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months or years. While short-term aerosols such as lead may affect

weather on a local scale, it is the aerosols that remain and accumulate

in the atmosphere that will have long-term effects on climate.

Figure 4. — The mean annual radiation and heat balance of the atmosphere,

relative to 100 units of incoming solar radiation, based on satellite measure-

ments and conventional observations.

Source : National Research Council. U.S. Committee for the Global Atmospheric Research

Program. Understanding Climatic Change : A Program for Action, Washington, National

Academy of Sciences, 1975, p. 18.

34-857 O - 79 - 13

158

Idso and Brazel reporting on their research results in the November



18, 1977 issue of Science magazine found that initial increases in

atmospheric dust concentration tend to warm the Earth's surface.

After a certain critical concentration has been reached, continued dust

buildup reduced this warming effect until, at a second critical dust

concentration, a cooling trend begins. But, they explain, this second

critical dust concentration is so great that any particulate pollution of

the lower atmosphere will have the inexorable tendency to increase

surface temperatures. The authors pointed out that if, and when, man-

generated, industrial pollution of the atmosphere as a source of par-

ticulates ever becomes climatologically significant, the resultant sur-

face temperature trend will definitely be one of warming, not cooling.

Thus, whereas many groups assigned to assess the problem have looked

on this aspect of intensified industrialization as acting as a "brake"

on the warming influence inferred lately of increased carbon dioxide

production, 20 just the opposite is actually the case — the two phenomena

could tend to complement each other. 21

Sources of atmospheric particulates: natural against manmade

Of course, not all aerosols in the Earth's atmosphere, or even a major

proportion, are attributable to human activity. In fact, dust from vol-

canic eruptions, sea salt from evaporated ocean spray, smoke from

lightning-caused forest fires (see fig. 5), debris from meteors which

burn up in the atmosphere, windblown dust or sandstorms, and organic

compounds emitted by vegetation are much larger sources of atmos-

pheric particulates than human activity. Scientists at Stanford Uni-

versity estimate that natural processes produce about 2,312 million

tons of aerosols a year, which amount to 88.5 percent of the total.

Man and his activities account for only 296 million tons, the remaining

11.5 percent. At present, it is unlikely that man's activities and man-

made aerosols will affect global temperatures. It is important to note,

however, that while aerosols from natural sources are distributed

fairly evenly across the planet, man, in contrast, contributes high con-

centrations mostly from industrial centers. Atmospheric scientists at

the National Oceanic and Atmospheric Administration's Atmospheric

Physics and Chemistry Laboratory found that the 296 million tons of

manmade aerosols are produced every year on only about 2.5 percent

of the surface of the globe. Within these limited areas, manmade

aerosols account for nearly 84 percent of the total. It follows, then,

that these aerosols may be expected to have noticeable effects on local

weather and urban climates.

20 See, generally, National Research Council. Geophysics Research Board, "Energy and

Climate," Washington, National Academy of Sciences, 1977, 281 pp.

21 Idso, Sherwood B. and Anthony J. Brazel, "Planetary Radiation Balance RB a Function

of Atmospheric Dust : Climatological Consequences," Science, vol. 198, Nov. 18, 1977, pp.

731-733.

159

Figure 5. — Not all aerosols in the Earth's atmosphere are attributable to human



activity. In this Landsat photo, smoke from a fire in the Seney National Forest,

upper peninsula of Michigan, serves as a source of atmospheric particulates.

Note the extent of the dust veil downwind of the source. ( Courtesy of National

Aeronautics and Space Administration. )

Atmospheric processes affected by particles

Everyday, particles of soot, smoke, dust, and chemicals from indus-

trial combustion and other activities are emitted into the urban atmos-

phere. About 80 percent of the solid contaminants are small enough to

remain suspended in the air, sometimes for several days. 22 Even though

these tiny particles reflect and scatter sunlight ostensibly keeping its

heat from reaching the ground, they also can act as a lid to prevent

the outflow of heat from the land surface to the atmosphere. In a sense,

this turbidity acts as an insulator. It reduces the amount of sunlight

received at the top of the city in the daytime and cuts down on a source

of heat. However, at night urban aerosol pollutants retard the depar-

ture of radiant energy from the heated city air, encasing the heat in

22 "Do Cities Change the Weather?" Mosaic, vol. 5, summer 1974, pp. 33, 34.

