Changing Climate and Resource use Efficiency in Plants

By Amitav Bhattacharya

Changing Climate and Resource Use Efficiency in Plants reviews the efficiencies for resource use by crop plants under different climatic conditions. This book focuses on the challenges and potential remediation methods for a variety of resource factors. Chapters deal with the effects of different climatic conditions on agriculture, radiation use efficiency under various climatic conditions, the efficiency of water and its impact on harvest production under restricted soil moisture conditions, nitrogen and phosphorus use efficiency, nitrogen use efficiency in different environmental conditions under the influence of climate change, and various aspects of improving phosphorus use efficiency.

The book provides guidance for researchers engaged in plant science studies, particularly Plant/Crop Physiology, Agronomy, Plant Breeding and Molecular Breeding. In addition, it provides valuable insights for policymakers, administrators, plant-based companies and agribusiness companies.

• Explores climatic effects on agriculture through radiation, water, nitrogen, and phosphorus-use efficiency
• Guides the planning and research of, and recommendations for, fertilizer application for different crops under various climatic conditions
• Discusses efficiency improvements for plant and molecular breeders seeking to maximize resource use

• Language: English
• Publisher: Academic Press
• Release date: Nov 1, 2018
• ISBN: 9780128168370

Amitav Bhattacharya

Amitav Bhattacharya, Principal Scientist from Indian Institute of Pulses Research - Retired, Kanpur, India. Amitav Bhattacharya earned his M. Sc. degree in Botany (with special paper in Plant Physiology) from Allahabad University, Allahabad, India and his Ph. D. degree in Plant Physiology from the Indian Agricultural Research Institute, New Delhi, India. He is now retired from his role as Principal Scientist at the Indian Institute of Pulses Research, Kanpur - 208024, India where he served approx. 41 years, actively engaged in conducting laboratory as well as field research on pulses (grain legumes). He has published extensively with both books and book chapters.

Book preview

Changing Climate and Resource Use Efficiency in Plants

Amitav Bhattacharya

Former Principal Scientist (Plant Physiology), Indian Institute of Pulses Research, Kanpur, Uttar Pradesh, India

Table of Contents

Cover image

Title page

Copyright

Preface

Chapter 1. Global Climate Change and Its Impact on Agriculture

Abstract

1.1 Climate Change

1.2 Crop Responses to Expected Climate Change Factors

1.3 Interactive Effects of Carbon Dioxide and Climate Change

1.4 Agricultural Greenhouse Gas Sinks

1.5 Climate Change and Resource Use Efficiency

1.6 Climate Change and Water-Use Efficiency

1.7 Climate Change and Radiation-Use Efficiency

1.8 Climate Change and Nitrogen-Use Efficiency

1.9 Implications of Fertilizers Under Changing Climate

1.10 Impact of Climate Change on Agriculture

1.11 Direct Impacts of Climate Change on Agriculture

1.12 Nonclimate Impacts Related to Greenhouse Gas Emissions: Impacts of Changes in Atmospheric Composition

1.13 Climate Change Impact on Water Availability

1.14 Climate Change Impacts on Crop Water Productivity

1.15 Agricultural Productivity and Food Security

1.16 Future Impact of Climate Change

References

Further Reading

Chapter 2. Radiation-Use Efficiency Under Different Climatic Conditions

Abstract

2.1 Solar Radiation

2.2 Effect of Interception of Radiation Leaf Area Index on Crop Growth and Production

2.3 Low Soil Moisture and Radiation-Use Efficiency

2.4 Radiation-Use Efficiency as Affected by Temperature

2.5 Light Interception Efficiency

2.6 Radiation-Use Efficiency Under Different Abiotic Factors

2.7 Canopy Structure, Row Orientation, and Radiation-Use Efficiency

2.8 Radiation-Use Efficiency and Crop Growth

2.9 Radiation-Use Efficiency and Crop Yield

2.10 Temperature and Radiation-Use Efficiency

2.11 Low Soil Moisture and Radiation-Use Efficiency

2.12 Elevated CO2 Concentration and Radiation-Use Efficiency

2.13 Genetic Variability in Radiation-Use Efficiency

2.14 Avenues for Genetic Modification of Radiation-Use Efficiency

References

Further Reading

Chapter 3. Water-Use Efficiency Under Changing Climatic Conditions

Abstract

3.1 Water-Use Efficiency

3.2 Properties of Water

3.3 Water at Equilibrium: Water Potential and Its Components

3.4 Models for Water-Use Efficiency

3.5 Plant Growth and Yield in Relation to Water-Use Efficiency

3.6 Carbon Isotope Discrimination and Water-Use Efficiency

3.7 Water-Use Efficiency Under High and Low Temperatures

3.8 Water-Use Efficiency Under Excess and Limited Water Conditions

3.9 Effect of Edaphic Factors on Water-Use Efficiency

3.10 Water-Use Efficiency in Relation to CO2 Concentration

3.11 Wind Velocity and Water-Use Efficiency

3.12 Water-Use Efficiency Under Diffused Light

3.13 Effect of Morphophysiological Trait on Water-Use Efficiency

3.14 Effect of Fertilizers on Water-Use Efficiency

3.15 Strategies for Improvement of Water-Use Efficiency

3.16 Soil Management for Higher Water-Use Efficiency

3.17 Breeding for High Water-Use Efficiency

3.18 Increasing Water-Use Efficiency Through Molecular Genetics

References

Further Reading

Chapter 4. Nitrogen-Use Efficiency Under Changing Climatic Conditions

Abstract

4.1 Importance of nitrogen

4.2 What is nitrogen-use efficiency

4.3 Role of nitrogen in plant growth

4.4 Plant and soil factors influencing nitrogen-use efficiency

4.5 Nitrogen-use efficiency under low soil moisture condition

4.6 Nitrogen-use efficiency under soil salinity conditions

4.7 Nitrogen-use efficiency under varying nitrogen levels

4.8 Genetic and environmental variations in nitrogen-use efficiency

4.9 Management effects on nitrogen-use efficiency

4.10 Approaches for increasing nitrogen through water-use efficiency

4.11 Maximizing nitrogen-use efficiency

References

Chapter 5. Changing Environmental Condition and Phosphorus-Use Efficiency in Plants

