Global change
1 Climate change: the physical science basis
1.1 Some basics of the climate system
1.1.1 Difference between weather and climate
Weather:
- refers to the atmospheric conditions at a specific time and location
- Includes variables such as: air temperature, precipitation, cloud cover, air pressure, wind speed and atmospheric humidity
- Can only observe short-term variations and doesn’t reflect long-term trends
- Highly chaotic and unpredictable beyond 10-14 days due to the complexity of atmospheric interactions
à illustrated by Lorenz butterfly effect= a small change in initial conditions (butterfly flapping its wings) can lead to
major differences in outcomes (like a storm forming days later)
Climate:
- Describes the average and extreme atmospheric, oceanic and cryospheric (ice-related) conditions over a longer time (30 years)
- Not chaotic, more stable and statistically predicable
- Based on aggregated weather data, easier to analyse and forecast long-term patterns
à example: predict average lifespan of population but not of an individual
à Climate is the statistic of the weather:
The climate represents long-term patterns and averages derived from daily weather data over extended periods, which focusses
on trends and is more predictable because it looks at the overall distribution of many weather events. Like analysing thousands
of dice rolls instead of trying to predict a single roll.
1.1.2 Interactions of the earth’s climate system
The earth’s climate system is made up of five major components:
- Atmosphere: layer of gases surrounding Earth (air, clouds, winds) (N2, O2, Ar, H2O, CO2, CH4, N2O, O3)
- Hydrosphere: all liquid water on Earth’s surface (oceans, rivers, lakes)
- Cryosphere: frozen water parts (glaciers, sea ice, snow, permafrost)
- Biosphere: all living organisms, vegetation and ecosystems
- Lithosphere: the solid, rocky outer layer of Earth (land surfaces, mountains)
à interactions (double arrows): two-way influences between components. A change in one part of the system leads to a
response in another, which can then feed back to the original system (feedbacks)
à Forcings (single arrows): one-way changes where a variation in one system forces a change in another, but not necessary
the other way around
Key Climate System Interactions:
- Solar Input (Forcing): Sun is main energy source; changes affect temperature, circulation, evaporation, and photosynthesis.
- Atmosphere Interactions:
à Atmosphere ↔ Ice: Warming melts ice → lowers albedo →
further warming.
à Atmosphere ↔ Hydrosphere (Oceans/Rivers):
Exchanges of heat, moisture, and momentum affect storms,
humidity, and water availability.
à Atmosphere ↔ Land: Surface type (soil, vegetation)
impacts energy absorption and local temperature.
à Atmosphere ↔ Biosphere: Plant processes
(photosynthesis, transpiration) affect carbon and water cycles;
climate affects plant health.
- Ocean Interactions & Feedbacks:
à Ocean ↔ Ice: Melting sea ice reduces salinity, disrupts
ocean circulation (thermohaline system), and raises sea level.
à Ocean changes (currents, chemistry) influence global
heat distribution and CO₂ uptake.
- Cryosphere Changes (Forcing Response):
à Melting of snow, glaciers, permafrost, and ice sheets affects
albedo, sea level, and ocean freshwater content.
à Volcanic Activity (Forcing): Eruptions release aerosols/gases, temporarily cooling Earth by blocking sunlight.
- Human Activities (Forcing):
àGreenhouse gas emissions, deforestation, and land use change alter atmosphere, land surface, and biosphere.
- Atmospheric Changes:
1
, à Composition: More greenhouse gases due to human activity.
à Circulation: Shifts in wind patterns, e.g., jet streams.
- Hydrological Cycle Changes:
Increased evaporation, stronger rainfall, shifting cloud patterns, and melting snow/ice due to warming.
- Land Surface Changes:
à Natural: Mountain building, erosion.
à Human: Urbanization, agriculture, and deforestation change albedo, water cycles, and carbon uptake.
1.1.3 Radiation balance of the Earth
Radiation balance of the earth= how the planet maintains a stable temperature
over time through the conservation of energy. The amount of energy the earth
receives from the sun (incoming radiation) must be equal to the amount it emits back
into space (outgoing radiation):
Based on Stefan-Boltzmann law= object energy emission increases rapidly with
temperature. Disturbed by changes in albedo, increase greenhouse gases and
variations in solar input.
