Lecture week 4 — anthropogenic climate change
Understanding climate change is not only about data and models, but about systems thinking —
how human demand, energy, land use, and behavior interact to create feedback loops that push
the planet beyond stability.
- The massive Co2 released by degrading and burning biomass and soils (drivers of CO2
emissions)
- The shrinking Co2 absorbation capacity of the ocean (state of the system and process)
- The GHG’s already in the atmosphere (state of the system and process)
The greenhouse e ect
1820 – Joseph Fourier: discovered that solar radiation alone cannot explain Earth’s temperature
— something in the atmosphere traps heat.
1903 – Svante Arrhenius: quanti ed that CO₂ variations regulate temperature — the rst to link
industrial emissions to potential warming.
1938–1960: temperature records con rmed rising trends.
1972: early climate models reinforced these ndings.
1988: the World Conference on the Changing Atmosphere led to the creation of the IPCC.
By the late 80’s, science was settled, what remaind is political will.
Greenhouse gases (GHG)
Main contributors and their approximate shares:
• CO₂ (carbon dioxide) — ~76%
• CH₄ (methane) — ~16%
• N₂O (nitrous oxide) — ~6%
• F-gases (CFCs, HFCs, etc.) — <2%
Each gas di ers in radiative forcing, i.e., how much energy it traps per square meter of Earth’s
surface, measured in W/m². Radiative forcing is the balance between incoming and outgoing
energy. When forcing is positive, the Earth warms; when negative, it cools.
Climate change is caused by:
• Oceanic processes (such as oceanic circulation)
• Biotic processes (e.g., plants)
• Variations in solar radiation received by Earth
• Plate tectonics and volcanic eruptions
• Human-induced alterations of the natural world
Hockey-stick graph: since 1950 carbon dioxide reached beyond a line it never did before.
Radiative balance
Positive forcing = GHG accumulation.
Negative forcing can be achieved through aerosols or geoengineering, like stratospheric sulfur
injection, which re ects sunlight — a controversial idea since it alters natural processes.
Greenhouse gases have a positive contribution to radiative forcing, aerosols have a negative
contribution. When increased greenhouse gases result in incoming energy being greater than
outgoing energy, the planet will warm due to increased radiative forcing. Reduced radiative
forcing: basis for geoengineering approach.
The physics
Dry air is about:
• 78% Nitrogen
• 21% Oxygen
• 0.93% Argon
• 0.04% CO₂
That 0.04% seems small — but its infrared absorption capacity makes it the main regulator of
Earth’s heat.
ff flff fi fi fi fi
, Temperature rise and climate disturbance
Wagner & Zeckhauser’s ‘bathtub analogy’:
Imagine a bathtub:
• Faucet = emissions (CO₂ entering the atmosphere)
• Drain = natural sinks (oceans, forests)
• Water level = atmospheric CO₂ concentration
Even if we slow the faucet, as long as more water enters than drains, the level keeps rising.
→ That’s why stabilizing emissions is not enough — they must drastically decrease.
Income → stock (account balance) → spendings
Global carbon budget
• Earth’s carbon sinks absorb ~21 Gt CO₂/year (9 from oceans, 12 from land).
• But 40 Gt CO₂/year are emitted.
• Net increase: ~19 Gt CO₂ annually — which stays in the atmosphere.
According to IPCC (2021):
• To have a 67% chance of staying under 1.5°C, humanity can emit only 200 Gt more CO₂.
• For 2°C, about 900 Gt remain.
At current rates (~40 Gt/year), this gives us <5 years for 1.5°C.
“The bathtub is nearly over owing. Every year we delay, the drain gets smaller and the tap harder
to close.”
Feedback loops
Key insight: CO₂ e ects are delayed.
The climate system reacts slowly — decades after the cause.
• Melting permafrost releases methane → ampli es warming.
• Ocean heating reduces CO₂ absorption → more remains in air.
• Forest dieback turns carbon sinks into sources.
These feedbacks are self-reinforcing. Once triggered, they’re di cult to reverse — what we call
tipping elements.
Tipping points
• Permafrost thaw
• Greenland & Antarctic ice sheet collapse
• Amazon rainforest dieback
• Disruption of thermohaline circulation
These can push the planet into a “Hothouse Earth” trajectory, a chain reaction beyond human
control.
ff fl fi ffi
Understanding climate change is not only about data and models, but about systems thinking —
how human demand, energy, land use, and behavior interact to create feedback loops that push
the planet beyond stability.
