Global cooling continued until there was a major reversal from cooling to warming indicated in the Southern Ocean at around 42–41 million years ago. Oxygen isotope analysis showed a large negative change in the proportion of heavier oxygen isotopes to lighter oxygen isotopes, which indicates an increase in global temperatures. This warming event is known as the Middle Eocene Climatic Optimum. The warming is considered to be primarily due to carbon dioxide increases, because carbon isotope signatures rule out major methane release during this short-term warming. The increase in atmospheric carbon dioxide is considered to be due to increased seafloor spreading rates between Australia and Antarctica and increased amounts of volcanism in the region. Another possible cause of atmospheric carbon dioxide increase could have been a sudden increase due to metamorphic release during the Himalayan orogeny; however, data on the exact timing of metamorphic release of atmospheric carbon dioxide is not well resolved in the data. Recent studies have mentioned, however, that the removal of the ocean between Asia and India could have released significant amounts of carbon dioxide. This warming is short lived, as benthic oxygen isotope records indicate a return to cooling at ~40 million years ago.