Climate Cycles and Cycles in Earth’s Orbit Cycles give predictability because they repeat at regular intervals. One of the major climate discoveries is that for the past 800,000 years, ice ages have repeated at more or less regular intervals roughly every 100,000 years and that these cycles match one of the cycles in the Earth’s orbit about the Sun. Earth’s orbit changes slowly over time as the Moon, Sun, and Jupiter tug on it. The orbital changes alter the amount and distribution of sunlight striking Earth. For example, at present, Earth is closest to the Sun on January 3 but 11,000 years ago, Earth was closest to the Sun in July and the North Hemisphere received 7% more sunlight per summer day than it does now. The critical factor that starts and ends ice ages appears to be the strength of the Sun in summer at high latitudes (about 65º) in the North Hemisphere because that is where the major ice sheets grow and shrink. When the summer Sun of the North Hemisphere is strong, winter snows melt entirely and ice sheets melt. Only when the summer Sun of the North Hemisphere is weak does weather in the summer remain cold enough so that the winter snows do not completely melt, and therefore can accumulate. The three cyclic variations of Earth’s orbit are: 1: Precession of the Equinoxes (Season when Earth is closest to the Sun) 2: Obliquity of the Ecliptic (Tilt of Earth’s Rotation Axis) 3: Eccentricity of Earth’s Orbit The next slides show sediments with cyclic variations due to the Ice Age cycles and an EKG showing cycles of the human heart.

1. Precession of the Equinoxes. As the Earth spins and circles the Sun it precesses or gyrates like a tilted top. As a result, the day Earth is nearest to the Sun slowly advances with respect to the equinoxes. Right now we are closest to the Sun on January 3, but in roughly 60 years it will be Jan 4. In about 5500 years Earth will be closest to the Sun on April 3, and 11,500 years from now it will be closest to the Sun on July 4. Precession has a complex cycle about 23,000 years long, but at least one simple climate impact--it made the Sahara a fertile grassland up to 5500 years ago! 2. Obliquity of the ecliptic. This is the fancy term for the tilt of the Earth’s axis of rotation. Right now the tilt is 23.47°, but it gets as small as 22° and as large as 24.5°, repeating with a cycle of about 41,000 years. The tilt is now decreasing and will reach a minimum about 10,000 years from now. The smaller the tilt, the smaller the differences between the seasons and the cooler the high latitudes in summer. 3. Eccentricity. Earth’s orbit is an ellipse that can be more circular or more eccentric. Eccentricity is how much the distance varies from average. The eccentricity of Earth’s orbit is now 1.67% because its distance from the Sun varies by 1.67% (2.5 million km) from the average of million km. Eccentricity varies from almost 0% to as much as 5%, repeating with two cycles, one about 108,000 years and the other, 412,000 years. It is now decreasing and will reach a minimum in about 25,000 years. Distance from the Sun also affects the length of the seasons. When Earth is closest to the Sun it moves fastest in its orbit, and the seasons are shorter. At present there are 186 days in the summer half of the year (March 21 to September 23) and only 179 days in the winter half when Earth is closest to the Sun. But 11,500 years from now, summer vacations in the North Hemisphere will be shorter.

Some sedimentary rocks have cycles of deposition, due to cycles of climate.

A normal EKG (electrocardiograph) for a 26 year-old male. This shows the complex but regular cycle of heartbeats. Ice age cycles are also complex but not quite so regular.

Record of the Vostok Ice Core including atmospheric CO 2, CH 4, 18 O (determined from trapped air bubbles in the ice) compared with solar Insolation in June at 65ºN latitude.

A 5.5 Million year record of 18 O of shells in a sediment core from the sea floor in the tropics. Because the temperature of tropical waters haven’t changed that much, this is essentially a record of the volume of the ice sheets on land. When the blue curve is lower (more enriched 18 O in the ocean) ice volume is larger. Note that ice volume began increasing about 3 million years ago and had high frequency variations. At first these variations repeated with rough cycles about 41,000 years but for the past 800,000 years, the cycles have slowed to about 100,000 years. The match of climate with a 100,000 year cycle is shown by the red dashed lines. It is certainly not perfect.

Eccentricity of Earth’s Orbit  100,000 and 400,000 year cycles

Obliquity of the Ecliptic = Tilt of the Earth’s Axis of Rotation  41,000 year cycle

Precession:  23,000 Year Cycle Polaris Vega Now 11,500 kY

11000 BPPRESENT

Past and Future Milankovitch Cycles We can predict past and future orbital parameters with great accuracy. ε is obliquity (axial tilt). e is eccentricity. ϖ is longitude of perihelion. esin( ϖ ) is the precession index, which together with obliquity, controls the seasonal cycle of insolation. Q AVG(day) is the calculated daily-averaged insolation at the top of the atmosphere, on the day of the summer solstice at 65 N latitude. Benthic forams and Vostok ice core show two distinct proxies for past global sea level and temperature, from ocean sediment and Antarctic ice respectively. Vertical gray line is current conditions, at 2 ky A.D.

Depth of Last Ice Age Post Ice Age Melting Ice Age Cycles and Summer Sunshine Insolation around the Summer Solstice at about 65º N latitude has been identified as a critical driver of Ice Ages. Ice Ages end when summer sunshine is large and cools to Ice Age conditions when summer sunshine is small. Summer in the Northern Hemisphere is now as cool as it will be for the next 50,000 years. Since it is apparently not cool enough to allow an ice age to start (possibly because we have added CO 2 and CH 4 ), it is unlikely that the next Ice Age will start for at least another 50,000 years. Would it be cool enough to have started an ice age now if we hadn’t increased CO 2 and CH 4 ? That is a tough question because the present minimum value of summer Insolation is nowhere near as low as 23,000 years ago. Next Chance for an Ice Age Right Now

When sunlight shines on ice, much is reflected but some is absorbed. After the absorbed heat warms the ice to the melting point it begins to melt the ice. Each extra 1 Wm -2 of absorbed sunlight will melt m of ice (with density, 900 kg m -3 ) per month. To estimate the time to melt an ice sheet 2 km thick assume that in the millennia after the last ice age that ended 20KBP, solar irradiance increased  20 W m -2 during the 5 spring and summer months (Apr – Aug). If the albedo of the dirty, melting ice was 50% then 10 W m -2 was used to melt the ice. Then the monthly melting rate was m and the annual melting rate was 5  m = 0.43 m yr -1. Absorbed Solar Irradiance Melts Ice Sheets How long should it take to melt the ice sheets?