The impact of astronomical forcing on the Late-Devonian greenhouse climate

1. The impact of astronomical forcing on the Late-Devonian greenhouse climate DE VLEESCHOUWER, David 1 ; CRUCIFIX, Michel²; BOUNCEUR, Nabila²; CLAEYS, Philippe 1 1Earth System Sciences, VrijeUniversiteitBrussel, Belgium; ² Centre de recherche sur la Terre et le climat Georges Lemaître, Earth and Life Institute, Université catholique de Louvain, Belgium DJF MAM JJA SON Precession and Obliquity? Like a spinning top, the Earth's orbit wobbles so that over the course of a precessional cycle, the North Pole traces a circle in space. This wobble causes the precession of the equinoxes. Earth's axial tilt or obliquity varies from 24.5 degrees to 22.1 degrees over the course of a 41,000-year cycle. The current angle is 23.4 degrees. Changes in axial tilt affect the distribution of solar radiation received at the earth's surface. When the angle of tilt is low, polar regions receive less insolation. When the tilt is greater, the polar regions receive more insolation during the course of a year. Output (obliquity = 23.5°; eccentricity = 0) Temperature DJF MAM JJA SON Methodology Fixed input parameters Precipitation DJF MAM JJA SON Palaeogeography Climate Simulator HadSM3 General Circulation Model (GCM) on a 96 x 73 grid. The atmospheric model is coupled to a simpel “slab ocean” rather than the full dynamic ocean because the morphology of the Devonian oceanic basins is unsure. NW-Europe General Circulation DJF Vegetation distribution Precession: MAX vs. MIN After Köppen clasification of climates under moderate astronomical forcing. Variable input parameters Experimental design 31 different astronomical configurations, i.e. different combinations of obliquity and precession (and eccentricity). JJA Obliquity max. pCO2 2180 ppm (7.8 x higher than pre-industrial pCO2) Precession max. Moderate forcing Other parameters Different Soil parameters Hydraulic conductivity, heat capacity, soil albedo, moisture content at saturation,... Different Vegetation parameters Canopy height, infiltration factor, root depth, albedo, resistance to evaporation, ... Precession min. Negative response to forcing in DJF in Euramerica Cooler dry season (JJA) in Euramerica Strongest response in Gondwana Obliquity min. Taken from closest modern analogue DJF MAM Climate Sensitivity How does the climate respond to astronomical forcing on the global scale? How does the climate respond to astronomical forcing at the palaeolocation of NW Europe? • Late-Devonian global mean annual temperature lies between 15-23°C (Present-day: 14°C). Mean annual precipitation between 83-101 mm/month (Present-day: 64 mm/month). • Global mean annual temperature and precipitationrespond almost identically to astronomical forcing • Coldest and driest climates occur during minimal obliquity and eccentricity. And slightly positive e sin(ω) values. Under these circumstances, the Earth is in its aphelion during austral winter (JJA), allowing for the most severe Gondwanan winters and for the growth of a thick and extensive snow cover, which can only be eliminated slowly in the subsequent spring and summer. • Precipitation during the wet season (DJF) at the paleolocation of Belgium is very intense (109-270 mm/month). • Precession is by far the mostimportant forcing parameter. • For the summer wet season (DJF), precipitation intensity at a precession maximum can be almost the threefold of precipitation intensity at a precission minimum. • Temperature shows a quadratic responseto precessional forcing. • In DJF, insolation in this region is minimal during a precession minimum. Still, temperatures seem to increase when moving towards the most negative values along the e sin(ω) -axis. This pattern is only partly due to the global response to astronomical forcing. More importantly, it is also enhanced by a less dense cloud cover (and lower precipitation intensity) during a precession minimum. In that case, direct incoming shortwave radiation is enhanced, and a consequent increase in surface temperature is generated. JJA SON Southward shift of the ITCZ during all seasons More intense wet season (DJF) in Euramerica Obliquity: MAX vs. MIN DJF MAM JJA SON Warmer climates during an obliquity max. Strongest response at the poles Sea ice formation is a strong positive feedback mechansim DJF MAM JJA SON A much warmer North Pole causes a northward ITCZ shift during all seasons, except DJF.