12 mid Flashcards
(69 cards)
1
Q
what is climate
A
The slowly varying aspects of the atmosphere-hydrosphere-land surface
system…typically characterized in terms of suitable averages of the climate system
over periods of a month or more
2
Q
How does climate differ from weather
A
- Climate and weather involve similar outcomes (temperature, precipitation, humidity,
cloudiness, wind, etc.), but climate is a long-term average - Averaging is crucial to distinguish the long-term climate signal (i.e., the trend) from
short-term weather noise (i.e., day-to-day, week-to-week, or year-to-year bouncing around)
3
Q
What is climate change
A
- Climate change: Any systematic
change in the long-term statistics of
climate elements (such as
temperature, pressure, or winds)
sustained over several decades or
longer. - Those long-term statistics are clearly
changing
4
Q
Why is it useful to be able to predict both climate and weather
A
- Both weather and climate are important,
for short-term and long-term planning,
respectively (see Dessler’s D-Day example) - Climate forecasting tells us, e.g., that we
will face a lot more years as hot or hotter
than 2024
5
Q
Dessler’s D-Day example
A
He compares the uncertainty surrounding the weather forecast for D-Day, which significantly impacted the mission’s success, to the uncertainty surrounding climate change projections. Dessler argues that, like the weather forecast for D-Day, understanding the full extent of climate change’s effects is uncertain, but the potential consequences are so significant that we cannot afford to ignore them
6
Q
What are the basic steps in the scientific process
A
- Step 1: scientists generate and test hypotheses (following generally accepted
procedures and rules of evidence) - Step 2: these studies/tests undergo peer review (quality control 1)
- Step 3: peer-reviewed conclusions are (not) replicated (quality control 2)
- Many results don’t make it through this “crucible of science” (e.g., 1989 cold fusion; 2023 room-
temperature superconducter) - Numerous independent teams are unlikely to make the same mistakes; faulty results typically not
replicated - Step 4: scientists test additional implications of theoretical claims
7
Q
What are some safeguards against individually
flawed studies
A
quality control; Numerous independent teams are unlikely to make the same mistakes; faulty results typically not
replicated
8
Q
Why should we have more faith in “consensus” beliefs
A
- A lot of scientific claims aren’t replicated, should be viewed with caution
- When numerous studies obtain the same results, a scientific consensus emerges
- Of 88,125 peer-reviewed studies, 99.9 percent agree that the climate is changing and that human
activity is the cause - We have more confidence in the aggregate consensus than in any individual study
- How do we know if there’s a consensus? We’re not experts
- The work of assessing the peer-reviewed literature has often been done by scientific bodies
9
Q
What are some alternatives to accepting the scientific consensus? Why are these alternatives
flawed
A
- Scientific (and other) institutions are not infallible, but they are much more reliable “cue
givers” than, e.g., politicians or TikTok influencers - This is worth keeping in mind in this era of declining trust in science and institutions
10
Q
What does the scientific consensus tell us about climate change (I’m asking here about whether and
how the climate is changing—don’t worry about the “why” for now.) On what evidence is this
consensus based
A
- The climate is changing: we know this from temperature anomalies
- The difference between the absolute temperature and some reference temperature
- Assertions about climate
change are always based
on anomalies rather than
absolute temperatures
11
Q
What does Dessler mean when he talks about “cherry picking” evidence
A
focusing on data that
supports your argument
12
Q
How does Dessler’s “cherry picking” evidence relate to the “warming pause” of the early 2000s
A
- In the early 2010s, we heard a lot about
the “warming pause” - E.g., Fox News on Sept 27, 2013: “[T]he
planet has largely stopped warming
over the past 15 years, data shows.” - This is an example of cherry picking
data (i.e., focusing on data that
supports your argument)—specifically, - Highlights the need to focus on long-
term trends
13
Q
What kinds of climatic changes have we observed over the Earth’s history
A
- Ice is melting
- Ocean temperatures are rising
- Sea levels are rising
14
Q
How does the climatic changes over the Earth’s history compare
with the record of the last ~11,000 years
A
- Over hundreds of thousands of years, global temperatures have varied by up to 10⁰C
- For the last 12,000 years, we’ve been in a narrower range (<1⁰C) (less than)
- For last 2,000 years (until recently), we’ve been in an even narrower range (<0.5⁰C) (less than)
15
Q
Why are the relatively small recent changes nonetheless
cause for concern
A
- The last ice age was ~6⁰C colder than today; Earth was a completely different planet
- A few degrees Celsius (our current warming path) will bring radical change
- Human civilization (settlement patterns, infrastructure, etc.), as well as current ecosystems, developed
in, and adapted to, a very stable climate - The speed of climate change matters greatly; current warming is unprecedentedly fast
16
Q
Different wavelengths of electromagnetic radiation have different properties. Why is this important for
understanding the climate and climate change? (Hint: how does Earth’s atmosphere “treat” different types of
radiation differently?)
