General information
13 contacturen Hoorcollege
26 contacturen Zelfwerk en -studie
Content
The course consists of about 6 lectures covering the broad theme of 'Global Change', often given by
invited speakers. Typical subjects of the presentations cover the history of the Earth, the palaeoclimate,
and various aspects of global warming (physical, economical, and political)
General competencies
Depending on the specific lectures given in this course, the following competences can be distinguished:
→Gaining insight in various mechanisms that have controlled climate and environmental changes during
the Earth's history.
→Be able to distinguish between anthropogenic and natural processes of climate and environmental
change.
→Be able to understand lectures given by active scientists from different disciplines.
→Be able to see relations between physical and human aspects of global change.
→Develop a critical attitude towards the societal discussion on global change.
→Gaining insight in the interplay between climate and environmental changes and demographic,
technological, socio-economic, and political developments and choices.
Assessment Information
The assessment consists of the following assignment categories:
The Written Examination determines 100% of the final grade.
Within the Written Examination category, the following assignments must be completed:
*Note: Written examination with 1 question per lecture. The final grade is the arithmetic mean of the
individual evaluation per question.
1
, Global Change: lecture 1 - Philippe Huybrechts
Overview:
1. Some basics of the climate system
2. Changes in human and natural drivers of climate
3. Observations of changes in climate: temperature
4. Attribution of climate change
5. Impacts of climate change: the cryosphere and sea level
6. Projections of future climate change
7. Limiting climate change
1.Some basics of the climate system
Climate change→most pressing problem of the century
Why? -connected to so many other problems (conflict, food, …)
The difference between weather and climate
→Climate is the statistic of the weather
weather Climate
=atmospheric condition at a given time and a = mean and extreme conditions of the
given place atmosphere, ocean, sea ice, etc… over a longer
period (e.g. 30 years)
-one component: atmosphere -statistic (mean, average)
-at given time -parameters of the atmosphere and all connected factors
-at given place -usually over a 30year period
-difficult to predict ahead “Lorenz butterfly” (very stochastic
variables, a lot of feedbacks, a lot of interactions)
*Air temperature, precipitation, clouds, air pressure, wind
speed, atmospheric humidity, …
-usually no further then 14days
Interactions are crucial to understanding the climate system
-Atmosphere: laag van gassen rond de
aarde die lucht, weersystemen en klimaat Cryosphere
ondersteunt.
-Hydrosphere: alle water op aarde, inclusief
oceanen, meren, rivieren en grondwater.
-Cryosphere: Alle bevroren waterdelen van
de aarde, zoals gletsjers, ijskappen en
sneeuw.
-Biosphere: Alle levende organismen op
aarde en de ecosystemen waarin ze leven.
-Lithosphere: buitenste, vaste laag van de
aarde, inclusief gesteenten en de aardkorst.
The Earth’s climate system
Big connected
system with
interactions
and feedback…
2
,Formulas
Radiation balance of the Earth
-energy received (from sun)
-energy reflected back to atmosphere/ absorbed by earth system
*imported equation: conservation of energy
1. Solar Input Adjusted by Albedo:
𝑄absorbed = 𝑄(1 − 𝛼)
Only ~70% of solar radiation is absorbed; the rest is reflected.
2. Outgoing Infrared Radiation: Stefan-Boltzmann based equation
𝑅emitted = 𝜀 𝜎 𝑇 4
Not all radiation escapes directly due to the greenhouse effect; ε accounts for atmospheric absorption.
3. Equilibrium Temperature:
At energy balance:
𝑄(1 − 𝛼) = 𝜀 𝜎 𝑇 4
Q = mean incoming solar radiation at the boundary of the atmosphere (340 Wm -2)
α = planetary albedo (0.30)
ε=effective emissivity of the atmosphere (0.61)
*Represents how efficiently the Earth-atmosphere system emits infrared radiation to space.
Accounts for the greenhouse effect: some outgoing radiation is absorbed and re-emitted by
greenhouse gases, so not all reaches space directly.
For Earth today: 𝜀 ≈ 0.61, meaning about 61% of emitted IR radiation actually escapes to space.
