15.1.2.2. Urbanization

Within developed regions of North America, one important feature is the high-value concentrated development center or corridor. Proliferation of these centers and corridors (e.g., U.S. Gulf coast, Boston-New York-Washington corridor) can lead to high damage costs from extreme events associated with weather phenomena (see Sections 15.2.5 and 15.2.7).

A second feature concerns extension of urban land use into previously undeveloped or less-developed areas, including agricultural lands, forested areas, wetlands, barrier islands, and other coastal margins. In the case of urban encroachment on forested areas, climate change effects on human use and value of forest ecosystems are likely to be significant but are very poorly understood (Binkley and Van Kooten, 1994). This is a challenge in considering traditional human uses of forests for wood products, recreational space, or environmental protection. A more recent phenomenon that must be considered, however, is the increase in human populations in many forested areas in the mid-latitude regions of North America. For example, in the Colorado Rocky Mountains in the United States, human population is expected to double in the next 20-40 years (Stohlgren, 1999). These population increases will amplify climate-induced stresses on forest habitat and species assemblages. Moreover, these increases will increase the exposure of human populations to climate-induced catastrophic events (e.g., fire, floods).

Table 15-2: Population of Mexican states along the U.S.-Mexican border (data from Instituto Nacional de Estadistica Geografia e Informatica, Mexico, http://www.inegi.gob.mx/poblacion/ingles/fipoblacion.html).
Population	1990	1992	1997	% change
Baja California Sonora Chihuahua Coahuila Nuevo Leon Tamaulipas Total population in Mexico	1,660,855 1,823,606 2,441,873 1,972,340 3,098,736 2,249,581 81,249,645	1,908,434 1,866,757 2,503,515 2,040,046 3,336,044 2,351,663 85,627,971	2,241,029 2,183,108 2,895,672 2,227,305 3,684,175 2,628,839 93,716,332	+35 +20 +19 +13 +19 +17 +15

15.1.2.3. Continental Free Trade

The North American Free Trade Agreement (NAFTA) between Canada, the United States, and Mexico has contributed to several changes in economic activity. One indicator is the recent increase in the population of Mexican states along the U.S.-Mexican border (see Table 15-2), which is at a faster rate than for Mexico as a whole. Such shifts may alter the relationship between regions and their climate, affecting the nature of future climate-related impacts and adaptation responses (e.g., see Section 15.3.2.9).

Society has long viewed climate as "constant"—that is, we expect year-to-year variations, but we expect that the chances of hurricanes, floods, or heat waves don't change with time. The probability of a 100-year event (defined by crossing some threshold—say, 5 cm of rain in an hour) should be a constant. As we have developed longer records of climate—from geologic records in ice cores and sediments to longer and longer instrumental records—we have learned that this assumption of constancy simply is not true. As we look back and see constant change, we should look to the future and expect additional changes to climate and to climate variability.

There have been significant changes (i.e., trends) in North America's climate over the past century. As a whole, North America has warmed by about 0.7°C (Carter et al., 2000), although this warming has been quite heterogeneous. For example, the southeastern United States cooled slightly over that same period, although recent decades have seen some warming. There also have been trends in precipitation. Annual precipitation over North America decreased by about 50 mm in the early years of the century and since then has steadily increased by ~70 mm. These trends, like those of temperature, have been fairly heterogeneous. The largest increases have been in the northern Atlantic and Pacific coastal regions. Some regions have experienced seasonal decreases in precipitation. Accompanying these trends in temperature and precipitation, which are fairly well measured, have been changes in cloud cover and humidity, which affect plant growth as well as people's perception of temperature. Inferences from atmospheric CO₂ and satellite data suggest that the northern growing season (the period of active plant growth) has lengthened appreciably over the past decades (Myneni et al., 1997), and data from Alaska show an unequivocal warming trend in the western Arctic (Bering Sea Impacts Study, 1999).

Over the course of the 20th century, there have been several extreme periods in the climate. The drought of the 1930s changed all of North American society. The damage to midcontinental agriculture and communities has consequences even today, through changes in land ownership, government policies, and farmer and merchant perceptions of risk. More recently, there have been major consequences of drought, flooding, tropical storms, and tornadoes. There is no clear evidence of trends in the meteorological risks from "heaviest rainfall category" events or that their frequency has changed over the period of instrumental measurements. However, evidence from the paleorecord documents changes over the past thousands of years that lie outside what our contemporary society can remember. For example, there have been several droughts in California, documented in tree-ring and lake-level data, of more than 80 years' duration (Meko et al., 1991). Evidence from the Rocky Mountain and Great Plains regions suggests that there have been long periods in the Holocene (the time since the last glaciation) when these regions were much drier than today (Woodhouse and Overpeck, 1998). These historical events did not happen as a result of anthropogenic impacts but are within the range of natural climate variability; they provide a caution against assuming that tomorrow's climates will necessarily be no less benign than today's.

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