Abstract

5.1 Importance of Phosphorus

5.2 Phosphorus Cycle

5.3 Phosphorus in Agriculture and Phosphorus-Use Efficiency

5.4 Assessment of Phosphorus-Use Efficiency

5.5 Role of Phosphorus in Plant Growth and Yield

5.6 Phosphorus Requirements of Different Crops

5.7 Factors of Phosphorus Availability

5.8 Phosphorus-Use Efficiency With and Without Nitrogen

5.9 Microbial Mobilization of Soil Phosphorus

5.10 Effect of Phosphorus Application on Phosphorus-Use Efficiency

5.11 Effect of Elevated Carbon Dioxide on Phosphorus-Use Efficiency

5.12 Improving Phosphorus-Use Efficiency

5.13 Genetics and Breeding for Phosphorus-Use Efficiency

5.14 Improving Phosphorus-Use Efficiency With Polymer Technology

References

Further Reading

Index

Copyright

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Preface

Amitav Bhattacharya, Former Principal Scientist (Plant Physiology), Indian Institute of Pulses Research, Kanpur, Uttar Pradesh, India

In 2013, the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report concluded that "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. The largest human influence has been the emission of greenhouse gases such as carbon dioxide, methane and nitrous oxide. Climatic model projections summarized in the report indicated that during the 21st century, the global surface temperature is likely to rise a further 0.3−1.7°C in the lowest emissions scenario and 2.6−4.8°C in the highest emissions scenario." These findings have been recognized by the national science academies of the major industrialized nations and are not disputed by any scientific body of national or international standing. Future climate change and associated impacts will differ from region to region around the globe. Anticipated effects include increasing global temperatures, rising sea levels, changing precipitation, and expansion of deserts in the subtropics. Warming is expected to be greater over land than over the oceans. Other likely changes include more frequent extreme weather events such as heat waves, drought, heavy rainfall with floods, and heavy snowfall due to shifting temperature regimes. Effects significant to humans include the threat to food security from decreasing crop yields.

In modern agriculture use of essential plant nutrients in crop production is very important to increase productivity and maintain the sustainability of the cropping system. Use of nutrients in crop production is influenced by climatic, soil, and plant factors, and the socioeconomic condition of farmers. Overall, nutrient-use efficiency by crop plants is lower than 50% under all agro-ecological conditions. Hence, a large part of the applied nutrients is lost in the soil–plant system. The lower nutrient-use efficiency is related to loss and/or unavailability due to many environmental factors and it not only increases the cost of crop production but is also responsible for environmental pollution. Nutrient-use efficiency is designated as nutrient-efficiency ratio, agronomic efficiency, physiological efficiency, agro-physiological efficiency, apparent recovery efficiency, and utilization efficiency. The environmental pollution of post-revolution, high-yielding agriculture, is often associated with the law of diminishing returns which states that the relationship between the amount of a production factor and the yield level is not linear, but levels off so that more and more external inputs are needed to push up yields to their potential level. However, this law is formulated for situations where all other production factors stay the same and this is obviously not the case when changes over time are considered.

This book is intended to give a review on the efficiencies for resource use by crop plants under different climatic conditions. The first chapter deals with the effects of different climatic conditions on agriculture. It is an important chapter as the effects of various climatic conditions on agriculture and the harvestable product are discussed. The second chapter deals with radiation-use efficiency under various climatic conditions. Radiation use is an integral part of plant life and any change in the efficiency of radiation use under the influence of climatic change affects the other physiological processes, as well as the quantum of harvest. The third chapter deals with the efficiency of another natural resource, that is, water. Water use is an integral part of plant life and any change in the efficiency of water use would decide the quantum of harvest under restricted soil moisture condition. The next two chapters deal with nitrogen- and phosphorus-use efficiency. In the light of escalating prices of nitrogenous and phosphatic fertilizers these two chapters are important enough. The fourth chapter deals with nitrogen-use efficiency in different environmental conditions under the influence of climate change. The importance of nitrogen for plant growth and harvestable product is well known and therefore the importance of this chapter is also self-explanatory. The fifth chapter is on various aspects of phosphorus-use efficiency, including strategies for improvement of this efficiency under different climatic conditions.

The book would be useful for students, teachers, and researchers engaged in plant science studies, particularly plant/crop physiology, agronomy, plant breeding, and molecular breeding. The book would be of great help for policymakers, administrators, plant-based companies, and agribusiness companies. Suggestions and advice are most welcome for improvements in the book structure and for additions of current technologies as well as methodologies. A feeling of successful would prevail in my efforts in publishing this book if I was to know how meaningful this text has been to readers. Suggestions from researchers, teachers, and students, who use the book, would be highly appreciated. I hope this book serves the needs of students, faculty members, and researchers. I sincerely hope the review chapters of this book meet the requirements of postgraduate students, as well as faculty members of plant physiology and will be glad to receive constructive criticism and suggestions from the faculty.

Chapter 1

Global Climate Change and Its Impact on Agriculture

Abstract

Global climate change is not a new phenomenon. The effect of climate change poses many threats; one of the important consequences is bringing about changes in the quality and quantity of water resources and crop productivity. It can be concluded that the Indian region is highly sensitive to climate change. The agriculture sector is the most prone sector as it will have a direct bearing on the lives of 1.2 billion people. India has set a target of halving greenhouse gas emissions by 2050. There is an urgent need for coordinated efforts to strengthen the research to assess the impact of climate change on agriculture, forests, animal husbandry, aquatic life, and other living beings. Climate change, the outcome of global warming, has now started showing its impacts worldwide. Climate is the primary determinant of agricultural productivity which directly impacts on food production across the globe. The agriculture sector is the most sensitive sector to climate changes because the climate of a region/country determines the nature and characteristics of vegetation and crops. An increase in the mean seasonal temperature can reduce the growing duration of many crops and hence reduce final yield. Food production systems are extremely sensitive to climate changes, like changes in temperature and precipitation, which may lead to outbreaks of pests and diseases, thereby reducing the harvest and ultimately affecting the food security of the country. The net impact of food security will depend on the exposure to global environmental change and the capacity to cope with and recover from global environmental change. Coping with the impact of climate change on agriculture will require careful management of resources like soil, water, and biodiversity. To cope with the impact of climate change on agriculture and food production, India will need to act at the global, regional, national, and local levels. It has long been concerned with how crop water use efficiency responds to climate change. Most of the existing research has emphasized the impact of single climate factors but paid less attention to the effect of developed agronomic measures on crop water use efficiency. Based on the long-term field observations/experiments data, investigation have been carried out on changing responses of crop water use efficiency to climate variables (temperature and precipitation) and agronomic practices (fertilization and cropping patterns) in semiarid areas.

