Saturday, February 16, 2008

Understanding Global Warming

1. Global Warming - The greenhouse effect

The Earth is heated by the sun; solar radiation passes through the atmosphere and is
at the Earth's surface (except for a small portion that is reflected back into space). This heat is readily lost; it is emitted from the surface as infra-red radiation. Fortunately, this infra-red radiation cannot escape the atmosphere as easily as the solar radiation can enter. Instead, some of it is 'trapped' by a number of gases; these gases act in a similar, yet different, way to the glass in a greenhouse - heat is allowed in but cannot get out - hence we have the 'Greenhouse Effect'. Were it not for this natural greenhouse effect the Earth would be at least 30°C cooler and life as we know it would not exist.The greenhouse effect (DoE)

The problem

Unfortunately, recent human activities are enhancing the natural greenhouse effect. The concentrations of certain greenhouse gases in the atmosphere are increasing, therefore more of the infra-red radiation emitted by the Earth's surface is being trapped. The planet is losing less heat and as a result we are beginning to experience 'Global Warming'. Going back to the greenhouse analogy, it is as if the panes of glass are gradually becoming thicker.

The greenhouse gases

The major greenhouse gases are: water vapour, carbon dioxide, methane, nitrous oxide, CFCs (and their replacements), and ozone. Some of these occur naturally, some result only from human activity, while others occur both naturally and as a result of human activity. The contribution of each gas to the greenhouse effect is a function of three factors:

1.Atmospheric lifetime - simply, how many years the gas remains active in the atmosphere.

2.Global warming potential (GWP) - a relative measure of how effective the gas is at preventing the passage of infra-red radiation (i.e. the Earth's heat). The GWP of CO2 is 1; methane, molecule for molecule, is eleven times as effective as CO2, therefore it has a GWP of 11.

3.Atmospheric concentration - usually measured in parts per million (ppm).

Water vapour
is the most important greenhouse gas, although its concentration in the atmosphere is not directly influenced by human activity. However, any increase in global temperatures will increase concentrations, because, as air gets warmer it can hold more water vapour. Hence, any enhanced greenhouse effect caused by other greenhouse gases will increase the global warming effect of water vapour. Carbon dioxide Carbon dioxide (CO2) exists naturally in the atmosphere; however, its concentration is increasing sharply as a result of human activities:

a.Fossil fuel burning: oil, gas, and coal are stores of carbon; when these are burnt, this carbon is released directly into the atmosphere as CO2
b.Deforestation: trees absorb CO2 from the atmosphere; if these trees are cut down they can no longer do this, therefore CO2 levels rise.

Since the beginning of the Industrial Revolution, atmospheric concentrations of CO2 have risen by almost 30%. This rise currently accounts for most of the enhanced greenhouse effect. CO2 has an atmospheric lifetime of about 100 years. Methane About a third of the current atmospheric methane is from natural sources, such as bogs and swamps. The remainder exists as a result of human activities such as cattle rearing, waste disposal, fossil fuel mining, and sewage treatment. It has a higher GWP than CO2, but remains in the atmosphere for less time - just 11 years. Nitrous Oxide A powerful and long-lived greenhouse gas, it occurs naturally in the environment, but concentrations are now rising, mainly as a result of fertiliser use. Nitrous oxide (N2O) should not be confused with nitrogen oxides (NOx) - see section on acid rain. CFCs and their replacements (see section on the ozone layer) CFCs and their replacements are synthetic; their contribution to global warming is entirely due to human activity. All are powerful greenhouse gases. CFCs and HCFCs are being phased out of use as a result of their effect on the ozone layer. They are likely to be replaced with HFC - a similar compound, but one which does not affect the ozone layer. Unfortunately HFC is still a very powerful (GWP 1500), albeit fairly short-lived, greenhouse gas. Ozone (see section on the ozone layer) Ozone is an important greenhouse gas, it occurs both at ground level and in the stratosphere. Ground-level ozone concentrations are increasing as a result of some industrial processes and spiralling transport use. Higher up in the atmosphere, the ozone layer is being depleted; the contribution of stratospheric ozone to the greenhouse effect, therefore, is diminishing. Unfortunately, depletion of the ozone layer is not a good thing. Also, any reduction in the greenhouse effect of the ozone layer is likely to be offset by the global warming effect of the CFCs and other gases responsible for depleting it in the first place. Sulphate aerosols These actually have a 'global cooling' effect. Sulphate aerosols are formed from sulphur-containing gases in the atmosphere; they reflect sunlight away from the Earth. The most recent evidence of this was following the eruption of the volcano Mt. Pinatabu in 1991, which spewed sulphur clouds into the upper reaches of the atmosphere. The following three years were cooler as a direct result. Unfortunately, the most important gas from which the sulphate aerosols are formed - sulphur dioxide - is also responsible for acid rain. In addition, sulphate aerosols have been linked to ozone layer depletion.

