The succession of exceptional years with record high temperatures, which characterized the 1980s, helped to generate widespread popular interest in global warming and its many ramifications. The decade included six of the warmest years in the past century, and the trend continued into the 1990s, with 1991 the second warmest year on record. All of this fuelled speculation especially among the media that the earth’s temperature had begun an inexorable rise and the idea was further reinforced by the results of scientific studies which indicated that global mean temperatures had risen by about 0. °C since the beginning of the century.
Periods of rising temperature are not unknown in the earth’s past. The most significant of these was the so-called Climatic Optimum, which occurred some 5,000-7,000 years ago and was associated with a level of warming that has not been matched since. If the current global warming continues, however, the record temperatures of the earlier period will easily be surpassed. Temperatures reached during a later warm spell in the early Middle Ages may well have been equaled already.
More recently, the 1930s provided some of the highest temperatures since records began, although that decade has been relegated to second place by events in the 1980s. Such warm spells have been accepted as part of the natural variability of the earth/ atmosphere system in the past, but the current warming is viewed in a different light. It appears to be the first global warming to be created by human activity. The basic cause is seen as the enhancement of the greenhouse effect, brought on by rising levels of anthropogenically-produced greenhouse gases.
It is now generally accepted that the concentrations of greenhouse gases in the atmosphere have been increasing since the latter part of the nineteenth century. The increased use of fossil fuels has released large amounts of CO2, and the destruction of natural vegetation has prevented the environment from restoring the balance. Levels of other greenhouse gases, including CH4, N2 O and CFCs have also been rising. Since all of these gases have the ability to retain terrestrial radiation in the atmosphere, the net result should be a gradual increase in global temperatures.
The link between recent warming and the enhancement of the greenhouse effect seems obvious. Most of the media, and many of those involved in the investigation and analysis of global climate change, seem to have accepted the relationship as a fait accompli. There are only a few dissenting voices, expressing misgivings about the nature of the evidence and the rapidity with which it has been embraced. A survey of environmental scientists involved in the study of the earth’s changing climate, conducted in the spring of 1989, revealed that many still had doubts about the extent of the warming.
More than 60 per cent of those questioned indicated that they were not completely confident that the current warming was beyond the range of normal natural variations in global temperatures (Slade 1990). The greenhouse effect is brought about by the ability of the atmosphere to be selective in its response to different types of radiation. The atmosphere readily transmits solar radiation which is mainly short-wave energy from the ultraviolet end of the energy spectrum allowing it to pass through unaltered to heat the earth’s surface.
The energy absorbed by the earth is reradiated into the atmosphere, but this terrestrial radiation is long-wave infrared, and instead of being transmitted it is absorbed, causing the temperature of the atmosphere to rise. Some of the energy absorbed in the atmosphere is returned to the earth’s surface, causing its temperature to rise also. This is considered similar to the way in which a greenhouse works allowing sunlight in, but trapping the resulting heat inside hence the use of the name ‘greenhouse effect’.
In reality it is the glass in the greenhouse which allows the temperature to be maintained, by preventing the mixing of the warm air inside with the cold air outside. There is no such barrier to mixing in the real atmosphere, and some scientists have suggested that the processes are sufficiently different to preclude the use of the term ‘greenhouse effect’. Anthes et al. (1980) for example, prefer to use ‘atmospheric effect’. However, the use of the term ‘greenhouse effect’ to describe the ability of the atmosphere to absorb infrared energy is so well established that any change would cause needless confusion.
The demand for change is not strong, and ‘greenhouse effect’ will continue to be used widely for descriptive purposes, although the analogy is not perfect. Without the greenhouse effect, global temperatures would be much lower than they are perhaps averaging only ? 17°C compared to the existing average of +15°C. This, then, is a very important characteristic of the atmosphere, yet it is made possible by a group of gases which together make up less than 1 per cent of the total volume of the atmosphere. There are about twenty of these greenhouse gases.
Carbon dioxide is the most abundant, but methane, nitrous oxide, the chlorofluorocarbons and tropospheric ozone are potentially significant, although the impact of the ozone is limited by its variability and short life span. Water vapour also exhibits greenhouse properties, but it has received less attention in the greenhouse debate than the other gases since the very efficient natural recycling of water through the hydrologic cycle ensures that its atmospheric concentration is little affected by human activities.