160


the city's closed atmospheric system. Certain aerosols may undergo

chemical change when they combine with water vapor in the presence

of solar radiation. There are many complicated processes that can

generate aerosol gas-to-particle conversions, and the particles can then

grow by surface chemistry and physical accretion. 23

Perhaps the most sensitive atmospheric processes which can be

affected by air pollutants are those involved in the development of

clouds and precipitation. The formation and building of clouds over

a city can be influenced by the presence of pollutants acting as nuclei

upon which water vapor condenses and by the hot dry air with which

these aerosols are swept into the base of the clouds (see fig. 6). The

structure of clouds with temperatures below 0° C (defined as cold

clouds) can be modified, and under certain conditions precipitation

from them altered, by particles which are termed ice nuclei. 24 The con-

centrations of natural ice nuclei in the air appear to be very low : Only

about one in a billion atmospheric particles which are effective as ice

nuclei at temperatures above about — 15° C have the potential for mod-

ifying the structure of clouds and the development of precipitation.

If the concentration of anthropogenic ice nuclei is about 1 in 100 mil-

lion airborne particles, the result may be an enhancement of precipita-

tion ; however, if the concentration is greatly in excess of 1 in 100 mil-

lion, the result may be a tendency to "overseed" cold clouds and reduce

precipitation. Certain steel mills have been identified as sources of ice

nuclei. Also of concern is the possibility that emissions from automo-

biles may combine with trace chemicals in the atmosphere to produce

ice nuclei. 25

23 Hobhs. P. V.. H. Harrison, E. Robinson, "Atmospheric Effects of Pollutants." Science,

vol. 183, Mar. 8, 1974. p. 910.

2i National Research Council. Committee on Atmospheric Sciences. "Weather and Climate

Modification : Problems and Progress," Washington, National Academy of Sciences, 1973,

pp. 41-47.

25 Hobbs, P. V., H. Harrison, E. Robinson, "Atmospheric Effects of Pollutants," p. 910.

161

Figure 6. — The formation and building of clouds can be influenced by the pres-



ence of pollutants acting as nuclei upon which water vapor condenses and by the

hot dry air with which these aerosols are swept aloft. In this Landsat photo,

excess particles as well as heat and moisture produced by the industries of Gary,

Ind.. favor the development of clouds downwind. The body of water shown is

the southern tip of Lake Michigan. (Courtesy of National Aeronautics and

Space Administration.)