A
- Crucially, Earth’s atmosphere is transparent to visible
photons but less so to infrared - Visible photons (from Sun) pass through the
atmosphere, increasing Earth’s internal energy - Infrared photons (from Earth) are blocked by the
atmosphere, so they don’t escape into space
17
Q
How is an object’s temperature related to the power it emits?
A
- Earth absorbs photons from the Sun, increasing
its internal energy - To maintain equilibrium, it (like all objects above
0 K) emits photons - We don’t see this radiation because the Earth isn’t hot
enough to produce visible light - If photons hit an object, they transfer their energy to that object; the object’s internal energy
increases; the object gets hotter (when sunlight hits you, you get warm) - If an object emits photons, its internal energy decreases; the object gets cooler (your coffee mug
feels warm because it’s emitting infrared photons and cooling down) - Whether an object’s temperature is increasing, decreasing, or staying the same depends on energy
(im)balance
18
Q
If an object’s Ein = Eout (power in/power out), what does this imply about the object’s temperature?
A
the temperature of that object is not changing
19
Q
What if Ein > Eout (more than)
A
internal energy/temperature increases for that object
20
Q
What if Ein < Eout (less than)
A
internal energy/temperature decreases for that object
21
Q
What is the Earth’s Ein in W/m 2 ? What two factors determine this value
A
Given our distance from the Sun, we receive 1,360 W/m^2 (solar constant or S). 2 facts are solar output and earth-sun distance.
22
Q
Why is a planet’s albedo (α) important for its climate
A
- The Earth doesn’t absorb all the energy that hits it: some is reflected back to space by
clouds, ice, etc. - A planet’s reflectivity is its albedo (α), the fraction of photons reflected back into space
23
Q
What, conceptually, is a planet’s equilibrium temperature
A
The Earth’s actual temperature is 288K (15⁰C)
24
Q
What assumptions does Dessler make about the atmosphere in his one-layer climate model
A
- Earth’s atmosphere is transparent to visible
photons, which pass through and are absorbed by
Earth’s surface - The atmosphere is opaque to infrared photons, all
of which are absorbed by atmosphere - The atmosphere is a blackbody that emits
photons based on internal energy equally in both
directions (upward and downward) - Photons emitted upward go into space (Eout);
photons emitted downward are absorbed by Earth’s surface
25
Dessler shows that a planet with a one-layer atmosphere is warmer than one with no atmosphere. Why? Why
does the addition of more layers warm the planet even further
* More atmospheric layers intensify the greenhouse effect
* Each additional layer traps more energy, reflects more energy
downward, requiring the surface to emit more energy (i.e., get
hotter) to maintain energy balance
26
Why do the relative temperatures of Mercury, Venus, Earth and Mars make sense, given what we know about
their solar constants, albedos and atmospheres
Mercury has much higher E in than Venus (higher S,
lower α). But Venus is much hotter due to powerful
greenhouse effect (very thick, mostly CO 2 atmosphere).
Mars is coldest due to low S, thin atmosphere.
