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Until the middle of the 20th century, the discipline of climatology was a stagnant field preoccupied with regional statistics. It had little to do with meteorology, which itself was predominantly a craft that paid scant attention to physical theory. The Second World War and Cold War promoted a rapid growth of meteorology, which some practitioners increasingly combined with physical science in hopes of understanding global climate dynamics. However, the dozen or so scientific disciplines that had something to say about climate were largely isolated from one another. In the 1960s and 1970s, worries about climate change helped to push the diverse fields into contact. Scientists interested in climate change kept their identification with different disciplines but developed ways to communicate across the boundaries (for example, in large international projects). Around the turn of the 21st century, the Intergovernmental Panel on Climate Change institutionalized an unprecedented process of exchanges; its reports relied especially on computer modeling, which became a center of fully integrated interdisciplinary cooperation.

A list was compiled of virtually all significant military confrontations between republics throughout history. By including regimes only marginally republican, some forty cases were found from ancient Greece to the 1990s; about half of these had significant combat. Detailed historical investigation of each case reveals consistent patterns. A striking lack of wars between well-established democracies prevailed not only among modern states but also among earlier regimes commonly described as democracies, for example in medieval Switzerland. A historically more numerous class of republic is the oligarchies, where those in power hold equal rights but deny such rights to other important groups (e.g. South Africa). Remarkably, oligarchies scarcely ever made war on other regimes of their own type. Oligarchies did commonly fight democracies. It also appears that both types of republic, unlike all other regimes, have tended to form durable peaceful leagues among themselves. These reliable and general observations are not consistent with explanations solely in terms of institutional structures, but they can be understood in terms of domestic political culture. Leaders who negotiated with fellow citizens as equals invariably treated foreign leaders in the same non-violent manner, provided that they perceived the foreigners too as political equals.

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People had long speculated that human activities might affect a region's climate. But a developed conjecture that humanity might change the climate of the entire planet first appeared in 1896: a calculation that carbon dioxide from fossil fuel combustion could gradually warm the globe. Scientists soon rejected the idea. Most people thought it incredible that climate could change globally except on a geological timescale, pushed by forces far stronger than human activity. In midcentury, a few scientists revived the hypothesis of global warming. Meanwhile, the exponential growth of human activity, especially chemical pollution and nuclear armaments, was showing that humanity really could affect the entire atmosphere. Moreover, during the 1960s research suggested that small perturbations might lead to an abrupt change in the climate system. Although nobody expected serious impacts until the distant 21st century, some began to frame global warming not just as a scientific puzzle but as an environmental risk, a security risk, a practical policy question, an international relations issue, and even a moral problem. In the late 1970s a scientific consensus began to take shape, culminating around the end of the century in unanimous agreement among government representatives on essential points, although many uncertainties remained. Meanwhile, increasing media warnings of peril made most of the literate world public aware of the issue, which had deep implications for the human relationship with nature. Skepticism persisted, correlated with aversion to regulation. The majority of the world public were now concerned, but disinclined to take action. 