impacted them. ... Historians continue to ignore the catastrophic impact of disease and its relationship to U.S. policy and action even when it is right before their eyes." Historian David Stannard says that by "focusing almost entirely on disease ... contemporary authors increasingly have created the impression that the eradication of those tens of millions of people was inadvertent—a sad, but both inevitable and "unintended consequence" of human migration and progress," and asserts that their destruction "was neither inadvertent nor inevitable," but the result of microbial pestilence and purposeful genocide working in tandem. He also wrote: In contrast, historian Russel Thornton has pointed out that there were disastrous epidemics and population losses during the first half of the sixteenth century “resulting from incidental contact, or even without direct contact, as disease spread from one American Indian tribe to another.” Thornton has also challenged higher Indigenous population estimates, which are based on the Malthusian assumption that “populations tend to increase to, and beyond, the limits of the food available to them at any particular level of technology." The European colonization of the Americas resulted in the deaths of so many people it contributed to climatic change and temporary global cooling, according to scientists from University College London. A century after the arrival of Christopher Columbus, some 90% of Indigenous Americans had perished from "wave after wave of disease", along with mass slavery and war, in what researchers have described as the "great dying". According to one of the researchers, UCL Geography Professor Mark Maslin, the large death toll also boosted the economies of Europe: "the depopulation of the Americas may have inadvertently allowed the Europeans to dominate the world. It also allowed for the Industrial Revolution and for Europeans to continue that domination." Biological warfare When Old World diseases were first carried to the Americas at the end of the fifteenth century, they spread throughout the southern and northern hemispheres, leaving the Indigenous populations in near ruins. No evidence has been discovered that the earliest Spanish colonists and missionaries deliberately attempted to infect the American natives, and some efforts were made to limit the devastating effects of disease before it killed off what remained of their labor force (compelled to work under the encomienda system). The cattle introduced by the Spanish contaminated various water reserves which Native Americans dug in the fields to accumulate rainwater. In response, the Franciscans and Dominicans