The fifth and early sixth centuries saw a sharp economic decline in the Afro-Eurasian world-system. The Silk Roads towards Afghanistan and northwestern India were shut as of the end of the fifth century. This situation favored the development of maritime routes between China and the Persian Gulf, although trade activity remained limited in the Indian Ocean at the beginning of the following century. The sixth century, however, saw a revival in trade within the Eurasian sphere. It was marked by the founding of a Turkish Empire in 552 in Central Asia – in this empire, the western khānate controlled Sogdiana – and the reunification of China by the Sui dynasty in 589. During the third century, the collapse of the Han and the downturn of the Roman Empire had triggered a global recession of the world-system; the reappearance of a Chinese empire under the Sui, succeeded by the Tang in the early eighth century, relaunched a phase of overall growth. This upturn coincided with a period of climatic changes that probably played a major role: milder conditions prevailed and rainfall increased in northern China and part of Central Asia.