global warming and the hydrological cycle - uc san...

Global Warming and the

Hydrological Cycle

Climate Change Projections

• Wet regions will become wetter • Dry regions will become drier • Precipitation will occur less frequently • Precipitation will be more intense

Why?

Concepts

• Radiation, albedo, and greenhouse effect • Surface energy budget • Saturation and temperature • Adiabatic expansion and compression • The hydrological and energy cycle • Effects of global warming

Visible light, infrared (IR), ultraviolet (UV), X-rays, γ-rays, microwaves, and radio are all forms of electromagnetic radiation Each type has a different wavelength (λ). The shorter the wavelength, the greater the energy of individual photons Shortest to longest wavelength: γ-rays, X-rays, UV, visible, IR, microwave, radio

Sun

Earth

Does infrared radiation easily go through glass?

Sun

Earth’s Energy Budget

Earth

• Earth absorbs visible and near IR radiation from the Sun • Earth emits thermal infrared radiation back to space • Certain constituents reduce emission to space

Solar Radiation • Solar radiation flux at the average distance of Earth’s

orbit (S0) is about 1367 W m-2

(This is equivalent to 22.8 60W light bulbs every square meter)

• This solar radiation intersects with πR2 area, where R is the radius of the Earth

Solar Radiation • The total energy rate for solar radiation intersecting

Earth is S0 πR2 and global surface area of Earth is 4πR2

• The average amount of solar radiation per m2 spread out over the Earth is S0 / 4 = 342 W m-2

(This is equivalent to 5.7 60W light bulbs every square meter)

Planetary Albedo • Planetary albedo αp is the global average fraction of

solar radiation reflected back to space. • Earth’s planetary albedo is 0.31 • The global average value of reflected solar flux is

αp S0 / 4 = 107 W m-2 • The global average value of absorbed solar flux is

(1 – αp) S0 / 4 = 235 W m-2

Various Albedo Values • Ocean albedo is about 0.1 (except for sunglint) • Sandy desert albedo is about 0.25-0.35 • Forest albedo is about 0.15 • Snow/ice albedo is about 0.6-0.9 • Cloud albedo is about 0.3-0.6

Greenhouse Effect • The reduction in the amount of energy leaving the Earth

caused by clouds and greenhouse gases acts like a blanket to keep the Earth warmer

• Clouds and greenhouse gases also emit IR radiation toward the surface (324 W m-2)

• The surface absorbs this additional radiation and thus is kept warmer

Greenhouse Effect • The strongest greenhouse gas is water vapor • The next strongest greenhouse gas is CO2 • Clouds (composed of liquid water droplets and ice

crystals) have a greenhouse effect and an albedo effect • High clouds have the strongest greenhouse effect

because they are coldest

Surface Energy Budget

• Energy transfer between the surface and the atmosphere is not purely radiative

• A major component of energy transport from surface to atmosphere is via water (latent heat)

• The hydrological cycle is closely tied to the energy cycle

Energy Cycle Game

Players • Sun • Earth’s Surface • Earth’s Atmosphere • Space

Game components • White chips (solar energy) • Red chips (thermal energy)

Rules for No-Atmosphere Game

Steps for each turn A. Sun takes 4 white chips from the bank B. Sun gives 4 white chips to Surface C. Surface exchanges with the bank 4 white chips for 4 red

chips D. Surface gives half (rounding down) of red chips to

Space E. Space returns red chips to bank

Play until Surface has the same number of red chips at

the end of every turn

Questions for No-Atmosphere Game

• What does each step physically represent? • At the end of the game, how many red chips does

Surface give to Space each turn? • At the end of the game, how many red chips does

Surface have at the end of each turn?