Effects of global warming

There is still much uncertainty over the degree by which temperatures will actually rise and what the effects of these rises will be. What is not in doubt is that the Earth's temperature has risen by 0.5°C over the past century, and that recent years have been among the hottest on record. The likely effects of global warming have serious implications for the environment:

Sea level - a 20cm rise by 2030 is expected to result from glacial melting and from the thermal expansion of the oceans as water temperatures rise. This may inundate some coastal regions, and increase the risk of serious flooding in many more. Internationally, populations throughout much of China, Egypt, Denmark and South-East Asia are most at risk - millions could lose their homes and livelihoods. In the UK, south-east England is most vulnerable to inundation, although floods, such as that which occurred in Towyn, mid-Wales, in 1990, are likely to increase in both frequency and severity throughout the country.

Agriculture and forestry - crop failure and famine will ensue throughout marginally productive regions of the world. In the UK, agricultural practices will need to adapt; milder weather will benefit some crops at the expense of others. Forestry will be affected: in Wales, native broadleaf trees could suffer as a result of droughts, whereas conifer plantations are likely to become more productive. More conifers will exacerbate some of the problems associated with acid-rain, and, simultaneously, have serious implications for the biodiversity of our woodlands.

Water resources - global rainfall patterns will change and the water management strategies of different regions will need to adapt. This will have serious global security implications, conflict over water resources could well follow. In the UK, droughts and floods will become more common, while rising temperatures will increase domestic water demand. Water quality may suffer owing to an increase in the incidence of algal blooms.

Terrestrial ecosystems - many plant and animal species will have problems adapting, others will not; this will influence the mix of species at different locations. Many will be at risk from extinction, whereas more tolerant varieties (often including weeds and pests) will thrive.

Climate Change Impacts on Welsh Birdlife

Human health - over the long term there is likely to be an increase in the spread of certain diseases; malaria, for instance, could reach parts of Europe, including the UK. Urban air pollution is likely to get worse and its associated ill-health effects exacerbated. Heat-induced deaths are likely to increase, especially among the elderly. Adverse weather conditions, such as storms and floods, will exact their own costs, both human and economic: lives will be lost, insurance premiums will rise.

Measures to curb global warming

The most important contributor to global warming is the increase in atmospheric CO2 levels due to human activity. To remedy the situation, the increase will not only need to be halted, but also reversed:
CO2 emissions can be cut by reducing the use of fossil fuels: cutting back on car use, investing in energy efficiency, implementing energy conservation measures, and utilising renewable resources such as wind, solar, and hydro power. Reducing fossil fuel use will, coincidentally, also reduce emissions of methane, nitrous oxide, and ozone
CO2 is consumed by plants and trees (known as 'carbon-sinks') as they grow, therefore levels in the atmosphere may be reduced by reversing deforestation and implementing reforestation programmes.
The need to take such measures was first recognised at the Rio Climate Change Convention in 1992, after which over 160 countries pledged to limit emissions of CO2 and to protect and enhance natural 'sinks' of CO2.

2. The Ozone layer

Ozone is a gas found throughout the atmosphere, but most highly concentrated in the stratosphere, between 10 and 50km above sea level, where it is known as the 'ozone layer'. Without the ozone layer, life on the Earth's surface would not be possible: it protects us from the damaging ultra-violet radiation of the sun; in particular it filters out UV-B radiation. Recent evidence has shown that certain parts of the ozone layer are becoming thinner, ozone 'holes' have developed. The consequence of any thinning of the ozone layer is that more UV-B radiation reaches the Earth's surface. UV-B radiation affects DNA molecules, causing damage to the outer surface of plants and animals. In humans it causes skin cancer, leads to eye disease, and is a general immuno-suppressant.

