Growth of Continental-Scale Metro-Agro-Plexes, Regional Ozone Pollution, and World Food Production

Three regions of the northern mid-latitudes, the continental-scale metro-agro-plexes, presently dominate global industrial and agricultural productivity. Although these regions cover only 23 percent of the Earth's continents, they account for most of the world's commercial energy consumption, fertilizer use, food-crop production, and food exports. They also account for more than half of the world's atmospheric nitrogen oxide (NOx,) emissions and, as a result, are prone to ground-level ozone (O3) pollution during the summer months. On the basis of a global simulation of atmospheric reactive nitrogen compounds, it is estimated that about 10 to 35 percent of the world's grain production may occur in parts of these regions where ozone pollution may reduce crop yields. Exposure to yield-reducing ozone pollution may triple by 2025 if rising anthropogenic NOx emissions are not abated.

degrees of freedom.
The correlation dimension calculations are prone to various artifacts (16), and in order to reduce this possibility we used a control time series of surrogate data (17) with the same characteristics (number of points, power spectrum) as the CZ model time series. We chose a time series from a linear Markov model built from the CZ model and driven by random forcing (18). The dimension estimate for the Markov model ( Fig. 2B) indicates that this time series is random and distinguishable from the low-order dimension found with the CZ model. This result is consistent with the suggestion that the irregularity of ENSO events (at least in the CZ model) is not due to random noise (such as ocean weather phenomena present in the CZ model).
We suggest that the natural oscillator of the equatorial Pacific ocean-atmosphere system can enter into nonlinear resonance with the seasonal cycle at several periods of the oscillator (mostly 2 to 5 years). The coexistence of these resonances results in chaotic behavior that is due to the jumping of the system among the different resonances. This is a feature of the quasi-periodicity route to chaos (12). Much additional work is needed to further examine the relevance of these ideas to teosre NOcaatrsisadt the observed ENSO characteristics and to clarify the spatial and temporal mechanisms 74 of the seasonal forcing of Pacific interannual variability. If this theory can be validated, the ENSO cycle might be established as an example of low-order chaos in a highly complex physical system. produces regular ENSO oscillations when run without the seasonal forcing term. Equation 1 was integrated by use of a high-accuracy, variable-order, variable-step Adams method using routine DO2CBF in Fortran Library Manual, Mark II (Numerical Algorithms Group, 1984). 12 Growth of Continental-Scale Metro-Agro-Plexes, Regional Ozone Pollution, and World Food Production W. L. Chameides,* P. S. Kasibhatla,t J. Yienger, H. Levy 11 Three regions of the northern mid-latitudes, the continental-scale metro-agro-plexes, presently dominate global industrial and agricultural productivity. Although these regions cover only 23 percent of the Earth's continents, they account for most of the world's commercial energy consumption, fertilizer use, food-crop production, and food exports. They also account for more than half of the world's atmospheric nitrogen oxide (NO,) emissions and, as a result, are prone to ground-level ozone (03) pollution during the summer months. On the basis of a global simulation of atmospheric reactive nitrogen compounds, it is estimated that about 10 to 35 percent of the world's grain production may occur in parts of these regions where ozone pollution may reduce crop yields. two factors: the development of high-input-high-yield agriculture, capable of feeding an increasingly urban population, and an urban-industrial infrastructure, heavily dependent on fossil fuels for the production and transport of manufactured goods (1).
The correlation between agriculture and fossil fuel burning is most pronounced in three regions of the northern mid-latitudes gions, intense urban-industrial and agricultural activities tend to cluster together into a single large network, or plexus, of lands affected by human activity. We use here the term continental-scale metro-agro-plexes (CSMAPs) as shorthand for these three regions to characterize their size and intermingling of agricultural and urban-industrial activities.
