Global warming potential is affected by the different atmospheric lifetimes of greenhouse gases before breakdown, so that the relative importance of gases for global warming depends on the future date to which effects are estimated. In addition, chemical reactions in the atmosphere convert some radiationally inactive compounds into greenhouse gases over time. The estimation of the global warming potential of currently emitted gases is quite uncertain due to incomplete knowledge of the relevant atmospheric chemistry.
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Although these estimates differ from the radiative forcing estimates in the table, the differences are not great in terms of the relative importance of the gases for the global warming phenomenon. Our analysis uses the estimates of radiative forcing because they are far less uncertain. Uncertainties for the future projections are very large. Total effects of greenhouse gases projected for varied by a factor of 5 from the "accelerated policies" scenario, which projected the lowest level of emissions, to the "business-as-usual" scenario, which projected the highest.
N 2 O emissions Mt N 2 O per year a. For example, Watson et al. In addition, N 2 O releases from unknown sources are probably larger than all anthropogenic releases. It is not clear how much of the unaccounted releases is anthropogenic. For instance, automobile fuel consumption can be analyzed as the product of number of automobiles, average fuel efficiency of automobiles, and miles driven per automobile; the determinants of each of these factors can be studied separately. Researchers might then investigate the social factors that affect change in the number of automobiles and their typical life span, such as household income, household size, number employed per household, and availability of public transportation.
More detailed analysis can be carried out until it no longer would provide information of high enough impact to meet some preset criterion. Again, there are many ways to ana-. The task of making such accounts, even for a single tree, is enormous. The work can be eased by using the impact criterion: analysts might reasonably choose to move from trunk to limb to branch to twig only until the contribution falls below a preset level of impact for the time period of concern. Data collection and substantive analysis of the thinnest twiglets can be deferred. Table presents a composite of the accounts of individual green.
Note: Production estimates are from Watson et al. Projections of future production are very sensitive to changes in economic growth, and relatively quick substitution is possible when alternative chemicals become available. CFC 22 production doubled between and e. Integration over other time horizons would change the relative potentials because of differing atmospheric residence times. Source: Shine et al. When CFC 11 and CFC 12 production shifted from aerosols to other applications after , the result was a longer lag time from production to entry into the atmosphere.
Note: U. However, the United States is responsible for approximately 20 percent of global CO 2 emissions. Source: Compiled from Tables , , and For interpretation of the data, see the note at Table For a policy-oriented analysis based on such an approach, see National Academy of Sciences, b.
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Accountings such as the one represented in Figure can help guide the research agenda for the human causes of global change. They are critically dependent, however, on analyses from the natural sciences to sketch the trunk and major limbs, that is, to identify the most important environmental effects of human action and the technologies that produce those effects.
Natural science can help social science by providing an improved picture of the trunk and limbs, and particularly by improving estimates of the uncertainties of their sizes. The uncertainties of some components are quite large see, for instance, Table Research that estimates the relative impacts of proximate human causes of global change on particular environmental changes of concern, specifying the uncertainty of the estimates, is essential for understanding the human dimensions of global change.
As tree diagrams move from the trunk out toward the branches and twigs, analysis depends more on social science. For each important environmental change, there are several possible accounting trees, each consistent with the data but highlighting different aspects of the human contribution.
Social science knowledge is needed to choose accounting procedures to suit specific analytic purposes. Whatever accounting system is used, social scientists conducting research on the human causes of global change should focus their attention on factors that are significant contributors to an important global environmental change. Because many different tree diagrams may be consistent with the same data, tree diagrams must be treated as having only heuristic, not explanatory, value.
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They are useful but not definitive accounts. A more serious limitation of tree-structured accounts is that they do not by themselves illuminate the driving forces behind the proximal causes of global change. Social forces that have only indirect effects on the global environment, and that may therefore be omitted from tree accounts, can have at least as. Consider, for instance, the rate of female labor force participation, which affects energy use in many different ways.
