Milankovitch Cycles

Milankovitch Cycles Linda Forbes – Unit A1

Climate Change • Building for the future • What sort of future could we be planning for? • How climate has varied in the past • Humankind’s effect on Earth’s patterns

Climate Change • In the beginning there were Ice Ages • Every 100,000 years (or so) the glaciers melted • Rising sea levels and warming of the climate for 10, 000 years • So how did this happen?

Milankovitch Cycles • Milankovitch (1879-1958) explained advance and retreat of polar ice caps were caused by changes in the Earth’s orbital cycles • These cycles influence amount of solar radiation striking different parts of the Earth at different times of year

History • But it wasn’t just his idea • Adhemar (1842) and Croll (1875) suspected it • Deep ice and ocean cores support theory

Earthly wobbles • It’s done by precession, eccentricity, and axial tilt of the Earth's orbit • Each has a different time period (think of biorhythms!) • Combined together have noticeable effect

History • Work by Hays, Imbrie and Shackleton • Paper "Variations in the earth’s orbit: pacemaker of the ice ages" in 1976

Precession • Earth's axis completes one full cycle of precession every 26,000 years • Results from nonspherical shape and equatorial bulge •

http://www.homepage.montana.edu/~geol445/hyper glac/time1/milankov.htm

Eccentricity • The elliptical orbit rotates, more slowly, leading to a 100,000 year cycle

•

http://www.homepage.montana.edu/~geol445/hyper glac/time1/milankov.htm

Axial tilt • Angle between Earth's rotational axis and the normal to plane of its orbit changes from 21.5 degrees to 24.5 degrees and back again on a 41,000 year cycle

http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm

Insolation (INcident SOLar radiATION) • Influence of cycles calculated by Berger (1991) and Laskar (1993) • Berger's solution for 65° N from present day to one million years ago. http://deschutes.gso.uri.edu/~rutherfo/milankovitch.html

So where are we now? • Northern Hemisphere • Peak summer insolation 9,000 years ago • Ice sheets melted • Less solar radiation since then in summers

And what happens next? • Next Ice Age was rumoured in 1960s • Turned out to be a false alarm • Carbon dioxide emissions changed pattern permanently?

Climate Change • Building for the future • What sort of future could we be planning for? • Ice Age or Dot Age?