Vanished sea leaves climatic legacy.

A map of Asia during the Oligocene period, 30 million years ago, is a mix of familiar and foreign, like a picture of one's face taken at birth. The broad cartographic features are recognizable, but the heart of the continent holds a strange sea, the Paratethys, which stretches from the Mediterranean to Siberia.

Since the Oligocene, the Paratethys has shriveled, leaving tiny geographic remnants that include the Black, Caspian, and Aral Seas. The disappearance of that once great body of water has also made a lasting mark on the Asian climate, according to new computer simulations.

Past studies have tended to link the emergence of today's climate in Asia to the birth of the Tibetan Plateau during the last 30 million years. In this scenario, the rise of the plateau redirected wind patterns, drying out central Asia and strengthening the south Asian monsoon. These studies ignored the role of the shrinking Paratethys.

Now, French researchers have assessed the various factors by using a complex atmospheric computer model to simulate the Asian climate in the Oligocene period and in the present. They tested the influence of the Paratethys and the Tibetan Plateau separately and together.

In the April 24 Nature, Gilles Ramstein of the Laboratoire de Modelisation du Climat et de I'Environnement in Gif-sur-Yvette and his colleagues report that the shrinking sea played just as important a role as the rise of Tibet. As the sea disappeared, it removed a large source of water from central Asia, helping both to dry the region and to enhance temperature extremes between summer and winter. At the same time, it intensified Asia's monsoon rains.