Two of the great questions of human history and economics are why some nations held far-flung empires and why some presently enjoy great wealth. One factor that should be included in the inevitably multifactorial answer to these questions is regular moderate precipitation (precipitation with an average rate between 30 and 120 mm for each month). Only a small proportion of Earth’s surface has regular moderate precipitation, and most of that area is in Europe and eastern North America. Strikingly, of the 13 nations that held geographically discontinuous multicontinental transoceanic empires, 12 overlap with regions of regular moderate precipitation. Similarly, of the 20 nations with the greatest per capita GDP in 2015, 16 coincide with regions of regular moderate precipitation. These relationships are presumably rooted in the greater success, or lesser inhibition, of human construction of infrastructure, husbandry of livestock, and cultivation of crops, some combination of which likely allowed industrialization, projection of geopolitical power, and accumulation of wealth. One instructive example is that of China, which has a climate superficially like that of Europe and eastern North America but no regions of regular moderate rainfall, and which neither developed an overseas empire nor is among the world’s nations with greatest per capita GDP. Furthermore, concentration of nations holding empires and wealth in the Northern Hemisphere and their absence from the south can be linked to the coincidence that the Southern Hemisphere’s latitudinal zone of regular moderate rainfall is over the Southern Ocean, where there is little land on which human societies could have enjoyed the benefits of that supportive climate.
a. The hypothesis
Why some peoples have ruled and/or exploited much of the world, and why other peoples have been ruled or exploited, is one of the great questions of history. Similarly, why various nations today are more prosperous than others is one of the great questions of economics. Answers have involved factors ranging across a broad spectrum from the natural environment to social structures to the influence of writing systems on language and thought (Table 1). One might conclude that multiple factors have interacted synergistically, as suggested by Jones (2003, p. xvii), but recent explanations are commonly accompanied by dismissals of all other answers, as in a chapter on “Theories that don’t work” in Acemoglu and Robinson (2012), a section explaining why “the standard answers … just fall apart” in Hoffman (2015, p. 4), and a paper claiming that its favored factor “trumps everything else” (Rodrik et al. 2004). Less broadly but equally clearly, a paper on economic development with the title “Institutions rule” (Rodrik et al. 2002) was quickly countered by a paper with the title “Institutions don’t rule” (Sachs 2003).
One reason for the abundance of explanations of the distribution of empire, power, and wealth is that claims can shift subtly but significantly. To take a popular example, the explanation provided by Crosby (1993) and Diamond (1997) hinged on the many usefully domesticable plants and animals native to the Fertile Crescent of southwestern Asia, and more generally of western Asia. That region indeed was the source of many important plants and animals that were eaten by or used by Europeans as they colonized the world. However, the model would imply that Syria, Iraq, and Iran would have gone on to be, or at least be among, the nations that had colonies in the Americas, Africa, southern and eastern Asia, and Australia. Discrepancies of this sort point to the need for clear definition of both the question(s) asked and the answer(s) given, as suggested by Abernethy (2000).
With the need for clear definition in mind, this paper proposes that two clearly defined questions can be answered in part, but certainly not entirely, by one clearly defined answer. The historical question is, “Why did some nations come to rule geographically discontinuous multicontinental transoceanic empires?” where “empire” is defined as by Abernethy (2000, p. 19) and clearly does not extend to nongeographic forms of imperialism, such as cultural imperialism. The economic question is, “Why do some nations presently have the greatest per capita gross domestic product (GDP), as reported by the International Monetary Fund?” The answer proposed here is that these patterns can be explained partly, but certainly not entirely, by regular moderate atmospheric precipitation, which is defined here as precipitation falling at an average rate between 30 and 120 mm for each month (with the additional restriction that the average temperature for each month be greater than −10°C). The nations categorized historically and economically by the two questions coincide strikingly with the regions defined climatically in the answer. That result has implications for not only how we think about history and economics but also how we categorize climate.
