Climate Change and Crops

Recent studies have shown that global atmospheric carbon dioxide (CO₂) has increased markedly due to human activities, including burning of fossil fuels and deforestation, and its current level (383 ppm) has far exceeded the natural range (180–300 ppm) seen over 6500 centuries (IPCC 2007). Rising of atmospheric CO₂ has caused the globally averaged surface temperatures to increase by 0.6 ± 0.2°C over the 20th century, while surface air temperature is estimated by models to warm 1.1–2.9°C “low scenario” or 2.4–6.4°C “high scenario” by the end of the 21st century relative to 1990 (IPCC 2007). A new global climate model predicts that in the coming decade, the surface air temperature is likely to exceed existing records (Smith et al. 2007). Global warming can be accompanied by shifts in precipitation patterns around the world (IPCC 2007).

Climate is changing naturally at its own pace, since the beginning of the evolution of earth, 4–5 billion years ago, but presently, it has gained momentum due to inadvertent anthropogenic disturbances. These changes may culminate in adverse impact on human health and the biosphere on which we depend. The multi-faceted interactions among the humans, microbes and the rest of the biosphere, have started reflecting an increase in the concentration of greenhouse gases (GHGs) i.e. CO₂, CH₄ and N₂O, causing warming across the globe along with other cascading consequences in the form of shift in rainfall pattern, melting of ice, rise in sea level etc. The above multifarious interactions among atmospheric composition, climate change and human, plant and animal health need to be scrutinized and probable solutions to the undesirable changes may be sought.

The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC 2007) emphatically shows that the Earth’s climate is changing in a manner unprecedented in the past 400,000 years. By the end of 2100, the mean planetwide surface temperatures will rise by 1.4–5.8.C, precipitation will decrease in the sub-tropics, and extreme events will become more frequent (IPCC 2007). However, changes in climate are already being observed – the last 60 years were the warmest in the last 1000 years and changes in precipitation patterns have brought greater incidence of floods or drought globally. These predicted changes are largely driven by increasing atmospheric concentrations of greenhouse gases, such as CO₂, CH₄ and N₂O. These changes will affect the agro-climatic conditions for food production systems worldwide. Just as the production ecosystems are influenced by these changes, the agricultural activities also contribute to these changes by unsustainable utilization of natural resources, altered biogeochemical cycling of nutrients, and emission of greenhouse gases.

Rice (O. sativa and O. glaberrima) is one of the world most important cereal food crops, particularly in Asia and increasingly so in Africa and Latin America. Rice provides a substantial portion of the dietary requirements of nearly 1.6 billion people, with another 400 million relying on rice for quarter to half of their diet (Swaminathan 1984). Rice is cultivated as far north as Manchuria in China (39° 53’N) and far south as New South Wales in Australia (28° 81’S) (Khush 2005), either as an upland (aerobic) or wetland (irrigated, rainfed and deepwater) crop. Upland rice cultivation covers 17Mha, while wetland rice is cultivated on 131Mha, contributing about 30 and 70%, respectively, of the total rice production in the world (Dubey 2001). Rice occupies 23% of the total cultivated area under cereals in the world, of which 89% is in Asia (FAO 2003). Hence, Asia produces 523MT of rice (91% of the world production) (Dubey 2001), on which nearly half of the world’s population depend for food and livelihood (Carriger and Vallee 2007). Since the world population is increasing at 1.17% annually, an annual increase in rice production by 0.6–0.9% is required until 2050 (Carriger and Vallee 2007) to meet the anticipated demand.

