Milancovitch cycles

Milancovitch Cycles explains variations in eccentricity, axial tilt, and precession of Earth. The climate changes caused by changes in the position of the Earth in comparison to the Sun. This explains the formation of ice ages.

1. Milankovitch Cycles and Earth’s Climate Change - By Chilukuri Lakshmi Sravani CC20MTECH11006

2. Overview Introduction -------------------------------------------------------3 Three Variations in Orbit of Earth around Sun ---------------4 Eccentricity --------------------------------------------------------5 Obliquity ----------------------------------------------------------8 Precession --------------------------------------------------------11 Ice Ages due to Milankovitch cycles -------------------------13 Stratigraphy ------------------------------------------------------14 Summary ---------------------------------------------------------15 2

3. Introduction The term “Milankovitch Cycles” is named after the Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he hypothesized that variations in eccentricity, axial tilt, and precession resulted in cyclical variation in the solar radiation reaching the Earth, and that this orbital forcing strongly influenced the Earth's climatic patterns. This theory explains Earth's long-term climate changes caused by changes in the position of the Earth in comparison to the Sun. This explained the ice ages occurring in the geological past of the Earth, as well as the climate changes on the Earth which can be expected in the future. Milutin Milanković , Wikipedia.org 3

4. Three Variations in Orbit of Earth around Sun Eccentricity- the change in shape of the orbit around the sun. Obliquity- changes in the angle that Earth's axis makes with the plane of Earth's orbit. Precession- the change in the direction of the Earth's axis of rotation Together, the periods of these orbital motions have become known as Milankovitch cycles. Image credit:  K. Cantner , AGI. 4

5. Eccentricity Eccentricity is a term used to describe the shape of Earth's orbit around the sun. The variation of Earth's orbit around the sun ranges from an almost exact circle (eccentricity = 0.0034) to a slightly elongated shape (eccentricity = 0.058). The current eccentricity of Earth is 0.0167. The eccentricity changes on a cycle taking approximately 1,00,000 years. • Perihelion - when the Earth is closest to the Sun (usually happens in January) • Aphelion - when the Earth is furthest from the Sun (usually happens in June) https://www.youtube.com/watch?v=X1XrtqubGPE https://www.quantamagazine.org/how-earths-climate-changes-naturally-and-why-things-are-different-now-20200721/ 5

6. Solar Insolation. The average solar irradiance received by earth on the top of its atmosphere is 1360 W/m2. Insolation on circular area received from sun is πR^2. This insolation is distributed over the surface of Earth which is 4 πR^2, where R is Radius of Earth. So, the average solar insolation at the top of the atmosphere over its entire spherical surface is 340 W/m2. The impact of the eccentricity includes change in the amount of solar energy from perihelion (around January 3) to aphelion (around July 4). The numbers for the eccentricity are not constant. The eccentricity changes in time. The period varies between 1,00,000 years to 4,00,000 years. https://sureshemre.wordpress.com/2014/05/03/difference-between-the-precession-of-the-equinoxes-and-the-precession-of-earths-axis/ 6

7. Perihelion For the minimum value of the eccentricity e the amount of solar radiation received by the Earth (insolation) is 3 % less than at present, and for the maximum value of e the radiation is 9 % greater than at present. Aphelion For the minimum value of the eccentricity e the amount of solar radiation received by the Earth (insolation) is 3 % greater than at present, and for the maximum value of e the radiation is 8 % less. The difference of the insolation at perihelion and at aphelion is 7 % at present, and 17 % for the maximum eccentricity. Effects of Eccentricity on Solar Insolation. https://www.researchgate.net/figure/Eccentricity-of-earths-orbit_fig2_328773252 7

8. Obliquity Obliquity is the variation of the tilt of the earth’s axis away from the orbital plane. Currently, the axis of rotation for the earth is tilted at 23.5°. However, this value changes from a minimum of 22.5° to a maximum of 24.5°. Earth takes 41,000 years to complete one cycle. https://sureshemre.wordpress.com/2014/05/03/difference-between-the-precession-of-the-equinoxes-and-the-precession-of-earths-axis/ https://www.quantamagazine.org/how-earths-climate-changes-naturally-and-why-things-are-different-now-20200721/ 8

