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Vol. 6 No. 1 - Millennium Issue - January 2000

Air Pollution, Global Climate Change and Agriculture

A Vignette of the Last 50 Years of A Millennium

By: Sagar V Krupa

At the outset, I would like to wish each of you much success, as we enter the year 2000. The next millennium has a form and a direction that few of us can fully imagine, although "futurists" continue to make predictions. Only time will tell.

The British scholar James Lovelock wrote in his book on "Gaia" that the "Atmosphere is the face of the earth". Clearly since the 1950s, the face of the earth has changed measurably. Human activity has led to increases in the concentrations of many trace gases in the atmosphere. Fundamentally such changes in the chemical composition are directly linked to the alterations in the physical climate of the earth. Consequently, any observed and or predicted changes in air temperature (global warming) and precipitation patterns are of utmost concern worldwide.

Emissions of chemical constituents into the atmosphere by human activity (air pollution) are nothing new. Air pollution has been known to exist since the ancient age of the nomads. The movement of nomads from place to place, never staying at any given location for any length of time was not accidental, it was due to the foul odors generated by plant, animal and human waste. The concerns, however, since the 1950s have been the amount and variety of air pollutants generated through urbanization and industrialization. While some trace gases such as carbon dioxide, sulfur dioxide and ozone are produced from both natural and human-made sources, others such as chlorofluorocarbon (CFC) compounds are entirely due to human activity.

Clearly, as with any other field of scientific endeavor, in the last 50 years, research on the effects of air pollutants on agriculture has made major advances. Emphasis has shifted from studying the local scale effects of primary pollutants such as sulfur dioxide, to long-range transport and regional scale effects of secondary pollutants such as ozone. Similarly, traditional views of examining the short-term visible injury on the foliage of sensitive crop species have changed to studying the long-term effects on growth and yield. Here, the use of artificial field fumigation methods came into vogue in the 1970s. Large-scale research efforts such as the National Crop Loss Assessment Network (NCLAN) in the US and the European Open-top Chambers Programme (EOTCP) became major contributors to our understanding of crop response to air pollution stress. These programs mainly addressed the deleterious effects of surface level ozone, an all-pervasive air pollutant toxic to plants. In 1996, the US EPA (Environmental Protection Agency) concluded that ozone caused $0.8 billion in crop loss annually in that country.

There can be some confusion about the prevalence of ozone. Life on earth has evolved around ozone. There is a naturally occurring beneficial ozone layer in the upper atmosphere (stratosphere) that filters sufficient levels of harmful solar radiation from arriving at the surface and causing skin cancer. This overall process is also responsible for converting absorbed radiation to air temperature and the mechanical energy of winds (jet streams) governing the global scale weather of the middle atmosphere. However, at the present time, there is much concern about the destruction of the beneficial ozone layer by the increasing emissions of human-made trace gases such as the chlorofluorocarbon and organ-bromine compounds, and consequently the predicted increase in the incidence of harmful ultraviolet-B radiation at the surface.

In contrast, ozone at the surface is deleterious and is produced primarily from emissions of fossil fuel combustion. At the global scale ozone concentrations at the surface are increasing due to population growth, urban density and consequent industrialization. So are the other trace gases such as carbon dioxide. Among others, ozone and carbon dioxide are "greenhouse gases" that normally trap the outgoing radiation from the surface leading to the warming of the air in the lower atmosphere. This is the natural "greenhouse effect" that sustains life on earth.

Some believe that "global climate change" is simply "global warming". In this context, whether increasing concentrations of the "greenhouse gases" will lead to climate warming, uniformly across all latitude is questionable. As my colleague J.P. Lodge Jr. stated, "sizable areas may well become warmer, cooler, drier, wetter or remain unchanged, so far as annual mean values are concerned". Probably any modification of the climate will manifest itself through changes not in mean values, but in deviations from those means, and in the frequency of severe weather such as high winds, thunderstorms and blizzards. Evidence also shows that nighttime temperatures over the US have increased measurably during the last several decades, without a corresponding increase in the daytime temperatures. Likely, increased cloud cover due to air pollution during the same period has an important role in this. The significance of this shift in daytime/nighttime temperatures on plant biology is not understood at the present time, although in addition to any alterations in growth patterns, such changes would most likely have an effect on the incidence of disease and insect pest epidemics.