Keywords

Global climate change; global warming; crop productivity; water use efficiency; radiation use efficiency; nitrogen use efficiency; food security; sustainable agriculture

Chapter Outline

1.1 Climate Change 1

1.1.1 Weather and Climate 3

1.2 Crop Responses to Expected Climate Change Factors 7

1.2.1 Light 8

1.2.2 Temperature 8

1.2.3 Precipitation 9

1.2.4 Wind 9

1.3 Interactive Effects of Carbon Dioxide and Climate Change 15

1.3.1 Photosynthetic and Productivity Interactions 15

1.4 Agricultural Greenhouse Gas Sinks 17

1.5 Climate Change and Resource Use Efficiency 17

1.6 Climate Change and Water-Use Efficiency 18

1.7 Climate Change and Radiation-Use Efficiency 20

1.8 Climate Change and Nitrogen-Use Efficiency 20

1.9 Implications of Fertilizers Under Changing Climate 22

1.10 Impact of Climate Change on Agriculture 24

1.11 Direct Impacts of Climate Change on Agriculture 25

1.11.1 Changes in Mean Climate 25

1.11.2 Climate Variability and Extreme Weather Events 26

1.11.3 Extreme Temperatures 26

1.11.4 Drought 27

1.11.5 Heavy Rainfall and Flooding 27

1.11.6 Tropical Storms 28

1.12 Nonclimate Impacts Related to Greenhouse Gas Emissions: Impacts of Changes in Atmospheric Composition 28

1.12.1 CO2 Fertilization 28

1.12.2 Ozone 29

1.13 Climate Change Impact on Water Availability 31

1.14 Climate Change Impacts on Crop Water Productivity 32

1.15 Agricultural Productivity and Food Security 34

1.16 Future Impact of Climate Change 36

References 37

Further Reading 50

1.1 Climate Change

Climate change is a change in the statistical distribution of weather patterns when that change lasts for an extended period of time (i.e., decades to millions of years). Climate change may refer to a change in average weather conditions, or in the time variation of weather around longer-term average conditions (i.e., more or fewer extreme weather events). Climate change is caused by factors such as biotic processes, variations in solar radiation received by Earth, plate tectonics, and volcanic eruptions. Certain human activities have been identified as primary causes of ongoing climate change, often referred to as global warming. Scientists actively work to understand past and future climate by using observations and theoretical models. A climate record—extending deep into the Earth’s past—has been assembled, and continues to be built up, based on geological evidence from borehole temperature profiles, cores removed from deep accumulations of ice, floral and faunal records, glacial and preglacial processes, stable-isotope and other analyses of sediment layers, and records of past sea levels. More recent data are provided by instrumental records. General circulation models (GCMs), based on the physical sciences, are often used in theoretical approaches to match past climate data, make future projections, and link causes and effects in climate change.

The most general definition of climate change is a change in the statistical properties of the climate system when considered over a long period of time, regardless of cause. Accordingly, fluctuations over a period shorter than a few decades, such as El Nino, do not represent climate change. On the broadest scale, the rate at which energy is received from the Sun and the rate at which it is lost to space determine the equilibrium temperature and climate of Earth. This energy is distributed around the globe by winds, ocean currents, and other mechanisms to affect the climates of different regions. Factors that can shape climate are called climate forcings or forcing mechanisms. These include processes such as variations in solar radiation, variations in the Earth’s orbit, variations in the albedo or reflectivity of the continents, atmosphere, mountain building in ocean's bed, continental drift and changes in greenhouse gas concentrations. There are a variety of climate change feedbacks that can either amplify or diminish the initial forcing. Some parts of the climate system, such as the oceans and ice caps, respond more slowly in reaction to climate forcings, while others respond more quickly.

Whether the initial forcing mechanism is internal or external, the response of the climate system might be fast (e.g., a sudden cooling due to airborne volcanic ash reflecting sunlight), slow (e.g., thermal expansion of warming ocean water), or a combination (e.g., sudden loss of albedo in the Arctic Ocean as sea ice melts, followed by more gradual thermal expansion of the water). Therefore, the climate system can respond abruptly, but the full response to forcing mechanisms might not be fully developed for centuries or even longer. Global climate change is a change in the long-term weather patterns that characterize the regions of the world. The term weather refers to the short-term (daily) changes in temperature, wind, and/or precipitation of a region. Weather is influenced by the Sun. The Sun heats the Earth’s atmosphere and its surface, causing air and water to move around the planet. The result can be as simple as a slight breeze or as complex as the formation of a tornado. Some of the Sun’s incoming long-wave radiation is reflected back to space by aerosols. Aerosols are very small particles of dust, water vapor, and chemicals in the Earth’s atmosphere. In addition, some of the Sun’s energy that has entered Earth’s atmosphere is reflected into space by the planet’s surface. The reflectivity of the Earth’s surface is called albedo. Both of these reflective processes have a cooling effect on the planet.

The real threat of climate change lies in how rapidly the change occurs. For example, over the past 130 years, the mean global temperature appears to have risen 0.6–1.2°F (0.3–0.7°C). Further evidence suggests that future increases in mean global temperature may occur at a rate of 0.4°F (0.2°C) each decade. During the history of the Earth, there have been changes in global temperatures similar in size to these changes. However, the past changes occurred at much slower rates, and thus they were spread out over long periods of time. The slow rate of change allowed most species enough time to adapt to the new climate. The current and predicted rates of temperature change, on the other hand, may be harmful to ecosystems. This is because these rates of temperature change are much faster than those of the Earth’s past. Many species of plants, animals, and microorganisms may not have enough time to adapt to the new climate. These organisms may become extinct.

Climate change and variability are concerns to human beings. The recurrent droughts and floods threaten seriously the livelihood of billions of people who depend on land for most of their needs. The global economy is adversely being influenced very frequently due to extreme events such as droughts and floods, cold and heat waves, forest fires, landslips, etc. Natural calamities, like earthquakes, tsunamis, and volcanic eruptions, though not related to weather disasters, may change the chemical composition of the atmosphere. This will, in turn, lead to weather-related disasters. An increase in aerosols (atmospheric pollutants) is due to emission of greenhouse gases (GHGs) such as carbon dioxide due to burning of fossil fuels, chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), etc. Ozone depletion and UV-B-filtered radiation, eruption of volcanoes, the human hand in deforestation in the form of forest fires and loss of wet lands are causal factors for weather extremes. The loss of forest cover, which normally intercepts rainfall and allows it to be absorbed by the soil, causes precipitation to reach across the land, eroding top soil and causing floods and droughts. Paradoxically, a lack of trees also exacerbates drought in dry years by making the soil dry more quickly. Among the GHGs, CO2 is the predominant gas leading to global warming as it traps long-wave radiation and emits it back to the Earth’s surface. Global warming is nothing but heating of the surface atmosphere due to the emission of GHGs, thereby increasing global atmospheric temperature over a long period of time. Such changes in surface air temperature and consequent adverse impact on rainfall over a long period of time are known as climate change. If these parameters show year-to-year variations or cyclic trends, it is known as climate variability.

