Ecc Climate Lecture

8/20/2019 Ecc Climate Lecture

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Energy and Climate Change

Module 1: Climate

Prof. Tony Broccoli

Dept. of Environmental [email protected]


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What I Do

I am an atmospheric scientist who studies climate and climate change.


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• What do we mean by climate?

• What processes affect Earth’s climate?

• How has climate changed in the past?

•

How do we project future climate?• What changes do we expect?


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Source: NASA/Goddard Institute for Space Studies


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What is climate?

Not the same as weather…

Weather: The state of the atmosphere at a particular time.


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What is climate?

• Formal definition: Climate is the synthesis of

weather, including averages, extremes, and

variations.

• “Climate is what you expect; weather is what you

get.” (Good point, but could be misleading.)

• “Climate informs us on what wardrobe to buy,

weather on what clothes to wear on a particularday.”


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Annual Average Temperature


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New Jersey annual mean temperature

Source: National Climatic Data Center


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Temperatures in New Brunswick


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The Climate System


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Temperature and Energy

• Temperature is a measure of the energy associated with

random molecular motions.

• Faster moving molecules → higher temperature

• Slower moving molecules → lower temperature

• No molecular motion → “absolute zero”

(-273 °C or -459 °F)

• Molecular motions at the atomic and subatomic scales are

associated with emission of energy in the form of

electromagnetic radiation


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The Electromagnetic Spectrum


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Kilauea lava lake


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Kilauea lava lake


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Earth’s Energy Balance:

Regulator of Global Climate

• Earth is heated by the sunlight that is absorbed by its

surface and atmosphere.

• Without a cooling mechanism, Earth’s temperature

would steadily increase.

• Earth’s cooling mechanism is the emission of energy

in the form of infrared radiation.

• These two processes form Earth’s energy balance,

which determines its temperature.


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Earth’s Energy Balance

If incoming solar energy equals outgoing infrared

energy, Earth’s temperature remains constant.

If incoming solar energy is greater than outgoing infrared

energy, Earth’s temperature will increase.

If incoming solar energy is less than outgoing infrared

energy, Earth’s temperature will decrease.


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Calculating Earth’s Temperature

• If Earth did not have an atmosphere, we could make

a very good estimate of its average temperature

using basic physics.

• To make this calculation, we have to know how much

energy is emitted by the Sun and how much sunlight

is reflected back to space by Earth.

• We also have to account for the shape of the Earth,which spreads the sunlight over a larger area.


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Calculating Earth’s Temperature (cont.)

• We also use a principle of physics, the Stefan-

Boltzmann Law, to determine how much infrared

radiation is emitted by an object with a given surface

temperature.

• Using this principle, we can calculate what Earth’s

temperature would have to be for the amount of

emitted infrared energy to equal the absorbed solar

energy.


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Calculating Earth’s Temperature (cont.)

• Based on this calculation, Earth’s average

temperature would be approximately -18 °C (or 0 °F).

• This calculated temperature is much lower than the

global average temperature of 15 °C (or 59 °F).

• What is the reason for this difference?


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Cross-Section of Atmosphere

The atmosphere is heated

primarily by sunlight

heating the surface.


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Cross-Section of Atmosphere

colder

warmer


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Greenhouse Gases

• The atmosphere contains certain gases, often called

“greenhouse gases,” that are transparent to sunlight

but absorb and re-emit infrared radiation.

• Water vapor, carbon dioxide (CO2), methane (CH4)

and nitrous oxide (N2O) are greenhouse gases found

in the atmosphere.

• Greenhouse gases affect Earth’s energy balance.


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The Greenhouse Effect

Less greenhouse gases above this level → easier for energy from this

level to escape to space

More greenhouse gases above this level → harder for energy from this

level to escape to space


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The Greenhouse Effect (cont.)

• Because Earth has an atmosphere that contains

greenhouse gases, it is harder for energy to escape

from Earth to space than it would be if there were no

greenhouse gases.

• This causes the surface temperature to be ~33 °C

higher than it would be otherwise.

• This is the natural greenhouse effect, which can beenhanced by adding more greenhouse gases.


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Geometry of Earth and Sun

• Although the total amount of solar energy absorbed

by Earth is important for determining the global

average temperature, there is a large variation in

temperature on the surface of Earth.

• The shape of Earth and the geometry of its orbit

around the sun are responsible for much of this

variation.


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Average Temperature for January and July


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Earth Rotates


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Diurnal Temperature Variations


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Earth Revolves Around the Sun


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The Tilt of Earth’s Axis

Northern

Hemispheretilted

toward Sun;

Southern

Hemisphere

tilted away

from Sun

Northern

Hemispheretilted away

from Sun;

Southern

Hemisphere

tilted

toward Sun


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Northern

Hemisphere

winter:

Solar radiation

decreasesrapidly toward

higher latitudes

Northern

Hemisphere

summer:

Small changes insolar radiation

Northern

midlatitudes:Maximum solar

radiation in

June; minimum

in December

Southern

midlatitudes:Maximum solar

radiation in

December;

minimum in

June

Deep tropics:

Seasonal variation of solar

radiation is very small

Ecc Climate Lecture

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