Whether climate warming occurs at a given location or not, increases in the concentrations of trace gases such as carbon dioxide and ozone per se will have a direct effect on crops. Here, we continue to conduct experiments to study the effect of one independent variable at a time. This has no realism. Most recent efforts, for example, to study the combined effects of elevated levels of carbon dioxide and ozone show that any outcome (positive or negative) will be species dependent. This has implications for future agricultural production. There is a slow, but progressive shift to examine the integrated effects of multiple, ambient growth regulating variables on crops, using chamber-less methods. The next millennium will usher much progress in this direction.

As our understanding of long-range transport of air pollutants increased, so did our concern for the occurrence of "acidic precipitation". Whether this is a new phenomenon or a newly re-discovered phenomenon is questionable. Indeed the famous chemist Robert Boyle (1692) referred to the "nitrous and salino-sulphurous spirits" of the air. Nevertheless, after significant amounts of financial expenditure and research in the 1980s, it was concluded that "ambient" precipitation does not cause direct adverse effects on crops.

During all these times, an aspect that has been largely overlooked relates to the effects of air pollutants on food quality, the plant produce that we consume. There is a clear disparity in the laws regulating the use of agricultural chemicals across nations. As fresh produce is exported and imported between nations, the food quality issue becomes very important. There is growing evidence to show that atmospheric deposition of persistent organic chemicals can accumulate in plant parts. A similar scenario can be envisioned for toxic metals. These are areas where our current knowledge is minimal.

There are many that believe that increased levels of carbon dioxide would result in increased crop yield. Although this would be an attractive outcome, leaving aside the presence of other air pollutants toxic to plants; water availability, improved cropping practices, demands for increased farm resources and the associated costs would be key limiting factors for food production in developing countries. With or without air pollution or climate change, there is already evidence of the critical role these factors play in limiting food production in Africa. Here and elsewhere, sudden shifts in cropping systems are not expected to occur nor are they realistic, because of the corresponding need in making rapid changes from production to marketing systems.

Reductions in the emissions of air pollutants are achieved through mitigation (post-mortem control of an observed problem), adaptation (societal change) and pollution prevention. The first and last approaches are expensive because of a need for additions or modifications to the existing industrial processes and production technologies. In comparison, the second approach is frequently constrained by cultural, religious and literary issues. For example, rice paddies constitute the second largest source for the "greenhouse gas", methane. To expect that the society will forego rice consumption in Asia is unrealistic, at least I cannot foresee this. This is analogous to the suggestion made by one scientist that populations of cattle (also a source for methane) in the US be retrenched and the consumption of dairy products be rationed.

In the final analysis, air pollution, climate change and agriculture are inherently linked to two very important factors: population growth and economic disparity. Environmental literacy is quite different from being simply literate. We, as environmental scientists have a lasting role in this education. Personally, I feel very challenged and rewarded in my recurring experiences since 1978, in sharing my knowledge on air pollution related matters with highly motivated groups of students in Mexico. In a small way, I think my contributions have made an impact. I encourage each of you to become involved with similar or other opportunities in environmental education.

Looking into the immediate future, in my personal opinion, the scientists within the Commission of the European Communities (CEC) will be a strong force in furthering our knowledge of air pollutant effects on agronomic and other plant communities. A commonality in purpose and a cohesive view in achieving a goal will be the determining factors. Without describing all the underlying reasons, it will suffice to state that scientists in other sectors of the world community are fragmented in this aspect. Thus, there are lessons to be learned from the CEC.

This is not to say that there is no critical mass or groups of scientists in certain disciplinary areas (e.g., ecology, but not necessarily plant toxicology) represented by the IGBP (International Geosphere - Biosphere Program). This major, international effort is reminiscent of the IBP (International Biological Program) in the 1960s. Numerous well-known and reputed scientists participated in IBP. At the end IBP never fulfilled all the expectations it was billed a priori to address. In my opinion, there was a clear lack of interdisciplinary input and the holistic integration of the multiple scientific fields of specialization involved. In the present context, the overall issues associated with "climate change" are no different.

In closing, I would like to quote W.H. Auden, the British-born, American author who in his "Commentary" wrote, "Little upon his little earth, man contemplates the universe of which he is both judge and victim". Hopefully, in the next millennium, we will successfully face this challenge.

Dr. Sagar V. Krupa is Professor at the Department of Plant Pathology, University of Minnesota, 495 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108, USA (FAX: + 1-612-625-9728: E-mail:[email protected]

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

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