Climate and weather have always had a powerful impact on human health and well-being. The world's climate has been relatively stable for thousands of years, with a strong temperate central tendency and a nearly constant atmospheric level of carbon dioxide (CO2). But there have been reports that levels of CO2, methane, and other greenhouse gases have been rising, a trend associated with changes in climate and other earth systems. Over the ages, human societies have altered local ecosystems and modified regional climates. Today, the human influence has attained a global scale. This reflects the recent rapid increase in population size, energy consumption, intensity of land use, international trade and travel, and other human activities. These global changes have heightened awareness that the long-term good health of populations depends on the continued stability and functioning of the biosphere's ecological, physical, and socioeconomic systems.