climate and geography: Asia and the Pacific 223 tains on its southern edge and run northward into the Arctic and Pacific oceans From west to east these rivers are the Ob, the Yenisei, the Lena, and the Amur The northern fringe of Siberia is tundra, sparsely inhabited by human beings South of this is a wide belt of ancient coniferous and deciduous forest Agriculture is difficult in Siberia for several reasons The summers are not particularly warm, and the winters are the coldest of any large landmass that is not actually glaciated (such as Antarctica or Greenland) Because of this, much of the land has permafrost at a depth of less than a yard; these lands are forested only by the Siberian larch, which has specially evolved a shallow root system to adapt to this condition Average rainfall is less than 20 inches (except in the Kamchatka Peninsula at the extreme eastern edge of Siberia, which is a temperate rain forest) Thus Siberia was inhabited in medieval times by tribal peoples who lived by hunting and gathering in the forest with only very limited agriculture The tribes were monarchical and loosely federated under the khan of Sibir, who submitted to the Mongols in the 13th century, but these arrangements probably made little practical difference in the lives of the widely scattered families and clans Inner Asia Inner Asia extends south from Siberia in the west as far as the chain of mountain ranges that separate the Indian Subcontinent from Asia proper and in the east to the fertile plains of China It includes the modern territories of Mongolia, western China (including Tibet), the former Soviet central Asian republics, Afghanistan, Kashmir, and northern Pakistan The climate varies from steppe to desert, probably with ever-increasing desertification throughout the Middle Ages resulting from natural climate changes and human activity (deforestation) The driest region is the Gobi Desert in Mongolia Farther west are a number of large, shallow freshwater lakes (Lake Balkash, the Aral Sea, and at the extreme western limit the Caspian Sea) These lakes are fed by large rivers flowing from the southern mountains, including the Amu Darya, the Syr Darya, and the Hari Rud Tibet is the southernmost part of inner Asia At the beginning of the Middle Ages (629–842), the Tibetan monarchy dominated inner Asia and even controlled the Chinese western capital of Xian But Tibetan power was broken through civil war, and Islamic invaders and Chinese empires divided the area between themselves In the 13th century conquerors from the northern extreme of inner Asia, the Mongols, unified virtually the whole Asian continent under their rule The only practical way of life in the grasslands and deserts of inner Asia is nomadic horse breeding Local tribes lived by controlling large numbers of horses through a continuous wandering of many hundred or thousands of miles each year The nomadic way of life was so different from that of settled agricultural civilizations like China and western Europe that, once he had conquered northern China, the Mongol ruler Genghis Khan considered massacring the entire population of farmers because he could not imagine what use they served in society Trained by their way of life on the steppe, the populations of inner Asia produced the finest cavalrymen in the world The steppe tribes usually had little trouble attacking and overcoming more settled populations when they wished The Great Wall of China was built and maintained as a not entirely successful means of containing them Iranian horsemen from inner Asia in the first millennium b.c.e had conquered the Near East and southern Russia and occasionally raided China But the stirrup was developed in this area during the early Middle Ages, making nomadic life on horseback much easier and cavalrymen still more effective as soldiers Thereafter, migrations of peoples from this area, such as the Huns, Magyars and Turks, led to the collapse of the western Roman Empire, the general collapse of civilization in western Europe known as the Viking Age (850–1500), the collapse of the Byzantine Empire (1453), and the conquest of much of eastern Europe by the Turks The last great wave of invading tribesmen from inner Asia consisted of the Mongols under Genghis Khan (ca 1162–1127 b.c.e.) Genghis Khan and his immediate successors conquered an empire that endured throughout the later Middle Ages and stretched from China to include all of inner Asia, Iran, the Near East as far as Baghdad, European Russia, and India Except for a call from the central Mongol government in China to turn resources inward, Mongol forces that were preparing to attack western Europe and had probed as far as the border of the Holy Roman Empire (1241–42) may well have succeeded, changing the entire course of world history The Mongol Empire was essentially a unification of the various states connected by trade along the Silk Road India, protected by its mountainous borders, had long been spared invasion from the inner Asian steppe, but in the 14th century Tamerlane (1336–1405), a Mongol nobleman, conquered northern India, and his successors in the Mongol Dynasty later overran nearly the entire subcontinent From late in the first millennium b.c.e a trade network developed in inner Asia that is now usually known as the Silk Road The name derives from its being the main way that silk reached western Europe from China It was a caravan trade between Xian, the western capital of China, and Roman or Byzantine port cities on the Mediterranean Desert tribesmen brought huge caravans of camels and other pack animals across two principal routes: one all the way overland across the whole of inner Asia to Iran and the second to ports in