27
symbol for solar constant
S
28
symbol for albedo
α
29
symbol for atmospheric layers
n
30
symbol for temperature
T
31
How do the solar constant (S), albedo (α) and atmospheric layers (n) relate to a planet’s temperature (T)
- An increase in S leads to an increase in T (more solar energy means higher E in)
- An increase in α leads to a lower T (more reflectivity means lower Ein)
- An increase in n leads to a higher T (more GHGs increase greenhouse effect)
32
Which components of Earth’s atmosphere are GHGs (greenhouse gasses)? Approximately how much of the atmosphere do they constitute
- water vapor (H2O) (around 0.4%)
- Carbon dioxide (CO2) is (now) 0.0412%
- CO 2 absorbs infrared photons; is the most important GHG that humans directly control
- Methane (CH 4) is the next most important GHG
- Now 1.87 ppm, but it’s extremely potent (32X more than CO 2), so it’s important
33
Why don’t we think the natural carbon cycle explains the recent increase in atmospheric CO2
* Earth has a natural carbon cycle that’s constantly
circulating carbon between the atmosphere, land
biosphere, ocean, and rock reservoir
* These natural processes move carbon on a scale that
dwarfs human carbon emissions
* How do we know climate change isn’t a natural
phenomenon?
* Basically, none of these processes can explain the
trend increase in atmospheric CO 2 over the last 200 years
34
How have humans perturbed the carbon cycle? How has this affected our atmosphere
* Every year, the atmosphere and land biosphere
exchange ~110 GtC (gigatons of carbon) through
photosynthesis and respiration (a lot, compared
with ~11 GtC from human emissions)
- This reflects the annual cycle of (mostly northern
hemisphere) plant growth and decay
- Photosynthesis: plants absorb CO2 and use it to make
carbohydrates and oxygen
- Respiration: animals and bacteria consume
carbohydrates and convert it to energy and CO2
35
How do we know fossil fuels are causing the increase in atmospheric CO 2 , rather than natural sources such as volcanoes or plants
* We burn fossil fuels (coal, oil, natural gas) for energy
* This transfers carbon from the rock reservoir into the atmosphere much faster (~100X)
than via natural processes
- For decades, the increase in atmospheric CO 2 has been ~44% of human emissions—i.e., human
emissions and atmospheric CO 2 are strongly correlated. There’s no reason to expect this if the
former doesn’t cause the latter. (Note: the remaining 56% is absorbed by the ocean and land
biosphere.)
- CO 2 can be chemically “fingerprinted” via radiocarbon dating. CO 2 being added to the atmosphere is
“radiocarbon dead” (i.e., no carbon-14); hence it came from plants that died a (very) long time ago
36
Why do we pay so much attention to CO2 from fossil fuels when plants and animals emit far more CO2 to the atmosphere
- Natural respiration simply releases CO 2 that was removed by photosynthesis a few months
earlier; this produces seasonal changes but no long-term increase in CO 2
- Burning fossil fuels releases CO 2 that had been safely sequestered in rock for millions of years;
this constitutes a net addition to the atmosphere
37
How long does CO2 remain in the atmosphere? (For concreteness, you can interpret this question as “How long does it take to remove 70
percent of a CO2 “pulse” from the atmosphere?”) What does this imply about the impact of current CO2 emissions
- Land biosphere and ocean mixed layer remove about 50% in 50 years
- The remaining processes are slower, so 28% remains after 500 years, and 14% after 10,000 years
- Upshot: CO 2 we emit today will be in the atmosphere for thousands of years (methane is removed much faster)
38
Let’s say we add an atmospheric “layer” by adding GHGs to the atmosphere. Will the Earth warm instantly in response? Why or why not
no because of thermal inertia, radiative forcing, energy redistribution, and positive feedback mechanisms take time to form
39
What is radiative forcing (RF)? What are the consequences of positive and negative RF, respectively
If RF (radiative forcing) is positive, the planet will warm; if it’s negative,
the planet will cool
* Fig 6.3: human GHG emissions since the industrial
revolution have caused RF = +2.5
- This is why the Earth is warming
* Note: some human emissions (aerosols) produce
negative RF, i.e., they have a cooling effect
40
How have emissions of carbon dioxide, methane, and sulfate aerosols affected the Earth’s RF