Decrease in Solar Luminosity

• Repeat game with Sun giving only 3 white chips to surface

Rules for With-Atmosphere Game

Steps for each turn A. Sun takes 4 white chips from the bank B. Sun gives 4 white chips to Surface C. Surface exchanges with the bank 4 white chips for 4 red

chips D. Surface gives half (rounding down) of red chips to

Atmosphere E. Atmosphere gives half (rounding down) of red chips

back to Surface F. Atmosphere gives the rest of red chips to Space G. Space returns red chips to bank

Rules for With-Atmosphere Game

Play until Surface has the same number of red chips at the end of every turn

Questions for With-Atmosphere Game

• What does each step physically represent? • At the end of the game, how many red chips does

Surface give to Atmosphere each turn, minus the number of red chips Atmosphere gives back?

• At the end of the game, how many red chips does Atmosphere give to Space each turn?

• At the end of the game, how many red chips does Surface have at the end of each turn?

Increase in Greenhouse Effect

• Repeat game with Atmosphere giving half of red chips rounded up back to the surface

• Surface still gives atmosphere half of red chips rounded down

Water Cycle Game

Players • Earth’s Surface • Earth’s Atmosphere

Game components • Blue chips (water)

Rules for Water Cycle Game

Before game starts Surface takes lots of blue chips from bank Steps for each turn A. Surface gives 2 blue chips to Atmosphere B. Atmosphere gives 2 blue chips to Surface

Play until bored

Questions for Water Cycle Game

• What does each step physically represent? • What is the significance of no exchanges with the bank

once the game starts?

Latent Energy Transport

• Energy is required to convert liquid water to vapor • When liquid water evaporates, the surface is cooled • Water vapor rises in the atmosphere • When water vapor condenses, the atmosphere is heated • Precipitation falls back to the surface

Latent energy demonstration

Energy+Water Cycle Game

Players • Sun • Earth’s Surface • Earth’s Atmosphere • Space

Game components • White chips (solar energy) • Red chips (thermal energy) • Blue chips (water)

Rules for Energy+Water Cycle Game

Steps for each turn A. Sun takes 4 white chips from the bank B. Sun gives 4 white chips to Surface C. Surface exchanges 4 white chips for 4 red chips D. Surface gives half (round down) of red chips to Atmosphere E. Surface gives an additional 2 red chips paired with 2 blue

chips to Atmosphere F. Atmosphere gives 2 blue chips (no red) back to Surface G. Atmosphere gives half (round down) of red chips to Surface H. Atmosphere gives the rest of red chips to Space I. Space returns red chips to bank

Rules for Energy+Water Cycle Game

Play until Surface has the same number of red chips at the end of every turn

Questions for Energy+Water Game

• What does each step physically represent? • What role does water play in this game? • At the end of the game, how many red chips does

Surface give to Atmosphere each turn, minus the number of red chips Atmosphere gives back?

• At the end of the game, how many red chips does Atmosphere give to Space each turn?

• At the end of the game, how many red chips does Surface have at the end of each turn?

Evaporation and Condensation

vapor

liquid

evaporation condensation

Saturation occurs when evaporation and condensation are in equilibrium

Saturation

• Saturation units are vapor pressure or specific humidity (SH) (g water per kg air)

• Relative humidity (RH) is the amount of water vapor as a percentage of saturation

• RH < 100% water evaporates • RH > 100% water condenses • RH = 100% equilibrium at saturation

Saturation and Temperature

From http://www.atmos.washington.edu/2002Q4/211/notes_water.html

Spec

ific

Hum

idity

SH

Evaporation and Condensation

vapor

liquid

evaporation condensation

Greater saturation vapor pressure at warmer temperature

Humidity and Temperature

From http://commons.wikimedia.org/wiki/Category:Snow and http://commons.wikimedia.org/wiki/Saguaro

Group Discussion Which has greater relative humidity? Which has greater specific humidity?

