease spread to Florida through hurricanes and infected all citrus crops in the state.

Bean leaf beetle is a pest that harms soybean by spreading pod mottle virus. This pest has spread from the southern United States to the central and northern midwest.

Source: Willoughby and Barnes, 2002; Wildlife Trust Fund, 2005; Epstein and Mills, 2005; Prichard and

Ameteau, 2005.

The influence of temperature changes The yield of crops will change greatly due to different temperatures (Figure 4). Even a slight warming will lead to a decline in rice yield, because rice is suitable for growing in a climate close to the maximum temperature difference between day and night. A recent study on the impact of global warming on rice yield in the Philippines shows that rice yield will decrease by 10% every time the daily average minimum temperature (at night) increases by one degree during the growing season.

The influence of carbon dioxide The increase of carbon dioxide concentration in the atmosphere can improve the net productivity of many crops, because the "fertility" of carbon can increase photosynthesis. The extent of this influence varies with different crops. Carbon dioxide has a positive fertilization effect on some crops, namely C3 crops. These crops include the main cereal crops in Europe and Asia, namely wheat and rice. On the other hand, C4 crops such as corn, sorghum, sugarcane and millet have no special response to carbon dioxide. Because C3 weeds may also respond well to this change, it is possible to inhibit the yield of C4 crops. C4 crops are the main food sources for agriculture in tropical Africa and Latin America.

Recent research on experimental crops of corn, wheat, soybean and rice planted in real environment in China, Japan and the United States shows that due to other environmental factors such as high ground ozone concentration, the fertility improvement brought by the increase of farmland carbon dioxide concentration may be only half of its theoretical best effect.

The decrease of land caused by sea level rise may reduce the fertile land area in some of the most densely populated areas in the world, especially those low-lying delta areas, such as the Nile, Mekong and Ganges-Brahmaputra river basins. For example, sea level rise 1m will cause the land in the lower Nile Delta to decrease by 5,800km2, directly affecting Egypt 15% of the habitable land. In Bangladesh, sea level rise 1 m will inundate nearly 30,000 square kilometers, affecting more than 65,438+03% of the country's residents. If this happens in Viet Nam, 40 000km2 of land will disappear and 23% of China residents will be affected (table 1). Even in those areas that are not submerged by seawater, the quality of soil will decline due to the salinization of soil and groundwater and the increased risk of tidal surge.