ϭ = Boltzmann constant (5.67 10-8 Wm-2K-4)
T = mean global surface temperature (in K)
Calculation of the global mean temperature
=above formula solved for T
How to change the climate? Factors in equation
→ Solar radiation (Q): More sunlight → warmer
Weaker Sun (Q: −1%):
T = 286.8 K = +14.1 °C
current radiative fluxes:
T = 287.5 K = +14.8 °C→minor effect
→albedo (α): Higher reflection → cooler (Ice, snow, clouds)
Higher planetary albedo during the ice ages (α = 0.38):
T = 279.4 K = +6.3 °C
→Greenhouse effect(ε): BIGGEST FACTOR Lower ε → warmer, higher ε → cooler
Without greenhouse effect (ε = 1):
T = 253.4 K = −18.6 °C
*Greenhouse effect: natural property of the atmosphere
=re-emission of sunlight at lower frequency in all directions in the planet’s atmosphere
!!!! Is needed for the current life on earth
3
,The greenhouse effect
The Earth’s temperature without greenhouse
effect: T = -19°C
The Earth’s temperature with natural greenhouse
effect: T = +15°C
Absorption spectra of greenhouse gases in the
near infrared
Earth’s Radiation vs. Planck Curve
→Smooth lines (Planck curves) represent the
ideal thermal radiation emitted by Earth’s surface
if it behaved like a perfect blackbody at a given
temperature. These curves come from Planck’s
law, which describes how much electromagnetic
radiation a body emits at each wavelength
depending on its temperature.
For Earth (~288 K average surface temperature):
The peak emission occurs in the infrared region (around 10 micrometers wavelength).
This is why greenhouse gases matter, they absorb infrared radiation.
Actual Emission (Jagged Line)
It deviates from the smooth Planck curve because Earth’s
atmosphere absorbs radiation at specific wavelengths.
What causes the dips?
Different gases absorb different infrared wavelengths:
→CO₂ (carbon dioxide) strongly absorbs around 15 μm,
close to Earth’s peak emission → very important for
warming.
→H₂O vapor (water vapor) absorbs across many infrared
bands.
→O₃ (ozone), CH₄ (methane), and others also absorb in
specific regions.
Water Vapor: The Most Important Greenhouse Gas
Water vapor contributes the largest share of the natural greenhouse
effect because: -It is abundant
-It absorbs across broad infrared ranges
However policy isn’t focused on H₂O?
Because water vapor is not directly controlled by
human emissions.
Its concentration depends mainly on temperature.
Warm air can hold more moisture (Clausius–
Clapeyron relation).
So water vapor acts as a feedback, not a forcing
→Positive feedback loop:
CO₂ (and other gases) warm the planet.
Hotter → more H₂O → stronger greenhouse effect →
hotter
Why Focus on CO₂ and Other Anthropogenic Gases
Because they are:
Anthropogenic (human-caused) forcings
Capable of changing Earth’s energy balance independently
CO₂ acts like a “control knob” for climate:
Increase CO₂ → temperature rises → water vapor increases → amplifies warming
So: CO₂ triggers the warming, water vapor amplifies it
4
, This figure shows how doubling atmospheric CO₂
changes Earth’s energy balance and surface
temperature in steps.
(a) Initial state — balanced climate
Earth receives about 240 W/m² of solar energy (S) and
emits the same amount as infrared heat (L). Because
incoming and outgoing energy are equal, the average
surface temperature is stable at about 15 °C.
(b) Immediately after CO₂ doubles
Incoming solar radiation is unchanged (CO₂ barely
affects sunlight), but more outgoing infrared radiation is
absorbed by the atmosphere. Therefore, less heat
escapes to space (L drops to 236 W/m²). This creates an
energy surplus (more in than out), meaning the planet
must warm.
(c) Direct warming (no feedbacks)
As the surface warms, it emits more infrared radiation (hotter objects radiate more). The temperature rises
by about 1.2 °C, which restores the balance so outgoing energy again equals incoming (240 W/m²). This is
the warming caused by CO₂ alone.
(d) Warming with climate feedbacks
Additional processes — especially increased water vapor, reduced ice cover, and cloud changes —
amplify the warming. The surface temperature rises further (to about +2.5 °C) until energy balance is again
restored.
In short: Doubling CO₂ first traps extra heat (energy imbalance), then Earth warms until it can radiate that
extra energy away, and feedback mechanisms make the final warming larger than the direct effect alone.
