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Vol. 16 No. 3 - July 2010

Global Climate Change and Sustainability of Food Security

By: Dr. Sagar Krupa^*

The climate of the earth, its chemical (e.g., carbon dioxide) and physical properties (e.g., air temperature), has been changing since the beginning of the 20^th century or since the onset of the industrial revolution. There is a great deal of concern about "global warming”. However, global climate change and global warming are not one and the same. "Global climate change” includes three critical issues: (1) changes in trace or greenhouse gas (GHG) concentrations at the surface, (2) thinning of the beneficial ozone layer in the stratosphere (15-50 km above the surface) and (3) changes in air temperature (warming) and other physical parameters at the surface (according to the World Meteorological Organization [WMO], by 2025 availability of fresh water supply will be the single largest limiting factor affecting global population). All three issues are interrelated and represent an integrated atmospheric system and thus, should not be viewed individually.

Clearly the chemical climate of the earth has been changing. GHG concentrations have increased. As a result, at least in the last 50 years, while the daytime temperatures in the United States have declined, nighttime temperatures have increased. Similarly spring and fall seasons have become warmer. Overall, these processes can prolong the crop growth season, induce rapid growth and maturity and reduce flowering and seed filling, for example in cereals. Any increases in air temperature will need to be coupled to the availability of moisture (frequency of drought).

As an independent phenomenon, as opposed to the thinning of the beneficial stratospheric ozone layer (filters harmful ultraviolet radiation from reaching the surface), increasing ozone levels at the ground (part of the smog) is the most important phytotoxic compound worldwide.

There is evidence that the beneficial effects of the rising carbon dioxide levels on crops are likely to be offset by the increasing ozone levels. Further, increasing nitrogen in the atmosphere is a part of global climate change and its deposition is expected to result in changes in the community composition of perennial plant populations and biodiversity. Nevertheless, virtually all our knowledge of the effects of global climate change on crop production and yield is based on studies that involve the impact of one variable at a time, although that has little resemblance to the real world conditions. Nevertheless, with world population rapidly increasing toward nine billion by the mid-21^st century, there is a need for doubling the level of current food production by 2030. Clearly that is a daunting challenge in sustaining global food security into the future.

Introduction

Life on earth has evolved under a naturally produced ozone layer that exists between 15-50 km above the surface (the stratosphere). That beneficial ozone layer filters significant levels of biologically harmful solar ultraviolet radiation from reaching the surface, and transforms that energy into heat and wind. At the surface, climate is the result of the interactions between its chemical (e.g., water vapor, carbon dioxide) and physical (e.g., radiation, temperature) parameters. The incoming solar radiation is absorbed by all surfaces (living, e.g., forests and non-living, e.g., buildings), those objects retain certain portion of the heat and the remaining energy is re-emitted into the atmosphere. The dynamics of the incoming shorter wavelength and the outgoing longer wavelength radiations are regulated by the presence of certain trace gases (e.g., water vapor, carbon dioxide, methane). These trace gases trap the outgoing heat, thus warming the atmosphere (the surface air temperature). This is a natural heating process that supports life on earth.

The physical mechanism of the heating of the ambient atmosphere is similar to the heating of the air inside a greenhouse where all surfaces absorb the radiation and that portion of the outgoing heat is trapped by the roof of the greenhouse, the "greenhouse effect". As a similarity, the trace gases in the ambient atmosphere that trap or block the outgoing radiation (serving like the roof of a greenhouse) are known as "greenhouse gases” (GHGs). Table 1 provides a summary of the contributions of various greenhouses gases to the "natural greenhouse effect”. It is very important to note that water vapor is the single largest contributor (62%) to the natural greenhouse effect and carbon dioxide contributes to only 22%.

Table 1. Contribution of greenhouse gases to the natural greenhouse effect at 33^oC

Modified from: German Bundestag, Protecting the Earth’s Atmosphere 1991

In addition to their role in the greenhouse effect, trace gases such as chlorofluorocarbons (CFCs), organo-bromines (OBs) and nitrous oxide participate in reactions driven by the sunlight leading to the destruction of ozone in the stratosphere and the thinning of the beneficial ozone layer there and consequently increased penetration of the harmful ultraviolet radiation to the surface (therefore, possible increases in the incidence of skin cancer?).

