Download ATS150 Introduction to Climate Change Spring 2010 Study Guide

ATS150
Introduction to Climate Change
Study Guide for Exam #2
Spring 2010
We’ll have the second exam in ATS 150 on Friday March 26, from 1 PM to 1:50, in
the regular classroom. The exam will count 25% of your grade for the course. You
will need to bring a pencil or pen to the exam, but you won’t need a “blue book.” The
format of the exam will be pretty much the same as the first exam.
I’m mainly trying to see that you understand concepts presented in the lectures. I
don’t want you to memorize a bunch of stuff from the notes or the textbook. The
exam will be “open notes,” meaning you can use printed copies of the lecture
notes, or books, or stuff you’ve written down during the exam. You may not
consult one another, or look stuff up on the internet. The exam will cover stuff we’ve
talked about in class. The textbook may or may not be helpful in studying (I hope
so), but I will not ask you anything on the exam that is in the book but has not
been covered in class. Also, there will be no calculations or math problems on
the exam.
The idea of this study guide is not to tell you precisely what questions will be on the
exam. Rather, I’m trying to tell you what topics from the class will be covered.
Basically, the exam covers all the lecture materials since the previous exam: “Energy
Budget of the Earth,” “Pressure, Wind, and Weather,” “Circulation of the Oceans,”
and “Climate Feedback Processes.” There won’t be any questions about climates of
the past, which we’ll begin in earnest next week.
Here’s a list of stuff to study:
I. Energy Budget of the Earth
a. Flows of energy between surface and atmosphere
 100 units of sunshine at top of atmosphere
 51 units absorbed at surface
 96 units of thermal (IR) absorbed at surface
 117 units of thermal IR emitted by surface
 7 units of rising hot air, 23 units of evaporated water at sfc
b. Geographic distribution of energy in and energy out
 Absorbed solar energy depends mostly on latitude/season
 Outgoing longwave radiation (OLR) much more “lumpy”
 Thermal emission (OLR) mostly from hot dry places
 Thermal emission (OLR) much weaker from cloudy places
 Net heating more than 100 W/m2 over tropical oceans
 Net cooling more than 150 W/m2 over polar regions
c. Effects of evaporating water and rising hot air
 Evaporating water takes a lot of energy (5 times as much to
evaporate 1 kg of water as to warm it from 0 to 100 C)
 Almost 4 times as much atmospheric heating by condensation
of evaporated water as from rising hot air
 Water evaporated from sunny subtropical oceans is carried
into deep tropics where it rains out (heats air)
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ATS150
Introduction to Climate Change
Study Guide for Exam #2
Spring 2010
II. Pressure, Wind, and Weather
a. Wind is “pushed around” by 5 forces (3 real, two imaginary)
 Real forces: Gravity, pressure differences (gradient), friction
 Imaginary forces: Coriolis and centripetal
 Heating lifts the air against gravity
 Lifted air pushes against adjacent air (pressure gradients)
 Combination of lifting and pushing produced by geographic
variations in heating and cooling causes planetary-scale
circulations of the atmosphere (wind) and oceans (currents)
 These circulations act to balance Earth’s energy budget,
moving heat from hot places (tropical surface) to cold places
(upper air and poles)
b. Pressure-gradient force
 Caused by different amounts of heating/cooling in different
places
 Air moves (wind blows) from high pressure to low pressure
c. Coriolis force
 Not a real force, but the apparent deflection of winds and ocean
currents that results from Earth’s spin underneath us
 Always pushes 90 to the right of motion in the Northern
Hemisphere (to the left in Southern Hemisphere)
 Strength of deflection is proportional to speed of motion
d. Geography of air circulation and patterns of winds
 “Hadley Cell:” rising hot air caused by tropical convergence
and rain forces air to blow toward Equator near surface,
away from Equator aloft (also causes rainforests!)
 “Trade Winds:” Inflowing surface air toward Equatorial
convergence is deflected to right in Northern Hemisphere
(NE Trades) and to left in Southern Hemisphere (SE Trades)
 “Subtropical subsidence:” sinking branch of Hadley Cell near
30 latitude in both hemisphere associated with deserts
 “Midlatitude westerlies:” warmed air flowing poleward is
deflected eastward (“westerly wind”) in both hemispheres
e. Polar vortex, jet streams, and winter storms
 Polar air extremely cold in polar night (because outgoing
thermal radiation is unopposed by solar heating)
 Thermal contraction of air in polar winter causes very strong
pressure gradient forces to try to “fill it in”
 “Jet Streams:” Coriolis force deflects air “falling” into polar
night to spin very fast in direction of Earth’s spin (toward
east, westerly jet stream winds)
 Polar vortex/jet stream blocks direct poleward heat flow
 Waves in jet stream (winter storms, warm/cold fronts) are the
main mechanism for mixing polar & subtropical air masses
III. Circulation of the Oceans:
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ATS150
Introduction to Climate Change
Study Guide for Exam #2
Spring 2010
a. Subtropical Gyres, western and eastern boundary currents
 “Ekman Transport:” Water is pushed by Coriolis force to right
of wind in Northern Hemisphere (left in SH)
 Ekman flow moves water toward tropics in midlats (because of
westerlies) and poleward out of tropics (because of easterly
Trade Winds, causing it to “pile up” in subtropics
 Gyres:” Coriolis force causes elevated water in subtropical
oceans to rotate clockwise in NH, counterclockwise in SH. The
subtropical gyres carry huge amounts of heat poleward!