Although these regions comprise only 23% of the Earth's continents, the three CSMAPs account for about 75% of the world's consumption of commercial energy and fertilizers and about 60% of the world's food crop production and food exports (2)(3)(4). They are also major source regions for atmospheric pollutants such as nitrogen oxides (NO, = NO + NO2). More than 50% of global NO, emissions originate in CSMAPs, and within CSMAPs anthropogenic emissions make up more than 75% of this total (Table 1). Anthropogenic emissions arise primarily from the burning of fossil fuels. However, microbial emissions from fertilized soils are also significant. Our calculations with the Geophysical Fluid Dynamics Laboratory (GFDL) threedimensional, globalchemical transport model (GCTM) suggest that photochemical smog has become ubiquitous in CSMAPs because of these NOX emissions, and as a result many of the world's most productive agricultural regions are probably exposed to harmful concentrations of ozone (03) (5)(6)(7).
Photochemical smog refers to the mix of noxious gases and particles produced near the Earth's surface from the photooxidation of VOC (hydrocarbons and other volatile organic compounds) and CO in the presence of NOx (8). Although photochemical smog produces several phytotoxins, we focus here on 03, a component of smog whose effects on vegetation have been well documented (9)(10)(11)(12). Concentrations of 03 tend to be highest in and around urban areas. However, suburban sprawl, growing numbers of automobiles and expanding roadway networks, as well as an increasing reliance on nitrogenous fertilizers, has greatly increased the spatial scale of photochemical smog in the CSMAPs. Regional 03 pollution, often associated with summertime high-pressure systems, can extend over thousands of kilometers and encompass agricultural as well as urban areas (13). The repetition of episodes of 03 pollution over a growing season produces a pattern of chronic exposure that ultimately reduces crop yields.
The specific relation between 03 dosage and crop yield is complex and depends on several parameters, such as the species and developmental stage of the crop, the environmental conditions, and the pattern and duration of 03 exposure (10, 1 1). In general, crop yield reductions of 5 to 10% result when 03 reaches some threshold concentration, and these reductions increase as the 03 concentration increases above the threshold. For cumulative exposures over a growing season, the threshold ranges from -50 parts per billion by volume (ppbv) for sensitive crops, such as several types of winter wheat, to 70 ppbv for insensitive crops, such as rice (6, 1 1).  (24). Estimates are given for the present (P) and for 2025 assuming slowly changing world (SCW) and rapidly changing world (RCW) scenarios (that is, scenarios with modest and rapid expansion in the global economy, respectively.) Emissions for 2025 are estimated from regional projections for fossil fuel and N fertilizer usage with an econometric model (35). "Other" refers to NO, emissions sources from natural (nonfertilized) soils (34), biomass burning (36), lightning (17), aircraft (37), and stratospheric N20 oxidation (38). For sources of fossil fuel emissions and fertilizer-induced soil emissions, see (16,34 We estimated the areal extent of 03 pollution and its correlation with agricultural activity from simulations of NO,, and total reactive nitrogen (NOY) (14) with the GFDL 3D GCTM (15-21). (A dearth of data on air quality on a continental scale precludes a direct determination from 03 observations.) On regional scales, 03 production is typically found to be limited by the availability of NO,, (22,23), and in rural areas, the summertime 03 concentration has been observed to vary linearly with the concentration of the products of NO, oxidation (19,21,23)-that is, 103] = a + b[NOY - NO,] (1) where a represents the nominal background 03 concentration in air not directly influenced by anthropogenic emissions (-35 ppbv), and b represents the number of 03 molecules produced for each NO, molecule emitted into the atmosphere. This latter parameter depends on the local concentrations of VOC and NO, and generally varies from 5 to 13. These results suggest that [NOY -NO] is a reasonable diagnostic for estimating 03 concentrations on regional scales. Figure 2 illustrates the average [NOY -NO,,] during the growing season (24) calculated for present-day NO, emissions as well as for those projected for 2025 under scenarios for a slowly changing world (SCW) and a rapidly changing world (RCW) ( Table 1). Here, the influence of anthropogenic emissions is apparent. For present-day emissions, [NOY -NO] concentrations in excess of 2 ppbv cluster in the CSMAPs, whereas concentrations outside the CSMAPs generally fall well below 1 ppbv. In the absence of anthropogenic NO,, emissions, we find that [NOY -NO] concentrations in the CSMAPs never exceed 0.5 ppbv, except in northeastern Chi-76 na where concentrations approach 1 ppbv. The increase in emissions projected for 2025 intensifies the pollution in the European and Asian CSMAPs and also produces pockets of [NOY -NO,, pollution in northern Africa, the Indian subcontinent, and South America.
