Researchers studying the rings of ancient trees in mountainous central Mongolia think they may have gotten at the mystery of how small bands of nomadic Mongol horsemen united to conquer much of the world within a span of decades, 800 years ago. The rise of the great leader Genghis Khan and the start of the largest contiguous empire in human history was propelled by a temporary run of nice weather.The rings show that exactly when the empire rose, the normally cold, arid steppes of central Asia saw their mildest, wettest weather in more than 1,000 years.