2. Changes in human and natural drivers
Drivers of climate change
• Changes of the atmospheric concentrations of carbon dioxide, methane, and nitrogen oxide
over the last 2000 years
-anthropogenic
-main human caused greenhouse gasses
→clear rise of all
greenhouse gasses during
industrial revolution
*bacteria in
microbiome excrete
N2O
=intergovernmental panel for climate change
*replantation of trees or eating vegetarian
does a little but most is caused by
industry→ company’s need to take
responsibility.
5
,CO2
→Carbon dioxide concentrations have increased by more than 50% since pre-industrial times
*recent measurements: only recently started recording (at Hawaiian volcano)
*older proof: fluctuations of CO2 in earth’s history& current rise through studying of geological layers
Evidence for human caused rise in CO2?
→The current atmospheric concentrations of CO2 have not been experienced for at least 2 million years
(high confidence)
Human Disruption of the Carbon Cycle (2010–2019)
Primary sources of anthropogenic CO₂ emissions -Fossil fuel combustion: 34.8 GtCO₂/year
-Deforestation and land-use change: 4.1 GtCO₂/year
Mechanism: How the Carbon Cycle Is Disrupted
Natural carbon cycle:
Carbon exchanges between reservoirs:
• Atmosphere
• Land biosphere
• Ocean
Normally, inputs ≈ outputs (balanced fluxes).
Human perturbation:
• Extra emissions increase carbon input to the
atmosphere
• Natural sinks cannot fully compensate
→Result: More CO₂ enters the atmospheric reservoir
than leaves it: system out of equilibrium
Distribution of Carbon Between Reservoirs
Fate of anthropogenic CO2 emissions (2014-2023 average)
Sources Sinks
Fossil fuels 90% Atmosphere 48%
35.6 GtCO2/yr *accumulation 19.2 GtCO2/yr
deforestation 10% Terrestrial biosphere 29%
4.1 GtCO2/yr *vegetation and soil 11.7 GtCO2/yr
absorption
Ocean 26%
*major long term sink 10.5 GtCO2/yr
via dissolution and
biological processes
Budget imbalance: -4%, -1.6 GtCO2/yr
Global Carbon Budget from 1850 to 2023
The cumulative contributions to the
global carbon budget from 1850
The carbon imbalance represents the
gap in our current understanding of
sources & sinks
6
,Atmospheric Oxygen Decline
Observed simultaneously: CO₂ concentration ↑ & O₂ concentration ↓
→ Combustion reaction: C + O₂ → CO₂
This chemical fingerprint directly implicates fossil fuel burning.
Role of aerosols (suspended particles in the air)
→negative feedback of warming: more aerosols= more clouds=more albedo= lower temperature
-direct effect: scattering and absorbing shortwave and longwave radiation
-indirect effect: modifying the radiative properties, amount, and lifetime of clouds
Aerosols have a net cooling effect on climate (negative radiative forcing)
Aerosols have a short lifetime (maximally a few years)
Radiative forcing is the change in Earth’s energy balance caused by a factor that affects how much
energy enters or leaves the climate system.
-positive radiative forcing= warming
IPCC AR6 (2021): Total anthropogenic radiative forcing for 2019 relative to 1750 is +2.72 [1.96 to 3.48]
Wm−2, and it has increased more rapidly since 1970 than during prior decades
*this already keeps in mind the positive and negative feedback loops
7
,3. Observations of changes in the climate system Temperature
Global Temperature Rise
-Earth’s surface temperature increased by ~1.3 °C (1880–2025)
-2024 was the warmest year recorded; 2025 the second warmest
-25 of the 26 warmest years occurred in the 21st century
Long-Term Historical Context
-Current warming is unprecedented in at least 2000 years
-Recent warming rate (last 50 years) is extremely rapid: spiralling
-Likely warmer than any multi-century period of the Holocene (last 11,700 years)
-Comparable to temperatures during the Last Interglacial (~125,000 years ago)
Past climate variations
• Little Ice Age: linked to volcanic
activity and low solar output
Current warming
• Much faster than natural changes
in the last 2000 years
• Occurring during the
Anthropocene (human-dominated
era)
Anomalies
Regional Differences
Warming is not uniform:
-Land warms faster than oceans (water moderates temperature)
-Urban heat-island effect increases city temperatures (e.g., Brussels/Uccle)
-Some regions show warming roughly double the global average
E.g: Uccle: total warming of almost 3°C since the mid-19th century= double of total average
Temporary Cooling or Slower Warming Periods
Example: 1940–1960
Cause: Global dimming due to air pollution aerosols
-Aerosols reflect sunlight
-Masked underlying greenhouse warming temporarily
Climate models show:
Natural factors alone cannot reproduce observed warming
Observations match simulations only when human influence is included
8
,4. Attribution of climate change
Attribution: Causes of Observed Warming
According to the IPCC AR6: Human influence is unequivocally the main cause of warming.
