After three hundred years of impressive gains in wealth and population, Europe's economy began to slow around 1300. Several factors accounted for the decline. One the most important, though perhaps the least dramatic to relate, was a shift in climate. The remarkably fair weather of the twelfth and thirteenth centuries took a decided turn for the worse in the fourteenth. Chronicler's comments, tree-ring examination, and pollen analysis all indicate that over the course of the fourteenth century Europe's average annual temperature declined approximately two degrees Celsius – which may sound like very little at first, but if one considers current projections about the possible effects of global warming, in which the average annual temperature shift is only one degree Celsius, a rather different impression emerges. As the temperature dropped, shortening the summer growing season and affecting the resilience of certain vegetable species, the wind and rain increased. This meant that crop yields declined precipitously and the agricultural economy began to contract. As food supplies dwindled, costs rose accordingly and cut into the amount of capital that people had available for other purchases or investments. This in turn added to the gradual construction of the commercial economy. Just as significant were changes in the geopolitics of the Mediterranean world. The decline of the Byzantine Empire, which had dominated the eastern Mediterranean, meant the interruption or trade routes to central and eastern Asia. The rise of new political powers signaled a new era in Mediterranean connections, one in which religious loyalty and ethnic fidelity mattered more than commercial ties. Consequently the movement of goods and services between east and west began to slow. European interest in circumnavigating Africa and exploring westward into the Atlantic Ocean, in fact, originated in the desire to avoid the roadblock in the eastern Mediterranean and to tap directly into the trade with eastern Asia that had long sustained Europe's economic growth. A more immediate cause of the sputtering economy was an observable absence: since the eleventh century there had been few significant changes in the technology of agriculture. Developments like the wheeled plow, the rotation of crops, and the use of natural fertilizer that had made possible the agricultural revolution of the past two hundred years had had no follow-up. Farming was still conducted in 1300 roughly the same way it had been done in 1100, but with a considerably larger population to feed, there was little surplus left to generate fresh capital. As a consequence, food production fell perilously close to subsistence level. Although the failure of agriculture to keep up with the growing population did not become a crisis until the fourteenth century, clear signs of the problem had already emerged by the middle of the thirteenth century, when occasionally low yields due to bad weather or social disruption revealed how perilous the balance between Europe's population and its food supply had become. Apart from territories beset by war, the tentativeness of the food supply became evident first on the farmlands most recently brought under cultivation during the economic depression of the twelfth century. The less established farmers of these lands frequently did not have the means to survive successive poor harvests. Tenant farmers unable to pay their rents thus began to slip into debt, and landlords who depended on rents for their income began to rely increasingly on urban financiers for credit. Even whole governments became entangled in the credit crisis, England being the most notable example. The cycle of indebtedness was hardly inevitable, but the string of bank failures and commercial collapses in the first half of the fourteenth century was striking. The famed Bardi and Peruzzi banks of Florence (the two largest financial houses of Europe) collapsed spectacularly in the 1340's. They were soon followed by the Riccardi bank of Lucca, whose massive loans had kept the English government afloat for years. Many more houses collapsed in turn. An important demographic trend resulted from and contributed to the economic malaise: large-scale migration of rural populations into the cities. Europe's overall population growth from 1050 to 1300 had been primarily due to an increase in the number of rural folk. But as economic forces made agrarian life more perilous around 1300, hard-pressed farmers and their families began to migrate to the cities in large numbers in search of work. Many cities doubled in size, and some even tripled, over the course of just one or two generations. Few were capable of absorbing such large numbers of people.