Physics of Energy and the Environment€¦ · Physics of Energy and the Environment Prof. Raghuveer Parthasarathy [email protected] Spring 2010 Physics 161:Physics of Energy & Environment

Physics 161:

Physics of Energy and the Environment

Prof. Raghuveer Parthasarathy

[email protected]

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.


Last time

• Climate / Radiative Equilibrium / Atmospheric infrared absorption

• What’s keeping us warm?

• The atmosphere! (Specifically, infrared absorption)


The Greenhouse Effect• Suppose there’s no IR absorption and the Earth’s surface and the atmosphere are each at equilibrium


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:


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!


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?


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!


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!


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.


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.


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.


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.


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.


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?


3 planets• Consider Venus, Earth, Mars.


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.


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)


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)


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?


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.”


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.


Positive feedback• Positive feedback: “Effect” acts to strengthen the “cause.”(Like audio feedback...)

• Positive feedback in climate: Many examples.


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...


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.


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)


* 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?)


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.)


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.


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)


Climate Forcing• Some factors that “force” the climate, and the amounts by which the forcings have changed since 1750:


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


Greenhouse Gases• Table 13.1


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


Greenhouse Gases• “Global warming potential”


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)


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


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


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.


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.


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


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)


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.)


Next: data• Will discuss

– Atmospheric CO2 (& other things)

– Temperature measurements (direct & indirect)


Documents

Physics of Energy and the Environment€¦ · Physics of Energy and the Environment Prof. Raghuveer Parthasarathy [email protected] Spring 2010 Physics 161:Physics of Energy & Environment