Precipitation from clouds that have temperatures above 0° C (warm

clouds) may be modified by particles which serve as cloud condensa-

tion nuclei (CCN). A source that produces comparatively low con-

centrations of very efficient CCN will tend to increase precipitation

from warm clouds, whereas one that produces large concentrations

of somewhat less efficient CCN might decrease precipitation. Modi-

fications in the structure of clouds and precipitation have been observed

162


many miles downwind of fires and pulp and paper mills. Large wood-

waste burners and aluminum smelters have also been identified as

major sources of CCN. 26

The La Porte tveather anomaly: urban climate modification

La Porte, Ind., is located east of major steelmills and other indus-

tries south of Chicago. Analysis of La Porte records revealed that,

since 1925, La Porte had shown a precipitation increase of between

30 and 40 percent. Between 1951 and 1965, La Porte had 31 percent

more precipitation, 38 percent more thunderstorms, and 246 percent

more hail days than nearby weather stations in Illinois, Indiana,

and Michigan. 27 Reporting on this anomaly at a national meeting of

the American Meteorological Society in 1968, Stanley Changnon, a

climatologist with the Illinois State Water Survey pointed out that

the precipitation increase in La Porte closely followed the upward

curve of iron and steel production at Chicago and Gary, Ind. Fur-

thermore, La Porte's runs of bad weather correlated closely with

periods when Chicago's air pollution was bad. Stated simply, Ohang-

non's theory was that if this effect did not occur by chance, then the

increase in precipitation comd be caused by the excess particles

as well as heat and moisture produced by the industries upwind

of La Porte. Pollutants from the industrial sources, it seemed, were

serving as nuclei to trigger precipitation, just as silver iodide crystals

are used to seed clouds in deliberate efforts of weather modification. 28

The discovery of the La Porte anomaly helped usher in considerable

scientific and public concern as to whether cities could measurably

alter precipitation and severe weather in and downwind of them. A

large urban-industrial center is a potential source of many conditions

needed to produce rainfall. These include its release of additional

heat (through combustion and from "storage" in surfaces and build-

ings) which lifts the air ; the mechanical mixing due to the "mountain

effects" of a city existing in flat terrain ; additional moisture released

through cooling towers and other industrial processes ; and the addi-

tion of many small particles (aerosols), which could serve as nuclei

for the formation of cloud droplets and raindrops.

The interest in whether urban emissions into the atmosphere could

trigger changes in weather and climate on a scale much larger than

the city itself led to climatological studies of other cities. Historical

data for 1901-70 from Chicago. St. Louis, Washington, D.C., Cleve-

land, Xew Orleans, Houston, Indianapolis, and Tulsa were studied in

an effort to discern whether cities of other sizes, different industrial

bases, and varying climatic-physiographic areas also experienced rain-

fall changes. The six largest cities — Washington, Houston, New

Orleans, Chicago, Cleveland, and St. Louis — all altered their summer

precipitation in a rather marked fashion: Precipitation increases of

LOto 30 percenl in and downwind of t heir urban locales, plus associated

increases in thunderstorm and hailstorm activity were documented.

16 National Research Council. Committee on Atmospheric Sciences, "Weather and Climate

Modification : Prohlems and Progress." p. 50.

» Lansford. Henry, "We're Changing the Weather hy Accident," Science Digest, vol. 74,

Dec. 1973, p. 21.

M Changnon. S. A., Jr.. "The La Porte Weather Anomaly — Fact or Fiction?" Bulletin of

the American Meterologlcal Society, vol. 49, January 19G8, pp. 4-11.

163

Tulsa and Indianapolis, cities of lower population and lesser physio-



graphic irregularities than the others studied, did not reveal any

precipitation anomalies. 29

The key questions that could not be answered conclusively at the

completion of these climatic studies were (1) whether the anomalies

found were real (or adequately measured) ; (2) if real, what was

causing the anomalies; and (3) whether and how extensive the anoma-

lies were around other cities. To this end, a major atmospheric pro-

gram dealing with inadvertent weather modification was initiated

by a group of scientists in 1971. The Metropolitan Meteorological

Experiment (METROMEX) was designed by four research groups

who received support from Federal agencies and one State (Illinois).

St. Louis was chosen as the site of extensive field investigations in this

first major field program aimed at studying the reality and causes of

urban rainfall anomalies suggested in the climatological surveys con-

ducted previously. 30

Although data analysis and report preparation continue (summer

1975 was the fifth and final year for field work), METROMEX data

thus far portray statistically significant increases in summer rainfall,

heavy (more than 2.5 cm) rainstorms, thunderstorms and hail in and

just east (downtown) of St. Louis. Examination of the rainfall yield of

individual showers, the spatial distribution of rain developments, and

areal distribution of afternoon rain clearly point to the urban-indus-

trial complex as the site for the favored initiation of the rain process

under certain conditions. 31

Writes climatologist Stanley Changnon :

