The greenhouse effect

Planet Earth's capacities for dispersing, diluting, and degrading most humangenerated pollutants are large, but limited. As pollution rates increase, the natural processes that absorb and assimilate pollutants are eventually overwhelmed, leading to rising concentrations of pollutants in the environment. Depending on the pollutants, this overloading can create local disruptions in human health and ecosystem sustainability or, eventually, even global effects, such as climate change.

The authors of this book focus on the largest and oldest of all human pollutant releases, carbon into the atmosphere. Since their mastery of fire, human beings have disrupted the global carbon cycle by burning wood and other biomass at greater rates than occur naturally. Being about half carbon, biomass upon combustion releases carbon dioxide, methane, carbon monoxide, and other carbon-containing pollutants, which must be transported and broken down by natural processes.

Some of this release has been the direct result of using wood and other biomass forms such as crop residues as fuel. Another part is due to the clearing of biomass so that the land could be used for farming or other human purposes. Throughout most of human history, however, it is thought that human biomass combustion did not create large disturbances in the atmosphere. That is, natural processes such as regrowth replaced sufficient portions of the burned biomass to prevent significant build-up of these carboncontaining pollutants in the atmosphere.

The industrial revolution in what are now the economically developed countries increased the combustion of fossil fuels, which are mostly carbon. Today, fossil fuel combustion is the major source of atmospheric carbon releases although biomass burning has also probably increased. The rates of release became such that in the second half of the twentieth century, it became clear that atmospheric levels of important carbon-containing gases, particularly carbon dioxide and methane, were steadily increasing over their natural levels.

The atmospheric concentrations of these gases are far from those thought to be toxic or otherwise of much acute concern. Their impact is more subtle, for they act to blanket Earth, keeping in more of the sun's warmth than otherwise would be the case. This effect is indisputable because humans have observed the warming due to natural levels and variations of these same gases. Indeed, without the existence of these natural amounts of greenhouse gases', Earth would be too cold for life.

The higher-than-natural rates of greenhouse gas releases resulting from human fossil fuel and biomass combustion are boosting carbon dioxide and methane levels at greater rates than has occurred in recent Earth history. Earth's natural systems may not be able to cope with the extra heat being absorbed except by an overall increase in temperature, that is, global warming. This warming may in turn be associated with significant disruptions in local weather, such as patterns of precipitation and cloudiness. It may also have global impacts through thermal expansion of the oceans and melting of glaciers to cause sea-level rise. It may even disrupt ocean current patterns and marine and terrestrial ecosystems.

The nature and magnitude of global warming and associated climate change and sea-level rise resulting from greenhouse gases released by human activities are not known with certainty. The global atmospheric/ocean/climate systems are extremely complex, so much so that even the largest computers can only model a small portion of them at one time. Some natural processes seem to reduce the effect. Extra heat, for example, leads to more evaporation, which leads to more clouds, which leads to more sunlight being reflected, which leads to less heat. In contrast, other processes may enhance the effect. Extra heat, for example, leads to more snow melting, which leads to less sunlight reflection, which leads to more heat. Thus, contemporary estimates of the global effects are imprecise and so uncertain as to be not usable for predicting effects at any one place and time.

Nevertheless, a growing number of scientists believe that there is a significant chance that damaging levels of global warming will occur sometime before the middle of next century if existing trends of greenhouse gas emissions are continued. The most authoritative source for this view is the scientific report of the Intergovernmental Panel on Climate Change (IPCC)

The IPCC consists of scientists from many countries working together under the aegis of the World Meteorological Society and United Nations Environment Programme and who have reviewed and summarized the available knowledge and uncertainties about greenhouse gases and climate change. As part of its conclusion, the IPCC predicts that unless emissions patterns change, there will be: a rate of increase in global mean temperature during the next century of 0.3°C per decade (with an uncertainty range of 0.2-0.5°C per decade); this is greater than that seen over the past 10,000 years. This will result in a likely increase in global mean temperature of about 1°C above the present value by 2025 and 3°C before the end of the next century.

Its collective judgement is that:

Rapid change in climate will change the composition of ecosystems; some species will benefit while others will be unable to migrate or adapt fast enough . . . The effect of warming on biological processes . . . may increase the atmospheric concentrations of natural greenhouse gasses.

IPCC's impact report concluded that this warming could cause major and mostly negative local impacts on agriculture, forestry, water resources, natural ecosystems, air quality, and coastal zones among other sectors important to humanity

Recognizing that there are still significant uncertainties, particularly about the timing of impacts, and that some countries, notably the United States, have not yet officially acknowledged a need to take immediate steps, this book nevertheless starts with the premise that the world community decided when it signed the Climate Change Convention that a serious effort must be made to reduce the probability and magnitude of adverse impacts from global warming. According to the IPCC, a successful response strategy is likely to be one which recognizes that:

• 'Climate change is a global issue, effective responses . . . require a global effort',
• '[Effective responses] may have considerable impact on humankind and individual societies',
• 'Industrialized countries and developing countries have a common responsibility in dealing with problems arising from climate change.

Although the developed countries have emitted far more greenhouse gases to date, based on the current relationships between economic development and greenhouse-gas emissions, and on the sorely needed increases in economic welfare that are required, large poor countries such as China, India, Indonesia, Nigeria, and Brazil will also turn into major greenhouse-gas contributors. Whether developing countries are engaged adequately in the implementation of a Climate Change Convention will be a major determinant of how quickly and completely the world responds to the dangers presented by human-enhanced greenhouse warming. Action in the Organization for Economic Cooperation and Development (OECD) and other developed countries alone can slow the rate and magnitude of global warming in the short term, but unless action is also started in developing countries, significant threat remains. The b As discussed in some detail in Chapters 2-4, when making international comparisons it is important to distinguish carefully between cumulative past emissions and present emission rates as well as between emissions per nation and per capita. fact - often discussed in this book - that there are attractive greenhouse amelioration projects in developing countries, however, does not necessarily mean that developing countries should pay for them.

The challenge facing humanity, therefore, is to find ways that the many benefits accompanying economic development can be attained by the world's poor without simultaneously emitting the amounts of greenhouse gases that have accompanied such economic development in the past; and to reduce dramatically the emissions from rich countries at the same time.