Post on 02-Apr-2021
Physics 161:
Physics of Energy and the Environment
Prof. Raghuveer Parthasarathy
raghu@uoregon.edu
Spring 2010
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
May 27, 2010
Lecture 17: Announcements
• Reading: Wolfson, Chapter 13.
• Homework: Problem Set 8. Due Today, May 27.
• Posted: Grade weighting options
• Final exam: June 8, 8.00‐10.00 am. Review session next week, TBA.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Last time
• Climate / Radiative Equilibrium / Atmospheric infrared absorption
• What’s keeping us warm?
• The atmosphere! (Specifically, infrared absorption)
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
The Greenhouse Effect• Suppose there’s no IR absorption and the Earth’s surface and the atmosphere are each at equilibrium
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
surface
atmosphere
radiated
from
surface
refle
cted
TE
TA
Arrows balance at each gray line
(power in = power out)
The Greenhouse Effect• Now “insert” atmospheric IR absorption, radiation
• Consequences:
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
surface
atmosphere
radiated
from
surface
refle
cted
radiated
from
atm
.
TE
TA
This is the Greenhouse Effect
The Greenhouse Effect• ... which means the surface temperature must be higher than it would have been without atmospheric absorption!
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Final Diagram
surface
atmosphere
radiated
from
surface
refle
cted
radiated
from
atm
.
TE
TA
The Greenhouse Effect• Suppose atmospheric infrared absorption increases: What happens to the thickness of these arrows?
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
surface
atmosphere
radiated
from
surface
refle
cted
radiated
from
atm
.
TE
TA
A. Thicker arrows
B. Thinner arrows
The Greenhouse Effect• Suppose atmospheric infrared absorption increases: What happens to the thickness of this arrow, and to TE?
• An important question!
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
surface
atmosphere
radiated
from
surface
refle
cted
radiated
from
atm
.
TE
TA
A. Thicker; higher TE
B. Thinner ; higher TE
C. Thicker; lower TE
D. Thinner ; lower TE
The Greenhouse Effect• Atmosphere: absorbs most of the infrared light!
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
surface
atmosphere
radiated
from
surface
refle
cted
radiated
from
atm
.
TE
TA
• Could balance equations.(PDF – show! )
IR absorption → Higher TEthan our simple (unrealistic) models!
Atmospheric IR absorption
• Why does the atmosphere absorb infrared (IR) radiation?
• Molecules with more than two atoms (H2O, CO2, ...) absorb IR. These are “greenhouse gases,” responsible for the Greenhouse Effect.
• Water is the dominant greenhouse gas, followed by CO2.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
A. Nitrogen and CO2
B. Oxygen and CO2
C. Molecules with more than two atomsD. Molecules with two atomsE. None of the above
Summary
• Radiative equilibrium: Total Power In = Total Power Out
• Simple model: Earth, Sun, no atmosphere – easy to calculate TE that maintains equilibrium
• Albedo: reflection at atmosphere. Again, calculate TE.
• Absorption of infrared light at atmosphere – an extra “layer” to the diagrams. Atmospheric IR absorption →TE higher to maintain equilibrium! Greenhouse effect.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
A problem...We’ve learned:
• We use a lot of fossil fuels
• CO2 is a necessary product of fossil fuel combustion. (We’ll discuss water soon)
• CO2 in the atmosphere absorbs IR radiation
• An atmosphere that absorbs more IR necessarily leads to a higher temperature for the Earth (radiative equilibrium)
• This sounds like a problem. It is.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Reasoning
• Each step of reasoning on the last slide follows “simply” from basic facts / concepts.
• Each concept is supported by a lot of scientific data, experimentatione.g. measuring IR absorption by gases in the lab, measuring atmospheric composition, etc.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
The greenhouse effect• (Earlier slides) CO2, radiative equilibrium, IR absorption, ... etc.
• Can we test these ideas on a planetary scale?
• (We’re presently doing this unintentionally, but that’s not what I mean.)
• Consider Venus, Earth, Mars.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
The greenhouse effect• Venus is hot; Mars is cold. Why?
• Obvious answer: Proximity to sun. Does this quantitatively explain the temperatures?
