Climate model sensitivity to changes in Miocene paleotopography.

The Middle Miocene Climatic Optimum (MMCO), which occurred between 17 and 15 Ma, was the last of a series of warming events to have punctuated the Cenozoic and is the warmest the planet has been since 35 Ma (∼6°C warmer than present at middle latitudes). Paradoxically, CO2 concentrations during the MMCO are reported at present-day values or less and an explanation for warming has yet to be established. We test the sensitivity of version three of the Community Atmosphere Model (CAM) and Community Land Model (CLM) to changes in Middle Miocene paleotopography using prescribed sea-surface temperatures. We create four separate global paleotopography datasets based on altered Andean and Tibetan Plateau elevations as well as changes to global sea-level. We find that the largest warming is achieved with a lowering of the Tibetan Plateau from 4700 to 2600 m, resulting in a local temperature increase of up to 9°C—consistent with the modern-day lapse rate measured in China—and a global mean ground temperature increase of 0.5°C. An additional effect of this lowering is to weaken the summer and winter monsoons, and we discuss implications such changes may have for coupled ocean-atmosphere simulations. We conclude that while changes to topography significantly affect local temperatures, they do little to reconcile the disagreement between temperature and CO2 measurements derived from the geological record. [ABSTRACT FROM AUTHOR]