The greenhouse effect is a warming process that balances Earth’s cooling processes. During this process, sunlight passes through Earth’s atmosphere as short-wave radiation. Some of the radiation is absorbed by the planet’s surface. As Earth’s surface is heated, it emits long-wave radiation toward the atmosphere. In the atmosphere, some of the long-wave radiation is absorbed by certain gases called GHGs. Each molecule of greenhouse gas becomes energized by long-wave radiation. The energized molecules of gas then emit heat energy in all directions. By emitting heat energy toward Earth, GHGs increase Earth’s temperature. Note that the warning mechanism for the greenhouse effect is NOT exactly the same as the warning mechanism of greenhouse walls. While GHGs absorb long-wave radiation then emit heat energy in all directions, greenhouse walls physically trap heat inside greenhouses and prevent it from escaping to the atmosphere. The greenhouse effect is a natural occurrence that maintains Earth’s average temperature at approximately 60°F. The greenhouse effect is a necessary phenomenon that keeps all Earth’s heat from escaping to the outer atmosphere. Without the greenhouse effect, temperatures on Earth would be much lower than they are now, and the existence of life on this planet would not be possible. However, too many GHGs in Earth’s atmosphere could increase the greenhouse effect. This could result in an increase in mean global temperatures as well as changes in precipitation patterns.

When weather patterns for an area change in one direction over long periods of time, they can result in a net climate change for that area. The key concept in climate change is time. Natural changes in climate usually occur over such long periods of time that they are often not noticed within several human lifetimes. This gradual nature of the changes in climate enables plants, animals, and microorganisms on Earth to evolve and adapt to the new temperatures, precipitation patterns, etc. However, the official definition by the United Nations Framework Convention on Climate Change (UNFCCC) is that climate change is the change that can be attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods. However, scientists often use the term for any change in climate, whether arising naturally or from human causes. In particular, the Intergovernmental Panel on Climate Change (IPCC) defines climate change as a change in the state of the climate that can be identified by changes in the mean and/or the variability of its properties, and that persists for an extended period, typically decades or longer.

1.1.1 Weather and Climate

Weather is the set of meteorological conditions such as wind, rain, snow, sunshine, temperature, etc., at a particular time and place. By contrast, the term climate describes the overall long-term characteristics of the weather experienced at a place. The ecosystems, agriculture, livelihoods, and settlements of a region are very dependent on its climate. The climate, therefore, can be thought of as a long-term summary of weather conditions, taking account of the average conditions as well as the variability of these conditions. The fluctuations that occur from year to year and the statistics of extreme conditions, such as severe storms or unusually hot seasons, are part of the climatic variability. The Earth‘s climate has varied considerably in the past, as shown by the geological evidence of ice ages and sea level changes, and by the records of human history over many hundreds of years. The causes of past changes are not always clear but are generally known to be related to changes in ocean currents, solar activity, volcanic eruptions, and other natural factors. The difference now is that global temperatures have risen unusually rapidly over the last few decades. There is strong evidence of an increase in average global air and ocean temperatures, widespread melting of snow and ice, and rising of average global sea levels. The IPCC Fourth Assessment Report concludes that global warming is unequivocal. Atmosphere and ocean temperatures are higher than they have been at any other time during at least the past five centuries, and probably for more than a millennium. Scientists have long known that the atmosphere’s GHGs act as a blanket, which traps incoming solar energy and keeps the Earth’s surface warmer than it otherwise would be, and that an increase in atmospheric GHGs would lead to additional warming.

Climate change is now affecting every country on every continent. It is disrupting national economies and affecting lives, costing people, communities, and countries dearly today and even more tomorrow. People are experiencing the significant impacts of climate change, which include changing weather patterns, rising sea levels, and more extreme weather events. The greenhouse gas emissions from human activities are driving climate change and continue to rise. They are now at their highest levels in history. Without action, the world’s average surface temperature is projected to rise over the 21st century and is likely to surpass 3°C this century—with some areas of the world expected to warm even more. The poorest and most vulnerable people are being affected the most. Affordable, scalable solutions are now available to enable countries to leapfrog to cleaner, more resilient economies. The pace of change is quickening as more people are turning to renewable energy and a range of other measures that will reduce emissions and increase adaptation efforts.

According to the United Nation’s Goals for Sustainable Development, climate change is a global challenge that does not respect national borders. Emissions anywhere affect people everywhere. It is an issue that requires solutions that need to be coordinated at the international level and it requires international cooperation to help developing countries move toward a low-carbon economy. To address climate change, countries adopted the Paris Agreement at the COP21 in Paris on December 12, 2015. The Agreement entered into force shortly thereafter, on November 4, 2016. In the agreement, all countries agreed to work to limit global temperature rise to well below 2°C, and given the grave risks, to strive for 1.5°C.

Implementation of the Paris Agreement is essential for the achievement of the Sustainable Development Goals, and provides a roadmap for climate actions that will reduce emissions and build climate resilience. Thanks to the IPCC we know:

• From 1880 to 2012, average global temperature increased by 0.85°C. To put this into perspective, for each 1°C of temperature increase, grain yields decline by about 5%. Maize, wheat, and other major crops have experienced significant yield reductions at the global level of 40 mega tons per year between 1981 and 2002 due to a warmer climate.

• Oceans have warmed, the amounts of snow and ice have diminished, and sea levels have risen. From 1901 to 2010, the global average sea level rose by 19 cm as oceans expanded due to warming and ice melted. The Arctic’s sea ice extent has shrunk in every successive decade since 1979, with 1.07 million km² of ice loss every decade.

• Given current concentrations and on-going emissions of GHGs, it is likely that by the end of this century, the increase in global temperature will exceed 1.5°C compared to 1850–1900 for all but one scenario. The world’s oceans will warm and ice melt will continue. Average sea level rise is predicted as 24–30 cm by 2065 and 40–63 cm by 2100. Most aspects of climate change will persist for many centuries even if emissions are stopped.

• Global emissions of carbon dioxide (CO2) have increased by almost 50% since 1990.

• Emissions grew more quickly between 2000 and 2010 than in each of the three previous decades.

• It is still possible, using a wide array of technological measures and changes in behavior, to limit the increase in global mean temperature to 2°C above preindustrial levels.

• Major institutional and technological change will give a better than even chance that global warming will not exceed this threshold.

Climate change and agriculture are interrelated processes, both of which take place on a global scale. Climate change affects agriculture in a number of ways, including through changes in average temperature, rainfall, and climate extremes (e.g., heat waves); changes in pests and diseases; changes in atmospheric carbon dioxide and ground-level ozone concentrations; changes in the nutritional quality of some foods; and changes in sea levels. Climate change is already affecting agriculture, with effects unevenly distributed across the world. Future climate change will likely negatively affect crop productivity in low-latitude countries, while effects in northern latitudes may be positive or negative. Climate change will probably increase the risk of food security for some vulnerable groups, such as the poor. Agriculture contributes to climate change by (1) anthropogenic emissions of GHGs, and (2) by the conversion of nonagricultural land (e.g., forests) into agricultural land. Agriculture, forestry, and land-use change contributed around 20–25% to global annual emissions in 2010. There are a range of policies that can reduce the risk of negative climate change impacts on agriculture, and to reduce greenhouse emissions from the agriculture sector.