9. The axial tilt causes seasons because the tilt determines the amount of solar energy deposited per square-meter on a given location on the Earth’s surface, especially in locations away from the equator. The axial tilt and the solar insolation are directly related. The more tilt means more severe seasons - warmer summers and colder winters; less tilt means less severe seasons - cooler summers and milder winters. https://www.researchgate.net/figure/The-present-day-orbit-of-the-Earth-around-the-Sun-the-beginning-and-length-of-the_fig1_291797677 9

10. Beam Spreading The energy present per square meter of sunlight spreads over a larger area as the angle of incidence increases. Thus the same amount of incident energy is spread over a larger area of the surface near the poles than at the equator. Attenuation T he distance travelled by rays of the sun to hit the earth’s surface near the poles is nearly 2.5 times that of the distance travelled at the equator. This results in increases attenuation of the solar radiation near the poles. L atitude has a very large effect on the mean temperature of various locations of the earth. The reason why this happens is due to two factors: Dennis L.Hartmann , Global Physical Climatology, Edition-2, Chapter 2 - The Global Energy Balance, 2016, Pages 25-48 10

11. Precession Precession is the change in orientation of the Earth's rotational axis. Earth’s spin axis traces a conical area in space around the rotation axis of the solar system.  This motion (Earth’s Precession) is similar to the precession of a spinning top. The precession of Earth’s axis is a  clockwise  motion. Earth’s Precession is very slow. It takes about 26 thousand years for the Earth’s axis to point to the same location in space. https://sureshemre.wordpress.com/2014/05/03/difference-between-the-precession-of-the-equinoxes-and-the-precession-of-earths-axis/ https://www.quantamagazine.org/how-earths-climate-changes-naturally-and-why-things-are-different-now-20200721/ 11

12. An extra day is added to February in each year divisible by four. The leap year system gets an extra tweak by excluding century years not divisible by 400 . If it weren’t for this leap year fix, June 21, 15014, would indeed become winter in the Northern Hemisphere. Instead, Northern Hemisphere residents will still be welcoming summer on that date. https://astronomy.com/magazine/ask-astro/2014/10/shifting-seasons 12

13. Ice Ages due to Milankovitch cycles http://www.zo.utexas.edu/courses/THOC/Milankovitch_Cycles.html An ice age is triggered when summer temperatures in the northern hemisphere fail to rise above freezing for years. This means that winter snowfall doesn't melt, but instead builds up, compresses and over time starts to compact, or glaciate, into ice sheets. The onset of an ice age is related to the Milankovitch cycles - where regular changes in the Earth's tilt and orbit combine to affect which areas on Earth get more or less solar radiation. When all these factors align so the northern hemisphere gets less solar radiation in summer, an ice age starts. Ice age have been happening roughly every 100,000 years - around 90,000 years of ice age followed by a roughly 10,000 year interglacial warm period. At the moment the Earth is in an interglacial period - a short warmer period between glacial (or ice age) periods. 13

14. Stratigraphy Vital to idea for stratigraphy is the amount of insolation(incoming solar radiation) at 65°N, a bit south of the Arctic Circle. At that latitude, insolation can vary seasonally by 25%. Reduction in summer insolation allow some winter ice to survive. ( i ) The oxygen isotopic(O18) composition of planktonic foraminifera (ii) Ts, an estimate of summer sea-surface temperatures at the core site, derived from a statistical analysis of radiolarian assemblages (iii) percentage of Cy- cladophora davisiana , the relative abundance of a radiolarian species not used in the estimation of Ts. It turns out these cycles are quite similar to cycles in the orbital parameter of the Earth around the Sun. Hays, J., Variations in the Earth's Orbit: Pacemaker of the Ice Ages , Vol-194, pp. 1121-1132 1977/01/01, Science (New York, N.Y.) High 18 O  low T Low 18 O  high T 14

15. Summary Past changes in climate have often been triggered by changes in orbital characteristics known as Milankovitch Cycles . Variations in earth’s eccentricity , obliquity , and precession alter the seasonality of solar radiation. It is the seasonality and location of insolation that impact the contrast between the seasons. • Low eccentricity • Low obliquity • Large Earth-Sun Distance in NH Summer Net Effect: Less seasonal contrast • High eccentricity • High obliquity (tilt) • Small Earth-Sun Distance in NH Summer Net Effect: More seasonal contrast Glacial Inter-Glacial globalchange.umich.edu/Vostok, M ilankovitch Cycles - Global Change 15

16. Thank you!! YouTube Video https://www.youtube.com/watch?v=iA788usYNWA 16