Atmospheric Turbidity and Nuclear Winter!
One of the most obvious indicators of atmospheric pollution is the presence of solid or liquid particles, called ‘aerosols’, dispersed in the air. These aerosols are responsible for phenomena as diverse as the urban smog’s that prevailed in the world’s major cities and the spectacular sunsets which often follow major volcanic eruptions.
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The concentration and distribution of particulate matter in the atmosphere is closely linked to climatic conditions. Some local or regional climates encourage high aerosol concentrations, as in Los Angeles, for example, with its combination of high atmospheric pressure, light winds and abundant solar radiation.
On a global scale, the mid-latitude Westerlies and their associated weather systems, already implicated in the distribution of acid rain, are responsible for the transportation of aerosols over long distances in the troposphere.
At a continual or even hemispheric scale, the relationship between atmospheric circulation patterns and the spread of particulate matter can be used to provide an early warning of potential problems following catastrophic events such as volcanic eruptions or nuclear accidents. In such situations the atmospheric aerosols are responding to existing climatic conditions.
1. Aerosol Types, Production And Distribution:
The total global aerosol production was estimated to be over 3 x 104 tonnes per annum and, on any given day, perhaps as many as 1 x 107 tonnes of solid particulate matter is suspended in the atmosphere. Under normal circumstances, almost all of the total weight of particulate matter is concentrated in the lower 2 km of the atmosphere in a latitudinal zone between 30°N and 60°N.
The mean residence time for aerosol in the lower troposphere is between 5 and 9 days, which is sufficiently short so that the air can be cleansed in a few days once emissions have stopped.
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The equivalent time in the upper troposphere is about one month, and in the stratosphere the residence time increases to two to three years. As a result, anything added to the upper troposphere or stratosphere will remain in circulation for a longer time and its potential environmental impact will increase.
Aerosol can be classified in a number of ways, but most classifications include such elements as origin, size and development—sometimes individually, sometimes in combination.
Most of the atmospheric aerosol content—perhaps as much as 90 per cent—is of natural origin, although anthropogenic sources may be dominant locally, as they are in urban areas. Dust particles created during volcanic activity or carried into the troposphere during dust storms are examples of natural aerosol.
Atmospheric aerosol comprise a very heterogeneous group of particles and the mix within the group changes with time and place.
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Following volcanic activity, for example, the proportion of dust particles in the atmosphere may be particularly high; in urban areas such as Los Angeles, photochemical action on vehicle emissions causes major increase in secondary particulate matter; over the oceans, 95 per cent of the aerosols may consist of coarse sea-salt particles.
Such variability makes it difficult to establish the nature of the relationship between atmospheric aerosols and climate.
It is clear, however, that the aerosols exert their influence on climate by disrupting the flow of radiation within the earth atmosphere system and there are certain elements which are central to the relationship.
The attenuation of solar radiation caused by the presence of aerosols provides a measure of atmospheric turbidity, a property which, for most purposes, can be considered as an indication of the dustiness or dirtiness of the atmosphere.
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Several things may happen when radiation strikes an aerosol in the atmosphere. If the particle is optically transparent the radiant energy passes through unaltered and no change takes place in the atmospheric energy balance.
More commonly, the radiation is reflected, scattered or absorbed— in proportion which depend upon the size, colour and concentration of particles in the atmosphere and upon the nature of the radiation itself.
Aerosol which scatter or reflect radiation increase the albedo of the atmosphere and reduce the amount of insolation arriving at the earth’s surface. Absorbent aerosols will have the opposite effect. Each process, through its ability to change the path of the radiation through the atmosphere, has the potential to alter the earth’s energy budget.
Among the atmospheric aerosols, desert dust and soot particles readily absorb the shorter solar wave lengths with soot—a particularly strong absorber across the entire solar spectrum. In addition to disrupting the flow of incoming solar radiation the presence of aerosols also has an effect on terrestrial radiation.