The formation of the ozone layer

Two billion years ago life on Earth was confined to a few micro-organisms living underwater. These organisms had the ability to photosynthesise - they existed on the basis of sunlight. As a by-product of the photosynthesis, oxygen was released into the atmosphere. Some of this oxygen reached the stratosphere, where it began to react with incoming ultra-violet radiation from the sun to form ozone. Over a period of several million years, the ozone layer was formed. By filtering out much of the harmful ultra-violet radiation of the sun, the formation of the ozone layer permitted the evolution of land-borne species. Life was no longer confined to the protection of the depths.

Depletion of the ozone layer

Atmospheric ozone is continually broken-down and reformed; until recently this process has been in a state of natural balance. This balance has been upset as a result of human activity; specifically, the manufacture and use of a group of synthetic chemical substances known as CFCs and HCFCs. These chlorine-containing compounds, used in aerosols, refrigeration, solvents and foam insulation, have drifted up into the stratosphere. Through a complex series of chemical chain-reactions, small amounts of these compounds are able to destroy very large quantities of ozone. The result is that ozone is being broken down more quickly than it is forming; parts of the layer are becoming thinner - 'holes' are developing. Over recent years, very large holes have developed over Antarctica (owing to climatic conditions, the atmosphere over the Antarctic is particularly susceptible to ozone depletion). There is now evidence that ozone layer is thinning, albeit less extensively, throughout the world.

Effects of ozone layer depletion

The likely effects of an increase in the amount of UV-B radiation reaching the earth's surface are extensive. The potential consequences for humans, animals, plants, and even building materials are serious: Human health - the greatest threat to human health is an increase in the incidence of skin cancer; each 1% loss of total ozone leads to a 3% to 5% increase in skin cancer cases. In addition, eye disease, including temporary disorders such as 'snow blindness', and more permanent conditions, such as cataracts, become more common. These problems are compounded by the fact that UV-B suppresses the immune system. Marine environment - there is likely to be a reduction in the numbers of phytoplankton, photosynthesising organisms in the sea that both, (a) absorb CO2 from the atmosphere, helping to stave off global warming, and (b) form the base of the marine food chain. UV-B also causes direct damage to young fish, shrimp and crab larvae, and other small animals. Further up the food chain, this means a reduction in fish stocks, marine animals, and seabirds. Agriculture and plant life - crop yields would suffer; exposure to high concentrations of UV radiation has been shown to stunt the growth and leaf development of most plant varieties. UV sensitive plants are also more susceptible to pests and disease. Material degradation - many plastics suffer when exposed to UV radiation. Photochemical reactions cause the properties of such materials to change, thus shortening their useful lives. It is perhaps worth noting that depletion of the ozone layer, in itself, does not exacerbate global warming. Any heating effect from increased UV-B light penetration is negated as a result of the reduction in the levels of stratospheric ozone - itself a greenhouse gas. However, increased global warming is caused by the gases responsible for depleting the ozone in the first place: CFCs and HCFCs.

Measures to protect the ozone layer

International concern over the seriousness of the problems associated with ozone layer depletion lead to the adoption of the Vienna Convention for the Protection of the Ozone layer, in 1985. This was followed up with the formation of the Montreal Protocol, to which over 140 countries agreed to the reduction and eventual phasing-out of the manufacture and use of most of the ozone depleting substances. Unfortunately, many countries in the developing world have as yet been unable to sign to any international agreement; while in the West, a (un)healthy black market trade in CFCs has developed. It is difficult to predict how far the ozone layer will become depleted before it begins to recover.

3. Acid Rain

During the 1980s, acid rain emissions became the major environmental issue. More recently, the problem has become overshadowed by concern over the ozone layer and global warming. This does not mean that acid rain has gone away; indeed, in Wales at present its effects are being felt as strongly as ever. There is evidence of damage to trees, peat erosion, reduced fish-stocks, and a general reduction in the biodiversity of our natural habitats. Coal and oil-fired power stations are responsible for the bulk of acid gas emissions. Their tall chimneys spew sulphur dioxide (SO2) and nitrogen oxides (NOx) into the skies where they are carried away with the wind. During the time they are airborne, these gases are converted into dilute sulphuric and nitric acids which later fall back to earth - often hundreds of miles away. Road transport is responsible for most of the remainder of acid gas emissions. Because petrol and diesel are fairly low-sulphur fuels, their SO2 emissions are limited; their NOx emissions, however, are high, making road transport the biggest contributor of this particular acid gas. The term 'acid rain' was first coined in 1872, after a link was made between sulphur dioxide emissions from the burning of coal in Manchester to the acidification of nearby rainfall. The term now tends to be used indiscriminately to describe all forms of pollution associated with the burning of fossil fuels. Not only rain, but also snow, sleet, and mist: collectively known as 'wet deposition'; and gases, dusts, and smog: collectively known as 'dry deposition'. Really, we are talking about acid pollution.

