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Climate Change

some astrophysical perspectives

Earth’s Changing Orbit • Earth’s axis precesses

Period = 26,000 yr

• Earth’s axis becomes more or less tilted Period = 40,000 yr

• Earth’s orbit becomes more or less elliptical Period = 100,000 yr

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We’re not certain how to combine the cycles, but there seems to be a tie between the combination and the regular cycle of hot and cool periods

Milankovitch Cycles3

Sedimentary layers in Welsh coast show variations every ~100,000 yr (from about 300 million years ago)

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Sunspots• Cooler regions in

Sun’s photosphere—still bright, but look dark by contrast!

• Huge plasma storms on Sun’s surface.

• Regions of intense magnetic fields.

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The Sun’s Magnetic Field• Magnetic fields apply a force on

charged particles that pushes them along the direction of the field lines.

• The charged particles in the ionized gas also push back on the magnetic field, altering its shape.

• Magnetic fields appear to play an important role in the “storms” we see on the Sun’s surface.

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Sunspot Structure

• The strong magnetic field should push the sunspot apart.

• Like a storm on Earth, the cool region draws in gas, which drags along magnetic field.

• The strong, tangled magnetic field blocks gas from rising from below

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Solar Prominences

Hot plasma can be ejected along the magnetic fields where they are strong.

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Prominence

The plasma moves along the magnetic field lines, and may get ejected into space (coronal mass ejection)

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Solar Flares

In regions of strong magnetic fields, sudden bursts of energy, sending out a pulse of energy and a spray of hot gas.

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Solar Wind

Plasma from Sun interacts with Earth’s

magnetic field to produce Aurora

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The Solar Cycle

• The number of sunspots goes up and down about once every 11 years on average.

• After each sunspot maximum, the Sun’s magnetic field reverses.

• Understanding the Sun may help us understand the Earth’s magnetic field.

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Magnetic Reversals• Geologists find that the Earth’s magnetic field

reverses once every ~250,000 years based on the magnetic field frozen in volcanic flows.

• We are overdue for a reversal—it’s been 800,000 years since the last reversal!

• Since 1900 Earth’s magnetic field is ~10% weaker and has shifted position by ~10˚

• During the time when Earth’s field reverses, we will not be well protected from cosmic rays.

• Cosmic rays may affect climate (cloud nuclei)

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Long Term Changes in the Sun• You might think that the Sun is darker when it

has more sunspots, but when it is more active it generates more power overall.

• This appears to affect Earth’s climate.

• In the 1600s there was a period with very few sunspots—and a “little ice age” on Earth.

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Longer Term Changes in the Sun• Occasional records piece together longer term changes

in the Sun’s energy output.• Can also look at trace isotopes carried to Earth by the

Solar Wind.• In medieval times the Sun was more luminous and

Greenland was green!

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The Sun’s Life History

16Very Long Term Changes of the Sun

• The Sun is about mid-way through its main hydrogen fusion phase, and will last another 5-6 billion years.

• Studies of other stars and computer models indicate stars grow steadily more luminous throughout this phase

• The Sun was only ~½ as luminous when it was born, and will be ~2 times more luminous in 5 billion years.

• In “just” 100 million years, it should be so luminous that Earth will undergo runaway greenhouse (like Venus)

• …but why then is Earth relatively cool now?