In the popular media, terms such as "Global climate change”, "Greenhouse effect” and "Global warming” have been used interchangeably. Great emphasis has been placed on global warming. That is certainly justified. However, it is very important to note that global climate change and global warming are not the same. Global warming is only a part of global climate change that is composed of a system of atmospheric processes and their products (Table 2). Both the atmospheric processes and their products have an impact on the environment and thus, global warming should be considered as only one of the critical issues within the overall climate change scenario.

Table 2. Global climate change: A system of atmospheric processes and their products

Is the global climate changing?

"Global climate change" includes three critical issues: (1) changes in trace or greenhouse gas (GHG) concentrations at the surface, (2) thinning of the beneficial ozone layer in the stratosphere and (3) changes in air temperature and other physical parameters at the surface. It is important to note that all three issues are interrelated and represent an integrated atmospheric system and thus, should not be viewed individually.

A. Is there a change in the global chemical climate?

Clearly the chemical climate of the earth has been changing (Table 3). The air concentrations of GHGs have been increasing since: (1) the onset of industrial revolution, (2) rapid growth in populations and (3) increases in the number of urban centers across the world, for example mega cities with populations of  >10 million (grew from 6 to some 38 in the last 50 years). The primary basis for the increases in the concentrations of carbon dioxide and surface level ozone from the past to the present is increased fossil fuel combustion. In contrast, increases in other GHGs are due to changes in: rice cultivation (methane), agriculture/nitrogen fertilizer use (nitrous oxide) and industrial processes including the use of refrigerants (CFCs). While carbon dioxide occurs at ppm (parts per million, 10^-6) levels, others such as methane occur at ppb (parts per billion, 10^-9) and still others such as the CFCs occur at ppt (parts per trillion, 10^-12) levels. More importantly, all trace gases do not have the same heating effect or global warming potential (GWP). For example, although nitrous oxide concentrations (320 ppb) are orders of magnitude lower than carbon dioxide (384 ppm), it is ~300 times more potent than carbon dioxide in heating the atmosphere (Table 3). Note that although water vapor contributes to about 60% of the natural greenhouse effect, no data are available in relation to other trace gases included in Table 3. One would readily conclude that the water vapor in the atmosphere is due to evaporation and evapo-transpiration. However, it should be remembered that water vapor is also produced by chemical reactions in the atmosphere that are driven by sunlight. In fact, global warming will largely enhance that process.

B. Is there an increase in the harmful solar ultraviolet radiation at the surface?

As noted previously, the thinning of the beneficial stratospheric (15-50 km above the surface) ozone layer will lead to increases in the harmful ultraviolet radiation (an average of 2% increase in radiation for every 1% loss in ozone) at the surface.  Because of the angle of the rotation of the earth (the Coriolus), the jet stream in the northern hemisphere moves from west to east and in the southern hemisphere from east to west. These two general circulation patterns collide over the poles creating a polar vortex (vórtice). Because of that, pollutants transported to the stratosphere accumulate over the poles and thus the observed ozone hole due to its destruction by nitrous oxide, CFCs etc. Atmospheric models predict the thinning of the beneficial stratospheric ozone layer across all latitudes. However, beyond the polar locations, there are no spectrally resolved data (because of the complexity and cost associated with its measurement) to demonstrate increases in ultraviolet radiation across all latitudes, except at the southern tip of South America, New Zealand and Southern Australia. In as much as stratospheric ozone filters the ultraviolet radiation, so does the ozone produced at the surface by urban activity, as in Mexico City.  Background surface ozone concentrations are increasing globally and combined with particulate matter, it acts as a limited radiation filter. However, the overall concern regarding the ultraviolet radiation is very valid at the global scale.