With an increase in the proportion of women in the labor force, there tend to be more automobiles and miles driven per household, increased travel by plane, and, because of the associated decrease in household size, increased per capita demand for residential space conditioning and household appliances see Schipper et al. Because these factors appear in different branches of Figure , the figure is not useful for representing the effect of female labor force participation on energy demand.
The broader social process—the changing role of women in many societies—has even wider effects on energy use, but is still harder to capture in the figure. Despite these limitations, the accounting tree is useful as a preliminary check on the likely impact of a major social variable.
When such a variable has a high impact, it is worth considering for inclusion in models of the relevant proximal causes of global change. Tree-structured accounting is also limited in that it can evaluate human activities against only some criteria of importance such as high and widespread impact , but not others such as irreversibility. Consideration of criteria of importance other than current impact may require detailed empirical analyses of factors that look small in an accounting of current human causes of environmental change.
An example, elaborated in the next section, concerns future CO 2 emissions from China. If per capita income grows rapidly there, Chinese emissions may increase enough to become tremendously important on a world scale. To make projections, it would be very useful to have detailed studies of the effects on emissions of increased income in other countries that have undergone recent spurts of economic growth, such as Taiwan and South Korea, even though these countries have no major impact on the global carbon dioxide balance.
As we have shown, all human activity potentially contributes, directly or indirectly, to the proximate causes of global change. This section presents three rather detailed cases of human action with high impact on important global environmental changes to explore what lies behind the proximate causes.
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Taken together, the cases illustrate human causes that operate through both industrial and land-use activities and in both developing and devel-. They illustrate how multiple driving forces interact to determine the proximate human causes of global change and why systematic social analysis is necessary for understanding how human actions cause it. In the section that follows, we discuss the interrelationships among the driving forces at a more theoretical level.
In , the head of the British Antarctic Survey, Joseph Farman, reported that his team had discovered a heretofore unobserved atmospheric phenomenon: a sudden springtime thinning of the ozone layer over Antarctica, allowing ultraviolet radiation to reach the ground much more intensely than was ordinarily the case Farman et al. Subsequent scientific investigations soon led to what is now the most widely accepted explanation of what was happening. Chlorine compounds derived mostly from chlorinated fluorocarbon gases CFCs , mass-produced by industrial societies for a variety of purposes, reacted in the stratospheric clouds over Antarctica during the cold, dark, winter months to produce forms of chlorine that rapidly deplete stratospheric ozone when the first rays of the Antarctic spring sunlight arrive Solomon, Massive destruction of ozone followed very quickly, until natural circulation patterns replenished the supply and closed what came to be known as ''the ozone hole.
To understand this event and the political controversies that followed in its wake, one has to reach back through almost a century's worth of history, long before CFCs existed.
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Until almost the end of the nineteenth century, refrigeration was a limited technology, based almost entirely on natural sources of supply. Urban Americans who could afford to drink chilled beverages relied on metropolitan ice markets, which cut ice from local ponds in the winter and stored it in warehouses for use during the warm months of the year. Breweries and restaurants were the heaviest users of this stored winter ice, which was sometimes shipped hundreds of miles to provide refrigeration. Boston ice merchants, for instance, were regularly delivering ice to consumers in Charleston, South Carolina, and even the Caribbean by the fourth decade of the nineteenth century Hall, ; Cummings, ; Lawrence, Given the expense and difficulty of obtaining this stored winter ice, food preservation was accomplished largely with chemical additives, the most common being ordinary table salt: sodium chloride.
In the United States, pork was the most popular form of preserved meat because of the ease with which its decay could be arrested by salt. Beef was much less popular in preserved form, so those who ate it preferred to purchase it freshly slaughtered from local butchers.