Climate change is any significant long-term change in the expected patterns of average weather of a region (or the whole Earth) over a significant period of time. It is about nonnormal variations to the climate, and the effects of these variations on other parts of the Earth. These changes may take tens, hundreds, or perhaps millions of year. But increases in anthropogenic activities such as industrialization, urbanization, deforestation, agriculture, change in land use patterns, etc., lead to the emission of GHGs due to which the rate of climate change is much faster. Climate change scenarios include higher temperatures, changes in precipitation, and higher atmospheric CO2 concentrations. There are three ways in which the greenhouse effect may be important for agriculture. First, increased atmospheric CO2 concentrations can have a direct effect on the growth rate of crop plants and weeds. Secondly, CO2-induced changes of climate may alter levels of temperature, rainfall, and sunshine that can influence plant and animal productivity (Mahato, 2014).

The greenhouse effect is a natural process that plays a major part in shaping the Earth’s climate. It produces the relatively warm and hospitable environment near the Earth’s surface where humans and other life-forms have been able to develop and prosper. However, the increased level of GHGs (carbon dioxide (CO2), water vapor (H2O), methane (CH4), nitrous oxide (N2O), HFCs, PFCs, sulfur hexafluoride (SF6), etc.) due to anthropogenic activities has contributed to an overall increase of the Earth’s temperature, leading to global warming. The average global surface temperature has increased by 0.74°C since the late 19th century and is expected to increase by 1.4–5.8°C by 2100 AD with significant regional variations (IPCC, 2007a,c,d). The atmospheric CO2 concentration has increased from 280 to 395 ppm, the CH4 concentration increased from 715 to 1882 ppb, and the N2O concentration from 227 to 323 ppb from the year 1750 to 2012. The global warming potential of these gases, that is, CO2, CH4, and N2O are 1, 25, and 310, respectively (Mahato, 2014) (Fig. 1.1).

Figure 1.1 Global carbon emissions from fossil fuels 1900–2011. Boden, T.A., Marland, G., Andress, R.J., 2015. Global Regional and National Fossil-Fuel CO2 Emmission. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy (Boden et al., 2015).

Earth receives energy from the Sun in the form of ultraviolet, visible, and near-infrared radiation. Of the total amount of solar energy available at the top of the atmosphere, about 26% is reflected to space by the atmosphere and clouds and 19% is absorbed by the atmosphere and clouds. Most of the remaining energy is absorbed at the surface of the Earth. Because Earth’s surface is colder than the photosphere of the Sun, it radiates at wavelengths that are much longer than the wavelengths that were absorbed. Most of this thermal radiation is absorbed by the atmosphere, thereby warming it. In addition to the absorption of solar and thermal radiation, the atmosphere gains heat by sensible and latent heat fluxes from the surface. The atmosphere radiates energy both upwards and downwards; the part radiated downwards is absorbed by the surface of Earth. This leads to a higher equilibrium temperature than if the atmosphere were absent (Fig. 1.2).

Figure 1.2 The solar radiation spectrum for direct light at both the top of Earth’s atmosphere and at sea level. Adopted from Rohde, R.A. as part of the Global Warming Art Project.

An ideal thermally conductive blackbody at the same distance from the Sun as Earth would have a temperature of about 5.3°C. However, because Earth reflects about 30% of the incoming sunlight, this idealized planet’s effective temperature (the temperature of a blackbody that would emit the same amount of radiation) would be about −18°C. The surface temperature of this hypothetical planet is 33°C below Earth’s actual surface temperature of approximately 14°C. The basic mechanism can be qualified in a number of ways, none of which affects the fundamental process. The atmosphere near the surface is largely opaque to thermal radiation (with important exceptions for window bands), and most heat loss from the surface is by sensible heat and latent heat transport. Radiative energy losses become increasingly important higher in the atmosphere, largely because of the decreasing concentration of water vapor, an important greenhouse gas. It is more realistic to think of the greenhouse effect as applying to a surface in the mid-troposphere, which is effectively coupled to the surface by a lapse rate. The simple picture also assumes a steady state, but in the real world, there are variations due to the diurnal cycle as well as the seasonal cycle and weather disturbances. Solar heating only applies during daytime. During the night, the atmosphere cools somewhat, but not greatly, because its emissivity is low. Diurnal temperature changes decrease with height in the atmosphere.

Within the region where radiative effects are important, the description given by the idealized greenhouse model becomes realistic. Earth’s surface, warmed to a temperature of around 255K, radiates long-wavelength, infrared heat in the range of 4–100 μm. At these wavelengths, GHGs that were largely transparent to incoming solar radiation are more absorbent. Each layer of atmosphere with GHGs absorbs some of the heat being radiated upwards from lower layers. It reradiates in all directions, both upwards and downwards, in equilibrium (by definition) the same amount as it has absorbed. This results in more warmth below. Increasing the concentration of the gases increases the amount of absorption and reradiation, and thereby further warms the layers and ultimately the surface below.

GHGs—including most diatomic gases with two different atoms (such as carbon monoxide, CO) and all gases with three or more atoms—are able to absorb and emit infrared radiation. Though more than 99% of the dry atmosphere is infrared transparent (because the main constituents—N2, O2, and Ar—are not able to directly absorb or emit infrared radiation), intermolecular collisions cause the energy absorbed and emitted by the GHGs to be shared with the other, noninfrared-active, gases.

All climate models indicate a rising trend in temperature. The precipitation pattern has changed with decreased rainfall over south and south-east Asia. More intense and longer droughts have occurred since 1970s. Perpetual snow cover has declined on both area and depth of snow cover. The global mean sea level is projected to rise by 0.18–0.59 m by the end of the century. Six of the 10 countries most vulnerable to climate change are in the Asia-Pacific. Bangladesh tops the list followed by India, Nepal, the Philippines, Afghanistan, and Myanmar. In Bangladesh, for example, about one-fifth of the nation’s population would be displaced as a result of the farmland loss estimated for a 1.5 m sea-level rise. The Maldives islands in the Indian Ocean would have one-half of their land area inundated with a 2 m rise in sea level (Fig. 1.3).

Figure 1.3 Global land air temperature (annual) 10-year moving averages.