Incoming >outgoing =warms
< = cools
à key understanding of global warming
1.1.4 Calculation of the global mean temperature
Based on energy balance equation: estimated global mean temperature based on solar input,
albedo, atmospheric emissivity and Boltzmann cst.
Current radiative balance (with real greenhouse effect, albedo = 0.3, emissivity = 0.61) à
This is the current global average temperature.
Without the greenhouse effect (ε = 1, meaning no trapping of heat):
àThe Earth would be a frozen planet without atmospheric insulation.
Weaker Sun (Q decreases by 1%):
à Only a small drop in global temperature; the Sun's output is very stable
Higher albedo (like during the Ice Ages, α = 0.38):
à Reflecting more sunlight results in a significantly colder planet
à CORE CONCEPT: 3 fundamental ways to change earth’s temperature
- Solar input(Q): hard to influence naturaly or artificialy (more ice and clouds= higher albedo= cooler earth
- Albedo (𝜶): influenced by ice cover, vegetation, cloudiness, desertification, deforestation
- Atmospheric emissivity (ε): (= captures amount of earth’s emited IR radiation escpaes into space and atmosphere)
affected by greenhouse gasses (lower ε = more trapped heat= warmer earth
1.1.5 The greenhouse effect
= a natural property of the Earth’s atmosphere, plays a vital role in maintaining habitable temperature on Earth. Sun emits
short-wave (SW) radiation, which passes through atmosphere, transparent to this type of energy. Half of this incoming solar
radiation absorbed by Earth’s surface, warming it. Earth then emits long-wave (infrared or IR) radiation back toward space.
Greenhouse gases (such as CO₂, H₂O, O₃, clouds) absorb large portion of this outgoing IR radiation.These gases re-emit the
absorbed radiation in all directions, including back toward the Earth’s surface. Process traps heat near the surface, acting
like a blanket, and keeps Earth warmer.
Impact: - With the natural greenhouse effect:
à Earth’s average surface temperature = +15°C
- Without the greenhouse effect
à Earth’s average surface temperature = –19°C
Essential for life — without it, the Earth would be an uninhabitable frozen planet.
When enhanced by human activities (like burning fossil fuels), the greenhouse effect becomes
stronger, leading to global warming.
1.1.6 Absorption spectra of greenhouse gases in the near
infrared
This graph shows how various greenhouse gases absorb Earth's emitted infrared (IR) radiation in the near-infrared region,
revealing their impact on Earth's energy balance and greenhouse effect.
2
, Smooth curves = Theoretical Planck curves representing blackbody radiation
from Earth’s surface at different temperatures (e.g., 300K, 275K, etc.).
Jagged line = The actual outgoing radiation measured from Earth.
The difference between the smooth and jagged lines = Radiation absorbed
by greenhouse gases in the atmosphere.
- CO₂: Major absorption around 15 μm (wavelength).Absorbs exactly where
Earth's IR emission is at its peak, making it especially effective in trapping
heat. Not the most abundant gas
- H₂O (Water vapor):Absorbs over broad ranges, the far ends of the
spectrum.The most abundant greenhouse gas and contributes strongly, but varies with humidity.
- O₃ (Ozone): Absorbs in a narrow band (~9.6 μm). Present in the atmospheric window, interrupting the radiation that would
otherwise escape directly.
- Chlorine, fluorine, and hydrocarbons (CFCs, HFCs): Also absorb IR radiation, present in trace amounts.
Very high global warming potential (GWP).
Atmospheric window: A region in the IR spectrum (~8–12 μm) where radiation escapes more easily into space.
Crucial for Earth's cooling. This window is partially blocked by O₃
Total absorption: Around 14–16 μm, nearly 100% of IR radiation is absorbed, mostly due to CO₂.
Shows why CO₂ is so central in climate change discussions, despite water vapor being more abundant.
1.1.7 Greenhouse gases in the atmosphere + water vapour is a feedback
Water Vapour is a Feedback, Not a Forcing:
à Forcing: direct change to the climate caused by an external factor (e.g., human-emitted CO₂).
à Feedback: A response to a change, which can amplify or reduce its effect.
- Water vapour reacts to warming, it doesn’t cause it on its own.