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Then, in the s, meatpackers began experimenting with ice-refrigerated railroad cars that could deliver dressed beef, slaughtered and chilled in Chicago, to consumers hundreds of miles away. Dressed beef, which was cheaper than fresh beef for a variety of reasons, soon took the country by storm, driving many wholesale butchers out of business and giving the Chicago packing companies immense economic power. The packers initially relied on complicated ice storage and delivery networks, cutting and storing millions of tons of winter ice along the railroad routes that delivered beef from Chicago to urban customers throughout the East.
websrv2-nginx.classic.com.np/la-importancia-de-la-alabanza-y-adoracin.php Their investment in ice storage technology contributed to dramatic shifts in the American food supply and was soon affecting foods other than meat. Fruits and vegetables from California and Florida and dairy products from metropolitan hinterlands throughout the East, were among the most important to benefit from the new ice delivery system Cronon, ; Yeager, ; Kujovich, ; Giedion, ; Clemen, ; Swift and Van Vlissingen, ; Neyhart, ; Unfer, ; Fowler, But natural ice was unreliable: two warm winters in and brought partial failures of the ice crop that encouraged the packers to turn to a more reliable form of refrigeration.
Although the principle of mechanical refrigeration, in which compressed gas was made to expand rapidly and so lower temperatures, had been known since the middle of the eighteenth century, its first application on a large commercial scale was not found until the second half of the nineteenth century Anderson, Urban brewers, especially in the warm climates of the South, were the first to make wide use of it.
As the meatpackers sought to solve their problems with erratic winter ice supply, they too adopted mechanical refrigeration on a large scale after By the first quarter of the twentieth century, the delivery of perishable foods throughout the United States—and international food shipments as well—had come to depend on mechanical refrigeration. By drastically lowering the rate at which food decayed and hence making perishable crops available to consum-. The most widespread early refrigeration technology depended on compressed ammonia gas, which easily produced desired drops in temperature for effective food storage.
But ammonia like other refrigerant gases such as sulfur dioxide and methyl chloride had serious problems. For maximum efficiency, it had to attain high pressures before being released, which increased the likelihood that the compression equipment might fail. Accidental explosions were frequent, and the toxic nature of the gas caused a number of fatalities. Toxicity and the need for large expensive compressors kept mechanical refrigeration from making headway with retail customers, who represented an immense potential demand.
That is why Thomas Midgely Jr. Midgely, working at the request of the General Motors Frigidaire division, developed the new chlorinated fluorocarbon as the perfect alternative to all other refrigerant gases then on the market. Nonflammable, nonexplosive, noncorrosive, and nontoxic, the various forms of Freon gas seemed the perfect technical solution to a host of environmental and safety problems. They also required less pressure to produce the desired cooling effect, so compressors could be smaller and less expensive.
Freon soon came to dominate the market for refrigeration and opened up new retail markets because of its diminished capital requirements. Previously, consumers had bought their refrigerated food at the store just before eating it, since efficient and reliable household refrigeration was not generally available. Now American households could own their own refrigerators, making it possible for the food industry to shift much of its marketing apparatus toward selling chilled food in retail-sized packages. Frozen foods burst onto the American marketplace in the s, as did fresh vegetables, dairy products, and other foods that are today accepted as ordinary parts of the national diet.
Although European countries were slower to adopt these technologies, they too eventually followed suit. No less importantly, the nontoxicity of Freon made it possible for refrigeration technology to be applied to the ambient cooling of buildings, so that air conditioning came to be an ever more important market for the gas. Air conditioning had been used in specialized industrial applications ever since Willis H.
Carrier's use of the technique for a climate-controlled lithography plant in The introduction of Freon meant that air conditioning suddenly became much cheaper and safer in a way that allowed it to. Air conditioning played a key role in the years following World War II in promoting urban growth in the region known as the Sun Belt, as well as in tropical areas around the globe. From Florida to Texas to southern California, the massive influx of new residents depended in no small measure on the ability of buildings to protect their occupants from summer heat.
Air conditioning became a fact of life in such places, so much so that it is hard to imagine urban life in the Sun Belt without it.