The warming may be more pronounced in the northern parts of India. The extremes in maximum and minimum temperatures are expected to increase under changing climate, few places are expected to get more rain while some may remain dry. Leaving Punjab and Rajasthan in the north-west and Tamil Nadu in the south, which show a slight decrease, on an average a 20% rise in all India summer monsoon rainfall over all states are expected. The number of rainy days may come down (e.g., Madhya Pradesh) but the intensity is expected to rise in most parts of India (e.g., the north-east). Gross per capita water availability in India will decline from 1820 m³/year in 2001 to as low as 1140 m³/year in 2050.

Corals in Indian Ocean will soon be exposed to summer temperatures that will exceed the thermal thresholds observed over the last 20 years. Annual bleaching of corals will become almost a certainty from 2050. Currently, the districts of Jagatsinghpur and Kendrapara in Odisha; Nellore and Nagapattinam in Tamil Nadu; and Junagadh and Porbandar districts in Gujarat are the most vulnerable to impacts of increased intensity and frequency of cyclones in India (NATCOM, 2004). The past observations on the mean sea level along the Indian coast show a long-term (100-year) rising trend of about 1.0 mm/year. However, the recent data suggest a rising trend of 2.5 mm/year in sea level along the Indian coastline. The sea surface temperature adjoining India is likely to warm up by about 1.5–2.0°C by the middle of this century and by about 2.5–3.5°C by the end of the century. A 1 m sea-level rise is projected to displace approximately 7.1 million people in India and about 5764 km² of land area will be lost, along with 4200 km of roads (NATCOM, 2004). Over 50% of India’s forests are likely to experience a shift in forest type, adversely impacting associated biodiversity, regional climate dynamics, as well as livelihoods based on forest products. Even in a relatively short span of about 50 years, most of the forest biomass in India seems to be highly vulnerable to the projected change in climate. Further, it is projected that by 2085, 68% of the forested grids in India are likely to experience shift in forest types (Fig. 1.4).

Figure 1.4 Greenhouse effect. Note: the image above expresses energy exchanges in watts per square meter (W/m²). Adopted from Wikipedia https://en.wikipedia.org/wiki/Greenhouse_effect.

1.2 Crop Responses to Expected Climate Change Factors

Cropping is practiced over a wide range of agroecosystems, field crops are being grown in climates ranging from very hot to very cold, and from very wet to very dry, across temperate, tropical, and semiarid zones. Cropping is highly sensitive to climate, crops have a limited environment in which they are productive and profitable. Global change, including climate change, means that the environmental limitations to crop growth will invariably be modified. This in turn affects the choice of crop species and cultivars and other farm management decisions. Climate change threatens to modify the envelope that characterizes different crop production systems, and the associated yield variability and production.

Climate is fundamental to crop growth. Moisture stimulates seeds to germinate, the time to emergence being temperature-dependent. The rates of growth of roots, stems, and leaves depend on the rate of photosynthesis, which in turn depends on light, temperature, moisture, and carbon dioxide. Temperature and day length also determine when the plants produce leaves, stems, and flowers and consequently the filling of grain or the expansion of fruit. The yield of grain crops depends on grain number and grain weight at harvest, which in turn depend on biomass at anthesis and the availability of moisture in the post anthesis period. Climate change and agriculture are interrelated processes, both of which take place on a global scale. Climate change affects agriculture in a number of ways, including through changes in average temperatures, rainfall, and climate extremes (e.g., heat waves); changes in pests and diseases; changes in atmospheric CO2 and ground-level ozone concentrations; changes in the nutritional quality of some foods; and changes in sea levels (Verheye, 2010).

Climate change is already affecting agriculture, with effects unevenly distributed across the world. Future climate change will likely negatively affect crop production in low-latitude countries, while effects in northern latitudes may be positive or negative. Climate change will probably increase the risk of food security for some vulnerable groups, such as the poor. Agriculture contributes to climate change by (1) anthropogenic emissions of GHGs, and (2) by the conversion of nonagricultural land (e.g., forests) into agricultural land. Agriculture, forestry, and land-use change contributed around 20%–25% to global annual emissions in 2010. There are a range of policies that can reduce the risk of negative climate change impacts on agriculture, and to reduce GHGs emissions from the agriculture sector.

Despite technological advances, such as improved varieties, genetically modified organisms, and irrigation systems, weather is still a key factor in agricultural productivity, as well as soil properties and natural communities. The effect of climate on agriculture is related to variabilities in local climates rather than in global climate patterns. The Earth’s average surface temperature has increased by 0.83°C since 1880. Consequently, agronomists consider any assessment has to individually consider each local area. On the other hand, agricultural trade has grown in recent years, and now provides significant amounts of food, on a national level to major importing countries, as well as comfortable income to exporting ones. The international aspect of trade and security in terms of food implies the need to also consider the effects of climate change on a global scale.

A 2008 study published in Science suggested that, due to climate change, southern Africa could lose more than 30% of its main crop, maize, by 2030. In South Asia losses of many regional staples, such as rice, millet and maize could top 10%. The IPCC has produced several reports that have assessed the scientific literature on climate change. The IPCC Third Assessment Report, published in 2001, concluded that the poorest countries would be hardest hit, with reductions in crop yields in most tropical and subtropical regions due to decreased water availability, and new or changed insect pest incidence. In Africa and Latin America many rain-fed crops are near their maximum temperature tolerance, so that yields are likely to fall sharply for even small climate changes; falls in agricultural productivity of up to 30% over the 21st century are projected. Marine life and the fishing industry will also be severely affected in some places.

Weather affects plants in many obvious ways, but also in ways we may not realize. While a tree snapped by a gust of wind is easy to associate with the event, large trees may not show the effects of drought until several years later. In addition to any direct effects, weather-related stress can make plants more susceptible to disease and insect problems. Weather is whatever is happening now—precipitation, temperature, wind, sun, and humidity. It is not the same as climate, which is historical weather or the average of weather conditions over a long period of time. Climate determines what will probably grow well in your area, but plants can still be damaged or killed by extreme weather. While we have no control over the weather, in some cases we can try to design and maintain the land to minimize the negative effects of weather on our plants.

1.2.1 Light

Solar radiation is the primary driver of plant photosynthesis, and the radiation levels in the upper atmosphere may be estimated mechanically as a function of latitude and time of year. Solar radiation at the crop surface differs from incident radiation in the upper atmosphere according to the daily atmospheric transmissivity, determined in part by cloud cover and that can vary substantially within and between years.

1.2.2 Temperature

Temperature has a major effect on photosynthesis and respiration, plant growth, and phnological development. Phenology is particularly important in cooler regions and higher altitudes. In general, atmospheric temperatures experienced by crops decrease by about 1°C for each 2°C increase in latitude, or for each 100 m increase in altitude. Cool temperate climatic plants typically have lower minimum, maximum, and optimal temperatures for growth than warm season C3 crops and tropical C4 crops. Temperature is important in controlling phnological changes in development from germination and seedling emergence, through vegetative growth to floral initiation to reproductive growth. Of course, variation in temperature tolerance is evident both within populations of single plants and between genotypes. Temperature also has a major influence on the rate of evaporation loss from soils and the leaf surface.