-Includes sulfate aerosols, mineral dust
(negative RF), black carbon aerosols (e.g., soot,
positive RF)
* Sulfate aerosols have direct and indirect
effects
- Direct: aerosols reflect light, increase albedo, cool
the Earth (RF = -0.3 W/m2)
- Indirect: aerosols serve as cloud condensation
nuclei, increase number of particles in clouds, again
increasing albedo (RF = -0.9 W/m2)
41
How do sulfate aerosols cool the planet? What are some examples of this? What are some policy implications
-But sulfate aerosols also cool the planet
- Implication 1: Reducing sulfur pollution will
reduce Earth’s albedo, worsen climate change
- Implication 2: We could use sulfate aerosols to
deliberately cool the Earth (solar radiation
management)
- We’ll talk about SRM later, but for now note that
volcanoes, pollution provide “proof of concept"
42
Under what conditions will Earth’s climate stabilize? What could we do to bring this about
- The climate will only stabilize when RF = 0, i.e., we must reduce E in or increase Eout
- Reducing E in: solar radiation management
- Increase Eout: reduce CO 2 abundance (not just emissions)
- This is an important point: simply reducing GHG emissions will not stop warming; it will just slow the
rate of increase. To stabilize the climate we need (net) zero emissions; to reverse climate change we need (net) negative emissions
43
What do we mean by climate sensitivity
* Climate sensitivity: how much the climate changes in response to a given increase in RF
E.g., if RF doubles, how much does the Earth warm
44
What do we mean by “climate feedbacks”? Can you give examples?
* It’s harder to calculate in reality because feedbacks can magnify or reduce the impact of RF,
increasing or decreasing climate sensitivity
* A feedback exists if a change in X leads, through one or
more channels, to further changes in X
Positive: an increase in X leads to changes that amplify the
initial increase in X
Negative: an increase in X leads to changes that offset the
initial increase in X
The climate system is full of feedbacks
- example: Water Vapor Feedback (Positive Feedback):
Mechanism: Warmer air holds more water vapor, which is itself a potent greenhouse gas.
Effect: More water vapor traps more heat, causing further warming
45
Do "climate feedbacks" tend (on average) tend to be positive or negative? How do feedbacks
affect Earth’s climate sensitivity?
* Some climate feedbacks are positive (ice-albedo,
water vapor)
* Some are negative (lapse-rate)
* Some could go either way (clouds, plant growth)
* When all known feedback effects are summed,
the net effect is positive
* Known feedbacks roughly double the Earth’s climate sensitivity (i.e., a given RF leads to twice as
much warming as it would without feedbacks)
* Which is to say: feedbacks are not our friend
* Note: we’re talking here about fast feedbacks. Slow feedbacks (melting Antarctic/Greenland ice
sheets, thawing permafrost, “weathering thermostat”) are harder to study, but could be very
important in the long run (and are probably positive)
Most climate feedbacks are positive
➡ They increase Earth’s climate sensitivity, making warming more severe than it would be from CO₂ alone.
46
What are some alternative (i.e., non-anthropogenic) explanations for recent climate change? Why are these explanations unconvincing? Why are
anthropogenic GHGs a better explanation?
Plate tectonics
Through plate tectonics, continents change shape and
location, with big climate implications (formation of ice
caps, ocean circulation, chemical weathering, etc.)
Alibi: plate tectonics are slow, require millions of years to
change the climate
* The Sun
The Sun’s energy is not constant: over its 5-billion-year
life, the Sun has become ~30% brighter
We have really good measures of solar radiation going
back to 1970s, less direct measures before that
Alibi: there’s (tiny) cyclical variation in solar radiation, but
no evidence of an upward trend
* Earth’s orbit and orientation
Changes in the Earth’s orbit can bring the Earth closer to
the Sun, increasing E in
Changes in the Earth’s tilt affect the distribution of
sunlight across the Earth, which affects climate
Such changes are crucial to explaining ice-age cycles
Alibi: these changes are very slow, require tens of
thousands of years to change the climate
* Greenhouse gases
* Most compelling explanation for several reasons:
Robust theory: well-understood physics predicts GHG-
warming link (since Svante Arrhenius in 1896!)
