In the 1930's and 1940's, African American industrial workers in the southern United States, who constituted 80 percent of the unskilled factory labor force there, strongly supported unionization. While the American Federation of Labor (AFL) either excluded African Americans or maintained racially segregated unions, the Congress of Industrial Organizations (CIO) organized integrated unions nationwide on the basis of a stated policy of equal rights for all, and African American unionists provided the CIO's backbone. Yet it can be argued that through contracts negotiated and enforced by White union members, unions—CIO unions not excluded—were often instrumental in maintaining the occupational segregation and other forms of racial discrimination that kept African Americans socially and economically oppressed during this period. However, recognizing employers' power over workers as a central factor in African Americans' economic marginalization, African American workers saw the need to join with White workers in seeking change despite White unionists' toleration of or support for racial discrimination. The persistent efforts of African American unionists eventually paid off: many became highly effective organizers, gaining the respect of even racist White unionists by winning victories for White as well as African American workers. African American unionists thus succeeded in strengthening the unions while using them as instruments of African Americans' economic empowerment.

Milankovitch proposed in the early twentieth century that the ice ages were caused by variations in the Earth’s orbit around the Sun. For some time this theory was considered untestable, largely because there was no sufficiently precise chronology of the ice ages with which the orbital variations could be matched.

To establish such a chronology it is necessary to determine the relative amounts of land ice that existed at various times in the Earth’s past. A recent discovery makes such a determination possible: relative land-ice volume for a given period can be deduced from the ratio of two oxygen isotopes, 16 and 18, found in ocean sediments. Almost all the oxygen in water is oxygen 16, but a few molecules out of every thousand incorporate the heavier isotope 18. When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it. Because heavier isotopes tend to be left behind when water evaporates from the ocean surfaces, the remaining ocean water becomes progressively enriched in oxygen 18. The degree of enrichment can be determined by analyzing ocean sediments of the period, because these sediments are composed of calcium carbonate shells of marine organisms, shells that were constructed with oxygen atoms drawn from the surrounding ocean. The higher the ratio of oxygen 18 to oxygen 16 in a sedimentary specimen, the more land ice there was when the sediment was laid down.

As an indicator of shifts in the Earth’s climate, the isotope record has two advantages. First, it is a global record: there is remarkably little variation in isotope ratios in sedimentary specimens taken from different continental locations. Second, it is a more continuous record than that taken from rocks on land. Because of these advantages, sedimentary evidence can be dated with sufficient accuracy by radiometric methods to establish a precise chronology of the ice ages. The dated isotope record shows that the fluctuations in global ice volume over the past several hundred thousand years have a pattern: an ice age occurs roughly once every 100,000 years. These data have established a strong connection between variations in the Earth’s orbit and the periodicity of the ice ages.

However, it is important to note that other factors, such as volcanic particulates or variations in the amount of sunlight received by the Earth, could potentially have affected the climate. The advantage of the Milankovitch theory is that it is testable; changes in the Earth’s orbit can be calculated and dated by applying Newton’s laws of gravity to progressively earlier configurations of the bodies in the solar system. Yet the lack of information about other possible factors affecting global climate does not make them unimportant.