- The massive Co2 released by degrading and burning biomass and soils (drivers of CO2
emissions)
- The shrinking Co2 absorbation capacity of the ocean (state of the system and process)
- The GHG’s already in the atmosphere (state of the system and process)
The greenhouse e ect
1820 – Joseph Fourier: discovered that solar radiation alone cannot explain Earth’s temperature
— something in the atmosphere traps heat.
1903 – Svante Arrhenius: quanti ed that CO₂ variations regulate temperature — the rst to link
industrial emissions to potential warming.
1938–1960: temperature records con rmed rising trends.
1972: early climate models reinforced these ndings.
1988: the World Conference on the Changing Atmosphere led to the creation of the IPCC.
By the late 80’s, science was settled, what remaind is political will.
Greenhouse gases (GHG)
Main contributors and their approximate shares:
• CO₂ (carbon dioxide) — ~76%
• CH₄ (methane) — ~16%
• N₂O (nitrous oxide) — ~6%
• F-gases (CFCs, HFCs, etc.) — <2%
Each gas di ers in radiative forcing, i.e., how much energy it traps per square meter of Earth’s
surface, measured in W/m². Radiative forcing is the balance between incoming and outgoing
energy. When forcing is positive, the Earth warms; when negative, it cools.
Climate change is caused by:
• Oceanic processes (such as oceanic circulation)
• Biotic processes (e.g., plants)
• Variations in solar radiation received by Earth
• Plate tectonics and volcanic eruptions
• Human-induced alterations of the natural world
Hockey-stick graph: since 1950 carbon dioxide reached beyond a line it never did before.
Radiative balance
Positive forcing = GHG accumulation.
Negative forcing can be achieved through aerosols or geoengineering, like stratospheric sulfur
injection, which re ects sunlight — a controversial idea since it alters natural processes.
Greenhouse gases have a positive contribution to radiative forcing, aerosols have a negative
contribution. When increased greenhouse gases result in incoming energy being greater than
outgoing energy, the planet will warm due to increased radiative forcing. Reduced radiative
forcing: basis for geoengineering approach.
The physics
Dry air is about:
• 78% Nitrogen
• 21% Oxygen
• 0.93% Argon
• 0.04% CO₂
That 0.04% seems small — but its infrared absorption capacity makes it the main regulator of
Earth’s heat.
ff flff fi fi fi fi
, Temperature rise and climate disturbance
Wagner & Zeckhauser’s ‘bathtub analogy’:
Imagine a bathtub:
• Faucet = emissions (CO₂ entering the atmosphere)
• Drain = natural sinks (oceans, forests)
• Water level = atmospheric CO₂ concentration
Even if we slow the faucet, as long as more water enters than drains, the level keeps rising.
→ That’s why stabilizing emissions is not enough — they must drastically decrease.
Income → stock (account balance) → spendings
Global carbon budget
• Earth’s carbon sinks absorb ~21 Gt CO₂/year (9 from oceans, 12 from land).
• But 40 Gt CO₂/year are emitted.
• Net increase: ~19 Gt CO₂ annually — which stays in the atmosphere.
According to IPCC (2021):
• To have a 67% chance of staying under 1.5°C, humanity can emit only 200 Gt more CO₂.
• For 2°C, about 900 Gt remain.
At current rates (~40 Gt/year), this gives us <5 years for 1.5°C.
“The bathtub is nearly over owing. Every year we delay, the drain gets smaller and the tap harder
to close.”
Feedback loops
Key insight: CO₂ e ects are delayed.
The climate system reacts slowly — decades after the cause.
• Melting permafrost releases methane → ampli es warming.
• Ocean heating reduces CO₂ absorption → more remains in air.
• Forest dieback turns carbon sinks into sources.
These feedbacks are self-reinforcing. Once triggered, they’re di cult to reverse — what we call
tipping elements.
Tipping points
• Permafrost thaw
• Greenland & Antarctic ice sheet collapse
• Amazon rainforest dieback
• Disruption of thermohaline circulation
These can push the planet into a “Hothouse Earth” trajectory, a chain reaction beyond human
control.
ff fl fi ffi