Supported by climate/geological record
Supported by climate models
Supported by other “fingerprints” of warming
47
seven broad categories of climate impacts covered in lecture and the readings. Specifically, you
should be able to state some central concerns within each category and explain how climate change contributes to these concerns
1. ecosystem & diversity
2. water resources
3. agriculture & find security
4. human health
5. coastal systems & sea level rise
6. economic & social systems
7. extreme weather events
48
What is attribution science? How does it help us attribute specific weather events to broader climate change
* Attribution science offers a more precise answer
(Increasingly sophisticated) models can tell us the likelihood
of an event with and without anthropogenic climate change
If the likelihood without climate change is very low (e.g., 1
in 30,000), this gives us more confidence that climate
change was a cause
49
What are some examples of abrupt climate change impacts
Rapid disintegration of West Antarctic or Greenland ice
sheets
Shutdown of Atlantic Meridional Overturning
Circulation (AMOC, related to Gulf Stream)
Thawing of permafrost
Rapid dying of Amazon rainforest
* These “tipping points” aren’t likely (anytime soon), but they
are possible and potentially disastrous
50
What does Dessler mean when he says that the impacts of climate change are non-linear? Can you give an example of non-
linear effects
* Although climate impacts are continuously worsening, their impact on people is non-linear
The impact can be quite small, until it’s not
E.g., increased rainfall/flooding isn’t a problem until it reaches your house
Lesson: don’t be complacent just because the (figurative) water hasn’t reached you yet
51
What is the general structure of global climate models? How do we use these models to anticipate climate effects? Can these
models tell us what future emissions will be?
* Global climate models (GCMs) are models of the
entire climate system
World is divided into multi-layer grid (latitude,
longitude, elevation)
Within each grid, all known physical processes
and their interactions are modeled (RF,
temperature/heat transfer, humidity,
evaporation, precipitation, plant growth,
melting/freezing of ice, cloud formation, ocean
and air currents, etc. etc. etc.)
Interactions across grids are also modeled
* These models are huge and take a lot of computing
power (and time) to run (this podcast provides a
good discussion)
* Climate modeling can tell us what is likely to happen for a given increase in GHGs
(purely physical question)
* It can’t tell us what our emissions will be in 20, 50 or 100 years (socio-economic-
political question)
* To predict our climate future, we have to know how human society will evolve
* We do not have one prediction but various scenarios embodied in shared
socioeconomic pathways (SSPs)
52
What does the IPAT identity tell us? How does it help forecast future emissions? How does it point us toward climate
solutions
- IPAT is an identity—i.e., it’s true by definition (there’s nothing to argue with here)
- IPAT exhaustively (completely)
accounts for carbon emissions
stemming from economic
activity (i.e., most
anthropogenic emissions)
* 𝐼 = 𝑃 × 𝐴 × 𝑇
I: Impact (carbon emissions)
P: population (number of people)
A: Affluence (GDP per capita in $)
T: Technology (Emissions intensity in carbon/$)
- Tells us what factors drive carbon emissions
* Tells us how we can (and probably can’t) reduce carbon emissions
53
What is an SSP? How are SSPs constructed, i.e., what is assumed
* We don’t know how these variables will evolve, so climate researchers consider
various shared socioeconomic pathways (SSPs)
Each pathway posits trends in population (P), GDP per capita (A), and carbon intensity (T)
Each trend is based on a plausible scenario (hence we can’t rule out any); the trends within each
pathway are internally consistent (i.e., they go together)
o E.g., scenarios in which the poor get richer are associated with lower population growth
(demographic transition)
Different combinations of P, A and T yield different pathways for emissions and global
temperature
54
How does our “choice” of SSP affect our medium-term climate future (e.g., the next 20 years)? How does it affect our long-
term future (e.g., the next 80, 200 or 1,000 years)
* SSPs present various plausible scenarios, but we
don’t know which will occur
Climate forecasts are “if-then” statements
When someone says “we’re on track for ____
degrees of warming,” this is a best guess based
on assumptions about where P, A and T will go
All emissions scenarios (especially the
optimistic ones) are based on technologies that
don’t yet exist
55
You come across an article that says “Right now the world is on track to warm by 2.5⁰C by 2100.” Is this forecast set in stone
(i.e., is it guaranteed to happen)? What developments could cause this forecast to be incorrect
No, the forecast that the world is "on track to warm by 2.5°C by 2100" is not set in stone—it's a projection based on current policies, emissions trends, and technological development. It represents what could happen if things continue more or less the way they are now, without major changes.