Evidence from climate simulations
Observed warming (1850–2019):
-Can only be reproduced when human factors are included
-Natural factors alone (solar, volcanic, internal variability) cannot explain it
Contribution to observed warming (~ +1.1 °C)
Warming factors
-Greenhouse gases: +1.5 °C
Cooling factors
-Aerosols (pollution): −0.4 °C → Masked 1/3 of warming
Net human influence
→+1.1 °C (matches observations of 1.3°C)
5. Impacts of climate change on the cryosphere and sea level
Cryosphere= all frozen parts of Earth
Includes:
-Snow cover
-Sea ice
-Glaciers
-Ice caps
-Ice sheets (Greenland, Antarctica)
→Importance:
• Controls Earth’s radiation balance (ice reflects sunlight)
• Influences weather and climate globally
• Melting land ice raises sea level
Acts as an early warning system for climate change
Observed Changes in the Cryosphere
Snow cover
-Northern Hemisphere spring snow cover decreasing
~1.1% decline per decade (1922–2018)
9
, Arctic sea ice
-Lowest extent since at least 1850
-Late-summer ice decline unprecedented in ~1000 years
Mountain glaciers
-Global retreat since late 19th century
-Synchronous worldwide retreat since 1950s
(unprecedented in ≥2000 years)
-Important freshwater source in dry regions
E.g glacier retreat in the Alps: Morteratsch Glacier
Polar Ice Sheets (Largest Sea-Level Potential)
Ice sheets
-gain mass through snowfall
-lose mass through melting
*Antarctic ice sheet
*Greenland ice sheet
The way ice is lost on both ice sheets is not the same
!!!Floating ice loss ≠ sea-level rise, Land ice loss = sea-level rise
10
13 contacturen Hoorcollege
26 contacturen Zelfwerk en -studie
Content
The course consists of about 6 lectures covering the broad theme of 'Global Change', often given by
invited speakers. Typical subjects of the presentations cover the history of the Earth, the palaeoclimate,
and various aspects of global warming (physical, economical, and political)
General competencies
Depending on the specific lectures given in this course, the following competences can be distinguished:
→Gaining insight in various mechanisms that have controlled climate and environmental changes during
the Earth's history.
→Be able to distinguish between anthropogenic and natural processes of climate and environmental
change.
→Be able to understand lectures given by active scientists from different disciplines.
→Be able to see relations between physical and human aspects of global change.
→Develop a critical attitude towards the societal discussion on global change.
→Gaining insight in the interplay between climate and environmental changes and demographic,
technological, socio-economic, and political developments and choices.
Assessment Information
The assessment consists of the following assignment categories:
The Written Examination determines 100% of the final grade.
Within the Written Examination category, the following assignments must be completed:
*Note: Written examination with 1 question per lecture. The final grade is the arithmetic mean of the
individual evaluation per question.
1
, Global Change: lecture 1 - Philippe Huybrechts
Overview:
1. Some basics of the climate system
2. Changes in human and natural drivers of climate
3. Observations of changes in climate: temperature
4. Attribution of climate change
5. Impacts of climate change: the cryosphere and sea level
6. Projections of future climate change
7. Limiting climate change
1.Some basics of the climate system
Climate change→most pressing problem of the century
Why? -connected to so many other problems (conflict, food, …)
The difference between weather and climate
→Climate is the statistic of the weather
weather Climate
=atmospheric condition at a given time and a = mean and extreme conditions of the
given place atmosphere, ocean, sea ice, etc… over a longer
period (e.g. 30 years)
-one component: atmosphere -statistic (mean, average)
-at given time -parameters of the atmosphere and all connected factors
-at given place -usually over a 30year period
-difficult to predict ahead “Lorenz butterfly” (very stochastic
variables, a lot of feedbacks, a lot of interactions)
*Air temperature, precipitation, clouds, air pressure, wind
speed, atmospheric humidity, …
-usually no further then 14days
Interactions are crucial to understanding the climate system
-Atmosphere: laag van gassen rond de
aarde die lucht, weersystemen en klimaat Cryosphere
ondersteunt.