Though warming has not been uniform across the planet, the upward trend in the globally averaged temperature shows that more areas are warming than cooling. Since 1880, surface temperature has risen at an average pace of 0.07°C every 10 years for a net warming of 0.95°C through 2016. Over this 136-year period, average temperature over land areas has warmed faster than ocean temperatures: 0.10°C per decade compared to 0.06°C per decade. The last year with a temperature cooler than the 20th-century average was 1976.

By 2020, models project that global surface temperature will be more than 0.5°C warmer than the 1986–2005 average, regardless of which carbon dioxide emissions pathway the world follows. This similarity in temperatures regardless of total emissions is a short-term phenomenon: it reflects the tremendous inertia of Earth’s vast oceans. The high heat capacity of water means that ocean temperature doesn’t react instantly to the increased heat being trapped by GHGs. By 2030, however, the heating imbalance caused by GHGs begins to overcome the oceans’ thermal inertia, and projected temperature pathways begin to diverge, with unchecked carbon dioxide emissions likely leading to several additional degrees of warming by the end of the century.

The concept of an average temperature for the entire globe may seem odd. After all, at this very moment, the highest and lowest temperatures on Earth are likely more than 55°C apart. Temperatures vary from night to day and between seasonal extremes in the northern and southern hemispheres. This means that some parts of Earth are quite cold while other parts are downright hot. To speak of the average temperature, then, may seem like nonsense. However, the concept of a global average temperature is convenient for detecting and tracking changes in Earth’s energy budget—how much sunlight Earth absorbs minus how much it radiates to space as heat—over time.

Generally, plants grow faster with increasing air temperatures up to a point. Extreme heat will slow growth and also increase moisture loss. The temperatures for optimal growth vary with the type of plant. Some annual flowers and vegetables are extremely sensitive to cold, and transplants should not be planted until temperatures are consistently warm. Extremely hot or cold soil temperatures can also hamper plant growth, as well as affect seed germination. Cool temperatures in fall trigger the plant to reduce growth and store energy. As temperatures approach freezing, growth stops and the plant (if perennial) becomes dormant. Plants are better able to withstand cold temperatures when dormant. A sudden cold snap in late fall before the plant has had a chance to harden off can do more harm than sustained cold temperatures in mid-winter.

Many plants require a chilling period of a certain number of days before growth resumes in spring. Plants native to areas further south with a shorter chilling requirement may resume growth during a warm period in winter and then be damaged when cold weather returns. Plants native to the area will generally not break dormancy. Wide temperature fluctuations can be hard on plants, particularly in winter. Warm days followed by freezing nights can cause bark injury on trees with thin, smooth bark. Alternate freezing and thawing of soil can result in heaving of shallow-rooted plants. Temperature (along with day length in some cases) can also trigger flowering in some plants, as well as affect how long flowers last. Extreme temperatures (too hot or too cold) can inhibit fruit set on tomatoes and other garden plants. Temperature as well as moisture level may affect the flavor of fruits and vegetables. Hot weather can cause cool-season vegetables to bolt, resulting in reduced production and changes in flavor.

An unexpected frost can cause special problems in spring or fall. Frost can damage cell walls or cell contents of actively growing plants. Frost is more likely in low-lying areas on a clear night with little wind. An early fall frost can be followed by a number of weeks of warm weather. Temperature can also have indirect effects on plants. A warm winter may result in a larger insect population the following season.

1.2.3 Precipitation

Precipitation comes in many forms—rain, snow, sleet, hail, and ice. The water available to plants for growth is affected by the amount and type of precipitation, as well as soil characteristics, temperature, and wind. The effects of too much or too little precipitation can be temporary or permanent, depending on the type of plant and how long the condition lasts. Water is necessary for virtually every function of plant growth. Lack of water damages plant cells, resulting in decreased growth, wilting, and leaf scorch, and eventually leaf drop and root damage.

Too much water reduces the amount of oxygen in the soil, resulting in root loss or injury. It can also make the plant more susceptible to many fungal diseases. Heavy rain can damage plants, compact soil, and cause erosion. Snow, in addition to providing moisture, can also insulate and protect plants from temperature extremes and fluctuations. However, the weight of heavy snow can break branches (especially on evergreens). Snow cover can also make it more difficult for wildlife to find food and result in more damage to landscape plants. Ice and hail, as well as deicing salts, can all cause damage to plants. Humidity refers to the amount of moisture in the air, and may or may not be associated with precipitation. High humidity reduces water loss from plants, and may increase the chance of disease.

1.2.4 Wind

Wind has a drying effect. This can dry out wet plants, reducing disease chances. However, it can also remove water faster than the plant can replace it. This can be a problem in summer when combined with high temperatures and low soil moisture. It can also be a special problem for evergreens in winter since they continue to lose moisture through their leaves or needles and are unable to replace it if the ground is frozen. Wind can disperse pollen, seeds, spores, insects, pathogens, salt, and noxious chemicals. Excess wind can do considerable damage to plants.

Climate change induced by increasing GHGs is likely to affect crops differently from region to region. For example, average crop yield is expected to drop down to 50% in Pakistan according to the UKMO scenario, whereas corn production in Europe is expected to grow up to 25% in optimum hydrologic conditions. More favorable effects on yield tend to depend to a large extent on realization of the potentially beneficial effects of carbon dioxide on crop growth and an increase in the efficiency of water use. A decrease in potential yields is likely to be caused by shortening of the growing period, a decrease in water availability, and poor vernalization.

Climate change scenarios include higher temperatures, changes in precipitation, and higher atmospheric CO2 concentrations, which may affect yield (both quality and quantity), growth rates, photosynthesis and transpiration rates, moisture availability, through changes of water use (irrigation) and agricultural inputs such as herbicides, insecticides, and fertilizers, etc. Environmental effects, such as frequency and intensity of soil drainage (leading to nitrogen leaching), soil erosion, land availability, and reduction of crop diversity may also affect agricultural productivity.

