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

DE VLEESCHOUWER, David 1 ; CRUCIFIX, Michel²; BOUNCEUR, Nabila²; CLAEYS, Philippe

11Earth System Sciences, Vrije Universiteit Brussel, Belgium; ² Centre de recherche sur la Terre et le climat Georges Lemaître, Earth and Life Institute, Université catholique de Louvain, Belgium

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.

Precession and Obliquity?

DJF MAM

JJA SON

DJF MAM

JJA SON

Precession: MAX vs. MIN

Negative response to forcing in DJF in EuramericaCooler dry season (JJA) in Euramerica Strongest response in Gondwana

Southward shift of the ITCZ during all seasonsMore intense wet season (DJF) in Euramerica

DJF MAM

JJA SON

Obliquity: MAX vs. MIN

A much warmer North Pole causes a northward ITCZ shift during all seasons, except DJF.

DJF MAM

JJA SON

Warmer climates during an obliquity max. Strongest response at the polesSea ice formation is a strong positive feedback mechansim

Climate SensitivityHow does the climate respond to astronomical

forcing at the palaeolocation of NW Europe?•Precipitation during the wet season (DJF) at the paleolocation of

Belgium is very intense (109-270 mm/month).

•Precession is by far the most important 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 response to 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.

How does the climate respond to astronomical forcing on the global scale?

•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 precipitation respond 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.

Palaeogeography

Vegetation distributionAfter Köppen clasification of climates under moderate astronomical forcing.

pCO22180 ppm (7.8 x higher than pre-industrial pCO2)

Other parametersDifferent 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, ...

Taken from closest modern analogue

Climate SimulatorHadSM3

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.

Experimental design31 different astronomical configurations, i.e. different combinations of obliquity and precession (and eccentricity).

Precession min.

Obliquity min.

Precession max.

Obliquity max.

MethodologyFixed input parameters

Variable input parameters

NW-Europe

Moderate forcing

Out

put

(obl

iqui

ty =

23.

5°; e

ccen

tric

ity =

0)

TemperatureDJF MAM JJA SON

Precipitation

DJF MAM JJA SON

General Circulation

JJA

DJF