Several developments could cause this forecast to be too high or too low, including
56
What, in general, are adaptation policies? Can you give some specific examples? What are some drawbacks of an adaptation-
only response to climate change
Because adaptation doesn’t require cuts in fossil fuels, the politics of
adaptation are less contentious than those of mitigation
Nonetheless, politics are affecting the basic research needed to
prepare for a warmer world
* A purely adaptive response would be very costly
Adaptation does not prevent negative climate impacts, including irreversible ones
“An ounce of prevention is worth a pound of cure”: it would be cheaper to spend a little on
mitigation now than much more on adaptation later
* Fails a “fundamental fairness test”
Adaptation requires resources
Rich countries and people—who (mostly) caused climate change—will be able to adapt
Poor countries and people—who (mostly) didn’t cause climate change—will be unable to adapt
A purely adaptive response leads to the most suffering among those who have done the least
environmental harm
* Recent climate negotiations have acknowledged this concern
57
What, in general, are mitigation policies? What parameters in the IPAT identity do they seek to influence? What kinds of broad
changes do these policies need to bring about
* Reducing GHG emissions
to prevent further climate
change
* As we discussed, emissions reductions will probably have to come from T (energy
intensity, carbon intensity)
We can (and should) make progress through energy efficiency, but there are limits to how fast
and far this can go
Stabilizing the climate requires us to reduce carbon intensity
To see how, let’s look at where our carbon emissions come from
58
What is solar radiation management? What are some pros and cons
a set of proposed techniques aimed at modifying the Earth's climate by altering the amount of sunlight that reaches the planet's surface
-pros:
Would definitely work (we’ve seen it before)
Would be very cheap (maybe $10 billion/year to offset GHG RF)
Would buy time to reduce emissions
- cons:
Addresses temperature but not other aspects of climate change (e.g., ocean acidification)
Because GHGs will continue to build, ceasing SRM (e.g., because of war or economic crisis) would
cause a “termination surge” (sudden spike in warming)
Possible unintended consequences, e.g., for weather patterns
Political and geopolitical nightmare
* For these reasons, SRM is being studied seriously as a possible accompaniment to GHG
reduction, but not as an alternative
59
What is carbon dioxide removal? What are some pros and cons
* What matters is net emissions of CO 2; we can reduce this two ways:
Reduce CO2 emissions (mitigation)
Remove CO2 from the atmosphere (direct air capture, DAC)
* Direct air capture can take various forms:
Plant trees
Big fans that remove CO2 from the air
Enhanced chemical weathering
* Trees are great, but not a solution (too much land, not permanently
sequestered), so interest in the latter approaches is rising
Pro: unlike SRM, CDR poses few dangers
Con: not yet technologically or economically feasible at scale
60
What are two common concerns about “geoengineering” policies? What responses to these concerns did we discuss in class
* Moral hazard: SRM/CDR reduce incentives to cut GHG emissions
True, but so do many policies we like (insurance, financial bailouts,
disaster relief)
We choose these policies despite moral hazard because sometimes
bad things happen and we want a “safety net”
* We shouldn’t play God
We already do: humans have utterly transformed the Earth
o Wilderness areas have vanished
o Human biomass is ~18X all other land mammals; humans +
livestock are ~46X other land mammals, ~16X land + marine
mammals
o We’ve been geoengineering for a long time (cities and roads,
fishing and hunting, agriculture, GHG emissions)
* Geoengineering proposals should (like all) face cost-benefit
analysis, but we shouldn’t pretend they are qualitatively new
61
We know that continued climate change will destroy a lot of coral reefs. If we want to incorporate this fact into the SCC, what
do we need to do?
