Decadal surface temperatures averaged over the Tibetan Plateau are derived for the last two millennia from ice core and tree-ring reconstructed surface temperatures at various locations on the Plateau. The temperature variations show that 1) the late 20th century is the warmest period in the past two millennia, 2) a persistent warming in Tibetan Plateau since the 17th century coincides with intensification of the Indian summer monsoon, and most importantly, 3) in the two millennia, epochs of cool/warm temperatures in the Tibetan Plateau coincided with weak/strong Indian summer monsoon. Comparisons between monsoon intensity variations and temperature fluctuations in other parts of the Eurasian continent suggest that the surface heating over the Tibetan Plateau has played a dominant role in changes of the Indian summer monsoon intensity.