• How would we figure this out?
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
3 planets• Consider Venus, Earth, Mars.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
This is the temperature we would calculate using the “no atmosphere” model we derived earlier
Actual temperature
Difference, due to greenhouse effect
3 planets• Consider Venus, Earth, Mars.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Venus: dense atmosphere, 96% CO2
Earth: just right
Mars: Mostly CO2atmosphere, but very thin
3 planets
• Consider Venus, Earth, Mars.
• Planetary temperatures agree with greenhouse predictions based on atmospheric compositions. (Also quantitatively agree)
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
SummaryWe’ve learned:
• We use a lot of fossil fuels
• CO2 is a necessary product of fossil fuel combustion. (We’ll discuss water soon)
• CO2 in the atmosphere absorbs IR radiation
• An atmosphere that absorbs more IR necessarily leads to a higher temperature for the Earth (radiative equilibrium)
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Climate Forcing• We’ve seen various “general” factors that control global temperature...– solar power incident on Earth
– Earth’s albedo
– atmospheric absorption of infrared radiation
• ... by controlling Earth’s radiative “energy balance”
• What specific things can affect these factors, and how?
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Feedback• The mechanisms of forcing can be complicated by feedback effects.
• Feedback: “output” influences “input” in a process
• E.g. Cruise control in a car: ...
• This is “negative feedback”
• Negative feedback: “Effect” acts to oppose the “cause.”
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Negative Feedback• Negative feedback in climate: Many examples.
• E.g. More CO2 in the atmosphere →more plant growth →more CO2 removed from the atmosphere by plants. →less CO2 in atm.
• This feedback means that CO2 has less of an effect on temperature than it would without plants present.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Positive feedback• Positive feedback: “Effect” acts to strengthen the “cause.”(Like audio feedback...)
• Positive feedback in climate: Many examples.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Positive feedback
• E.g. ice‐albedo feedback. Suppose TE rises, so sea ice melts. Liquid water is less reflective than ice.
• Do you expect TE will: A. Rise
B. Fall
• Less reflection (lower albedo); less solar power reflected so TE rises, so more sea ice melts, so less reflection...
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Water Vapor• Now is a good time to return to water vapor, the dominant greenhouse gas.
• H2O is also a combustion product. Why don’t we worry (much) about H2O in the atmosphere?
• Water vapor and liquid water coexist on Earth, in equilibrium with each other. At constant temperature, the amount of vapor is fixed. If we add more, it condenses into liquid water. So generating more water vapor is much less of a problem than generating CO2.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Water Vapor
• Generating more water vapor is much less of a problemthan generating CO2...*
• .. but it’s still a (smaller) issue. Can you think of how increased H2O emission might affect climate? (There is more than one mechanism)
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
* for more details, see http://www.realclimate.org/index.php/archives/2005/04/water‐vapour‐feedback‐or‐forcing/ a very good climate blog
Water Vapor• Can you think of how H2O emission might affect climate?
• One mechanism: If TE rises for other reasons, equilibrium water vapor amount rises... (A positive or negative feedback?)
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
A. positive
B. negative... more H2O in atmosphere →more IR absorption → greater TE...
A very important feedback mechanism!
Water Vapor• Can you think of how H2O emission might affect climate?
• Another mechanism: If TE rises, water vaporfraction rises →more clouds... (A positive or negative feedback? Clouds are reflective.)
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
A. positive
B. negative... more clouds →more reflected solar power, less solar power gets to Earth → lower TE → fewer clouds
Also important.
Feedback and Forcing
• To consider the climate forcing caused by any factor, need to consider feedback mechanisms in addition to “direct” effects.
• Quantitatively very complicated. (Though the basic concepts are fairly “simple.”) We’ll discuss climate modeling later.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Climate Forcing
• Some factors that “force” the climate (next slide), and the amounts by which the forcings have changed since 1750 (i.e. since the start of the Industrial Age)
• Positivemeans→ warming
• Negativemeans → cooling• Don’t bother memorizing the magnitudes
• (Units: W/m2 – think of it as equivalent to getting some extra power per piece of area of the Earth’s surface)
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Climate Forcing• Some factors that “force” the climate, and the amounts by which the forcings have changed since 1750:
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Greenhouse Gases• Several greenhouse gases. What makes some more important than others?