Effects of acid pollution

Dry deposition affects the environment directly. It combines with fog and dust to produce smog, a major killer in UK cities prior to the Clean Air Act of 1956, and still a serious contributor to respiratory problems among susceptible people. It damages buildings, historical and modern: eroding away limestone and marble, and penetrating concrete structures to corrode the steel reinforcement within. Deposition on farmland and forestry affects plant growth and reduces crop yields. Because sulphur and nitrogen are both plant nutrients their deposition can affect the competitive abilities of different plant species: plants with a high nutrient demand thrive at the cost of those with a lower nutrient demand.

The Acid Cycle (Source: NSCA) Wet deposition shares all of the same direct effects of dry deposition. In addition, it has a particularly potent acidifying effect on lakes, rivers, and streams. Direct deposition in the water is compounded by the inflow of groundwater from nearby acidified soils; the effects on aquatic life are twofold:

1.Acidification is further intensified; the more the pH decreases the more the damage, below 4.5, most aquatic life is gone.

2.Toxic metals, such as aluminium, leached by the acid in the soil, are introduced to the water. Aluminium has a direct effect on the respiratory systems of fish, causing their gills to clog-up. The presence of toxic metals in water supplies presents a health problem to all animal life, including humans.

The severity of the effects of wet deposition depends on a number of area-specific factors:

a.Soil type - the more acid the soil is naturally, the more at risk the ecosystem it supports is from the effects of acid rain. The geology of Snowdonia National Park is slightly acidic.

b.Soil cover - the thinner the soil layer the more readily it is affected by acidification. Over many of Wales' slate hills, soil cover is at most a few centimetres.

c.Mix of plant species - acidification tends to be exacerbated in areas with conifer plantations, such as the forestry land in mid and north Wales. Conifers are particularly effective scavengers of pollutants, the unfortunate effect of this is that more acid pollution can pass into the soil beneath them. The acid then leaches out toxic metals which pass into groundwater.

d.Rainfall levels - the more acid rain that falls, the greater the damage: Wales is at risk owing to high rainfall levels.

e.Altitude - upland areas are more affected than lowland areas: (1) through higher rainfall, and (2) through increased contact with acid clouds and mists.

Curbing acid emissions

Over recent years, the UK's and other countries' governments have taken serious steps to reducing acid gas emissions: desulphurisation plant is now fitted to many power stations, catalytic converters are compulsory on new motor vehicles. Unfortunately, both of these measures reduce the efficiency at which fuel is being used, resulting, for a given energy output, in higher emissions of CO2 . The real solution is to cut back on the use of fossil-fuels by reducing our dependency on cars, and by only using as much electricity as we need. In the short term, switching to 'cleaner' fossil fuels, such as natural gas, can certainly help reduce SO2 emissions, but not, unfortunately, the equally important NOx emissions.


Global warming, acid rain, ozone layer depletion, and ground-level ozone pollution all pose a serious threat to the quality of life on Earth. They are separate problems, but, as has been seen, there are links between each. The use of CFCs not only destroys the ozone layer but also leads to global warming. Power stations can cause global warming, ozone layer depletion and acid rain: their CO2 emissions cause global warming; while their SO2 emissions are either converted into sulphuric acid, which causes acid rain, or become sulphate aerosols, which deplete the ozone layer. Measures to reduce SO2 emissions from power stations tend, for a given power output, to increase their CO2 emissions. The situation is complex; but some things are clear. By reducing our dependence on fossil fuels we help prevent both acid rain and global warming. With appropriate reforestation we can help reduce the effects of acid rain, while at the same time increasing the uptake of CO2 from the atmosphere. Regarding ozone layer depletion, the answer is simple: stop the manufacture and use of CFC's and HCFC's, a measure which, fortunately, most governments are taking steps to ensure.