|
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] |