- Strongest natural greenhouse gas: H₂O Vapour: 60%, CO₂: 26%, O₃: 8%, CH₄ + NO₂: 6%
àabsorbs the most outgoing longwave radiation, significantly contributing to warming.
- Why do we focus on CO₂? à Humans cannot directly control water vapour levels in the atmosphere.
What we can change is the CO₂ concentration
CO₂ increases atmospheric temperature, which leads to: → More evaporation, More water vapour in the air
, Stronger greenhouse effect, More warming, Positive feedback loop
-Clausius-Clapeyron Relationship: warmer air can hold more water vapour.
à temperature increases, atmosphere stores more water vapour which in turn amplifies the warming.
Summary of the Feedback Loop:
Initial CO₂ increase (due to human activity)
àWarming of the atmosphere, Increased evaporation → more water vapour, More greenhouse trapping from water
vapour, Further warming
1.1.8 Effect of doubling the CO2 concentration
The Earth's climate system must maintain energy balance between:Incoming shortwave radiation (S) from the Sun and
Outgoing longwave radiation (L) from Earth
Panel Description CO₂? Temperature (Ts) Radiation (W/m²)
(a) Pre-industrial balance Normal 15°C S = L = 240
(b) CO₂ doubled → less outgoing radiation x2 15°C S = 240, L = 236
(c) Earth warms to restore balance x2 15°C + 1.2°C L rises back to 240
(d) Feedbacks kick in (mainly water vapour) x2 + feedbacks ~15°C + 2.5°C Radiative balance maintained
: - (a), Earth is in radiative equilibrium: 240 W/m² incoming = 240 W/m² outgoing
à Average global temperature = 15°C
- When CO₂ concentration is doubled (b):More longwave radiation is absorbed by the atmosphere and Only 236 W/m²
escapes, while 240 W/m² continues coming in
àResult: Energy imbalance → warming begins
- As Earth warms (c), outgoing radiation increases (since emission depends strongly on T).
Eventually, at +1.2°C, reach 240 W/m² again -à equilibrium is restored.
- Now feedbacks are triggered (d): Water vapour feedback à Warmer air holds more water vapour → More greenhouse
effect → More warming
3
, - Final outcome: +2.5°C warming, on average. = climate sensitivity the expected global temperature rise if CO₂ doubles.
Radiative Forcing: The imbalance caused by increased CO₂
Radiative Equilibrium: Balance between incoming and outgoing radiation
Climate Sensitivity: Average global surface temperature increases expected when CO₂
concentration doubles (typically +2–3°C)
Feedbacks: Secondary effects triggered by temperature change (e.g., water vapour, ice
albedo)
1.2 Changes in human and natural drivers
1.2.1 Changes atmospheric concentration CO2, CH4, N2O
The concentrations greenhouse gases – CO₂ (carbon dioxide), CH₄ (methane), and N₂O (nitrogen oxide) – remained fairly
stable over the past 2000 years until 1750, the start of the Industrial Revolution. Since then, there's been a sharp increase in
all three gases, primarily due to human activities (=anthropogenic).
Causes of the Increase:- CO₂:Burning of fossil fuels, Cement production, Deforestation
- CH₄ Agriculture, Waste management, released from landfills
- N₂O: Use of fertilizers, Livestock farming
CO₂ concentration has risen by nearly 50% since pre-industrial times (measured Mauna
Loa Hawaii).The current CO₂ level 415 ppm (parts per million)( rise 100 ppm in 50 years).
There is a seasonal fluctuation: in winter, CO₂ levels rise due to fewer leaves and less
photosynthesis.
Consequences: The rise in gases leads to an increase in global temperature
(the greenhouse effect).
1.2.2 Human perturbation of the carbon cycle
The natural carbon cycle is typically balanced, with carbon moving between the land, ocean, and atmosphere.
However, human emissions have tipped this balance, causing a net accumulation of CO₂ in the atmosphere. Net increase
in the atmosphere: 18.6 GtCO₂/year in the atmosphere as a result of the imbalance
The primary anthropogenic (human-caused) source of CO₂ in the atmosphere is the combustion of
fossil fuels (88% of human-made CO2)(12% deforestation) such as coal, oil, and natural gas.
When these fuels are burned, carbon (C) reacts with atmospheric oxygen (O₂) to form carbon
dioxide (CO₂): C + O₂ → CO₂
à measurable increase in atmospheric CO₂ and a simultaneous decrease in O₂
concentrations.Clear inverse relationship between CO₂ and O₂.
