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Abstract:
During the Devonian period (419 to 359 million years ago), life on Earth witnessed decisive evolutionary break-throughs, most prominently the colonisation of land by vascular plants and vertebrates. At the same time, it is also a period of major marine extinction events coinciding with marked changes in climate. There is limited knowledge about the causes of these changes and their interactions. It is therefore instructive to explore systematically how the Devonian climate system responds to changes in critical boundary conditions. Here we use coupled climate-model simulations to investigate separately the influence of changes in orbital parameters, continental configuration and vegetation cover on the Devonian climate. Variations of Earth's orbital parameters affect the Devonian climate system, with the warmest climate states at high obliquity and high eccentricity, but the amplitude of global temperature differences is smaller than suggested by an earlier study based on an uncoupled atmosphere model. The prevailing mode of climate variability on decadal to centennial timescales relates to surface air temperature fluctuations in high northern latitudes which are mediated by coupled oscillations involving sea-ice cover, ocean convection and a regional overturning circulation in the Arctic. Furthermore, we find only a small biogeophysical effect of changes in vegetation cover on global climate during the Devonian, and the impact of changes in continental configuration is small as well. Assuming decreasing atmospheric carbon dioxide concentrations throughout the Devonian, we then set up model runs representing the Early, Middle and Late Devonian. Comparing the simulations for these timeslices, the temperature evolution is dominated by the strong decrease in atmospheric carbon dioxide. In particular, the albedo change due to the in- crease in land vegetation alone cannot explain the temperature rise found in Late Devonian proxy data which remains difficult to reconcile with reconstructed falling carbon-dioxide levels. Simulated temperatures are significantly lower than estimates based on oxygen-isotope ratios, suggesting a lower δ18O ratio of Devonian seawater.