b. What the hypothesis is not
Any explanation of human affairs in terms of climate runs the risk of being considered “climatic determinism” and thus “environmental determinism.” Determinism has been defined as the notion that “for everything that ever happens there are conditions such that, given them, nothing else could happen” (Taylor 1967) because any other sequence of events “would break a law of nature” (Blackburn 2005), and as “the theory that examination of one or more definable factors allows for a complete explanation and prediction of the characteristics of society or the individual” (Drislane and Parkinson 2016). The complete, certain, and inevitable power of prediction and/or explanation implied by these definitions would, in itself, raise questions when applied to something as complex and interactive as human history. However, environmental determinism in particular has been further and more thoroughly dismissed by cultural geographers because early advocates of climatological explanations of economic development like Semple (1911) and Huntington (1915, 1922, 1924) extended their theses to the work ethics and intelligence of different races [as discussed, to name only a few, by Spate (1952), Peet (1985), Glantz (2003), Hulme (2011), and Livingstone (2011)]. Thus, environmental determinism was “shunned” (Coombes and Barber 2005), Blaut (1999) wrote that “most geographers think of the theory of environmental determinism as a musty, fusty relic of the past,” and Sluyter (2003) condemned Diamond’s (1997) book Guns, Germs, and Steel as “junk science,” “a pernicious book,” and “absurd.” According to one online article, “environmental determinism … has long been discredited in academic circles” (Wikipedia 2016), and even climatologists writing about modern anthropogenic climate change now speak of “the determinist fallacy” (Hulme 2011, p. 247). With regard to history and empire, Livingstone (2011) wrote that “environmental determinism has been read as a legitimating ideology underwriting imperial impulses” and thus providing “a scientific justification for the inevitability of imperial expansionism.” Similarly, Frenkel (1992) wrote that “geography, empire, and environmental determinism went hand in hand during the late nineteenth and early twentieth centuries.”
In light of the concerns discussed above, two points about this paper must be made clear. First, no claim to complete explanation or inevitability is made here, and instead this paper only suggests that one carefully defined climatological factor should be included in the complex interaction of multiple natural and human factors that have influenced human history, acknowledging “the inseparable nature of environmental and cultural influences” (Coombes and Barber 2005). In its appreciation of climate, this paper is akin to others published in this journal in the last six years that linked changing climate to humans’ health (Greene et al. 2011; Steinweg and Gutowski 2015), livelihoods (Carr et al. 2016; Kolawole et al. 2016), and technologies (Coffel and Horton 2015; Ferranti et al. 2016): none of those papers made claims of complete explanation, but nonetheless they assumed that climate was relevant to the human experience. In this respect, this paper and those have a surprising companion in that of Blaut (1999), who, despite his condemnation of Diamond’s environmental determinism, reached the climatologically based conclusion that the Khoi people of southern Africa “did not adopt Xhosa agriculture … [because] … almost all of the area in South Africa that the Khoi occupied before the Europeans arrived is just too dry to support rain-fed agriculture” (Blaut 1999, p. 396).
Second, with regard to the concerns discussed above, no claim is made in this paper that climate endowed any people with better genes, a superior intellect, an enhanced worth ethic, a superior culture, or a greater moral claim to power. Legitimation of imperialism is neither the intended goal nor the resultant inference: climate in no way justified imperialism, whether it be territorial, economic, military, or cultural. Instead, the implications of this paper are exactly the opposite: that the peoples who accumulated more wealth and exerted geopolitical over others did not do so because of their superiority of any sort but instead because, at least as a contributing factor, their acquisitiveness was made possible (not inevitable, but possible) or facilitated by the climate of the lands from which they came.
2. Context: Geographic patterns of empire, wealth, and rainfall
a. Geographically discontinuous multicontinental transoceanic empires
At least 10 and arguably as many as 13 nations have claimed or administered geographically discontinuous multicontinental transoceanic empires (Table 2). The 10 are Sweden, Denmark, Germany, the Netherlands, Belgium, France, Spain, Portugal, Great Britain, and the United States. Of those 10, five held concession zones in the Chinese port city of Tianjin, commonly known then as Tientsin (Johnstone 1937).
