In the late 1960s and early ’70s, climatologists envisioned the start of a new ice age because it was becoming clear that a cycle of planetary cooling was detected in the post-1940s record. The Central Intelligence Agency of the United States commissioned studies of the global political stresses that would ensue with a 1 °C (1.8 °F) temperature decline. The central question revolved around the locations of the political hot spots in a cooling world. The results of these studies were published under the title The Weather Conspiracy. In the late 1970s, Harvard University’s Center for International Affairs addressed these issues in a book by English diplomat and environmentalist Crispin Tickell titled Climatic Change and World Affairs. Tickell sounded a warning:

A shift of 2 °C in mean temperatures leads either to ice ages or to melting of the polar ice caps, either of which would destroy much of present civilization.

In the late 1970s the global warming concerns arising from the burning of fossil fuels were still a decade away, but the Harvard report provided the impetus to research the possible links between the burning of fossil fuels and global warming. In academia during this time, climatologists and historians began working together to reexamine the past connections between climate and history.

Since prehistoric times, humans have altered the land cover of the continents to suit their economic and cultural enterprises. In so doing, they have recast the two-way balance of mass and energy exchange between the atmosphere and the biosphere. Since the characteristics of Earth’s climate in the absence of cities, agriculture, and other human land uses are unknown, comparing the dynamics of the present Earth-atmosphere system with that of preurban and preagricultural times is very difficult.

In any case, humans are subject to the same climatic variations and changes that affect other life-forms. Food and fibre resources are climatically sensitive and vary in price accordingly. The cost of hauling coal by railroad depends on the temperature of the rails and on the weight gain that occurs when coal is wetted by rainfall. The consumption of fossil fuels for heating and cooling purposes is also climate-dependent. Damage from hurricanes, floods, droughts, snowstorms, tornadoes, and other weather phenomena have real costs. Climate conditions also affect decisions concerning travel and leisure.

As an exceptionally adaptable species, humans are found in all corners of the planet save the coldest polar regions, highest mountain peaks, and lowest ocean troughs. As a result, there remain few places on Earth that are not in some respect aptly classified as human-dominated ecosystems. No region is untouched by human influence. The release of waste products from domestic and economic enterprises (burning fossil fuels, synthetic chemical use, trash production, etc.) alters the composition of the atmosphere, and gases and particulates related to these activities travel to all parts of the globe. In contrast, land clearing and development often permanently alter the surface of the planet and modify patterns of both surface heating and local weather. Economically, these enterprises are purposeful actions; however, they are inadvertent when it comes to the realized changes in the environment. Air pollution, ozone depletion, acid precipitation, global warming, desertification, smog production, and deforestation are but a few of the human impacts on the climate system that arise from the alteration of the mass and energy exchange with the atmosphere.

The Gaia hypothesis, introduced at the beginning of this section, remains controversial in the scientific community. Nevertheless, the question of whether Homo sapiens possesses the capacity and the will to maintain the Earth-atmosphere system in a sort of relative homeostatic balance is intriguing. The Montreal Protocol on Substances That Deplete the Ozone Layer, which has resulted in the phasing out of chlorofluorocarbons (CFCs), a group of industrial compounds that react with and disassociate ozone molecules, is a collective adaptive response by humans to a perceived and predicted threat to life from stratospheric ozone depletion. The Environmental Protection Acts ratified by the United Kingdom and Australia and the Kyoto Protocol to the United Nations Framework Convention on Climate Change are some examples of attempts to combat deleterious environmental change associated with the release of additional carbon dioxide into the air. If humans are to maintain the Earth-atmosphere system, it is through the social institutions of research and education that they will recognize environmental threats, come to wise decisions, and attempt to exercise stewardship of the planet.