Large metropolitan areas in North America, home to 65% of the nation's population, have created major changes in their climates over the past 150 years. The rate and amount of the urban climate change approximate those being predicted globally using climate models. Knowledge of urban weather and climate modification holds lessons for the global climate change issue. First, adjustments to urban climate changes can provide guidance for adjusting to global change. A second lesson relates to the difficulty but underscores the necessity of providing scientifically credible proof of change within the noise of natural climatic variability. The evolution of understanding about how urban conditions influence weather reveals several unexpected outcomes, particularly relating to precipitation changes. These suggest that similar future surprises can be expected in a changed global climate, a third lesson. In-depth studies of how urban climate changes affected the hydrologic cycle, the regional economy, and human activities were difficult because of data problems, lack of impact methodology, and necessity for multidisciplinary investigations. Similar impact studies for global climate change will require diverse scientific talents and funding commitments adequate to measure the complexity of impacts and human adjustments. Understanding the processes whereby urban areas and other human activities have altered the atmosphere and changed clouds and precipitation regionally appears highly relevant to the global climate-change issue. Scientific and governmental policy development needs to recognize an old axiom that became evident in the studies of inadvertent urban and regional climate change and their behavioral implications: Think globally but act locally. Global climate change is an international issue, and the atmosphere must be treated globally. But the impacts and the will to act and adjust will occur regionally.

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