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Global warming: why the controversy?

In the span of less than one decade, the issue of an enhanced greenhouse effect and consequent global warming has moved from relative public obscurity to a position of prime importance on the agendas of all levels of government, from municipal to national and international. Terms such as 'greenhouse effect', 'carbon dioxide', and 'climate change' have become household words, while environmental groups stridently lobby for aggressive action to reduce emissions of greenhouse gases. Yet some scientists argue that the evidence for global warming due to an enhanced greenhouse effect is weak at best, and that mitigative measures undertaken now would be premature, perhaps even misdirected. This apparent contradiction in perceptions raises a number of questions. What are the facts and teh uncertainties? What do we really know about the potential for global warming, and how would such warming affect use? Given the scientific uncertainties, what is an appropriate response at this time, and which actions should be delayed pending better scientific understanding of the issue?

Recognizing the need for developing a consensus on the state of scientific understanding on this issue, in 1988 the World Meteorologicl Organization and the United Nations Environment Programme (both agencies of the United Nations) jointly established an Intergovernmental Panle on Climate Change (IPCC). The work of the IPCC involved more than 70 countries, was conducted by the leading scientific experts from the broad range of disciplines implicated in the issue, and was extensively peerreviewed. Its efforts culminated in 1990 with the release of a series of comprehensive reports on the background science, the potential impacts, and the policy implications of global warming. These reports represent the most reliable and up-to-date statement of what we do and don't know about global warming, and a good basis against which to assess appropriate policy response. The following review of the science of global warming is consistent with the IPCC conclusions.

Greenhouse Gases and the 'Greenhouse


The term 'greenhouse effect' describes an atmospheric process by which 'greenhouse gases'allow short-wave incoming solar energy to penetrate through the atmosphere relatively unimpeded, but absorb and re-radiate in all directions the long-wave heat radiation flowing from the earth's surface and lower atmosphere towards space. This phenomenon effectively traps heat in the lower atmosphere in a manner somewhat like a thermal blanket. Without this natural process, the planetary surface would be some 33 [degrees]C colder, a frozen mass unsuitable for life.

Greenhouse gases collectively represent less than 1% of the volume of all gases in the earth's atmosphere. The concentration of water vapour, the most significant and abundant of all greenhouse gases, depends mainly on availability of surface water and temperature, and hence local climate. Thus it is regionally variable (from zero to as much as 4% of local atmospheric composition by volume) and responsive to climate variations. The second most abundant greenhouse gas is carbon dioxide, well mixed throughout the atmosphere at about 0.03% concentration by volume, and relatively constant with time. Other naturally occurring greenhouse gases include methane, nitrous oxide and oxone. Proxy data for concentrations of carbon dioxide, methane and nitrous oxide during the past centuries clearly indicate that their values were relatively constant until the beginning of the industrial era (around 1800 AD). Since then, the concentrations of [CO.sub.2] have increased by 26%, those of methane have doubled, and nitrous oxide has increased by almost 8% (see Figure 1). In the northern hemisphere, low-level oxone is also believed to be increasing at about 1% per year. Meanwhile, trace concentration of totally man-made greenhouse gases, such as chlorofluoro-carbons (CFCs), are now becoming measurable in the atmosphere, and are increasing rapidly. CFCs are very potent greenhouse gases, and are already a major environment mental concern because of their effect on the protective ozone layer in the stratosphere.

There is clear evidence that these increases in atmospheric concentrations of greenhouse gases are primarily caused by releases into the atmospheric resulting from human activity. The annual quantities of such releases are quite well documented for CFCs and for most sources of [CO.sub.2], but less well for other greenhouse gases. For 1990, global estimates of anthropogenic greenhouse gas emissions into the atmosphere are as follows: [CO.sub.2], about 26000 million metric tonnes (MT), primarily from the combustion of fossil fuels (75%) and tropical deforestation (25%); methane, about 300 MT from agricultural activities (about 50%), energy use (25%) and other human sources; nitrous oxide, about six MT from fertilizer use and energy consumption; and CFC s collectively about one MT. While low-level ozone is not released directly by human activities, it is produced chemically by interactions with other non-greenhouse gases that are (see also p.20 in this issue). Global emissions of these 'greenhouse gas precursors' (principally oxides of nitrogen, carbon monoxide and volatile organic compounds) for 1990 are collectively estimated at 286 MT. By comparison, Canadian emissions of greenhouse gases vary from 1.2% of global estimates for methane to about 2% for nitrous oxide, and up to 5% for the greenhouse gas precursors.

The effect on the natural greenhouse effect of additions of a particular greenhouse gas to the atmosphere depends on a variety of factors, including" the infrared absorption properties of that gas; the extent to which its background concentration has already saturated the radiative absorption window in the atmosphere within which it is active; the length of time the added gas remains in the atmoshphere; and the duration over which the effect is integrated. Hence the effect of adding one kg of methane will differ from that of adding one kg of [CO.sub.2] or one kg of CFCs. In order to facilitate the assessment of the relative significance on the greenhouse effect of current changes in the greenhouse gas concentrations, an index based on estimates of the above variables for each gas is often used. Known as the Global Warming Potential (GWP) index, it is used to estimate the effect of the emission of one mass unit of a given gas in terms of the number of identical units of [CO.sub.2] required to have an equivalent effect. GWP estimates for the various greenhouse gases, integrated over 100 years, vary from 21 for methane to 290 for nitrous oxide, and as high as 7300 for CFC-12. Using these estimates and the estimates for 1990 level emissions of each greenhouse gas noted above, both the net total contributions to possible global warming in terms of equivalent emissions of [CO.sub.2], and the relative role of each gas, can be calculated (see Figure 2). However, because of uncertainties in both GWP values and emission estimates, these results should still be used with caution.