In addition to those 10 empires, Russia ruled an empire that was by definition geographically discontinuous and multicontinental and transoceanic, but the last of those three qualifications hinged on the 82 km of the Bering Strait, less than the width of Lake Michigan. Russia additionally held a concession in Tianjin (Johnstone 1937). Italy ruled an empire that was not truly transoceanic because its African holdings were only across the Mediterranean Sea, but it additionally held a concession in Tianjin. The Austro-Hungarian Empire was multicontinental and transoceanic only in the sense that it too held a concession in Tianjin, making its inclusion among geographically discontinuous multicontinental transoceanic empires the most tenuous of the 13 nations. Inclusion of Austria–Hungary, Italy, and Russia among a list of 13 is further supported by their participation, with the 10 nations listed above, in the Berlin West African Conference of 1884–85 (Wang 1998; Craven 2015), which is commonly considered to have established the procedural foundation for the subsequent “scramble” for colonies in Africa (e.g., Anghie 2004).
Other past empires meet some but not all of the requirements of the category discussed above (Table 3). Of modern candidates, the Empire of Japan was geographically discontinuous and stretched across the western Pacific Ocean but was not multicontinental, certainly not in the sense of holding territories in Africa or South America. Earlier empires, most notably the Roman Empire, occupied parts of two or even three of the continents surrounding the Mediterranean, but they were neither transoceanic nor significantly discontinuous.
Although no nation presently holds a geographically discontinuous multicontinental transoceanic empire (at least in name), several nations explicitly claim or hold distant islands (Table 4). In addition to nations listed in Table 2, one of the most striking claimants to distant islands is Australia, which administers islands more than 3800 km away, greater than the distance across Australia itself. Reinhard (2011) used the term “sub-imperialism” to characterize Australia’s role in administering parts of the more general British empire, probably more with regard to history than to the present, but Australia’s present distant claims in the Pacific, Indian, and Southern Oceans suggest the geographic configuration, if not the economic rewards, of empire.
b. Per capita gross domestic product
The 20 nations with the greatest per capita GDP for 2015 range from Luxembourg (USD $103,187) to France (USD $37,728) (IMF 2015) (Table 5). Those nations include four major producers of petroleum (Qatar, Norway, the United States, and Canada), Australia, Singapore, and 14 European nations in addition to oil-rich Norway. Nine of the 20 are among the 13 nations that held geographically discontinuous multicontinental transoceanic empires (Table 2). At the other extreme, the 20 nations with the smallest per capita GDP for 2015 are all in Africa or southern Asia, and all have a per capita GDP for 2015 of less than USD $710.
c. Variability of atmospheric precipitation
1) Variation in atmospheric precipitation through time and space
Atmospheric precipitation varies through time at scales ranging at least from seasonal to millennial (giving wet and dry seasons, and wetter and drier periods of history and prehistory), and through space from region to region (giving ecosystems that range from deserts to rain forests). This paper largely uses the worldwide geographic variation in precipitation synthesized by New et al. (1999) from rain gauge data collected from 1960 to 1990. Because this paper reaches conclusions about processes across time that go back at least a few centuries if not two or three millennia, one might question the relevance of the modern distribution to those past processes. However, some reassurance can be found in the fact that variation through time is generally less than variation through space. For example, tree-ring records indicate that rainfall in Europe in April–June varied by 90 mm, between 165 and 255 mm, over the period from 700 to 2000 CE (Büntgen et al. 2011). That variability in time is small on a planet on which geographic variation of rainfall for the same three months presently ranges from 3 mm in southwestern Egypt to almost 1600 mm in the state of Meghalaya in northeastern India. Similarly, tree-ring records indicate that annual rainfall in central China varied by 115 mm, from 265 and 380 mm, from 1600 to 2000 CE (Yi et al. 2010), whereas annual rainfall presently varies with location across just China, rather than the world, from less than 50 to almost 2000 mm.
Also reassuring is the fact the most studies of long-term climate change suggest changes in ecosystems that are significant but not extreme. For example, Railsback et al. (2016) concluded that conditions at one location in northern Namibia varied, during the period of extreme global-scale temperature change from the Last Glacial Maximum to the thermal maximum of the present Holocene interglacial, from a savanna ecosystem to a semidesert ecosystem, and thus between relatively dry ecosystems but not to the extremes of either full desert or forest (and certainly not rain forest). These comparisons between temporal and spatial variations in rainfall suggest the former at any one location is dwarfed by the latter across the globe, which will be the focus of the next section and more generally of this paper.