• Amount in atmosphere: CO2 “wins”
• How well they absorb IR radiation– Methane, others, absorb more IR per molecule than CO2 does
– Next slide: “Global warming potential” (roughly, forcing per kg gas, relative to CO2)
• Lifetime
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Greenhouse Gases• Table 13.1
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Greenhouse Gases• Many greenhouse gases. What makes some more important than others?
• Amount
• How well they absorb IR radiation– Methane, others, absorb more IR than CO2 does
– Next slide: “Global warming potential” (forcing per kg gas, relative to CO2)
• Lifetime: How long they stay in the atmosphere– CO2 “wins” (next slide)
– Methane, e.g., breaks down over ≈ 10 years
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Greenhouse Gases• “Global warming potential”
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
The Carbon Cycle
• CO2: The largest anthropogenic (human‐caused) contribution to climate forcing
• Carbon (& many other things) “cycle” through atmosphere / land / ocean / organisms
• Let’s look at reservoirs and flows of Carbon (not just in CO2, but in any chemical)
• Reservoirs: where C is stored
• (Don’t memorize the numbers; understand key parts of the diagram)
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Reservoirs: (where’s the C?)Atmosphere: mostly CO2
Land: Organisms (mostly plants) + SoilOceans: A lot. dissolved CO2, plants, minerals. Deep ocean: C doesn’t circulate much
Amounts in Gigatonnes; Flows in Gt/year
Flows: Carbon cycles among reservoirsPhotosynthesis: Plants take CO2 from atm.Respiration: Plants, animals using stored chemical energy; releases CO2 (like combustion)Decay: Releases organisms’ CO2.Very, very, very tiny amount turns into new fossil fuels
Amounts in Gigatonnes; Flows in Gt/year
Flows: Carbon cycles among reservoirsOceans: Near surface, lots of CO2 exchange.Oceans: Deeper ocean: diffusion and upwelling upward, “rain” of dead microorganisms downwardVolcanoes: very long timescale CO2 release to atm
Amounts in Gigatonnes; Flows in Gt/year
The Carbon Cycle
• Carbon is stored in the atmosphere, surface biota (organisms & biomass), and the ocean surface
• Cycles back and forth between atmosphere & other reservoirs
• Rapid: “Typical” CO2 molecule spends about 5 years in the atmosphere before photosynthesis or dissolution in oceans removes it.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
The Carbon Cycle• 5 Year cycle? What about 1000 year lifetime?
• 5 Years ≈ cycling time. Lifetime a separate issue: If I add extra C to the atm., how long does the excess amount (not those exact C atoms) remain?– e.g. you & I give each other $100 every day. I get an extra $5 tomorrow, but we continue to exchange $100 every day. The lifetime of my extra $5 is forever – it doesn’t matter that we exchange $100/day.
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Land & Ocean SurfaceAtmosphere
The Carbon Cycle• Extra carbon in atmosphere:
– short term removal of some (terrestrial, ocean; e.g. due to plant uptake, feedback effects)
– But a lot remains for a very long time (> 1000 yrs.), until deep ocean uptake is significant
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
The Carbon Cycle• Extra carbon in atmosphere:
– short term removal of some (terrestrial, ocean; e.g. due to plant uptake, feedback effects)
– But a lot remains for a very long time (> 1000 yrs.), until deep ocean uptake is significant
• Carbon’s long lifetime is a key difference between C & other forcing agents (e.g. methane, N2O)
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Next: an important “meta‐slide”
Next: data• Two “pieces” to understanding climate change:
• 1 Evidence for global warming, climate change. We haven’t discussed this at all yet.
• 2 Physics of climate– Fossil fuel use → CO2 generation– The greenhouse effect influences Earth’s temperature
• Just 2 alone should lead us to worry about climate change! (A point typically not understood by people who quibble over minor details of temperature data.)
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010
Next: data• Will discuss
– Atmospheric CO2 (& other things)
– Temperature measurements (direct & indirect)
Physics 161:Physics of Energy & Environment R. Parthasarathy Spring 2010