varieties to growing seasons, and architectural styles matched to patterns of temperature and precipitation. As climates change, societies face new challenges of varying severity. The indigenous peoples of the Colorado Plateau centuries ago and, more recently, the farmers of the “dust bowl” in the 1930s experienced climatic shifts that dramatically transformed their economies, settlement patterns, and governance structures. And as societies and their technologies change, the trajectory and effects of climate change are affected. For example, the technology of air conditioning as indoor climate control has fostered the development of the U.S. sunbelt, a region now facing climate-driven challenges of droughts and depleted water supplies. Air conditioning has also increased U.S. greenhouse gas emissions.
The climatic changes of the past 10,000 years have occurred in a context of remarkable stability in the average temperature of Earth, which experienced variations of less than 1° Celsius in this period. Since the advent of the industrial revolution (about the mid-nineteenth century), when fossil fuels became the primary source of energy for economic growth and societal development, the climate state has been changing from this stable condition. It is expected soon to reach a global average temperature unprecedented in recorded history, as depicted in Figure 1-1.
The Intergovernmental Panel on Climate Change (IPCC) (2007a) projects that the planet will warm substantially in the coming decades as a result of the current concentrations of greenhouse gases and expected future emissions. The National Oceanic and Atmospheric Administration (NOAA) reports that in 2007, the global concentration of atmospheric CO2 (carbon dioxide) emissions increased by 2.4 parts per million by volume (ppmv) to a level of 385 ppmv (see http://www.noaanews.noaa.gov/stories2008/20080423_methane.html).
If emissions continue at the 2007 level for a generation, atmospheric CO2 concentrations would increase to about 450 ppmv, a level that would, if maintained over time, lead to a stabilized global average temperature 2° Celsius (or slightly more) higher than preindustrial levels, according to the best available scientific estimates (Intergovernmental Panel on Climate Change, 2007a). Although scientists cannot predict the precise level of temperature change, there is now a clear consensus that the resulting temperature will be considerably higher than anything experienced in the past 10,000 years. Even if the rate of emissions could be globally reduced to near zero, the result would still most likely be a global average temperature of about 1° Celsius higher—still more than ever in recorded history—due to the heat-absorbing capacity of the oceans.