It is a well-known fact that an atmosphere with higher CO2 concentration will result in higher net photosynthetic rates. Higher CO2 concentrations may also reduce transpiration as plants reduce their stomatal apertures, the small openings in the leaves through which CO2 and water vapor are exchanged with the atmosphere. The reduction in transpiration could be 30% in some crop plants (Kimball et al., 1983). However, stomatal response to CO2 interacts with many environmental (temperature, light intensity) and plant factors (e.g., age, hormones) and, therefore, predicting the effect of elevated CO2 on the responsiveness of stomata is still very difficult (Rosenzweig and Hillel, 1995). For every 75 ppm increase in CO2 concentration rice yields will increase by 0.5 t/ha, but yield will decrease by 0.6 t/ha for every 1°C increase in temperature (Sheehy et al., 2006). CO2 enrichment has generally shown significant increases in rice biomass (25%–40%) and yields (15%–39%) at ambient temperature, but those increases tended to be offset when temperature was increased along with rising CO2 (Ziska et al., 1996a,b; Moya et al., 1998). Yield losses caused by concurrent increases in CO2 and temperature are primarily caused by high-temperature-induced spikelet sterility (Matsui et al., 1997a). Increased CO2 levels may also cause a direct inhibition of maintenance respiration at night temperatures higher than 21°C (Baker et al., 2000). In rice, extreme maximum temperature is of particular importance during flowering, which usually lasts 2–3 weeks. Exposure to high temperature for a few hours can greatly reduce pollen viability and, therefore, cause yield loss. Spikelet sterility is greatly increased at temperatures higher than 35°C (Osada et al., 1973; Matsui et al., 1997b) and enhanced CO2 levels may further aggravate this problem, possibly because of reduced transpirational cooling (Matsui et al., 1997a).

A key mechanism of high-temperature-induced floret sterility in rice is the decreased ability of the pollen grains to swell, resulting in poor thecae dehiscence (Matsui et al., 2001). Significant genotypic variation in high-temperature-induced floret sterility exists. Variation in solar radiation, increased maintenance respiration losses, or differential effects of night versus day temperature on tillering, leaf-area expansion, stem elongation, grain filling, and crop phenology have been proposed as possible causes (Peng et al., 2004; Sheehy et al., 2006). In a recent climate change study, there was the first evidence of possible genotypic variation in resistance to high night temperatures (Counce et al., 2005). High CO2 levels and/or temperature are likely to affect crop development rates.

Warming will accelerate many microbial processes in the soil–floodwater system, with consequences for the carbon and nitrogen cycles. Crop residue decomposition patterns may change. Increased soil temperature may also lead to an increase in autotrophic CO2 losses from the soil caused by root respiration, root exudates, and fine-root turnover. Climate change impacts will also impact on rice production through rising sea level. Most studies project decreased yields in nonirrigated wheat and in rice, and a loss in farm-level net revenue of between 9% and 25% for a temperature increase of 2–3.5°C. Aggarwal and Mall (2002) observed that a 2.0°C increase resulted in a 15%–17% decrease in grain yield of rice and wheat. Fungal and bacterial pathogens are also likely to increase in severity in areas where precipitation increases. Under warmer and more humid conditions, cereals would be more prone to outbreaks of pests and diseases, thereby reducing yield.

According to Allen (1991), it has been shown that the water requirement of plants is linearly related to the biomass production of plants and a linear relationship has been established (Allison et al., 1958; Arkley, 1963; Chang, 1968; Hanks et al., 1969; Stanhill, 1960). Fig. 1.5 shows the linear relationship between biomass produced and rainfall plus irrigation water used by Sart sorghum and Starr millet in Alabama, as adapted from data of Bennett et al. (1964).

Figure 1.5 Linear relationship between biomass production and water use for two forage crops. Adapted from Bennett, O.I., Doss, B.D., Ashley, D.A., Kilmer, V.J., Richardson, E.C., 1964. Effects of soil moisture regime on yield, nutrient content, and evapotranspiration for three annual forage species. Agron. J., 56: 195–198.

De Wit (1958) examined the relationships among climatic factors, yield, and water use by crops. He found the following general linear relationship to be true, especially in semiarid climates:

(1.1)

where Y = yield component (e.g., total above-ground biomass or seed production), T = cumulative actual transpiration, Tmax = maximum possible cumulative transpiration, m = constant dependent on yield component and species, especially on differences among photosynthetic mechanisms.

Pan evaporation was used to represent Tmax, which is proportional to climatic factors, especially air vapor pressure deficit (VPD):

(1.2)

where es = the saturation vapor pressure at a given air temperature, ea = the actual vapor pressure that exists in the air.

Combining these relationships, we see that yield is proportional to cumulative transpiration divided by VPD:

(1.3)

where k is a constant with units millibars · g (dry matter) · g−1 (water). Like m, k depends on yield component, species, and photosynthetic mechanisms.

Thus, one can see that theory predicts that yield will be proportional to cumulative transpirational water use, divided by VPD. There are several ways of calculating the VPD; it can be computed by aggregating seasonal daytime average VPD, or by using approximation methods based on daily maximum and minimum temperatures (Jensen, 1974). As pointed out by Tanner and Sinclair (1983), the maximum es can be computed from the daily maximum temperature, and ea can be estimated from the daily minimum temperature. Tanner and Sinclair (1983) estimated that the effective daytime es falls at a point two-thirds to three-quarters of the distance between the es computed at the daily maximum temperature and the ea computed at the daily minimum temperature. The effective daytime VPD values then must be averaged over the growing season of the crop.

The levels of carbon dioxide and climate are interlinked during most of the Earth’s history (Ahn and Brook, 2009; Goodwin et al., 2009). Present global carbon dioxide levels are 38% higher than the levels around 2.1 million years ago (Horisch et al., 2009). Human activities of the modern era, such as deforestation, fossil fuel burning, draining of wet lands, adaptation of modern technology in farming and livestock rearing, etc., are the main reasons for the present degradation of the global environment. Initially, during the preindustrial era the impact of such activities on the climate and environment were invisible and insignificant (Rao et al., 2011). But with the increasing urbanization and industrialization, the impact began to show on the world climate and reached such a level at present that it is no more tomorrow’s crisis but very much today’s crisis (Alexander and Nasheed, 2009).

Atmospheric carbon dioxide is known to affect plant yield. Kimball (1983) reviewed 430 observations of carbon dioxide enrichment studies conducted prior to 1982 and reported an average yield increase of 33%, plus or minus 6%, for a doubling of the carbon dioxide concentration. This value has been generally confirmed by many other studies since that time. The yield increases seem to apply for both biomass accumulation and grain yield. Thus, plants may grow larger and, considering Fig. 1.5, they may use more water as the global carbon dioxide concentration increases.

Transpirational water use is clearly related to ground cover (Jensen, 1974; Doorenbos and Pruitt, 1977). Daily water use soon after crops are planted on bare soil is typically only 10%–20% of water use after effective ground cover is reached. Water use rises sharply as the crop’s leaf area increases. Similarly, water use drops 60%–70% when hay crops such as alfalfa are cut. As leaf regrowth occurs, transpiration rates recover rapidly as the ground cover of leaves is restored. Ground cover can