* Climate change will cause diverse damages, e.g.:
Property damage from floods and wildfires; lost crops from droughts
Lost species and ecosystems, both economically important (coral, bees) and less so (polar bears,
penguins)
Adaptation costs
Health costs; loss of human life
* To sum these damages, we need to put them in a common metric ($$)
This is (relatively) easy for some damages (property, crops, adaptation), harder to do for others
(human lives), even harder for others (polar bears)
* Every SCC estimate involves these “translations” into $$
Things with little economic value (e.g., polar bears) get little weight
Poorer humans are “worth”
62
What is the discount rate? Why do we need to know it to calculate the SCC?
* Many climate damages are in the future
How much should we care about these future costs?
The discount rate provides one answer: we discount future costs at some percent r per year
Higher values imply more discounting, i.e., we care about the future less
o r = 0: we’re not discounting at all, i.e., the future matters as much as the present
o r = 100: we’re discounting the future entirely, i.e., the future doesn’t matter at all
If our discount rate is high, future climate damages “don’t count” much, so we shouldn’t pay
much now to prevent them
* Why discount? Don’t future generations matter as much as us?
Basically, we expect future people to be richer (because of economic growth), so $1 means less
to them than to us (growth discounting)
Maybe it makes sense to defer costly action now, let rich future generations deal with the mess
In this framework, our discount rate depends on predictions about future growth
63
How does the discount rate affect our willingness to adopt costly climate mitigation policies now
In short, the lower the discount rate, the more we’re inclined to spend now to prevent climate harm later. Economists and policymakers often debate what discount rate is appropriate, precisely because it so strongly influences climate strategy
64
The Obama/Biden and Trump administrations employed very different estimates of the SCC. What caused their estimates to
differ so much? Why do these differences make sense, given what we know of these administrations’ climate policy goals
Recent climate research suggests it may be $185/tonne CO 2 (in 2022 $)
The Obama administration estimated $45/tonne; Biden initially employed $51/tonne (Obama
estimate after accounting for inflation), later updated to $190/tonne ($185 updated for inflation)
Trump 1.0 estimated $1/tonne CO 2; Trump 2.0 is trying to eliminate the concept altogether
That’s a pretty big range. Where do the differences come from?
o Obama/Biden SCC includes global costs; Trump SCC includes only costs within the US
o Obama/Biden assumed r ≈ 3%; Trump assumed r ≈ 7%
* Not surprisingly, those who want climate action employ higher cost estimates and
lower discount rates
65
Some people would argue that it’s impossible to construct an objectively valid estimate of the SCC. If this is true, are there
nonetheless reasons to try
* Both future costs and discount rates are hard to estimate (and value-laden), so it’s
possible to justify a range of estimates
* Methodological “fiddling” can justify almost any climate action or inaction
* Is this all just values masquerading as science?
Quite possibly
But transparency is a virtue: requiring governments to calculate SCCs forces them to state their
logic and assumptions explicitly, and allows others to disagree
* In any case, the SCC is central to climate policy debates, so it’s something you should understand
66
What are “command and control” regulations? Can you give some examples that help reduce carbon
emissions? What are some “pros” of regulations? What are some “cons”?
* The government can tell private actors (businesses,
individuals) what they can/can’t/must do
* These “command and control” regulations can affect carbon
emissions:
Many states have adopted renewable portfolio standards
(RPSs) requiring utilities to generate a specified % of power
from renewable/clean energy sources
Obama’s (never implemented) Clean Power Plan required
states to reduce CO2 emissions on a set schedule
Biden’s EPA imposed strict limits on auto tailpipe emissions
California’s Air Resources Board mandated that all light /
medium-duty vehicles sold in CA must be zero-emission by
2035
Bay Area Air Quality Management District required new water
heaters and furnaces to be zero-emission by ~2030
Many governments impose fuel efficiency standards
Regulation pros
* Relatively effective (e.g., RPSs
and emissions standards have
done a lot to reduce emissions)
* Easy to understand; relatively
popular (though less so than,
e.g., green subsidies)
cons
* They are often inefficient, i.e., they achieve goals at
significant economic cost
* They infringe on individual liberty
* “Regulatory capture” leads to regulations that serve special
interests, not the public interest
* Government regulators don’t have enough information to
micromanage the economy
How do we know we’re regulating the right activities in the
right ways?