2) Earth’s modern geographic variation in atmospheric precipitation
Atmospheric precipitation is seasonal, rather than temporally uniform, across most of Earth’s continents (Granger 1987) (Fig. 1). Regions of winter precipitation but dry summers are found east of oceans and seas in midlatitude regions. Westernmost North America, southern Chile, southwestern-most Africa, and southwestern-most Europe and northwestern-most Africa are the noteworthy regions associated with the oceans, and the region around the Mediterranean Sea and east from there to westernmost China is the fifth major region. Regions of the opposite pattern, with summer precipitation but drier cool seasons, are most pronounced in the tropics, where migration of the intertropical convergence zone (ITCZ) southward and northward brings monsoonal rain in summer, in some places in great abundance, but leaves those regions dry in winter (Chang 1987). Additionally, regions in which summer rainfall dominates (but winter precipitation is not unusual) extend poleward from the subtropics into the northernmost parts of North America and Asia. In the equatorial zone, precipitation is abundant all year but is commonly greatest just after the equinoxes, when the ITCZ has recently passed from north to south or vice versa. In contrast, precipitation is scarce all year in the major deserts, most notably the Sahara, and in the polar regions (Fig. 1).
As the previous paragraph implies, regions of year-round moderate precipitation are not common. In this paper, “moderate” precipitation is defined as ranging from 30 to 120 mm month−1 (a range compatible with Fig. 3b of Brown and Lall 2006). Despite that fourfold range, only two large regions have moderate precipitation all 12 months of the year. The largest is in Europe, extending eastward from Ireland to western Romania and southward from southern Scandinavia to northern Spain (Fig. 2). The second of the two regions is in eastern North America, in the eastern United States and southeastern-most Canada. Successively smaller regions include eastern Australia and eastern Europe from northern Moldova through Ukraine to western Russia (although this fourth region is colder than those in western Europe, the United States, and Australia). Except for eastern Australia, all four of these regions of little intra-annual variability of precipitation are also regions of little interannual variability, as shown by Fig. 3 of Sokol Jurkovic and Pasaric (2013).
3. Results: Geographic relationships between climate, empire, and wealth
This paper’s most important finding is that, of the 13 nations that held geographically discontinuous multicontinental transoceanic empires, 12 overlap with the regions of regular moderate precipitation discussed above (Fig. 2). Only Portugal, which among these 13 is the nation that initially and most consciously promoted overseas exploration and economic exploitation (Russell 2000), is outside the regions of regular moderate precipitation. However, paleoclimatological studies indicate that climate in Portugal (Abrantes et al. 2005; Bartels-Jónsdóttir et al. 2015), and more generally in the Atlantic-facing regions of the Iberian Peninsula (e.g., Railsback et al. 2011), was wetter during the Little Ice Age, the climatic period in which European exploration and colonization began. Other research has documented this sort of relevance of past climate change to the timing of individual empires of lesser geographic extent than those considered here (e.g., Weiss et al. 1993; Cullen et al. 2000; Büntgen et al. 2011; Pederson et al. 2014).
The relationship of geopolitical domination with regions of regular moderate precipitation also extends to nations that have not held empires but that presently claim distant islands. Australia, the nation most notable for the distance of its territorial claims, includes one of the four largest regions of regular moderate precipitation (Fig. 2; Tables 4, 6). Additionally, Argentina, Chile, and New Zealand, all of which claim distant islands (Table 4), have regions of regular moderate precipitation (Fig. 2; Table 6). In contrast, almost all coastal nations without regions of regular moderate precipitation make no such distant claims (Table 6).
Like past empire and present overseas claims, present economic development also shows a strong relationship to regular moderate precipitation. Of the 20 nations with the greatest per capita GDP in 2015, 16 coincide with regions of regular moderate precipitation (Fig. 2; Table 5). The other four (Qatar, Norway, Switzerland, and Singapore) are either major petroleum producers or hubs of international business and finance. In contrast, the world’s poorest nations are those with great seasonal and interannual variability of rainfall, as was documented thoroughly by Brown and Lall (2006).
a. Causal linkages between climate, empire, and wealth
The relationship between regular moderate atmospheric precipitation and the empires and prosperity of nations is presumably best understood in terms of a favorable climate’s facilitation of human activity in general and specifically of agriculture (Lee 1957; Kamarck 1976), which provided sufficient surplus food and freedom from famine (Appleby 1981) to allow persons freed from agriculture to develop knowledge and technologies that led to societal wealth and dominance in world affairs. As Bandyopadhyaya (1980, p. 57) observed, “industrialization is the hard core of economic development,” but “industrialization presupposes agricultural development” [as also suggested by Jones (2003, p. 225)].