Table 3. Past (19^th Century), present (21^st Century) and the rate of change in some ambient greenhouse gas concentrations

*Global Warming Potential

** Chlorofluorocarbon, no longer produced

*** Not applicable, because of the short life time

Modified from: Report of the Intergovernmental Panel on Climate Change (IPCC), 2007

C. Is there a global warming?

There is a continuing debate regarding the question of global warming. While global warming is of great international concern, some believe that it is a product of natural phenomena such as sunspots or solar flares. Nevertheless, as the famous atmospheric scientist, James P. Lodge once stated: Most researchers believe that the climate will simply become warmer worldwide. While this may be true on a global mean basis, this is by no means necessarily true for a given spot on the earth or even for a given nation or continent. Instead, sizable areas may well become warmer, cooler, drier or wetter or remain unchanged, in so far as annual means are concerned. Probably any modification of the climate will manifest itself through changes not in mean values, but in the deviations from those means and in the frequency of severe weather conditions such as high winds, thunderstorms and blizzards.

Climate models predict an increase in air temperature of 0.5 to 4.5 C with the doubling of the carbon dioxide concentrations. Independent of the problems associated with the use of average values and the uncertainties associated with modeling, the air temperature appears to have increased by 0.7 C in the last 150 years. At non-urban or rural sites in the United States, the difference between day and night temperatures has declined since about 1950. While the daytime temperatures have declined, nighttime temperatures have increased. Similarly spring and fall seasons have become warmer.

D. Is atmospheric nitrogen deposition, a part of global climate change?

Clearly excess additions of nitrogen into the environment and their adverse effects on life are of major concern. A classic example is the flow of excess nitrogen from the Midwest down the Mississippi River into the Gulf of Mexico resulting in hypoxia. Hypoxia is a phenomenon where sea life such as fish and shrimp are starved of oxygen and die, because algal blooms or growth stimulated by the excess nitrogen supply exhaust the normal oxygen availability in the waters. However, the issue here is nitrogen in the atmosphere as opposed to surface waters.

Leaving aside nitrous oxide (a greenhouse gas), other major nitrogen species in the atmosphere consist of nitric oxide and nitrogen dioxide (known together as the oxides of nitrogen, these are not greenhouse gases), nitrates and ammonia (although there are other species such as nitric acid, organic nitrogen molecules, they are not considered to be the main components). Oxides of nitrogen are mainly produced during fossil fuel combustion (particularly transportation) and serve as the building blocks for the formation of ozone (part of smog as in Mexico City) at the surface. As opposed to the beneficial stratospheric ozone layer, ozone at the ground level, in addition to its negative impacts on human health,  is the most important phytotoxic compound worldwide.

Ammonia is a cooling gas and is a product of agriculture, particularly Concentrated Animal Feed Operations (CAFOs) and nitrate is produced by chemical reactions in the atmosphere. Clearly total nitrogen deposition has increased over the years; in some parts of Europe it’s input is as high as 70 kg/hectare/year. In comparison, in the United States nitrogen fertilizer is applied at the rate of ~100 kg/hectare/year to grow corn. Increasing nitrogen in the atmosphere is a part of global climate change. The consequences of excess atmospheric nitrogen deposition on the terrestrial ecosystems are discussed in the section to follow.

Is food security sustainable under global climate change?

Since roughly 2000, the world has been consuming more food than it has been producing. After years of drawing from stockpiles, in 2007 the reserves fell to 61 days of global consumption, the second lowest on record. According to the great 18^th century British scholar, Robert Malthus, while human population increases at a geometric rate, doubling about every 25 years if unchecked, agricultural production increases arithmetically, much more slowly. In 1943 as many as four million people died in the "Malthusian correction" known as the Bengal Famine. For the following two decades, India had to import millions of tons of grain to feed its people.

Consequently a group of international agricultural research centers helped to produce more than double the world's average yields of corn, rice, and wheat between the mid-1950s and the mid-1990s, an achievement called "the green revolution”. It is very important to note that the "green revolution” could out-produce the prior wheat cultivars as long as there was plenty of water and synthetic fertilizer and minimal impacts from diseases and insects and competition from weeds. To that end, for example, the Indian government subsidized canals, fertilizer availability, and the drilling of tube wells for irrigation and gave farmers free electricity to pump the water. Today, the miracle of the green revolution is over in northern India: yield has essentially flattened since the mid-1990s. Over-irrigation has led to steep drops in the water table, now tapped by 1.3 million tube wells, while thousands of hectares of productive land have been lost due to salinity and water-logging. Forty years of intensive irrigation, fertilization, and pesticides have not been kind to the loamy gray fields of Punjab, India, nor, in some cases, to the people themselves. Still there are some that believe that these problems are due to the abuse of the land management practices for gaining increased crop yields. Nevertheless, with world population rapidly increasing toward nine billion by the mid-21^st century, there is a need for doubling the level of current food production by 2030.