The greater frequency of rain initiations over the urban and industrial areas

appears to be tied to three urban-related factors including thermodynamic

effects leading to more clouds and greater in-cloud instability, mechanical and

thermodynamic effects that produce confluence zones where clouds initiate, and

enhancement of the [raindrop] coalescence process due to giant nuclei. Case

studies reveal that once additional [rainstorm] cells are produced, nature, cou-

pled with the increased likelihood for merger with more storms per unit area,

takes over and produces heavier rainfalls. Hence the city is a focal point for

both rain initiation and rain enhancement under conditions when rain is likely. 31

Recapitulating, METROMEX researchers have found that rain,

thunderstorms and hail can actually maximize within cities and nearby

areas, particularly in those downwind. Such locations may have more

storms, and they are more intense, last longer and produce more rain

and hail than storms in surrounding regions. Apparently, air heated

and polluted by a city can move up through the atmosphere high

enough to affect clouds. This urban-modified air clearly adds to the

strength of convective storms and increases the severity of precipita-

tion. Urban climatic alterations are summarized in table 1.

29 Huff, F. A. and S. A. Changnon, Jr., "Precipitation Modification by Major Urban Areas,"

Bulletin of the American Meteorological Society, vol. "54, December 1973, pp. 1220-1232.

30 Changnon. S. A., F. A. Huff, and R. G. Semonin, "Metromex : An Investigation of

Inadvertent Weather Modification," Bulletin of the American Meteorological Society, vol.

52, October 1971, pp. 958-967.

si "METROMEX Update," Bulletin of the American Meteorological Society, vol. 57, March

1976, pp. 304-308.

32 Changnon, S. A., R. G. Semonin and F. A. Huff, "A Hypothesis for Urban Rainfall

Anomalies," Journal of Applied Meteorology, vol. 15, June 1976, pp. 544-560.

164

Table 1. — Some urban climatic alterations 1



Comparison with rural environs

Radiation :

Global 10 to 20 percent less.

Ultraviolet :

Low sun 30 to 50 percent less.

High sun 5 to 10 percent less.

Temperature :

Annual mean 1 to 2° C higher.

Maximum difference 3 to 10° C higher.

Winter minima 1 to 3° C higher.

Cloudiness :

General cloud cover 5 to 10 percent more.

Fog:

Winter 100 percent more.



Summer 20 to 30 percent more.

Precipitation :

Totals :

Summer 10 percent more.

Winter 5 percent more.

Relative humidity : Annual mean 4 to 6 percent less.

Evapotranspiration : Total amount 30 to 60 percent less.

Dew : Amounts 50 to 80 percent less.

Wind speed : < 3 m sec -1 40 percent less.

Speeds :

3 — 6 m sec 20 percent less.

> 6 m sec 10 percent less.

Thunderstorms : Number of days 5 to 10 percent more.

1 After Helmut Landsberg, University of Maryland.

CARBON DIOXIDE AND WATER VAPOR

The constituent gases of the atmosphere that are important vari-

ables affecting the distribution of temperature within the atmosphere

are carbon dioxide and water vapor. Capable of absorbing important

quantities of infrared radiation, they both have a role in modifying

the vertical distribution of temperature in the atmosphere by con-

trolling the flux of infrared radiation. The absorption of incoming

solar radiation by these gases is so small that their concentration has

no appreciable effect on the amount of incoming solar radiation reach-

ing the Earth's surface. Carbon dioxide and water vapor are, how-

ever, opaque to major portions of the long- wave radiation emitted by

the Earth's surface. The greater the content of these gases the greater

the opacity of the atmosphere to infrared radiation and the higher its

temperature must be to radiate away the necessary amount of energy

to maintain a radiation balance. It is this absorption of long-wave

radiation emitted by the Earth, with the subsequent reradiation of

additional infrared radiation to the ground and consequent elevation

of air temperatures near the surface that is known as the "greenhouse

effect."