Globally, soils contain approximately 1500 Pg (1 Pg = 1Gt = 10¹⁵ g) of organic carbon (C) (Batjes 1996), roughly three times the amount of carbon in vegetation and twice the amount in the atmosphere (IPCC 2001). The annual uxes of carbon dioxide (CO₂) from atmosphere to land (global Net Primary Productivity [NPP]) and land to atmosphere (respiration and re) are of the order of 60 Pg Cyr⁻¹ (IPCC 2001). during 1990s, fossil fuel combustion and cement production emitted 6.3 ± 1.3 Pg Cyr⁻¹ to the atmosphere, while land-use change accounted for 1.6 ± 0.8PgCyr⁻¹ (Schimel et al. 2001; IPCC 2001).Atmospheric C increased at a rate of 3.2 ± 0.1PgCyr⁻¹, the oceans absorbed 2.3 ± 0.8PgCyr⁻¹ and therewas an estimated terrestrial sink of 2.3 ± 1.3PgCyr⁻¹ (Schimel et al. 2001; IPCC 2001). The amount of carbon stored in soils globally is, therefore, very large compared to gross and net annual uxes of carbon to and from the terrestrial biosphere, and the pools of carbon in the atmosphere and vegetation. Human intervention, via cultivation and disturbance, has also decreased the soil carbon pools relative to the store typically achieved under native vegetation. Historically, these processes have caused a loss of soil C between 40 and 90 Pg C globally (Paustian et al. 1998; Houghton et al. 1999; Lal 1999). Hence, increasing the size of the global soil carbon pool by even a small proportion has the potential to sequester large amounts of carbon, and thus help mitigate climate change.

A rapid increase in atmospheric concentrations of the three main anthropogenic greenhouse gases (GHGs), like carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), is evident from measurements taken over the past few decades as well as ice-core records spanning many thousands of years (IPCC 2007). The global increases in CO₂ concentration are due to fossil fuel and land-use change, while those of CH₄ and N₂O are primarily from agriculture (Cole et al. 1997; IPCC 2007). Despite large annual exchanges of CO₂ between the atmosphere and agricultural lands, the net flux is approximately balanced (IPCC 2007). Arable and permanent crops occupy 1,540Mha in 2003 which is about 12% of the Earth’s land surface (FAOSTAT 2006). In 2005, agriculture contributes about 47 and 58% of total anthropogenic emissions of CH₄ and N₂O, respectively, with a wide range of uncertainty in the estimates of both the agricultural contribution and the anthropogenic total.

Methane (CH₄) is a trace gas with an important role in both troposphere due to the reactions with the hydroxyl radical and the formation of organic radicals, and the stratosphere, because of its participation in the chlorine and water vapor chemistry. Methane is also an important greenhouse gas, with a relative contribution of about 20% for the global greenhouse effect (Wuebbles and Hayhoe 2002). Many works have evidenced that an increase of the methane concentration began around 1800 with the industrial era, and the consequent increase of human activities, including the utilization of fossil fuels, cattle raising and rice plantations (Blake and Rowland 1988; Stern and Kaufmann 1996; Dlugokencky et al. 1998; Etheridge et al. 1998).

Nitrous oxide, popularly known as a laughing gas has emerged as an important gas for environmental sustainability. In the troposphere, N₂O is a chemically inert gas, but acts as a potential greenhouse gas. The greenhouse effect of N₂O was first reported by Yung et al. (1976). Global warming potential of N₂O, over a time horizon of 100 years, is measured 296 times that of CO₂ (Ramaswamy et al. 2001). Therefore, an increasing trend of atmospheric N₂O is a serious concern, although N₂O emission is very low as compared to CO₂, the most abundant greenhouse gas in the atmosphere (IPCC 1996a). Nitrous oxide emission from 1750 to 2000 has caused an atmospheric radiative forcing of 0.15Wm^-2 or 6% of the enhanced radiative forcing by well-mixed greenhouse gases during this time, equivalent to 2.43Wm^-2. While other greenhouse gases, like CO₂, CH₄ and halocarbons, have contributed 1.46, 0.48 and 0.34Wm^-2, respectively. Thus, global average surface temperature (the average of near surface air temperature over land and sea surface temperature) has increased by 0.6 ± 0.2°C over 20th century (IPCC 2001).

