Humans have had a profound impact on carbon cycling in the atmosphere, the terrestrial biosphere, and, to a lesser extent, the oceans. To understand how humans will continue to interact with these reservoirs in the future, one must understand the drivers responsible for how humans interact with the environment.
Energy use has historically been viewed as an essential commodity for economic growth. The paradigm of "grow or die" historically meant increasing demands for additional resources and a commensurate increase in the amount of pollutants released. Over the past 30 years, however, a complex debate has ensued about how to decouple economic growth and resource consumption. Since this debate started, many scholars, policy makers, and nongovernmental organizations have asked themselves how to reshape this paradigm of development. Answers have ranged from doing nothing (business-as-usual approach) to slowing and even stalling economic growth, so as to not exceed the Earth's carrying capacity. Carrying capacity here refers to the ability of natural resources and ecosystems to cope with anthropogenic pressures, such as use of renewable natural resources, emission of pollutants, and modification of ecosystem structures without crossing "critical thresholds of damage beyond which [these resources] lose their ability for self-renewal and slide inexorably into deeper degradation" (Board on Sustainable Development Policy Division 1999).
Another more tenuous response is based on the idea that one could keep economic growth as a development goal and at the same time find mechanisms aimed at reducing the amount of material input and emissions or aimed at finding replacements for nonrenewable or dangerous resources (e.g., fossil fuels). This proposal has resulted in environmental policies and instruments promoting technological innovations that seek to increase the efficiency of economic activities and to decarbonize economies.
Energy intensity (the ratio of total domestic energy primary consumption to gross domestic product or physical output) in the United States, Japan, and other developed countries has increased far slower than economic growth. China, too, has made significant gains, and the growth rate of its energy consumption has been half that of its gross domestic product (GDP) since the early 1980s. While the energy use per unit of GDP has fallen or stabilized since 1970, energy use per capita has increased in most of the developed countries (Sathaye, Chapter 22, this volume). This statistic may be misleading however, because there has also been a change in consumption and production patterns away from materially intensive commodities toward less-intensive services, and production of many of the materially intensive commodities has moved out of the developed countries. This does not necessarily mean that the developed countries are using fewer resources, but rather that the regional distribution of this consumption has changed. Countries belonging to the former Soviet Union (FSU) have also experienced decreases in energy intensity but for very different reasons. Because of sociopolitical changes, economic activity declined substantially in these countries, resulting in a dramatic decline in energy use and associated carbon emissions.
Over the past century and a half, the use of biomass has successively given way to expanded use of coal, oil, and natural gas as the primary fuels to supply energy (Figure 2.3). Over the past two decades, the use and share of nuclear and other forms of renewable energy have increased. The carbon intensity (PgC/gigajoule [GJ]) of each successive fuel, beginning with coal, is lower, and this decline has led to the decarbonization of the global fuel mix over the past century. Critical inventions, such as the steam and internal combustion engines, vacuum tubes, and airplanes, have accompanied and fostered the use of successive fuels and electricity.
The speed of energy consumption and land use changes has increased during the past two centuries. Three spatial and historical variations of carbon-relevant social tendencies need to be considered in developing an understanding of where we are today.
• Historically, industrialized countries have been the main releasers of carbon from combustion of fossil fuels. Although developing countries are expected to increase their share of emissions, the relocation strategies of corporations based in industrialized regions may actually significantly contribute to increased emissions in these developing countries. In 1925, for example, Australia, Japan, the United States, and Western Europe were responsible for about 88 percent of the world's fossil-fuel carbon dioxide emissions (Houghton and Skole 1990). Data from 1950-1995 indicate that 26 countries fell above the average cumulative emissions figure. These countries include almost all developed, or Annex I countries, plus Brazil, China, India, and several other larger developing countries that have seen significant growth in foreign investments (Claussen and McNeilly 1998).
• Until the 20th century, most of the conversion from forest area to cultivated land occurred in the developed countries. During the past few decades, however, most of the deforestation has occurred in tropical forests.
• Urbanization, a key driver of energy and land use, was primarily a feature of industrialized countries until the middle of the 20th century. The largest and demograph-ically most dynamic urban agglomerations, like London, New York, and Tokyo, were in developed countries. As of the beginning of the 21st century, however, most urban agglomerations are situated in developing countries. These developing-country urban agglomerations have different production systems and living standards than the urban centers in developed countries. These differences affect the carbon emissions of various urban areas (Romero Lankao, Chapter 19, this volume).
Although technology is perceived as the answer to decarbonizing economies, other societal factors work as constraints and windows of opportunity for that purpose (e.g., institutional settings and economic dynamics; see Raupach et al., Chapter 6, this volume). In addition, new technological paradigms of production and consumption patterns emerge only over decades. The main energy and production components of the engineering epoch, for instance (1850-1940), took at least 20 years to develop. Hence, it will likely take decades to set up alternative energy sources and materials aimed at decarbonizing industrial and agricultural activities. Despite efforts aimed at decoupling
economic growth from its carbon impacts, CO2 emissions from the combustion of fossil fuels and from land use change have been increasing. Many features of the development trends need to be studied and better understood before opportunities for modifications in life styles, technologies, institutions, and other drivers of carbon emissions can be fully addressed.
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