Increases in atmospheric c

record indicates

Man adds carbon dioxide to the atmosphere through the combustion

of fossil fuels, and this addition is superimposed on the natural ex-

changes between the atmosphere, the biosphere, and the world ocean.

Since the use of energy has increased exponentially since the beginning

165


of industrialization around 1860, it is not surprising that the best

estimate of carbon dioxide production, which results from fossil fuel

combustion and cement manufacture, shows the same exponential

trend (see fig. 7).

The concentration of carbon dioxide in the atmosphere has in-

creased steadily from a preindustrial value of about 295 parts per

million in 1860 to a current value of 330 parts per million (+ 12

percent). Since the beginning of accurate and regular measurements

in 1958, observed atmospheric carbon dioxide concentrations have in-

creased some 5 percent from 315 parts per million to the current yearly

average value of 330 parts per million as indicated in figure 8.

Figure 7. — The annual world production of carbon dioxide from fossil fuels (plus

a small amount from cement manufacture) is plotted since the beginning of

the industrial revolution. Except for brief interruptions during the two world

wars and the Great Depression, the release of fossil carbon has increased at a

rate of 4.3 percent per year. (Data for 1860-1959 from C. D. Keeling, "Indus-

trial Production of Carbon Dioxide from Fossil Fuels and Limestone," Tellus,

vol. 25, 1973, p. 174 ; data for 1960-71 from R. M. Rotty, "Commentary on and

Extension of Calculative Procedure for Carbon Dioxide Production," Tellus,

vol. 25, 1973, p. 508.)

Source : Baes. 'C. F.. et al. "The Global Carbon Dioxide Problem," Oak Ridge National

Laboratory, 1976. (ORNL-5194.)

166

Figure 8. — Monthly average values of the concentration of carbon dioxide in the



atmosphere at Mauna Loa Observatory, Hawaii, are plotted since the beginning

of accurate and regular measurements in 1958. Variations in photosynthesis and

other seasonal effects produce the annual cycle. Mean annual concentrations

are well above the preindustrial level (290-300 ppm), and the secular increase

is quite apparent.

Source: Baes, C. F., et al. "The Global Carbon Dioxide Problem," Oak Ridge National

Laboratory, 1978. (ORNL-5194.)

The seasonal variation of the record of carbon dioxide measurements

made at Mauna Lao is obvious and regular, showing an October mini-

mum with increases in the later autumn and winter months and a maxi-

mum in May. However, of greater importance to possible climatic

changes is the continued year-to-year rise. Both the seasonal variation

and the annual increase have been confirmed by measurements at other

locations around the globe.

Predicting future atmospheric carbon dioxide levels

Projecting the worldwide needs for energy, even with the present

problems, indicates a long-term global growth in the consumption of

fossil fuels and the associated production of carbon dioxide. Insofar as

possible impact on the climate is concerned, it is the amount of carbon

dioxide remaining in the atmosphere that is most important. In addi-

tion to the atmosphere, the ocean and both land and marine biospheres

serve as reservoirs for carbon dioxide. Based on estimates of preindus-

trial levels of atmospheric carbon dioxide of 290-295 parts per million

and the 1958 to present Mauna Loa data, between 58 and 64 percent of

the carbon dioxide produced from burning fossil fuels remains in the

atmosphere. Cumulative production of carbon dioxide is plotted in

figure 9. The upper set of points indicates the increase in the carbon

dioxide fraction of the atmosphere that would have occurred if all car-

167

bon dioxide produced since 1860 from fossil fuels and cement remained



airborne. The lower set of points represents the observed increase based

on an assumed value of 290-295 parts per million in 1860. The differ-

ence between the two sets of points presumably indicates the amount of

carbon dioxide being taken up by the world ocean and possibly the

biosphere and placed in long-term storage. Nearly half of the carbon

dioxide produced from fossil fuels and cement seems to have found its

way into reservoirs other than the atmosphere.



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