-Hydrosphere: alle water op aarde, inclusief
oceanen, meren, rivieren en grondwater.
-Cryosphere: Alle bevroren waterdelen van
de aarde, zoals gletsjers, ijskappen en
sneeuw.
-Biosphere: Alle levende organismen op
aarde en de ecosystemen waarin ze leven.
-Lithosphere: buitenste, vaste laag van de
aarde, inclusief gesteenten en de aardkorst.
The Earth’s climate system
Big connected
system with
interactions
and feedback…
2
,Formulas
Radiation balance of the Earth
-energy received (from sun)
-energy reflected back to atmosphere/ absorbed by earth system
*imported equation: conservation of energy
1. Solar Input Adjusted by Albedo:
𝑄absorbed = 𝑄(1 − 𝛼)
Only ~70% of solar radiation is absorbed; the rest is reflected.
2. Outgoing Infrared Radiation: Stefan-Boltzmann based equation
𝑅emitted = 𝜀 𝜎 𝑇 4
Not all radiation escapes directly due to the greenhouse effect; ε accounts for atmospheric absorption.
3. Equilibrium Temperature:
At energy balance:
𝑄(1 − 𝛼) = 𝜀 𝜎 𝑇 4
Q = mean incoming solar radiation at the boundary of the atmosphere (340 Wm -2)
α = planetary albedo (0.30)
ε=effective emissivity of the atmosphere (0.61)
*Represents how efficiently the Earth-atmosphere system emits infrared radiation to space.
Accounts for the greenhouse effect: some outgoing radiation is absorbed and re-emitted by
greenhouse gases, so not all reaches space directly.
For Earth today: 𝜀 ≈ 0.61, meaning about 61% of emitted IR radiation actually escapes to space.
ϭ = Boltzmann constant (5.67 10-8 Wm-2K-4)
T = mean global surface temperature (in K)
Calculation of the global mean temperature
=above formula solved for T
How to change the climate? Factors in equation
→ Solar radiation (Q): More sunlight → warmer
Weaker Sun (Q: −1%):
T = 286.8 K = +14.1 °C
current radiative fluxes:
T = 287.5 K = +14.8 °C→minor effect
→albedo (α): Higher reflection → cooler (Ice, snow, clouds)
Higher planetary albedo during the ice ages (α = 0.38):
T = 279.4 K = +6.3 °C
→Greenhouse effect(ε): BIGGEST FACTOR Lower ε → warmer, higher ε → cooler
Without greenhouse effect (ε = 1):
T = 253.4 K = −18.6 °C
*Greenhouse effect: natural property of the atmosphere
=re-emission of sunlight at lower frequency in all directions in the planet’s atmosphere
!!!! Is needed for the current life on earth
3
,The greenhouse effect
The Earth’s temperature without greenhouse
effect: T = -19°C
The Earth’s temperature with natural greenhouse
effect: T = +15°C
Absorption spectra of greenhouse gases in the
near infrared
Earth’s Radiation vs. Planck Curve
→Smooth lines (Planck curves) represent the
ideal thermal radiation emitted by Earth’s surface
if it behaved like a perfect blackbody at a given
temperature. These curves come from Planck’s
law, which describes how much electromagnetic
radiation a body emits at each wavelength
depending on its temperature.
For Earth (~288 K average surface temperature):
The peak emission occurs in the infrared region (around 10 micrometers wavelength).
This is why greenhouse gases matter, they absorb infrared radiation.
Actual Emission (Jagged Line)
It deviates from the smooth Planck curve because Earth’s
atmosphere absorbs radiation at specific wavelengths.
What causes the dips?
Different gases absorb different infrared wavelengths:
→CO₂ (carbon dioxide) strongly absorbs around 15 μm,
close to Earth’s peak emission → very important for
warming.
→H₂O vapor (water vapor) absorbs across many infrared
bands.
→O₃ (ozone), CH₄ (methane), and others also absorb in
specific regions.
Water Vapor: The Most Important Greenhouse Gas
Water vapor contributes the largest share of the natural greenhouse
effect because: -It is abundant
-It absorbs across broad infrared ranges
However policy isn’t focused on H₂O?