Being at a lower energy level, the earth’s surface radiates energy at the infrared end of the spectrum. Aerosols—such as soot, soil and dust particles—released into the boundary layer absorb infrared energy quite readily, particularly if they are larger than 1.0 µm in diameter and, as a result, these changes tend to raise the temperature of the troposphere.
2. Atmospheric Turbidity—Natural and Man-Made Sources:
The large volumes of particulate matter thrown into the atmosphere during periods of volcanic activity are gradually carried away from their sources to be redistributed by the wind and pressure patterns of the atmospheric airculation.
Dust ejected during the explosive eruption of Krakatoa in 1883 encircled the earth in about two weeks following the original eruption and within 8 to 12 weeks had spread sufficiently to increase atmospheric turbidity between 35°N and 35°S.
The diffusion of dust from the Mount Agurg eruption in 1963 followed a similar pattern and, in both cases, the debris eventually spread pole wards until it formed a complete veil over the entire earth.
The volume of particulate matter produced by human activities cannot match the qualities emitted naturally. Estimates of the human contribution to total global particulate production vary from as low as 10% to more than 15% with values tending to vary according to the size-fraction included in the estimate. Human activities may provide as much as 22 per cent of the particulate matter finer than 5 µm.
The development of the Arctic Haze in recent years is generally considered to be an indication of continuing anthropogenic aerosol loading of the atmosphere. The haze is not of local origin. It is created in mid- latitudes as atmospheric pollutions, which is subsequently carried pole wards to settle over the Arctic.
Another anthropogenic activity is the war. For example during Gulf war in Feb ’91, over 600 oil-wells were set alight by the retreating Iraqi army.
These wells continued to burn for several months, kept alight by oil and gas brought to the surface under pressure from the underlying oil-fields. During the time they added massive amount of smoke, SOx, CO2, unburned HC and NOx to the atmosphere. Most of these products were confined to the lower half of the troposphere, with the top of the plume never exceeding 5 km.
3. Aviation and Climate Change:
The effect of aviation on the atmosphere is the well known contrail (condensation trail), which is line shaped ice crystal clouds formed behind aircraft from the initial emission of water vapour and particles from the engines.
The potential effects on climate of contrails and their spreading into cirrus cloud coverage continues to exercise scientists in terms of characterizing their microphysical properties, optical effects, occurrence and ultimate effect on climate.
In addition, emission if NOx from aircraft were considered to be a potential problem in terms of stratospheric ozone (O3) depletion. In the early 1990s, a number of publications dealt with potential impacts of aviation on climate, both NOx effects on O3, and contrails.
It is further known that the recent supersonic aviation affects radiative forcing (RF) in a number of ways and induce climate change processes. It is also known that sulphate and black carbon particles (soot) are also involved in the formation of contrails along with water vapour.
In fact CO2 emission from aviation growth ranges from 2-2.5 per cent over past few year, but the major effects lies on change in overall RF induced by contrails and cirrus cloud formation.
Although it is not possible to assess at this stage that real environmental consequences of future supersonic air transport present, knowledge indicates that there exists a real possibility of serious decrease in the atmospheric ozone shield due to the catalytic action of oxides of nitrogen, emitted in the exhaust of supersonic aircraft.
4. Nuclear Winter:
Despite recent agreements between the superpowers to limit the spread of nuclear weapons, the specter of nuclear war continues to hover over the world, as it has done for the past 45 years or so.
Terms such as thermonuclear device, first strike, fallout and ionizing radiation have become part of the modern lexicon. Added to these now is nuclear winter, perhaps the final blow for any survivors of a nuclear exchange (Fig. 19.1).
Environmental problems such as acid rain, ozone depletion and atmospheric turbidity would be intensified significantly by the multiple nuclear explosions which are a prerequisite for nuclear winter.
Nuclear winter differs from these other issues, however, it is a potential problem rather than an existing one and, as a result, provides no elements capable of direct measurement. It has been necessary, therefore, to develop a theoretical approach using statistical and computerised modelling techniques.