 “Western Boundary Currents:” (part of gyres; e.g. Gulf
Stream) Fast-flowing currents on western sides of oceans
(east coasts) that carry warm water poleward
 “Eastern Boundary Currents:” (part of gyres; e.g. California
Current) slow-flowing cold return flow on eastern sides of
oceans (west coasts) that carry cold water toward tropics
 “Coastal upwelling:” equatorward flow along eastern
boundaries (west coasts) causes Ekman flow offshore, so
very cold water is forced to surface from depth. Causes cold
deserts, low clouds and fog (e.g., Baja, Namibia, Peru)
b. Equatorial Oceans and El Nino
 No Coriolis force at Equator, so water Trade Winds push warm
surface water westward across equatorial oceans
 “Equatorial upwelling:” caused by diverging surface water as
NE Trades push water NW and SE Trades push water SW
 Warm water “piles up” and becomes very deep. “Warm pool”
(Sea-Surface Temp, SST > 30 C) the size of Siberia in Western
Pacific and Indian Ocean, hundreds of meters deep! (source
of energy for Indian Monsoon and torrential rains)
 Cold water forced to rise in Eastern Pacific because warm
water pushed away, very productive fishery on desert coast
 “El Nino:” Occurs when Trade Winds relax, warm water
sloshes eastward, caps EQ upwelling in eastern Pacific.
Monsoon rains often fail, torrential rains in east (esp Peru).
Changes in rainfall heating reduce EW pressure gradient,
reinforce weakening of Trade Winds. Weather ensues!
c. Thermohaline (heat-salt) Circulation and Conveyor Belt
 Hot dry NE Trade Winds blowing off of Sahara evaporate lots
of fresh water from subtropical Atlantic, leaving salt behind
 Salty North Atlantic water gets very cold near Greenland
 Cold salty N. Atlantic water very dense, sinks like a rock! Forms
“North Atlantic Deep Water” which slowly fills ocean bottoms
 NADW flows south along bottom of Atlantic all the way to
Antarctic, then around & round and back up through Pacific
and Indian Oceans. Surface water slowly returns in Atlantic.
 Thermohaline “conveyor belt” takes about 1000 years to cycle
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ATS150
Introduction to Climate Change
Study Guide for Exam #2
Spring 2010
 This is the only way the deep ocean ever “sees” the atmosphere
 Moves a HUGE amount of heat poleward, releases 1/3 as much
heat to North Atlantic region as received from sunlight there!
IV. Climate Feedback Processes
a. Climate Forcing, Response, and Sensitivity
 Define climate sensitivity as strength of response (e.g., change
in surface temperature) for a given change in forcing (e.g.,
change in heating by solar radiation)
 Without feedback, climate sensitivity is about 0.25 K per 1 Wm-2
b. Feedback happens when the response changes the forcing
 Changes in radiation change the surface temperature
 Surface temperature changes the radiation
 Changes can either reinforce/amplify the forcing (positive
feedback) or counteract/damp the forcing (negative
feedback)
 Positive feedback can amplify either warming or cooling (e.g.,
can make cooling stronger as in Ice Ages)
 Negative feedback can counteract either warming or cooling
(e.g., can resist temperature changes over time)
c. Kinds of negative climate feedback (stabilizes climate)
 Longwave radiation feedback: warm surface radiates more
energy, so warming is reduced (or vice versa).
 Low clouds reflect more sunlight. Warming can evaporate
more water from oceans, making more low clouds to shade
surface and reflect more sunlight to space (resists warming).
Low clouds also emit OLR to space, but at almost same temp
as surface so extra reflected solar radiation “wins out” over
extra OLR.
d. Kinds of positive climate feedback (amplifies climate changes)
 Water vapor feedback (very strong!). Warming evaporates
more water from oceans, water vapor greenhouse effect adds
more longwave radiation to surface, extra warming
evaporates more water (enhances warming or cooling)
 Ice-albedo feedback. Cooling causes snow and ice cover to
expand, increases albedo, enhances cooling. Warming cuases
snow and ice cover to contract, reduces albedo, enhances
warming. Very important in allowing ice ages.
 High-cloud feedback. High clouds reflect sunlight like low
clouds, but high clouds are very cold so OLR to space is much
less than surface. Reduced OLR “wins out” over reduced
solar. If warming produces extra high clouds, warming will
be enhanced.
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