Assessment

Global issues

A number of global and local environmental effects occur as a result of emissions by man. Although we have all heard plenty about global warming, acid rain, ozone depletion and smog, much of the information in the public arena is either incorrect or confused. This section provides an overview of the global issues, which have had a major driving force for environmental legislation, economic instruments and best practice.

Global warming top

The terms “global warming” and “greenhouse effect” are wrongly used interchangeably. The greenhouse effect is the mechanism which allows life to survive on earth by maintaining a suitably high average temperature. Global warming is the enhancement of the greenhouse effect due to compounds emitted by man. If this results in a significant average global temperature increase, it will be a major cause of concern.

Various bands of gases exist around the earth. The lowest of these is the troposphere which extends to between 9 and 20 km above the earth's surface. The next layer is the stratosphere between 9 and 60 km which is followed by the ionosphere stretching out to around 1,000 km above the surface. The outer layer is known as the exosphere .

Some of the gases in the lower layers produce an insulating effect. They allow energy from the sun (UV or short-wave radiation) to pass through and reach the earth. The earth then radiates back this energy in the form of infra red or long-wave radiation. Some of this is absorbed by the layer of gases and thus the temperature of the lower atmosphere and the earth is increased.

The gases that allow short-wave radiation to pass through, but absorb long-wave radiation, are known as the “greenhouse gases” . Without the effect that they produce, the average global temperature would be around 30°C lower - and life as we know it would not exist.

The main greenhouse gas is water vapour, but others include carbon dioxide, methane, chlorofluorocarbons (CFCs). Global warming is a real concern because the level of such gases in the atmosphere is increasing as a result of mans activities. It is therefore feared that the average global temperature will increase - leading to climate change, shifting of rain patterns and crop bands and the melting of the polar ice caps.

Around 50% of the enhancement of the greenhouse effect is due to carbon dioxide (CO2 ). This is released from the burning of fossil fuels such as oil, natural gas and coal. It is also released from deforestation and the burning of wood - though inversely, it is absorbed by growing trees. The industrialised nations account for around 20% of world population but 80% of CO2 emissions. Hence the concern at the Earth Summit in Rio in 1992 and the Earth Summit II in the US 1997, for major reductions in CO2 emissions by nations such as the US . The latter emits 22 tonnes of CO2 p.a. per person compared to just 2 tonnes in China and 0.7 in India.

Much has been made of the importance of CFCs in the global warming process. Such compounds have a massive global warming potential - 1 kg of CFC 12 has the same global warming effect as 7,300 kg of CO2. As a result replacements are being formulated for their industrial uses, e.g. in foams, as propellants and refrigerants.

Huge quantities of greenhouse gases have been released since the industrial revolution. However, their total global warming effect has not yet been felt due to the natural time lag involved in the heating up of oceans and from complex feedback mechanisms. It is therefore believed that the global warming effects from the past 50 years emissions have not yet manifested themselves. When / if they do, it will be too late to take action - the damage will already have been done.

The conclusion that has generally been reached is that global warming is not proven but it is strongly suggested by many factors. If the release of greenhouse gases by man continues unchecked, there is a strong possibility of catastrophic effects in the next century. Nations are therefore entering into voluntary agreements to bring their emissions of greenhouse gases under control.

Acid rain top

All rain is acidic, normally around pH 5. This acidity results from carbon dioxide dissolving in rain droplets as they fall through the atmosphere, leading to a weak solution of carbonic acid.

If the atmosphere also contains oxides of sulphur and nitrogen, they too will dissolve in falling rain, leading to increased acidity. Thus rain can have a pH as low as 4 (which is 10 times more acidic than pH 5 due to the logarithmic nature of the pH scale). In addition, acidic gases and particulate matter can be deposited on the earth. These are then washed into lakes and rivers by rainfall. These wet and dry processes are collectively known as “acid deposition” and they result in the acidification of water resources.

Sulphur dioxide and nitrogen oxides are the main gases that contribute to acid deposition. Nearly all fossil fuels release sulphur upon combustion. 70% of UK sulphur dioxide emissions are caused by coal fired power stations. The latter also contribute a significant amount of nitrogen oxides (NOx), which are formed when air is heated to more than 650°C. Road transport is the single biggest contributor of NOx in the UK .

The main European areas of production for these gases are central Europe and the British Isles . However, due to prevailing wind conditions, acid deposition tends to occur in areas in a north-easterly direction, e.g. Scandinavia . The effects of such acid deposition may be neutralised in areas with a natural buffering capacity, eg an underlying layer of chalk which neutralises the acidity. Unfortunately, much of Scandinavia is underlain by granite, which has a poor buffering capacity and the acidification of water resources is the result. This leads to a reduction in species and fragile ecosystems due to the degree of environmental stress. Acidification also affects the productivity of soils, causing stress to plant life and killing off necessary soil bacteria and earthworms.

Ozone depletion top

Ozone (O3) is a naturally occurring gas. At ground level it is harmful to health (see section on summer time smog). However at stratospheric levels (9 to 60 km above the earth) ozone is beneficial as it absorbs harmful ultra-violet radiation. This stratospheric ozone is commonly known as the “ozone layer” and its depletion can lead to problems such as skin cancer, premature ageing of the skin and damage to eye tissue.

The reduction of ozone in this layer is being brought about by chemicals containing chlorine or bromine that are released by man. These substances destroy stratospheric ozone and have led to an annual springtime “hole” in the ozone layer above Antarctica .

Chlorofluorocarbons (CFCs) are particularly severe ozone depleters. Due to their chemical stability, CFCs can be transported into the stratosphere where one chlorine atom can destroy up to 100,000 ozone molecules. For this reason, hydrochlorofluorocarbons (HCFCs) have been introduced as substitutes, as their ozone depletion potential is generally 2 to 10% of that of one of the main CFCs. Hydrofluorocarbons (HFCs) are even more preferable as they lack chlorine and therefore do not deplete ozone. However, they still contribute to global warming.

Although no international legislation exists to control ozone depletion, over 100 countries have signed the Montreal Protocol which limits the production of ozone depleting substances.

Summer time smog top

At ground level, ozone is an undesirable substance. Tropospheric ozone is formed by a photochemical reaction (i.e. one requiring light) between volatile organic compounds (VOCs) and nitrogen oxides (NOx). This results in a brown atmospheric haze known as summer time or photochemical smog.

Such an occurrence can lead to irritation of the eyes and nose - especially in those that are most sensitive, e.g. the elderly, young or those that are particularly active. High levels of tropospheric ozone can also damage trees and crops.

As mentioned previously, NOx is derived from combustion at high temperatures - with transport being the biggest cause in the UK .

VOCs are a class of carbon containing compounds which evaporate easily. They are released from many sources including solvents and petrol usage - as well as being given off naturally by vegetation. They are an important pollutant from a furniture manufacturing perspective as the sector is estimated to emit around 10,500 tonnes of VOC each year from wood coating operations.

Winter time smog top

Air temperature normally decreases with height. However, during cold winter nights, “inversions” may form in the atmosphere - where there is a layer of cold air at ground level, above which is warmer air. For example, in a valley, cold air sinks towards the lower ground - pushing warm air above it. This leads to a lid of air which traps pollutants such as particulates and sulphur compounds at ground level. These may lead to the irritation of eyes and nose.

top