Once emitted, CO₂ is distributed into three main carbon sinks: atmosphere 47% (18.9 GtCO₂/year) very long time,
driving global warming., terrestrial biosphere (plants, soils, forests) 31% (12.3 GtCO₂/year) plants use CO₂ during
photosynthesis, a natural CO₂ sink , ocean 26% (10.4 GtCO₂/year) regulating the climate by dissolving CO₂ into seawater.
Despite these sinks, the sources and sinks don’t perfectly match, resulting in a budget imbalance of about 4% (1.6
GtCO₂/year). This mismatch highlights gaps in our understanding of how carbon moves through
Earth's systems.
Not all emitted CO₂ is absorbed. There is a 4% imbalance in the carbon budget, meaning around 4%
of the anthropogenic CO₂ remains unaccounted for, contributing further to atmospheric CO₂
buildup, feedback loop that accelerates global warming.
1.2.3 Role of aerosols (suspended particles in the air)
Aerosols=tiny suspended particles in the air that can be natural or anthropogenic in origin.
Sources of Aerosols: - Anthropogenic sources: Burning of fossil fuels and biomass (soot).
- Natural sources: Desert dust, sea salt (from ocean spray), and volcanic eruptions.
Effects of Aerosols on Climate:
- Direct Effect: Aerosols scatter and absorb sunlight, reducing the amount of solar radiation reaching Earth’s surface
→ cooling effect.
- Cloud Albedo Effect (1st Indirect Effect):More aerosols = more cloud droplets = brighter clouds that reflect more sunlight
→ increased albedo and cooling.
- Drizzle Suppression (2nd Indirect Effect):Smaller droplets reduce rainfall → clouds last longer and reflect sunlight for a
longer time → extended cooling.
- Increased Cloud Height:Aerosols cause clouds to form higher in the atmosphere → may affect how much heat gets trapped
or reflected.
- Increased Cloud Lifetime:Clouds live longer due to less rain formation → they keep reflecting sunlight for more time
→ reinforces cooling.
4
1 Climate change: the physical science basis
1.1 Some basics of the climate system
1.1.1 Difference between weather and climate
Weather:
- refers to the atmospheric conditions at a specific time and location
- Includes variables such as: air temperature, precipitation, cloud cover, air pressure, wind speed and atmospheric humidity
- Can only observe short-term variations and doesn’t reflect long-term trends
- Highly chaotic and unpredictable beyond 10-14 days due to the complexity of atmospheric interactions
à illustrated by Lorenz butterfly effect= a small change in initial conditions (butterfly flapping its wings) can lead to
major differences in outcomes (like a storm forming days later)
Climate:
- Describes the average and extreme atmospheric, oceanic and cryospheric (ice-related) conditions over a longer time (30 years)
- Not chaotic, more stable and statistically predicable
- Based on aggregated weather data, easier to analyse and forecast long-term patterns
à example: predict average lifespan of population but not of an individual
à Climate is the statistic of the weather:
The climate represents long-term patterns and averages derived from daily weather data over extended periods, which focusses
on trends and is more predictable because it looks at the overall distribution of many weather events. Like analysing thousands
of dice rolls instead of trying to predict a single roll.
1.1.2 Interactions of the earth’s climate system
The earth’s climate system is made up of five major components:
- Atmosphere: layer of gases surrounding Earth (air, clouds, winds) (N2, O2, Ar, H2O, CO2, CH4, N2O, O3)
- Hydrosphere: all liquid water on Earth’s surface (oceans, rivers, lakes)
- Cryosphere: frozen water parts (glaciers, sea ice, snow, permafrost)
- Biosphere: all living organisms, vegetation and ecosystems
- Lithosphere: the solid, rocky outer layer of Earth (land surfaces, mountains)
à interactions (double arrows): two-way influences between components. A change in one part of the system leads to a
response in another, which can then feed back to the original system (feedbacks)
à Forcings (single arrows): one-way changes where a variation in one system forces a change in another, but not necessary
the other way around
Key Climate System Interactions:
- Solar Input (Forcing): Sun is main energy source; changes affect temperature, circulation, evaporation, and photosynthesis.
