Last modified: 2011-08-16
Abstract
Predicting the response of Earth’s climate to anthropogenic greenhouse gas emissions using climate models is a way of providing policy makers with the information they need to make society-wide decisions about the future climate change and its impacts on civilisation. Understanding the predictive ability of these climate models requires testing them against periods in Earth history where the climate was significantly different from today. The Miocene represents such a time period. To test the predictive ability of climate models the time period being studied needs to be fully understood, both in terms of its model boundary conditions (mountain height, continent distribution, ocean circulation and global vegetation) and in terms of its climate. By studying the global distribution of vegetation for geological time periods it is possible to provide a necessary boundary condition for palaeoclimate studies, a means to assess the ability of climate models and a greater understanding of global climate.
Focussing on the Middle to Late Miocene (15.97 – 5.33 Ma) a 617 palaeobotanical site database has been constructed, translating fossil vegetation into biomes. From the palaeobotanical database it can be seen that the Middle and Late Miocene were worlds both warmer and wetter than the pre-industrial era. During the Langhian (15.97 – 13.65 Ma) the world was considerably warmer than it has been since. Cool – temperate mixed forest occupied the high latitudes, warm – temperate mixed forest dominated the modern temperate latitudes and the tropics were characterised by extensive tropical evergreen broadleaf forest. The Seravallian (13.65 – 11.61 Ma) biome distribution reflects a cooling from the Langhian, with a reduction in the distribution of warm – temperate mixed forests and a change in the maximum poleward extent of biomes.
By the Tortonian (11.61 – 7.25 Ma) global cooling had further changed biome distribution. The northern high latitudes were now inhabited by cold evergreen needleleaf forests and the mid latitudes continued to dry out. The Messinian (7.25 – 5.33 Ma) saw further expansion of the cold evergreen needleleaf forests at the expense of the temperate biomes. Within the mid latitudes the warm – temperate mixed forests became regionally extinct in western North America and deserts were now present in the region of the modern Sahara and Taklimakan.