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Including a section on climate change in a book on Urban Planning may seem like a bit of a stretch, but both changes in climate and how we respond to them will have many effects on what we do and what happens at the local level. Climate change is likely to be the overarching environmental problem, and the great majority if not virtually all other environmental problems will have to be addressed within its framework.

At the global level changes in climate, particularly changes in rainfall, are likely to have big effects on world agriculture, with major consequences for war and peace, political stability, and mass migration. Within the United States, climate change may prove negative for some parts of the nation and positive for others. For example, warming and possibly drying may be a net negative for the Southwest. At the same time, it may produce what some regard as climate improvement for, say, Minnesota and other parts of the north-central region. If, as some climatologists expect, global warming increases the intensity of tropical storms, it will likely prove negative for the Gulf Coast region. Changing patterns of rainfall affect the spatial pattern of agriculture. In all of these ways global warming may affect the distribution of population in the United States and perhaps change the patterns of regional growth shown in Table 2-1.

What we choose to do to address the problem of climate change will affect the work of the planner in many ways. Transportation uses more than a quarter of U.S. energy. Thus energy planning and transportation policy are inextricably linked and, as discussed in Chapter 13, transportation technology and the pattern of land use are closely tied. Housing is a major consumer of energy, and so choices about energy and about housing development are also closely tied. Choices about environmental policy will have to be considered both in terms of purely local issues as well as the larger issue of global warming. For example, some dams have been torn down and some additional ones are slated to go because of local ecosystem considerations such as the survival of particular species of fish. But producing electricity by hydropower is very benign in greenhouse terms compared to producing it by the burning of fossil fuel. There is clearly a trade-off between two desirable but contradictory goals.

The biggest force behind global warming is the increasing level of carbon dioxide in the earth's atmosphere. As Figure 15-1 shows, there is no sign that the rate of increase is slowing. While there is near-universal agreement among climate scientists about the fact of warming, there is considerable uncertainty about just how fast it is likely to proceed. There is also much uncertainty about what should be and what will be done. One overarching question on which there is no general agreement is how much to spend on reducing greenhouse gas emissions versus how much to spend on mitigating the effects of increasing temperatures and shifting weather patterns.2 Within the context of reducing the rate of warming there is disagreement over how much to focus on the largest single cause, carbon dioxide emissions, versus how much to focus on smaller causes such as methane or fine carbon particles ("black carbon"). Within the context of carbon dioxide there are a large number of energy choices, including sequestration of carbon dioxide from coal

Atmospheric carbon dioxide in parts per million, 1960 to 2007

FIGURE 15-1 Atmospheric carbon dioxide in parts per million, 1960 to 2007.

Note: Preindustrial revolution CO2 levels are estimated from ice core samples to be in the 260-280 per million (ppm) range.

Source: Measured at Mauna Loa Observatory, Hawaii.

burning, direct removal of carbon dioxide fr om the atmospher e, solar and wind technologies, and expanded use of nuclear energy. One now hears a small but incr easing number of voices suggesting that beyond emission reductions we should consider positive steps to incr ease the earth's albedo (the percentage of incoming solar energy that the planet reflects back into space), a subject that is beyond the scope of this book but that the inter ested reader can easily look up by googling the wor d geoengineering or climate engineering.

Although there is a gr eat deal that the United States can do about global warming, much of the pr oblem is out of U.S. hands. At present the United States produces approximately a quarter of the world's total carbon dioxide emissions and that share is shrinking. In about 2006 or 2007 China passed the United States as the world's lar gest carbon dioxide emitter and the gap is gr owing rapidly. This country has very little leverage over China in this matter Over 1.3 billion Chinese want their automobiles (China is now the world's largest automotive market with 22 million vehicles sold in 2013), central heating, and electric appliances just asAmericans do. What was just said about China may, a decade from now, apply to India. It has a population of 1.2 billion, its birth rate, though lower than it once was, is still considerably higher than either ours or China's, and it is experiencing very rapid economic growth.

When a nation cuts back on its emissions of greenhouse gases it benefits the entire world, but it captures only part of the total benefit. Thus simple economic theory would suggest that the nations of the world are, in total, likely to underinvest in greenhouse gas reduction, the tragedy of the commons problem noted above.

At the December 2015 Paris climate summit many nations made nonbinding commitments to reduce carbon dioxide emissions. China's was to halt the rise in its emissions by 2030. Perhaps a near-universal understanding of the seriousness of the problem will enable the world to walk away from the tragedy of the commons this once. In the United States the question of reducing greenhouse gas emissions is tangled up with the question of political ideology. When Americans are questioned on the subject of global warming a majority of them say that they believe it is happening, but in some surveys at least half of those who do believe that it is happening attribute it to causes other than human activity.3 Belief in the reality of global warming shows little if any correlation with educational level, but it is very strongly correlated with political position. Democrats, as a group, are much more likely to believe it is happening and that human activity is behind it than are Republicans. Perhaps the difference is explained by the implications of believing that the problem is serious, since if it is, that implies the need for considerable amounts of regulation and for major public expenditures.

Reductions in greenhouse gas emissions are expensive, whether it is an expense laid on firms by regulation or an expense laid on government through subsidy programs and direct expenditures. Either way, at this writing it runs afoul of powerful concerns over tax cuts, budget deficits, and our ability to compete economically in world markets.

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