- Atmosphere Interactions:
à Atmosphere ↔ Ice: Warming melts ice → lowers albedo →
further warming.
à Atmosphere ↔ Hydrosphere (Oceans/Rivers):
Exchanges of heat, moisture, and momentum affect storms,
humidity, and water availability.
à Atmosphere ↔ Land: Surface type (soil, vegetation)
impacts energy absorption and local temperature.
à Atmosphere ↔ Biosphere: Plant processes
(photosynthesis, transpiration) affect carbon and water cycles;
climate affects plant health.
- Ocean Interactions & Feedbacks:
à Ocean ↔ Ice: Melting sea ice reduces salinity, disrupts
ocean circulation (thermohaline system), and raises sea level.
à Ocean changes (currents, chemistry) influence global
heat distribution and CO₂ uptake.
- Cryosphere Changes (Forcing Response):
à Melting of snow, glaciers, permafrost, and ice sheets affects
albedo, sea level, and ocean freshwater content.
à Volcanic Activity (Forcing): Eruptions release aerosols/gases, temporarily cooling Earth by blocking sunlight.
- Human Activities (Forcing):
àGreenhouse gas emissions, deforestation, and land use change alter atmosphere, land surface, and biosphere.
- Atmospheric Changes:
1
, à Composition: More greenhouse gases due to human activity.
à Circulation: Shifts in wind patterns, e.g., jet streams.
- Hydrological Cycle Changes:
Increased evaporation, stronger rainfall, shifting cloud patterns, and melting snow/ice due to warming.
- Land Surface Changes:
à Natural: Mountain building, erosion.
à Human: Urbanization, agriculture, and deforestation change albedo, water cycles, and carbon uptake.
1.1.3 Radiation balance of the Earth
Radiation balance of the earth= how the planet maintains a stable temperature
over time through the conservation of energy. The amount of energy the earth
receives from the sun (incoming radiation) must be equal to the amount it emits back
into space (outgoing radiation):
Based on Stefan-Boltzmann law= object energy emission increases rapidly with
temperature. Disturbed by changes in albedo, increase greenhouse gases and
variations in solar input.
Incoming >outgoing =warms
< = cools
à key understanding of global warming
1.1.4 Calculation of the global mean temperature
Based on energy balance equation: estimated global mean temperature based on solar input,
albedo, atmospheric emissivity and Boltzmann cst.
Current radiative balance (with real greenhouse effect, albedo = 0.3, emissivity = 0.61) à
This is the current global average temperature.
Without the greenhouse effect (ε = 1, meaning no trapping of heat):
àThe Earth would be a frozen planet without atmospheric insulation.
Weaker Sun (Q decreases by 1%):
à Only a small drop in global temperature; the Sun's output is very stable
Higher albedo (like during the Ice Ages, α = 0.38):
à Reflecting more sunlight results in a significantly colder planet
à CORE CONCEPT: 3 fundamental ways to change earth’s temperature
- Solar input(Q): hard to influence naturaly or artificialy (more ice and clouds= higher albedo= cooler earth
- Albedo (𝜶): influenced by ice cover, vegetation, cloudiness, desertification, deforestation
- Atmospheric emissivity (ε): (= captures amount of earth’s emited IR radiation escpaes into space and atmosphere)
affected by greenhouse gasses (lower ε = more trapped heat= warmer earth
1.1.5 The greenhouse effect
= a natural property of the Earth’s atmosphere, plays a vital role in maintaining habitable temperature on Earth. Sun emits
short-wave (SW) radiation, which passes through atmosphere, transparent to this type of energy. Half of this incoming solar
radiation absorbed by Earth’s surface, warming it. Earth then emits long-wave (infrared or IR) radiation back toward space.
Greenhouse gases (such as CO₂, H₂O, O₃, clouds) absorb large portion of this outgoing IR radiation.These gases re-emit the
absorbed radiation in all directions, including back toward the Earth’s surface. Process traps heat near the surface, acting
like a blanket, and keeps Earth warmer.
Impact: - With the natural greenhouse effect:
à Earth’s average surface temperature = +15°C
- Without the greenhouse effect
à Earth’s average surface temperature = –19°C
Essential for life — without it, the Earth would be an uninhabitable frozen planet.
