What Causes an Ice Age?

What changes the energy budget? INCOMING ENERGY solar output cycles sunspots Earth’s tilt shape of Earth’s orbit OUTGOING ENERGY reflection from surface reflection from clouds absorption by ground Greenhouse effect water & air circulation

Solar Activity Cycles Our sun goes through cycles of increased and decreased output every 11 years Sunspots appear and disappear in response to the changing activity

Maunder Minimum Edward Maunder noticed that in the 17th cent. there was a long time with no sunspots (1645 – 1715) “Little Ice Age” The Little Ice Age was caused by the cooling effect of massive volcanic eruptions, and sustained by changes in Arctic ice cover, scientists conclude

Milankovic Cycles Milutin Milankovic studied the shape of Earth’s orbit over time, and how that could affect the amount of sunlight that reaches us Three variables Eccentricity (degree of roundness) Obliquity (how far the poles are tilted) Precession (which way the poles point) www.youtube.com/watch?v=gc-ZJKayhWo

Eccentricity Earth’s orbit is nearly circular, but not quite… therefore seasons are not equal quarters Autumn 89.8 days Summer 93.6 days Winter 89.0 days Spring 92.8 days

As the orbit gets more oval, the seasons become even more unequal

Obliquity Earth’s tilt determines which regions get direct sunlight, and which get their light at a slant Arctic / Antarctic Circle Long periods of weak light in summer Long periods of dark in winter Current tilt = 23.5° more tilt, ice caps and tundra expand less tilt, ice caps and tundra retreat

Precession Like a spinning top, Earth’s axis slowly rotates AD 2015: north star is Polaris 3000 BC: north star was a Draconis 12,000 BC: north star was Vega

Why do we care? Tilt determines what season it is TOWARD SUN = SUMMER AWAY FROM SUN = WINTER FALL WINTER SUMMER SPRING FALL WINTER SUMMER SPRING

Summary:

Cycles overlap to make Earth really cold every 40,000 years or so, and super-cold every 400,000 years

Once solar energy gets here, what happens next? Reflected by atmosphere Reflected by ground surface Absorbed by ground surface Absorbed by atmosphere

Reflection from Ice or Clouds If ice caps shrink they reflect less sunlight, and Earth warms up If ice caps grow, they reflect more sunlight, and cool the Earth

Absorption by Atmospheric Gases Some gases hold in the heat Water Vapor Carbon Dioxide Nitrous Oxide Methane Trapped heat keeps our planet warm

Blue waters are very cold Circulation Patterns Global ocean currents carry heat from the equator to the poles Orange & Yellow If currents get out of balance, the poles get colder and colder Blue waters are very cold

Global Wind Patterns Polar, Tropical, and Temperate cells circulate the air Rising air at 0° & 60° makes stormy LO pressure regions Falling air at 30° N & S creates HI pressure and dry arid climates

the Jet Streams Fast winds form at high altitude where air cells come together Usually around 30 and 60 latitude, but they can drift north or south

When the jet stream is shifted by a storm, cold polar winds push farther south and warm tropical air comes farther north