Virtually all our knowledge of the effects of global climate change on crop production and yield is based on studies that involve the impact of one variable at a time, although that has little resemblance to the real world conditions as described in the previous section (#2). In principle, increases in atmospheric carbon dioxide concentrations will stimulate plant biomass production and yield, particularly in C3 plants (mostly vegetation from the temperate climate, e.g., wheat). For decades artificial exposure to elevated levels of carbon dioxide has been used to produce intensively managed high yielding horticultural crops in greenhouses. Plant uptake of increasing levels of carbon dioxide is dependent on the availability of more nitrogen. The impact of excess nitrogen on surface waters and hypoxia has been mentioned previously. In addition excess atmospheric deposition of nitrogen is known to change native, perennial plant community structure and biodiversity by favoring nitrogen-loving species and suppressing others.

Where carbohydrate to protein ratio is not balanced, the excess carbon assimilated by the plant will be converted to unwanted starch. That has implications in disease and pest incidence. For example, insects will have to feed on more foliage to obtain the required protein levels or migrate to new species that do not accumulate starch, C4 plants (mostly vegetation from the tropical climate, e.g., corn, that do not respond to elevated carbon dioxide levels, as much as the C3 plants do).

While increasing carbon dioxide concentrations can stimulate crop production, it should be remembered that any increases in the levels of surface ultraviolet radiation and ozone would counteract that beneficial effect. A predominant number of studies show that the negative effects of elevated surface level ozone offset the stimulatory effect of carbon dioxide.

In addition to the direct effects of trace gases on crops, greenhouse gas-induced increases in air temperatures will have a significant effect. There is evidence to show that night-time increases in temperature has an impact on flowering and seed filling in cereals and on blast disease incidence in rice. Increases in air temperature will affect precipitation patterns. C4 (tropical plant species) are adapted for heat and water limitation (drought). Thus, the soybean growing regions of the Midwestern United states might shift to the cultivation of sorghum. Similarly, warmer spring and fall will prolong the growth season, but induce rapid crop growth and early maturity, thus resulting in decreased yields.

Two billion people (one third the global population) already live in the driest parts of the globe, and climate change is projected to slash yields in these regions even further. No matter how great their yield potential, plants still need water to grow. And in the not too distant future, every year could be a drought year for much of the world. According to the World Meteorological Organization (WMO), by 2025 availability of fresh water supply will be the single largest limiting factor affecting global population.

Conclusions

In the final analysis, global climate change is much more complicated than simply global warming.  The interactive effects of the multiple climate variables and their impacts on crop production preclude deterministic or definitive predictions. The associated uncertainties are too large and must be assessed at local scales.

More recently, due to the demand for alternative sources of energy, the use of cropping systems has been diverted from food to the production of bio-fuels such as ethanol. As more grain has been diverted to the production of bio-fuels for vehicles, annual worldwide consumption of grain has risen from 815 million metric tons in 1960 to 2.16 billion in 2008. Since 2005, the mad rush to bio-fuels alone has pushed the growth of grain consumption from about 20 million tons annually to 50 million tons, according to the Earth Policy Institute.

Recent climate studies show that extreme heat waves are very likely to become common in the tropics and subtropics by the end of the century. Himalayan glaciers that now provide water for hundreds of millions of people, livestock, and farmland in China and India are melting faster than expected. In the worst-case scenario, yields for some grain crops could decline by 10 to 15% in South Asia by 2030. Projections for southern Africa are worse. In a region already racked by water scarcity and food insecurity, the all-important corn harvest could drop by 30 to 47%.  Meanwhile the population clock keeps going, with 2.5 more children being born every second (National Geographic, December 2009).

^* Professor Emeritus, Department of Plant Pathology, University of Minnesota, St. Paul, MN 55108, USA. E-mail: [email protected]

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.