Because water vapor is not directly controlled by
human emissions.
Its concentration depends mainly on temperature.
Warm air can hold more moisture (Clausius–
Clapeyron relation).
So water vapor acts as a feedback, not a forcing
→Positive feedback loop:
CO₂ (and other gases) warm the planet.
Hotter → more H₂O → stronger greenhouse effect →
hotter
Why Focus on CO₂ and Other Anthropogenic Gases
Because they are:
Anthropogenic (human-caused) forcings
Capable of changing Earth’s energy balance independently
CO₂ acts like a “control knob” for climate:
Increase CO₂ → temperature rises → water vapor increases → amplifies warming
So: CO₂ triggers the warming, water vapor amplifies it
4
, This figure shows how doubling atmospheric CO₂
changes Earth’s energy balance and surface
temperature in steps.
(a) Initial state — balanced climate
Earth receives about 240 W/m² of solar energy (S) and
emits the same amount as infrared heat (L). Because
incoming and outgoing energy are equal, the average
surface temperature is stable at about 15 °C.
(b) Immediately after CO₂ doubles
Incoming solar radiation is unchanged (CO₂ barely
affects sunlight), but more outgoing infrared radiation is
absorbed by the atmosphere. Therefore, less heat
escapes to space (L drops to 236 W/m²). This creates an
energy surplus (more in than out), meaning the planet
must warm.
(c) Direct warming (no feedbacks)
As the surface warms, it emits more infrared radiation (hotter objects radiate more). The temperature rises
by about 1.2 °C, which restores the balance so outgoing energy again equals incoming (240 W/m²). This is
the warming caused by CO₂ alone.
(d) Warming with climate feedbacks
Additional processes — especially increased water vapor, reduced ice cover, and cloud changes —
amplify the warming. The surface temperature rises further (to about +2.5 °C) until energy balance is again
restored.
In short: Doubling CO₂ first traps extra heat (energy imbalance), then Earth warms until it can radiate that
extra energy away, and feedback mechanisms make the final warming larger than the direct effect alone.
2. Changes in human and natural drivers
Drivers of climate change
• Changes of the atmospheric concentrations of carbon dioxide, methane, and nitrogen oxide
over the last 2000 years
-anthropogenic
-main human caused greenhouse gasses
→clear rise of all
greenhouse gasses during
industrial revolution
*bacteria in
microbiome excrete
N2O
=intergovernmental panel for climate change
*replantation of trees or eating vegetarian
does a little but most is caused by
industry→ company’s need to take
responsibility.
5
,CO2
→Carbon dioxide concentrations have increased by more than 50% since pre-industrial times
*recent measurements: only recently started recording (at Hawaiian volcano)
*older proof: fluctuations of CO2 in earth’s history& current rise through studying of geological layers
Evidence for human caused rise in CO2?
→The current atmospheric concentrations of CO2 have not been experienced for at least 2 million years
(high confidence)
Human Disruption of the Carbon Cycle (2010–2019)
Primary sources of anthropogenic CO₂ emissions -Fossil fuel combustion: 34.8 GtCO₂/year
-Deforestation and land-use change: 4.1 GtCO₂/year
Mechanism: How the Carbon Cycle Is Disrupted
Natural carbon cycle:
Carbon exchanges between reservoirs:
• Atmosphere
• Land biosphere
• Ocean
Normally, inputs ≈ outputs (balanced fluxes).
Human perturbation:
• Extra emissions increase carbon input to the
atmosphere
• Natural sinks cannot fully compensate
→Result: More CO₂ enters the atmospheric reservoir
than leaves it: system out of equilibrium
Distribution of Carbon Between Reservoirs
Fate of anthropogenic CO2 emissions (2014-2023 average)
Sources Sinks
Fossil fuels 90% Atmosphere 48%
35.6 GtCO2/yr *accumulation 19.2 GtCO2/yr
deforestation 10% Terrestrial biosphere 29%
4.1 GtCO2/yr *vegetation and soil 11.7 GtCO2/yr
absorption
Ocean 26%
*major long term sink 10.5 GtCO2/yr
via dissolution and
biological processes
Budget imbalance: -4%, -1.6 GtCO2/yr
Global Carbon Budget from 1850 to 2023
The cumulative contributions to the
global carbon budget from 1850
The carbon imbalance represents the
gap in our current understanding of
sources & sinks
6
,Atmospheric Oxygen Decline
Observed simultaneously: CO₂ concentration ↑ & O₂ concentration ↓
→ Combustion reaction: C + O₂ → CO₂
This chemical fingerprint directly implicates fossil fuel burning.