Climatic Effects of an Enhanced

Greenhouse Effect

While the net consequences of the natural greenhouse effect on the earth's average surface temperature is measurable and clearly very large, the implications of an enhanced greenhouse effect as discussed above are much more difficult to assess. Information about natural fluctuations in both the atmospheric concentrations of the primary greenhouse gases and the concurrent surface polar temperatures, obtained for the past 160000 years from ice cores extracted from the polar ice caps, provide some useful clues. However, while the data suggest a strong relationship between changes in the greenhouse effect and long-term climate change, they are inadequate for assessing the processes and feedbacks involved. The normal methods of testing scientific theories in a laboratory can be applied to the study of some of the small-scale climate processes, but are quite impossible for physically simulating the total climate system of the planet. Scientists have turned to mathematical models of the earth's climate system to find answers to the climate change dilemma. These computer models use basic laws of physics, like the laws of conservation of mass, energy, moisture, etc., together with advanced mathematical analysis techniques, to simulate theoretically the processes of the climate system. Models are first tested against real climate data to ensure that they reasonably duplicate earth's current climate, then are used to experiment with the effects of changes to the system, including that of an enhanced greenhouse effect.

The most advanced climate models available today are the three-dimensional General Circulation Models (GCMs). These models are very complex, and include many internal feedback processes involving oceans, clouds, sea ice, surface moisture and other parameters. They require the largest computers currently available for experimental use. Yet they are still relatively crude in simulating the real climate system, both because of scientific uncertainties about the internal processes and the limitations of computing power. Hence, experimental results with these models, particularly with respect to the rate and the regional characteristics of climate change, have large error margins and must be used with caution. Unfortunately, it may take at least another decade of accelerated research before the regional and precipitation characteristics of experimental results can be accepted with a much higher degree of confidence.

Based on the evidence available to date, the IPCC concluded that, in a business-as-usual world, the net effect of increasing concentrations of greenhouse gases would likely result in a global warming rate of 0.2 to 0.5 [degrees] C per decade (best guess of 0.3 [degrees] C per decade). Such changes are considered to be very rapid compared to natural historical rates of change, and are expected to cause major shifts in global distribution of rainfall and available soil moisture. Average global sea levels are also likely to rise between 3 and 10 cm per decade. The warming is expected to be amplified in polar regions, and most models suggest (although with low confidence levels) that the interior regions of the northern hemispheric continents will become drier. However, despite global temperature trends that suggest that a warming of about 0.5 [degrees] C has taken place during the past century (see Figure 3), irrefutable proof that such a warming is already due to an enhanced greenhouse effect (rather than natural causes) is still lacking.

Implications of Global Warming

Numerous studies have attempted to evaluate how global warming might affect ecosystems and huma society. Although the large uncertainties in the projections for change preclude definitive assessment of regional impacts, these have helped to identify some major concerns. Some consequences are potentially beneficial. Direct effects of higher [CO.sub.2] concentrations on plants, for example, promote more rapid grwoth and higher drought tolerance. Warmer temperatures will improve growing seasons in cooler climatic regions, while polar regions would become milder and more accessible to marine transportation. However, both ecosystems and human societies are generally well-adapted to the climate regime within which they are found. Hence, they may have considerable difficulty adjusting to a new and distinctly different environment, particularly if the transitions are rapid. Global warming at rates similar to those projected by climate models is potentially very disruptive and problematic. Perhaps most significant are the implications of sea level rises. With almost 50% of the world's human population close to ocean coastlines, loss of densely populated land to higher tides, and the direct effects of unprecedented storm surges can be catastrophic. This is particularly for low-lying countries such as Bangladesh, Egypt, the Maldives and The Netherlands. Global redistribution of rainfall with decrease drought stress in some areas and turn others into deserts, thus altering the global pattern of food production and distribution. Slow forest response to climate shifts may result in large-scale dieback along the warm margins of current ecosystems. Increased heat stress and disease, degradation of water quality and more frequent severe tropical storms may also have major implications for human health and life. The ability adequately adapt to these changes, with minimum cost, will depend very much on whether the changes can be properly anticipated, and on how fast the changes take place.

Responsibility to the Challenge

In recent international meetings convened to discuss possible policy response to the risk of global warming, policy-makers have repeatedly agreed that the precautionary principle should appy. That is, "where there are threats of serious or irreversible damage, lack of full scientific certainty should not be used as a reason for postponing cost-effective measures to prevent such environmental degradation" (Ministerial Declaration, Second World Climate Conference). At the same time, it is recognized that uncertainties are major obstacles to developing action strategies that go beyond 'cost-effective' measures. Hence, there is general consensus that strategies for controlling global warming should be cautious and phased, beginning with actions that appear achievable at minimal net costs but recognizing that more costly measures may be required once international agreement on pursuing such measures is reached, and/or improved scientific understanding justifies them. Negotiations to develop coordinated international response strategies have already begun. Late in 1990, the UN General Assembly agreed to establish an Intergovernmental Negotiating Committee (INC) to develop an international framework convention on global warming. That convention is expected to be ready for possible signature by countries at the UN Conference on Environment and Development, scheduled to be held in Brazil in 1992.
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Author:Hengeveld, H.G.
Publication:Canadian Chemical News
Date:Aug 1, 1991
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