Nuclear winter also differs from other issues in that its effects are catastrophic rather than gradual. The effects of acid rain or ozone depletion may become apparent after only years or even decades, with nuclear winter the effects are felt within days or, at most, weeks of the initial explosions.
Thus there is little time to respond once the nuclear devices have been launched. If nuclear proliferation is to be avoided, it is important that nuclear conflict be avoided also. As a result of political developments, especially the breakup of USSR in 1991, however, the conflict seems less likely and the issue of nuclear winter is regarded as irrelevant by some.
Nevertheless, the nuclear powers retain sufficient warheads to create nuclear winter many times over. A broad understanding of the potentially dire consequences of nuclear winter may help to ensure that they are never used.
The nuclear winter hypothesis was based on the assumption that smoke and dust thrown into the atmosphere during a nuclear war would increase atmospheric turbidity to such an extent that a high proportion of in-coming solar radiation would be prevented from reaching the lower atmosphere and the earth’s surface. The net effect would be to drive land temperatures down.
The initial effect of a nuclear ground-burst would be the injection of large amounts of dust into the atmosphere, through the destruction of soil aggregates, the vaporization of soil and rock and the incorporation of existing surface dust.
Simulations using the TTAPS’ (Turco, Toon, Ackerman, Pollack and Sagan — these investigators studied the process initially) baseline scenario resulted in an injection of more than 960 million tons of dust into the atmosphere of which 80% would reach the stratosphere.
Such fine dust particles convected into the upper atmosphere would remain in place for more than a year, contributing to global cooling by scattering in-coming solar radiation.
The environmental consequences of nuclear war received little attention in the TTAPS scenario, but a contemporary study by a group of life-scientists indicated that the net effect of a large scale nuclear war would be the global disruption of the biosphere and the disruption of the biological support systems of civilisation.
The severity of the impact has been mitigated in line with the successive modifications of the nuclear winter hypothesis, but the nuclear scenario is still considered serious.
The report of the Scientific Committee on Problems of the Environment concerning the ecological and agricultural impact of nuclear war sponsored by the International Council of Scientific Unions (ICSU) identified indirect biological effects as a major threat to humanity (Fig. 19.2).
As a result of nuclear winter, it is likely that vegetation in tropical regions would suffer significant damage, whatever the season. Tropical plant systems flourish under conditions which include mild stable temperatures.
They are susceptible to moderate declines in temperature and do not develop the resistance to cold which helps temperate plants to survive. Enrhich et al (1983) have suggested that forests in tropical regions might largely disappear if low light levels and low temperatures were to become widespread.
Marine ecosystems would be more capable of dealing with temperature extremes, because of the moderating influence of the oceans. Tropic webs would be seriously disrupted by the reduced productivity of phytoplankton during the low light period, however, and the increased storminess in coastal areas would cause damage to shallow-water ecosystems.
Overall, it is considered the aquatic ecosystem have the potential for a more rapid recovery than terrestrial systems.
The damage to vegetation in natural ecosystems would be paralleled by damage to cultivated plant species. Tropical food crops viz., rice, banana, maize can be damaged by temperatures falling to 7-10°C for as little as a few days and even that moderate chilling would be sufficient to cause crop failure. Many tropical Third World countries already experience food shortages, which would be aggravated by the effects of chilling.
In addition to the low light levels, subfreezing temperatures and violent storms, the human survivors of the initial conflict would face continued radioactive fallout, high levels of toxic air pollution and enhanced levels of ultraviolet radiation.
Combined with lack of food and drinking water, psychological stress and the disruption of support systems such as transportation, communications and medical care, this would probably ensure that fatality rates remained high even years after the conflict.
So it is very much debatable whether the earth is getting warmer day by day or cooler due to increasing atmospheric turbidity and nuclear winter processes. There is no realistic mode available so far to predict these future events.