Our results suggest that a sizable portion of the world's food crops are presently exposed to high [NOY -NOx] (Fig. 3). If the empirical formula of Trainer et al. (23) relating 03 concentration to [NOY -NOX] (with a slope of 8.5) were appropriate for all CSMAPs, our calculations would imply that 9 to 35% of the world's cereal crops are presently exposed over the growing season to average 03 concentrations above the threshold of 50 to 70 ppbv (25). These percentages would drop to 6 to 30%, if there were no emissions from fertilizers, and to 0%, if there were no emissions from fossil fuels and fertilizers. Typical 03 exposure-yield relations (6) suggest that the losses in crop yields from the current exposure are of the order of a few percent of the world's production of cereals and other crops (26).
Industrialized countries typically restrict agricultural supply through policies to eliminate surplus (27). Thus, a loss of a few percent in food production could easily be made up by adopting compensating measures in these countries to maintain productivity at an appropriate level to meet demand. Such compensating measures might include the use of additional fertilizers, pesticides, and herbicides or the cultivation and irrigation of relatively marginal lands, with associated economic and environmental costs. ger in poor countries, where the price elasticity of demand for staple foods remains high. Clearly, the full economic, environmental, and human costs of the effect of 03 pollution on crops is a complex issue that will require careful assessment. Moreover, although the current loss in crop yields because of 03 pollution appears to be only a few percent of the total, this will likely change in the coming decades.
The NO, emissions projected for 2025 not only intensify pollution over two of the CSMAPs but also enhance pollution in agricultural regions of the developing world. For 2025, we estimate that as much as 30 to 75% of the world's cereals may be grown in regions with 03 above the 50 to 70 ppbv threshold, which suggests that the agricultural losses may increase significantly. Additionally, this increased pollution effect will be occurring at a time when growing populations in developing nations will be straining food production capacities. Benefits may therefore result from reducing use of fossil fuels, limiting losses of nitrogen fertilizers from soils, implementing NO, emission controls, and developing 03-resistant crops. Enhanced networks for monitoring air quality throughout the developed and developing worlds to assess the extent and severity of 03 pollution on continental scales will aid in evaluating the benefits of these mitigating strategies.
Thin films of disk-shaped molecules are expected to display anisotropic optical and transport properties, leading to applications in optical display or sensor technologies. Bilayer Langmuir-Blodgett films of monomeric triphenylene mesogens have been studied by atomic force microscopy. The triphenylene cores of the constituent molecules tend to promote the formation of columnar structures in the plane of the substrate and along the direction of deposition of the film. Atomic force microscopy images of bilayer Langmuir-Blodgett films revealed two types of structure, one corresponding to an aligned columnar structure and the other to an unusual square lattice, which may result from the superposition of columnar structures in adjacent layers that intersect at near right angles. Annealing such bilayers near the melting point of the bulk compound improved the structural ordering by reducing the angular spread of orientations associated with the well-developed columnar structure in some areas and by producing a more distinct square lattice in other areas of the sample.
Langmuir-Blodgett (LB) films (1, 2) have long been of interest both as model systems for two-dimensional (2D) physics and for their promise in technological applications. Although most research in this area has concentrated on LB films of amphiphilic SCIENCE * VOL. 264 * 1 APRIL 1994 rod-like molecules, disk-shaped mesogens exhibiting columnar liquid crystalline phases (3) have been shown to form Langmuir (4) and LB films (5,6). The electronic conductivity of doped bulk discotic mesophases is highly anisotropic, with most of the conduc-