Any change in the volume of the greenhouse gases will disrupt the energy flow in the earth/atmosphere system, and this will be reflected in changing world temperatures. This is nothing new. Although the media sometimes seem to suggest that the greenhouse effect is a modern phenomenon, it is not. It has been a characteristic of the atmosphere for millions of years, sometimes more intense than it is now, sometimes less. Three of the principal greenhouse gases—CO2, methane (CH4) and the CFCs—contain carbon, one of the most common elements in the environment, and one which plays a major role in the greenhouse effect.
It is present in all organic substances, and is a constituent of a great variety of compounds, ranging from relatively simple gases to very complex derivatives of petroleum hydrocarbons. The carbon in the environment is mobile, readily changing its affiliation with other elements in response to biological, chemical and physical processes. This mobility is controlled through a natural biogeochemical cycle which works to maintain a balance between the release of carbon compounds from their sources and their absorption in sinks.
The natural carbon cycle is normally considered to be self-regulating, but with a time scale of the order of thousands of years. Over shorter periods, the cycle appears to be unbalanced, but that may be a reflection of an incomplete understanding of the processes involved or perhaps an indication of the presence of sinks or reservoirs still to be discovered (Moore and Bolin 1986). The carbon in the system moves between several major reservoirs.
The atmosphere, for example, contains more than 750 billion tones of carbon at any given time, while 2,000 billion tones are stored on land, and close to 40,000 billion tones are contained in the oceans (Gribbin 1978). Living terrestrial organic matter is estimated to contain between 450 and 600 billion tones, somewhat less than that stored in the atmosphere (Moore and Bolin 1986). World fossil fuel reserves also constitute an important carbon reservoir of some 5,000 billion tones (McCarthy et al. 1986).
They contain carbon which has not been active in the cycle for millions of years, but is now being reintroduced as a result of the growing demand for energy in modern society being met by the mining and burning of fossil fuels. It is being reactivated in the form of CO2, which is being released into the atmospheric reservoir in quantities sufficient to disrupt the natural flow of carbon in the environment. The greatest natural flow (or flux) is between the atmosphere and terrestrial biota and between the atmosphere and the oceans.
Although these fluxes vary from time to time, they have no long-term impact on the greenhouse effect because they are an integral part of the earth/atmosphere system. In contrast, inputs to the atmosphere from fossil fuel consumption, although smaller than the natural flows, involve carbon which has not participated in the system for millions of years. When it is reintroduced, the system cannot cope immediately, and becomes unbalanced. The natural sinks are unable to absorb the new CO2 as rapidly as it is being produced. The excess remains in the atmosphere, to intensify the greenhouse effect, and thus contribute to global warming.
The burning of fossil fuels adds more than 5 billion tones of CO2 to the atmosphere every year, with more than 90 per cent originating in North and Central America, Asia, Europe and the republics of the former USSR. Fossil fuel use remains the primary source of anthropogenic CO2 but augmenting that is the destruction of natural vegetation which causes the level of atmospheric CO2 to increase by reducing the amount recycled during photosynthesis. Photosynthesis is a process, shared by all green plants, by which solar energy is converted into chemical energy. It involves gaseous exchange.
During the process, CO2 taken in through the plant leaves is broken down into carbon and oxygen. The carbon is retained by the plant while the oxygen is released into the atmosphere. The role of vegetation in controlling CO2 through photosynthesis is clearly indicated by variations in the levels of the gas during the growing season. Measurements at Mauna Loa Observatory in Hawaii show patterns in which CO2 concentrations are lower during the northern summer and higher during the northern winter. These variations reflect the effects of photosynthesis in the northern hemisphere, which contains the bulk of the world’s vegetation (Bolin 1986).
Plants absorb CO2 during their summer growing phase, but not during their winter dormant period, and the difference is sufficient to cause semi-annual fluctuations in global CO2 levels. The clearing of vegetation raises CO2 levels indirectly through reduced photosynthesis, but CO2 is also added directly to the atmosphere by burning, by the decay of biomass and by the increased oxidation of carbon from the newly exposed soil. Such processes are estimated to be responsible for 5-20 per cent of current anthropogenic CO2 emissions (Waterstone 1993).
This is usually considered a modern phenomenon, particularly prevalent in the tropical rainforests of South America and South-East Asia (Gribbin 1978), but Wilson (1978) has suggested that the pioneer agricultural settlement of North America, Australasia and South Africa in the second half of the nineteenth century made an important contribution to rising CO2 levels. This is supported to some extent by the observation that between 1850 and 1950 some 120 billion tones of carbon were released into the atmosphere as a result of deforestation and the destruction of other vegetation by fire (Stuiver 1978).