With regard to general human activity, rainfall that is not highly seasonal precludes the regular and often extreme seasonal flooding that commonly immobilizes and sometimes devastates regions of tropical monsoonal climate (Bhalme and Mooley 1980; Kinosita 1983). At the same time, regular precipitation provides dependable water resources, both for drinking and for mechanical power (Biswas 1984b). With regard to the latter, Claiborne (1970, 364–365) argued that the technological development of northwestern Europe could be traced to a hydroclimate that allowed mills and industry driven by water wheels, which provided a precursor and transition to the hydrocarbon-driven Industrial Revolution in that region [as further suggested by Jones (2003, pp. 4, 54)].
Regular moderate atmospheric precipitation is significant to agriculture, and thus to sustenance of human activity, by supporting both animal husbandry and cultivation of crops (e.g., Kalff et al. 1985; Kgosikoma and Batisani 2014). With regard to animal husbandry, regular precipitation provides drinking water for livestock (e.g., Masike and Urich 2009) and growth of vegetation sufficient for pasturage (McGinty et al. 1991; Kgosikoma and Batisani 2014) while precluding flooding that disrupts continuity of herds (Biosecurity Queensland 2012). The opposite is seen in regions of seasonal drought, commonly exacerbated in ENSO years, where livestock must be sold at a loss, slaughtered, or left to die of thirst. Drought also promotes raiding of livestock and thus greater intensity of violence in pastoral societies (Ember et al. 2012).
With regard to cultivation of crops, regular rainfall recharges soil moisture and groundwater (Hutchinson et al. 1958; Gardner 1965), even in seasons when rainfall is not needed to support growing crops. Moderate precipitation allows infiltration of water to the soil, whereas little rainfall allows desiccation of vegetation and intense rainfall leads to much runoff, both of which promote erosion of topsoil (Gardner 1965; Biswas 1984a, 10–13). Regular rainfall also maintains vegetative ground cover that precludes erosion by wind. The intense but very seasonal rainfall typical of the tropics exacerbates both desiccation and erosion, allowing each year a dry period in which vegetative cover diminishes followed by heavy rain that erodes topsoil intensely (Bandyopadhyaya 1980, 61–62). In those regions, heat combines with seasonally intense rainfall to promote degradation of soil organic matter, lessening retention of moisture in the soil (Bandyopadhyaya 1980, p. 60) and thus compounding the problem of seasonally scarce rainfall. In the longer term, excessive rather than moderate rainfall leads to the leaching from soil of plant nutrients (Gardner 1965; Biswas 1984a, p. 13), which are among the soil elements and compounds most readily removed in weathering (Railsback 2003). These factors resulting from excessive rainfall favor growth of crops that are poor in protein and rich in carbohydrate (Bandyopadhyaya 1980, p. 61) and that are thus less advantageous for human nutrition, as was pointed out by Diamond (1997, pp. 138, 149). Given the linkage between nutrition and human development (Wahlqvist 1981; Gómez-Pinilla 2008; Jackson and Beaver 2015), and the greater susceptibility to infectious disease resulting from malnutrition (Schaible and Kaufmann 2007), it would hardly be surprising that populations advantaged by climate and thus nutrition would have had greater capacity for economic development and geopolitical domination.