Nitrous oxide (N₂O) is one of key greenhouse gases that cause global warming. It continues to rise at a rate of approximate 0.26% per year and has reached a concentration of 319 ppb (10^-9 mol mol^-1) in 2005 (IPCC 2007a). Agriculture accounts for about 60% of global anthropogenic N₂O emissions. Globally, agricultural N₂O emissions have increased by nearly 17% from 1990 to 2005 (IPCC 2007b), and are projected to increase by 35–60% up to 2030 due to increased nitrogen fertilizer use and increased animal manure production (FAO 2003). The emissions of N₂O that result from anthropogenic N inputs, occur through a direct pathway (i.e. directly from soils to which the N is added), and through two indirect pathways: volatilization of compounds, such as NH₃ and NO_X and subsequent redeposition, and through leaching and runoff. Relative to the indirect pathways, the direct emission contributes most to the agricultural N₂O sources (Zheng et al. 2004). Thus, a good estimate of direct N₂O emission from agricultural fields will help assess its global source strength.

Ozone (O₃) is a naturally occurring chemical present in both the stratosphere (the ‘ozone layer’, 10–40 km above the earth) and in the troposphere (0–10 km above the earth). While stratospheric O₃ protects the Earth’s surface from solar UV radiation, tropospheric O₃ is the third most important greenhouse gas (after CO₂ and CH₄) (Denman et al. 2007; Solomon et al. 2007). It contributes to greenhouse radiative forcing, causing a change in the balance between incoming solar radiation and outgoing infrared radiation within the atmosphere that controls the Earth’s surface temperature. Besides its role as a direct greenhouse gas, O₃ has been identified as one of the major phytotoxic air pollutants. The adverse effects of O₃ on plants were first identified in the 1950s (Hill et al. 1961), and it is now recognized as the most important rural air pollutant, affecting human health and materials, as well as vegetation (WGE 2004).

In their ecosystem, plants have to cope with a plethora of potentially unfavourable conditions. Stress factors affecting plant’s fitness not only derive from natural sources, such as adverse temperature fluctuations (heating, chilling and freezing), high irradiance (photoinhibition, photooxidation), osmotic imbalance (salinity and drought), hypoxia/anoxia (flooding), mineral (macro- and micronutrient) deficiency, wounding, phytophagy and pathogen attack, but also from anthropogenic activities. The latter include xenobiotics employed in agriculture (herbicide, pesticides and fungicides), environmental (air, soil and water) pollutants and increased UV radiations. Particularly, many atmospheric pollutants, belonging to greenhouse gases, may increase the greenhouse effect, a natural warming process that prevents heat from diffusing to the outer atmosphere, thus balancing Earth cooling processes. Without the natural greenhouse effect, temperature on Earth would be much lower than it is now, and the existence of life would have not been possible. However, the rising emissions of greenhouse gases due to anthropogenic activities, namely carbon dioxide (CO₂), chlorofluorocarbons (CFCs), nitrous oxide (N₂O), tropospheric ozone (O₃) and water vapour, may cause a short-term increase of the mean global temperature on the planet surface with consequent changes in precipitation patterns (Krupa and Kickert 1989). In this scenario, life on the earth depended from the co-evolution between atmosphere and biosphere, because the gradual and long-term climate changes enabled living organism adaptation to the new temperatures, precipitation patterns and other climate conditions (Voronin and Black 2007).

The Earth’s climate has always been changing. Recent observations indicate that the changes are occurring faster and greater than those in the past. Consequently, the main scientific concern is with the ability of organisms to cope with these changes, especially terrestrial plants and crops that are sessile.