Humidity and Temperature

From http://commons.wikimedia.org/wiki/Category:Snow and http://commons.wikimedia.org/wiki/Saguaro

RH = 100%, SH = 3 g/kg RH = 15%, SH = 6 g/kg

Saturation and Temperature

From http://www.atmos.washington.edu/2003Q3/101/webnotes.html

Soda can demonstration

Soda Can Demonstration


atmospheric pressure at sea level

heat to boiling

Soda Can Demonstration


atmospheric pressure at sea level

rapid cooling

Adiabatic Expansion

Why does temperature usually decrease with height? • When an air parcel rises, it goes to a level with less

atmospheric pressure • Since pressure is less, the air parcel expands • In order to expand, it must work to push the surrounding

air out of the way

Adiabatic Expansion

• If there is no energy input into the system (adiabatic), then the energy for the work of expansion must come from internal energy

• Internal energy is energy from the motion of molecules (temperature)

• Less internal energy means the air parcel cools down

Adiabatic Compression

• When an air parcel subsides, it goes to a level with more atmospheric pressure

• Since pressure is greater, the air parcel is compressed • Compression increases the internal energy of the air

parcel • The air parcel warms up

Bicycle tire demonstration

The Hydrological Cycle

Warm, moist air rises and cools Water vapor condenses and precipitates

Cold, dry air subsides and warms

Air is very dry (g/kg) due to precipitation loss

Air moistens from surface evaporation





Air moistens from surface evaporation

Water loss

Water gain

The Energy Cycle

How does the rising air maintain positive buoyancy so it can continue rising?

latent heat from condensation How does the subsiding air maintain negative buoyancy

so it can continue sinking? net radiative cooling (more emitted than absorbed)





Air moistens from surface evaporation Energy gain

from latent heating (“furnace”)

Energy loss from radiative cooling (“radiator fin”)

Water and Energy Balance

Approximately… precipitation = surface evaporation latent heating = atmos. radiative cooling Implication… the magnitude of radiative cooling controls the

magnitude of precipitation

Global Warming and Saturation Sp

ecifi

c H

umid

ity

SH

As temperature warms, saturation specific humidity strongly increases More water can evaporate into the atmosphere

start student activity

Global Warming and Saturation

• More evaporation from dry regions means dry regions get drier

• More water vapor in the atmosphere means more condensation and precipitation in wet regions

Saturation and Radiative Cooling Sp

ecifi

c H

umid

ity

SH

As temperature warms, net radiative cooling of the lower atmosphere weakly increases

Rad

iativ

e C

oolin

g (W

m-2

)

Global Warming Effects

Strong increase in net radiative cooling

Weak increase in net radiative cooling

Weak increase in precipitation

Middle atmosphere

Lower atmosphere

Strong increase in water vapor

Water and Energy Balance

Approximately… latent heating = atmospheric radiative cooling Implication… a small increase in radiative cooling means there can

only be a small increase in precipitation

Global Warming and Precipitation

• Weak increase in global average precipitation • Strong increase in water vapor available to be rained out Implication… Precipitation events become stronger but less frequent

finish student activity

show graph



• Net (up – down) IR flux is 66 W m-2 • Evaporative heat flux is 78 W m-2

• Absorbed solar flux is 168 W m-2

Primary surface energy balance at many locations: solar heating evaporative cooling



• Net (up – down) IR flux is 66 W m-2 • Evaporative heat flux is 78 W m-2

• Absorbed solar flux is 168 W m-2

Primary surface energy balance at many locations: solar heating evaporative cooling What might happen if surface solar heating is weakened?

Regional Anthropogenic Aerosol

From http://environment.newscientist.com/

Regional Anthropogenic Aerosol

From http://www.sciencemuseum.org.uk/antenna/sootyclouds/ and http://scrippsnews.ucsd.edu/Releases/?releaseID=860

Solar Heating and Precipitation

Large energy gain from latent heating Large energy

loss from radiative cooling

Large energy loss from evaporative cooling

Large energy gain from solar heating

Aerosol “Dimming” and Precipitation

Small energy gain from latent heating

Large energy loss from radiative cooling

Small energy loss from evaporative cooling

Small energy gain from solar heating

Reflection back to space

Solar heating from absorption by soot aerosol

Less precipitation

global warming and the hydrological cycle - uc san...

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