The burning of fossil fuels produced only half that much CO2 over the same time period. Current estimates indicate that the atmospheric CO2 increase resulting from reduced photosynthesis and the clearing of vegetation is equivalent to about 1 billion tones per year (Moore and Bolin 1986), down slightly from the earlier value. However, the annual contribution from the burning of fossil fuels is almost ten times what it was in the years between 1850 and 1950. Although the total annual input of CO2 to the atmosphere is of the order of 6 billion tonnes, the atmospheric CO2 level increases by only about 2. billion tonnes per year.
The difference is distributed to the oceans, to terrestrial biota and to other sinks as yet unknown (Moore and Bolin 1986). Although the oceans are commonly considered to absorb 2. 5 billion tonnes of CO2 per year, recent studies suggest that the actual total may be only half that amount (Taylor 1992). The destination of the remainder has important implications for the study of the greenhouse effect, and continues to be investigated.
The oceans absorb the CO2 in a variety of ways—some as a result of photosynthesis in phytoplankton, some through nutritional processes which allow marine organisms to grow calcium carbonate shells or skeletons, and some by direct diffusion at the air/ocean interface (McCarthey et al. 1986). The mixing of the ocean waters causes the redistribution of the absorbed CO2. In polar latitudes, for example, the added carbon sinks along with the cold surface waters in that region, whereas in warmer latitudes carbon-rich waters well up towards the surface allowing the CO2 to escape again.
The turnover of the deep ocean waters is relatively slow, however, and carbon carried there in the sinking water or in the skeletons of dead marine organisms remains in storage for hundreds of years. More rapid mixing takes place through surface ocean currents such as the Gulf Stream, but in general the sea responds only slowly to changes in atmospheric CO2 levels. This may explain the apparent inability of the oceans to absorb more than 40-50 per cent of the CO2 added to the atmosphere by human activities, although it has the capacity to absorb all of the additional carbon (Moore and Bolin 1986).
The oceans constitute the largest active reservoir of carbon in the earth/atmosphere system, and their ability to absorb CO2 is not in doubt. However, the specific mechanisms involved are now recognized as extremely complex, requiring more research into the interactions between the atmosphere, ocean and biosphere if they are to be better understood (Crane and Liss 1985). Palaeoenvironmental evidence suggests that the greenhouse effect fluctuated quite considerably in the past.
In the Quaternary era, for example, it was less intense during glacial periods than during the interglacials (Bach 1976; Pisias and Imbrie 1986). Present concern is with its increasing intensity and the associated global warming. The rising concentration of atmospheric CO2 is usually identified as the main culprit, although it is not the most powerful of the greenhouse gases. It is the most abundant, however, and its concentration is increasing rapidly. As a result, it is considered likely to give a good indication of the trend of the climatic impact of the greenhouse effect, if not its exact magnitude.
Svante Arrhenius, a Swedish chemist, is usually credited with being the first to recognize that an increase in CO2 would lead to global warming (Bolin 1986; Bach 1976; Crane and Liss 1985). Other scientists, including John Tyndall in Britain and T. C. Chamberlin in America (Jones and Henderson-Sellers 1990), also investigated the link, but Arrhenius provided the first quantitative predictions of the rise in temperature (Idso 1981; Crane and Liss 1985). He published his findings at the beginning of this century, at a time when the environmental implications of the Industrial Revolution were just beginning to be appreciated.
Little attention was paid to the potential impact of increased levels of CO2 on the earth’s radiation climate for some time after that, however, and the estimates of CO2 -induced temperature increases calculated by Arrhenius in 1903 were not bettered until the early 1960s (Bolin 1986). Occasional papers on the topic appeared, but interest only began to increase significantly in the early 1970s, as part of a growing appreciation of the potentially dire consequences of human interference in the environment. Increased CO2 production and rising atmospheric turbidity were recognized as two important elements capable of causing changes in climate.
The former had the potential to cause greater warming, whereas the latter was considered more likely to cause cooling (Schneider, 1987). For a time it seemed that the cooling would dominate (Ponte 1976), but results from a growing number of investigations into greenhouse warming, published in the early 1980s, changed that (e. g. Idso 1981; Schneider 1987; Mitchell 1983). They revealed that scientists had generally underestimated the speed with which the greenhouse effect was intensifying, and had failed to appreciate the impact of the subsequent global warming on the environment or on human activities.