Role of aerosols (suspended particles in the air)
→negative feedback of warming: more aerosols= more clouds=more albedo= lower temperature
-direct effect: scattering and absorbing shortwave and longwave radiation
-indirect effect: modifying the radiative properties, amount, and lifetime of clouds
Aerosols have a net cooling effect on climate (negative radiative forcing)
Aerosols have a short lifetime (maximally a few years)
Radiative forcing is the change in Earth’s energy balance caused by a factor that affects how much
energy enters or leaves the climate system.
-positive radiative forcing= warming
IPCC AR6 (2021): Total anthropogenic radiative forcing for 2019 relative to 1750 is +2.72 [1.96 to 3.48]
Wm−2, and it has increased more rapidly since 1970 than during prior decades
*this already keeps in mind the positive and negative feedback loops
7
,3. Observations of changes in the climate system Temperature
Global Temperature Rise
-Earth’s surface temperature increased by ~1.3 °C (1880–2025)
-2024 was the warmest year recorded; 2025 the second warmest
-25 of the 26 warmest years occurred in the 21st century
Long-Term Historical Context
-Current warming is unprecedented in at least 2000 years
-Recent warming rate (last 50 years) is extremely rapid: spiralling
-Likely warmer than any multi-century period of the Holocene (last 11,700 years)
-Comparable to temperatures during the Last Interglacial (~125,000 years ago)
Past climate variations
• Little Ice Age: linked to volcanic
activity and low solar output
Current warming
• Much faster than natural changes
in the last 2000 years
• Occurring during the
Anthropocene (human-dominated
era)
Anomalies
Regional Differences
Warming is not uniform:
-Land warms faster than oceans (water moderates temperature)
-Urban heat-island effect increases city temperatures (e.g., Brussels/Uccle)
-Some regions show warming roughly double the global average
E.g: Uccle: total warming of almost 3°C since the mid-19th century= double of total average
Temporary Cooling or Slower Warming Periods
Example: 1940–1960
Cause: Global dimming due to air pollution aerosols
-Aerosols reflect sunlight
-Masked underlying greenhouse warming temporarily
Climate models show:
Natural factors alone cannot reproduce observed warming
Observations match simulations only when human influence is included
8
,4. Attribution of climate change
Attribution: Causes of Observed Warming
According to the IPCC AR6: Human influence is unequivocally the main cause of warming.
Evidence from climate simulations
Observed warming (1850–2019):
-Can only be reproduced when human factors are included
-Natural factors alone (solar, volcanic, internal variability) cannot explain it
Contribution to observed warming (~ +1.1 °C)
Warming factors
-Greenhouse gases: +1.5 °C
Cooling factors
-Aerosols (pollution): −0.4 °C → Masked 1/3 of warming
Net human influence
→+1.1 °C (matches observations of 1.3°C)
5. Impacts of climate change on the cryosphere and sea level
Cryosphere= all frozen parts of Earth
Includes:
-Snow cover
-Sea ice
-Glaciers
-Ice caps
-Ice sheets (Greenland, Antarctica)
→Importance:
• Controls Earth’s radiation balance (ice reflects sunlight)
• Influences weather and climate globally
• Melting land ice raises sea level
Acts as an early warning system for climate change
Observed Changes in the Cryosphere
Snow cover
-Northern Hemisphere spring snow cover decreasing
~1.1% decline per decade (1922–2018)
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, Arctic sea ice
-Lowest extent since at least 1850
-Late-summer ice decline unprecedented in ~1000 years
Mountain glaciers
-Global retreat since late 19th century
-Synchronous worldwide retreat since 1950s
(unprecedented in ≥2000 years)
-Important freshwater source in dry regions
E.g glacier retreat in the Alps: Morteratsch Glacier
Polar Ice Sheets (Largest Sea-Level Potential)
Ice sheets
-gain mass through snowfall
-lose mass through melting
*Antarctic ice sheet
*Greenland ice sheet
The way ice is lost on both ice sheets is not the same
!!!Floating ice loss ≠ sea-level rise, Land ice loss = sea-level rise
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