The indisputable linkage of favorable hydroclimate to agricultural development documented above provides one critical explanation of why some nations and peoples came to be geopolitically, economically, and culturally dominant. Many other factors have undoubtedly been important too, but they commonly only ask for further causes. For example, the importance of European weaponry and other technology is inescapable (Fieldhouse 1967; Parker 1988; Abernethy 2000), but one must then ask why Europe was able to develop those technologies when others did not. The importance of Eurasian domesticable plants and animals is undisputable (Crosby 1993; Diamond 1997), but one must then ask why only the far western end of the Eurasian landmass hosted nations that eventually possessed overseas empires, and not nations like Persia/Iran closer to the natural source of those domesticable plants and animals. Geographic access to the oceans was clearly important (Faye et al. 2004; Collier 2007), in that the first 9 of the 13 largely European nations ranked in Table 2 had ports on the Atlantic Ocean, whereas the last 3 did not, and landlocked Switzerland is not in the list at all—but one must then ask why other nations with oceanic ports, from Morocco to Iran to China and from Mexico to Chile, did not come to possess overseas empires. Religious motivations were important (Pagden 1998; Goucher et al. 1998; Russell 2000; Harrison 2005; Strong 2006), but one must ask why other regions with religions promoting geographic hegemony (most notably Islam) failed to develop transoceanic empires. Institutions, like those of Great Britain, were important (Acemoglu et al. 2002; Easterly and Levine 2003; Dollar and Kraay 2003; Rodrik et al. 2004; Acemoglu and Robinson 2012), but one must ask why the institutions of British origin that led to wealth and geopolitical power in North America did not do the same in the Caribbean and Africa [as Engerman (2003) asked]. Questions like these combine with the relationship documented in Fig. 2 and Tables 2, 5, and 6 to suggest that regular moderate precipitation and its facilitation of societal development are necessary (not sufficient, but necessary) conditions for eventual empire and large-scale economic dominance.
b. Climate and regional aspects of human history
1) The climatological and historical anomaly of China
The insignificance of China to Fig. 2 and Table 2 is striking both with regard both to climate and to empire. From the climatic perspective, China's absence from the regions of regular moderate rainfall might be surprising to persons familiar with the widely accepted Köppen–Geiger classification of climate (Köppen 1900; Geiger 1954; Kottek et al. 2006; Peel et al. 2007). The Köppen–Geiger system is to climatologists the “apotheosis” of climate classifications (Robinson and Henderson-Sellers 1999, p. 121) and the “best representation of climate as an independent explanatory variable” in studies of economic development (Brown and Lall 2006) (see also Table 1 and Fig. 3 of Sachs 2001). In the Köppen–Geiger system, eastern China is classified as type Cf, a warm temperate fully humid climate, the same type as that of western Europe and the eastern United States. However, rainfall across eastern China is much more seasonal, largely as a result of the East Asian monsoon (e.g., Yihui and Chan 2005) (Fig. 3). For example, average monthly precipitation in June in northern Jiangxi Province, near the center of eastern China’s Cf region, is 5.5 times that for December, much greater than any ratio between two months in the northeastern United States or western Europe. Even more strikingly, the monthly average precipitation for July in Beijing (outside the Köppen–Geiger region of Cf climate) is 36 times that for December. Thus, although the Köppen–Geiger system is correct in categorizing the eastern United States and eastern China with regard to temperature and annual amount of precipitation, the much greater seasonality of precipitation in eastern China suggests a significantly more challenging climatic regime with regard to the sustenance of agriculture. Those challenges were in fact sufficient to lead to differences in culture, in that control of water and water resources seems to have led to a more collectively oriented rice-growing culture in southeastern China compared to more individualistic wheat-growing cultures to the north and elsewhere (Talhelm et al. 2014).
The more challenging nature of climate and agriculture in eastern China than in Europe may at least partly explain why China, despite having one of most long-lived and far-reaching “continental empires” (Reinhard 2011), deliberately chose not to develop an overseas empire and thus is not among the empires in Table 2. Ming Dynasty China demonstrated the technological capacity for overseas domination with the seven voyages to the Indian Ocean of an armada led by “Grand Director” Zheng He from 1405 to 1433. During those voyages, which Dreyer (2007) categorizes as “power projection,” Zheng's armada collected tributes, carried on commerce, carried out military actions to enforce Chinese foreign policy, and brought back the King of Ceylon to Beijing as a prisoner (Dreyer 2007). In the words of Zheng He's Mingshi biography, “Those [rulers and chieftains] who did not submit were pacified by force” (as translated by Dreyer 2007, p. 188). This series of voyages ended in 1433, probably for multiple reasons, but Dreyer (2007), like Levathes (1994, 177–180) before him and Hoffman (2015, p. 171) after, concluded that the fiscal burden of maintaining the armada led by Zheng was sufficiently great that further voyages were opposed by factions in the Ming court who saw greater need for funding elsewhere. The Chinese opening to overseas empire was thus deliberately closed during the decades in which Portugal, a much smaller nation, deliberately and successfully developed oceanic navigation and the capacity for empire (Russell 2000). That coincidence may also be linked to changing climate, in that the Little Ice Age led to less rainfall in eastern China with weakening of the East Asian monsoon (Zhou et al. 2011; Rehfeld et al. 2013) and thus stress on the Ming state (Brook 2010) at the same time that it allowed a wetter climate in western Iberia (Abrantes et al. 2005; Railsback et al. 2011; Bartels-Jónsdóttir et al. 2015).