Seven percent of the electromagnetic radiation emitted from the sun is in the range of 200–400 nm. As it passes through the atmosphere, the total flux transmitted is greatly reduced, and the composition of the UV radiation is modified. Short-wave UV-C radiation (200–280 nm) is completely absorbed by atmospheric gases. UV-B radiation is often defined as 280–320 nm. However, the legal definition provided by the International Commission on Illumination sets the UV-B radiation range as 280–315 nm. UV-B radiation is maximally absorbed by stratospheric ozone and thus, only a very small proportion is transmitted to the Earth’s surface, whereas UV-A radiation (315–400 nm) is hardly absorbed by ozone. In the past 50 years, the concentration of ozone has decreased by about 5%, mainly due to anthropogenic pollutants, such as chlorofluorocarbons, releasing Cl atoms that catalytically remove ozone molecules from the atmosphere. The surface concentration of ozone has risen from less than 10 ppb prior to the industrial revolution to a day-time mean concentration of approximately 40 ppb over much of the northern temperate zone. If current global emission trends continue, surface ozone might rise over 50% by this century. Ozone depletion is particularly severe over the Antarctic continent, where a dynamically isolated air mass cools down to extremely low temperatures during the austral winter, facilitating ozone photo-destruction and formation of the so called springtime “ozone hole”. Depletion of stratospheric ozone has increased solar ultraviolet- B radiation at high- and mid-latitudes in both Southern and Northern hemispheres (Frederick et al. 1994).

Radiative forcing of Earth’s atmosphere is increasing at unprecedented rates, largely because of increases in the concentrations of atmospheric trace gases, mainly carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) – collectively known as greenhouse gases (GHG). Concentrations of CO₂, CH₄ and N₂O have increased markedly as a result of human activities since 1750 and now far exceeded pre-industrial values as determined from ice cores spanning thousands of years (Table 15.1). The atmospheric concentrations of CO₂ and CH₄ in 2005 have exceeded the natural range over the last 650,000 years (IPCC 2007). The global atmospheric concentration of CO2 has increased at an annual growth rate of 0.5%, while that of CH₄ at 0.6% and nitrous oxide at 0.25%. Agriculture plays a major role in the global fluxes of each of these gases and is considered as one of the major anthropogenic sources (Fig. 15.1). Agriculture comprises several activities, contributing to GHG emissions and globally, the most significant activities identified include (i) deforestation and other land-use changes as a source of CO₂, (ii) rice-based production systems (including rice-wheat rotation) as sources of CH₄ and N₂O (and also source of CO₂ due to burning of agricultural residues) and (iii) animal husbandry as a source of CH₄.

Rising trend of earth’s surface temperature is today a global threat to mankind. This trend is directly linked to an increasing atmospheric abundance of various greenhouse gases, like CO₂, CH₄ , N₂O etc. emanating from man-made activities (IPCC 2007). Among these gases, CH₄ is the most abundant carbon species present in the atmosphere (mixing ratio ~ 1.8 ppm). Being a highly radiatively active gas, it is a major component of the natural gas after CO₂, accounting for about 20% of the global greenhouse effect (Wuebbles and Hayhoe 2002). Being highly reactive, CH₄ also affects the chemistry and oxidation capacity of the atmosphere by influencing the concentrations of tropospheric ozone, hydroxyl radicals and carbon monoxide (Cicerone and Oremland 1998). Ozone formation further amplifies the methane AQ1 induced greenhouse effect by approximately 70% (Moss 1992). Global atmospheric concentration of CH4 has increased from a pre-industrial value of about 715 ppb to 1745 ppb in 1998, and to 1774 ppb in 2005 (IPCC 2007). Once emitted, CH₄ remains in the atmosphere for approximately 8.4 years before removal (Dentener et al. 2003). Although atmospheric abundance of CH₄ is far less than 0.5% of CO₂, but on molecule to molecule basis, it is approximately 23 times more effective in absorbing infrared radiations than CO2 (IPCC 2007). Dlugokencky et al. (2003) observed that atmosphericmethane had been at a steady state of 1751 ppbv between 1999 and 2002 (Fig. 16.1). However, over the last two decades, the concentration of CH4 in the troposphere is reportedly increasing at the rate of ~ 0.7% each year and is anticipated to modify the global climate, affecting terrestrial ecosystem both functionally and structurally (Houghton et al. 1996).