When enhanced by human activities (like burning fossil fuels), the greenhouse effect becomes
stronger, leading to global warming.
1.1.6 Absorption spectra of greenhouse gases in the near
infrared
This graph shows how various greenhouse gases absorb Earth's emitted infrared (IR) radiation in the near-infrared region,
revealing their impact on Earth's energy balance and greenhouse effect.
2
, Smooth curves = Theoretical Planck curves representing blackbody radiation
from Earth’s surface at different temperatures (e.g., 300K, 275K, etc.).
Jagged line = The actual outgoing radiation measured from Earth.
The difference between the smooth and jagged lines = Radiation absorbed
by greenhouse gases in the atmosphere.
- CO₂: Major absorption around 15 μm (wavelength).Absorbs exactly where
Earth's IR emission is at its peak, making it especially effective in trapping
heat. Not the most abundant gas
- H₂O (Water vapor):Absorbs over broad ranges, the far ends of the
spectrum.The most abundant greenhouse gas and contributes strongly, but varies with humidity.
- O₃ (Ozone): Absorbs in a narrow band (~9.6 μm). Present in the atmospheric window, interrupting the radiation that would
otherwise escape directly.
- Chlorine, fluorine, and hydrocarbons (CFCs, HFCs): Also absorb IR radiation, present in trace amounts.
Very high global warming potential (GWP).
Atmospheric window: A region in the IR spectrum (~8–12 μm) where radiation escapes more easily into space.
Crucial for Earth's cooling. This window is partially blocked by O₃
Total absorption: Around 14–16 μm, nearly 100% of IR radiation is absorbed, mostly due to CO₂.
Shows why CO₂ is so central in climate change discussions, despite water vapor being more abundant.
1.1.7 Greenhouse gases in the atmosphere + water vapour is a feedback
Water Vapour is a Feedback, Not a Forcing:
à Forcing: direct change to the climate caused by an external factor (e.g., human-emitted CO₂).
à Feedback: A response to a change, which can amplify or reduce its effect.
- Water vapour reacts to warming, it doesn’t cause it on its own.
- Strongest natural greenhouse gas: H₂O Vapour: 60%, CO₂: 26%, O₃: 8%, CH₄ + NO₂: 6%
àabsorbs the most outgoing longwave radiation, significantly contributing to warming.
- Why do we focus on CO₂? à Humans cannot directly control water vapour levels in the atmosphere.
What we can change is the CO₂ concentration
CO₂ increases atmospheric temperature, which leads to: → More evaporation, More water vapour in the air
, Stronger greenhouse effect, More warming, Positive feedback loop
-Clausius-Clapeyron Relationship: warmer air can hold more water vapour.
à temperature increases, atmosphere stores more water vapour which in turn amplifies the warming.
Summary of the Feedback Loop:
Initial CO₂ increase (due to human activity)
àWarming of the atmosphere, Increased evaporation → more water vapour, More greenhouse trapping from water
vapour, Further warming
1.1.8 Effect of doubling the CO2 concentration
The Earth's climate system must maintain energy balance between:Incoming shortwave radiation (S) from the Sun and
Outgoing longwave radiation (L) from Earth
Panel Description CO₂? Temperature (Ts) Radiation (W/m²)
(a) Pre-industrial balance Normal 15°C S = L = 240
(b) CO₂ doubled → less outgoing radiation x2 15°C S = 240, L = 236
(c) Earth warms to restore balance x2 15°C + 1.2°C L rises back to 240
(d) Feedbacks kick in (mainly water vapour) x2 + feedbacks ~15°C + 2.5°C Radiative balance maintained
: - (a), Earth is in radiative equilibrium: 240 W/m² incoming = 240 W/m² outgoing
à Average global temperature = 15°C
- When CO₂ concentration is doubled (b):More longwave radiation is absorbed by the atmosphere and Only 236 W/m²
escapes, while 240 W/m² continues coming in
àResult: Energy imbalance → warming begins
- As Earth warms (c), outgoing radiation increases (since emission depends strongly on T).
Eventually, at +1.2°C, reach 240 W/m² again -à equilibrium is restored.
- Now feedbacks are triggered (d): Water vapour feedback à Warmer air holds more water vapour → More greenhouse
effect → More warming
3
, - Final outcome: +2.5°C warming, on average. = climate sensitivity the expected global temperature rise if CO₂ doubles.