2) Alfred W. Crosby’s neo-Europes and regions of regular moderate rainfall
Crosby (1993, 2–3) used the term “neo-Europe” to denote regions outside Europe in which the human population is dominantly of European descent [in parallel with the “settlement colonies” of Howe (2002), p. 74]. Crosby explicitly recognized four regions that account for more than 99% of the population of the five regions that he ultimately characterized as neo-Europes (Table 7). Those four regions are the four largest regions outside Europe that are characterized by regular moderate rainfall as defined in this paper (Fig. 2). Crosby noted that the climate of those four neo-Europes was “temperate,” a term pertaining largely to temperature, and he inferred that these regions favored European domesticated plants and animals. However, rainfall more clearly distinguishes those regions from other seeming candidates for extensive European settlement. For example, Crosby went on to discuss why South Africa, with its temperate climate, did not become a neo-Europe, and he concluded that it was a special case resulting from the exceptional persistence and reproduction of the native people there (Crosby 1993, p. 146). However, consideration of atmospheric precipitation does not require that South Africa be a special case, because its seasonal rainfall [both winter-dominated near Cape Town and summer-dominated through much of the rest of the country, as shown in Fig. 1 of Brook et al. (2015)] would predict that southern Africa would not be attractive to Europeans and their styles of agriculture and expectations of agricultural productivity.
3) The irrelevance of the Southern Hemisphere’s zone of regular moderate rainfall to human history
This paper has focused on the question of why affluent and powerful nations developed in tightly constrained regions in the Northern Hemisphere, but one might more usefully ask why advantaged nations did not arise in the Southern Hemisphere. Figure 1 provides an answer because, in the zone from 45° to 60°S symmetrical with the Northern Hemisphere’s zone of regular moderate precipitation, regular and moderate precipitation falls only on islands and continental extremities. That latitudinal zone in the Southern Hemisphere is the Southern Ocean, where plate tectonics has left virtually no land and thus little opportunity for humans to take advantage of a supportive hydroclimate. If Australia, India, and Africa had been passing through the latitudinal zone from 45° to 60°S when intelligent beings evolved, ready to develop agriculture and move toward industrialization, the distribution of affluent and powerful nations might have been very different than the one that developed as the result of Holocene climatic and plate tectonic configurations, and “Global South” (Dirlik 2007; H. Pagel et al. 2014, poster presentation) might signify something very different than what it has come to mean economically and geopolitically.
Historically, of the 13 nations that held geographically discontinuous multicontinental transoceanic empires, 12 overlap with regions of regular moderate precipitation, which are relatively scarce across Earth’s surface (Fig. 2). At present, economic development also shows a strong relationship to regular moderate precipitation in that, of the 20 nations with the greatest per capita GDP in 2015, 16 coincide with regions of regular moderate precipitation (Fig. 2, Table 5). These relationships are presumably rooted in the greater success (or lesser inhibition) of construction and maintenance of roads and structures, husbandry of livestock, and cultivation of crops, all of which in turned allowed the development of, and continue to support, industrialization and projection of geopolitical power. The linkage of geographically discontinuous multicontinental transoceanic empires to regular moderate rainfall not only provides at least a partial explanation of the origins of those empires but also helps explain why such empires were not ruled by China or nations of the Southern Hemisphere.
The manuscript was improved by the suggestions of Clayton Max Condit of the Idaho Science Center and of two reviewers.