* These concerns incline many people (and most economists)
toward carbon pricing
67
What is carbon pricing? Why should this help reduce carbon emissions? What are two ways to price carbon
* The parts maker will charge you the marginal cost of
producing the parts (labor, inputs, equipment, etc.)
* But producing the parts generates pollution, imposing an
environmental cost on society. Who pays for this?
Society pays, but not the parts maker or consumer
The environmental cost is a negative externality, i.e., a cost not
included in the price of the good/service
If negative externalities are not “priced in,” “dirty”
goods/services will be too cheap – they won’t accurately reflect
the good/service’s true cost – and will be overproduced
* The goal of carbon pricing is to price in the environmental
cost of GHGs
* The simplest way to price carbon is with a carbon tax
Impose a tax on fossil fuels proportional to their carbon content (ideally reflecting the SCC),
making fossil fuels more expensive
o Note: this is an “upstream” approach; taxes could also be levied “midstream” (utilities) or “downstream”
(industries/households/vehicles)
* Another way to price carbon is with a cap-and-trade system
Establish an overall cap on emissions
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What, according to economists, are the main advantages of carbon pricing? Are extant carbon pricing policies sufficient to meet common climate goals (e.g., 2C)
* It’s efficient, i.e., reduces emissions at the lowest economic cost
This is because emissions are reduced more by economic agents with low abatement costs (see
Dessler’s example from Table 12.1)
* Carbon pricing also solves the government’s informational problem
When fossil fuels become more expensive, producers and consumers naturally shift toward less
carbon-intensive energy, goods and services
The government doesn’t have to decide who should produce/consume how much of what, pick
winners and losers
* Carbon pricing is liberal, i.e., doesn’t infringe liberty
Producers and consumers can do what they want, taking the carbon price into account
69
What are carbon offsets? What conditions must be met for them to work? How well have they
worked in practice
* Sometimes we have to burn fossil fuels,
e.g., when we fly
* Wouldn’t it be great if we could pay
someone to “undo” our carbon
emissions?
This is the idea behind carbon offsets
o You (as an individual, business or
government) emit carbon
o You pay someone else to remove that
carbon from the atmosphere by planting
trees, direct air capture, etc.
o If you buy enough valid carbon offsets,
your activity becomes carbon-neutral
* Carbon offsets are enshrined in the Kyoto
Protocol’s Clean Develoment Mechanism; there
is also a private market for offsets
* For offsets to matter, they must involve actions that would not have occurred in the
offset’s absence
* Put differently, offsets must lead to additional carbon reductions
Let’s say a rancher promises to preserve a forest, or a government promises to build a solar plant,
if you give them money
The additionality condition is met if (and only if) the money causes the rancher/government to
take these actions
If the rancher/government would have done these things anyway, the offset doesn’t help the
climate—it just transfers money from you to the offsetter
* Many studies show that in practice, additionality is rarely met
* Another problem: many offsets are not permanent (e.g., even if trees are planted,
they will eventually die), so they don’t help in the long run
Are offsets a scam?
* At present, the answer is “mostly yes”
If you are thinking of buying an offset when, e.g., buying an airline ticket or renting a car, you
should probably save your money
* Yet, we don’t want to give up on the idea
In the long run, we need to achieve CDR at scale
To incentivize CDR technologies, we need a market for negative emissions (i.e., offsets)
Some forms of CDR are easily verifiable and permanent
New CDR technologies + better institutions to monitor/verify additionality might eventually make
offsets an important part of the climate solution