Radiative Forcing: The imbalance caused by increased CO₂
Radiative Equilibrium: Balance between incoming and outgoing radiation
Climate Sensitivity: Average global surface temperature increases expected when CO₂
concentration doubles (typically +2–3°C)
Feedbacks: Secondary effects triggered by temperature change (e.g., water vapour, ice
albedo)
1.2 Changes in human and natural drivers
1.2.1 Changes atmospheric concentration CO2, CH4, N2O
The concentrations greenhouse gases – CO₂ (carbon dioxide), CH₄ (methane), and N₂O (nitrogen oxide) – remained fairly
stable over the past 2000 years until 1750, the start of the Industrial Revolution. Since then, there's been a sharp increase in
all three gases, primarily due to human activities (=anthropogenic).
Causes of the Increase:- CO₂:Burning of fossil fuels, Cement production, Deforestation
- CH₄ Agriculture, Waste management, released from landfills
- N₂O: Use of fertilizers, Livestock farming
CO₂ concentration has risen by nearly 50% since pre-industrial times (measured Mauna
Loa Hawaii).The current CO₂ level 415 ppm (parts per million)( rise 100 ppm in 50 years).
There is a seasonal fluctuation: in winter, CO₂ levels rise due to fewer leaves and less
photosynthesis.
Consequences: The rise in gases leads to an increase in global temperature
(the greenhouse effect).
1.2.2 Human perturbation of the carbon cycle
The natural carbon cycle is typically balanced, with carbon moving between the land, ocean, and atmosphere.
However, human emissions have tipped this balance, causing a net accumulation of CO₂ in the atmosphere. Net increase
in the atmosphere: 18.6 GtCO₂/year in the atmosphere as a result of the imbalance
The primary anthropogenic (human-caused) source of CO₂ in the atmosphere is the combustion of
fossil fuels (88% of human-made CO2)(12% deforestation) such as coal, oil, and natural gas.
When these fuels are burned, carbon (C) reacts with atmospheric oxygen (O₂) to form carbon
dioxide (CO₂): C + O₂ → CO₂
à measurable increase in atmospheric CO₂ and a simultaneous decrease in O₂
concentrations.Clear inverse relationship between CO₂ and O₂.
Once emitted, CO₂ is distributed into three main carbon sinks: atmosphere 47% (18.9 GtCO₂/year) very long time,
driving global warming., terrestrial biosphere (plants, soils, forests) 31% (12.3 GtCO₂/year) plants use CO₂ during
photosynthesis, a natural CO₂ sink , ocean 26% (10.4 GtCO₂/year) regulating the climate by dissolving CO₂ into seawater.
Despite these sinks, the sources and sinks don’t perfectly match, resulting in a budget imbalance of about 4% (1.6
GtCO₂/year). This mismatch highlights gaps in our understanding of how carbon moves through
Earth's systems.
Not all emitted CO₂ is absorbed. There is a 4% imbalance in the carbon budget, meaning around 4%
of the anthropogenic CO₂ remains unaccounted for, contributing further to atmospheric CO₂
buildup, feedback loop that accelerates global warming.
1.2.3 Role of aerosols (suspended particles in the air)
Aerosols=tiny suspended particles in the air that can be natural or anthropogenic in origin.
Sources of Aerosols: - Anthropogenic sources: Burning of fossil fuels and biomass (soot).
- Natural sources: Desert dust, sea salt (from ocean spray), and volcanic eruptions.
Effects of Aerosols on Climate:
- Direct Effect: Aerosols scatter and absorb sunlight, reducing the amount of solar radiation reaching Earth’s surface
→ cooling effect.
- Cloud Albedo Effect (1st Indirect Effect):More aerosols = more cloud droplets = brighter clouds that reflect more sunlight
→ increased albedo and cooling.
- Drizzle Suppression (2nd Indirect Effect):Smaller droplets reduce rainfall → clouds last longer and reflect sunlight for a
longer time → extended cooling.
- Increased Cloud Height:Aerosols cause clouds to form higher in the atmosphere → may affect how much heat gets trapped
or reflected.
- Increased Cloud Lifetime:Clouds live longer due to less rain formation → they keep reflecting sunlight for more time
→ reinforces cooling.
4
