Chapter 1: the physical science basis
Some basics of the climate system
Weather Atmospheric condition at given time & place
Has chaotic component (‘Lorenz butterfly’)
Climate Mean and extreme conditions of the atmosphere, ocean, sea ice, etc… over a
longer period (e.g. 30 years)
Different spheres interact: lithosphere, atmosphere, hydrosphere, cryosphere, biosphere
The Earth’s climate system
Radiation balance on Earth: Q(1-α) = εσT4
Incoming radiation = outgoing radiation
↔ calculation of the global mean T:
4
𝑄(1−𝛼)
𝜀𝜎
Greenhouse effect
- Solar radiation (in form of visible light) → Earth absorbs & warms up → emits energy back to
space (in form of infrared) → greenhouse gas molecules re-emit IR in all directions → warms
Earth’s surface + lower atmosphere
- = Natural phenomenon BUT human is adding extra gasses ® more IR is trapped
• Earth’s T without greenhouse effect: -19° C
• Earth’s T with natural greenhouse effect: +15° C
Absorption spectra of greenhouse gases in the near infrared
- Smooth lines = Planck curves = theoretical lines that show
how much heat Earth would radiate if there was no
atmosphere. Surface warmer → more heat.
- Jagged line = actual radiation emitted by the Earth.
Difference is heat trapped in atmosphere
- Valleys = areas in spectrum where specific gases in
atmosphere absorb the heat radiation
- If we add more gases ® valleys deeper
1
,Contribution to natural greenhouse effect: 60% H2O vapour, 26% CO2, 8% O2, 6% rest (CH4 & NO2)
® H2O = largest contributor → BUT is not cause of human-driven climate change because:
- Short lifespan: condenses and rains out after few days (CO2 stays much longer in atmosphere)
- Feedback effect, not forcing: T↑ ® water vapour↑ ® T↑↑ ® …
Effect of doubling CO2-concentration:
a Initial state
b CO2 x2 ® greenhouse effect traps more heat ® L↓
c TS↑ + restoring of radiative balance
d Climate feedback (e.g. H2O↑) ® TS↑
Changes in human and natural drivers of climate
Anthropogenic Sources of Greenhouse Gas Emissions
CO2 Fossil fuels, cement, deforestation
CH4 Agriculture waste
NO2 Fertilizers, livestock
- Result detailed measurements in Hawaii: CO2 concentrations have
increased by more than 50% since pre-industrial times
Result atmospheric CO2 during last 3,6 million years: current cCO2 = first
time since 2 million years ago
- Humans have tipped natural carbon cycle out of balance:
• blue arrows = natural annual CO2-transfers (photosynthesis…)
• brown arrows = anthropogenic annual (largest = fossil fuel
combustion)
Main human activity contributing to greenhouse effect = fossil fuel
combustion (71%)
- Increase in CO2 = parallel with decrease in O2 → because CO2 = C + O2
Fate of Anthropogenic CO2-emissions (2014-2023 The cumulative contributions to the global carbon
average) budget from 1850 to 2023
2
,Aerosols
- Direct effect: scatter and absorb radiation
- Indirect effect: modify radiative properties, amount and lifetime of clouds
® They have a net cooling effect ® but a short lifetime (few years)
Radiative forcing
What? A measure of the net change in energy balance of Earth system (in W/m2)
1750-2019: anthropogenic radiative forcing = +2,72 W/m2 (mainly increased from 1970)
Observations of changes in climate: temperature
- Warming Earth’s surface 1880-2025:
• + 1,3° C
• 1st warmest year = 2024; 2nd warmest year = 2025
• 25 of the 26 warmest years in 21st century
- Climate spiral = animation with monthly temperature anomalies
- Warming in Belgium since mid-19th century: +3° C
- Warming in last 1000 years = “hockeystick”
- Comparison on high scale: global mean surface T last decade =
• Highest T of Holocene
• Comparable to T of last interglacial period
Attribution of climate change
- Observed warming only reproduced in simulations including human influence ® human in cause
- Observed warming is caused by greenhouse gas emissions ® 1/3 = masked by aerosols
Impacts of climate change: the cryosphere and sea level
- Cryosphere mostly located in remote places but has global consequences:
• Changes in snow & ice ® affect global radiation balance + weather & climate everywhere
• Melting of land ice ® sea level↑
- Snow cover extent (SCE) = 2D surface area of Earth covered by snow at a specific time
→ SCE in Northern Hemisphere (1922-2018): decreased 1.1% per decade
- Current Arctic sea ice coverage levels annual mean and late-summer values since at least 1850:
are the lowest
3
, - Importance glaciers:
• Freshwater storage & supply (especially in summer in arid regions)
• Reflect solar energy back to space (albedo)
Polar ice sheets
Greenland ice sheet Antarctic ice sheet
- Volume: 7,4 m sea level equivalent - Volume: 58,3 m sea level equivalent
- Maximum thickness: 3300 m - Maximum ice thickness: 4776 m
- First ice: ͂7 m/y ago - First ice: ͂35 m/y ago
- Full ice sheet: 3 m/y ago - Permanent ice sheet since: 14 m/y ago
1. Evolution of Greenland ice sheet:
Small central thickening & much larger marginal thinning (mainly along major glaciers in SE and
W)
2. Evolution of Antarctic ice sheet:
- Small central thickening & peripheral thinning (mainly in Amundsen
Sea sector of West Antarctic Ice Sheet) => overall mass lost
- Causes of mass losses of West Antarctic outlet glaciers:
• Mainly: ice-shelf basal melt
• Locally: strong surface melt followed by ice-shelf disintegration
(Ice shelf = a floating sheet of ice permanently attached to a land mass)
3. Statistics for 2 ice sheets (up to 2021)
- Since 1990s: mass lost at increasing rate. Why?
• Increased surface melting
• Increased marine ice discharge in response to a warming ocean
- Mass-loss rate 1992-1999 compared to 2010-2019
Ocean warming (thermal expansion) 39%
• Antarctic ice sheet: x3 Land water storage 17%
• Greenland ice sheet: x6 Changes in glaciers 17%
Greenland ice sheet 17%
Current rate of sea level rise: 3,7 mm/y ® contributions: Antarctic ice sheet 10%
4
Some basics of the climate system
Weather Atmospheric condition at given time & place
Has chaotic component (‘Lorenz butterfly’)
Climate Mean and extreme conditions of the atmosphere, ocean, sea ice, etc… over a
longer period (e.g. 30 years)
Different spheres interact: lithosphere, atmosphere, hydrosphere, cryosphere, biosphere
The Earth’s climate system
Radiation balance on Earth: Q(1-α) = εσT4
Incoming radiation = outgoing radiation
↔ calculation of the global mean T:
4
𝑄(1−𝛼)
𝜀𝜎
Greenhouse effect
- Solar radiation (in form of visible light) → Earth absorbs & warms up → emits energy back to
space (in form of infrared) → greenhouse gas molecules re-emit IR in all directions → warms
Earth’s surface + lower atmosphere
- = Natural phenomenon BUT human is adding extra gasses ® more IR is trapped
• Earth’s T without greenhouse effect: -19° C
• Earth’s T with natural greenhouse effect: +15° C
Absorption spectra of greenhouse gases in the near infrared
- Smooth lines = Planck curves = theoretical lines that show
how much heat Earth would radiate if there was no
atmosphere. Surface warmer → more heat.
- Jagged line = actual radiation emitted by the Earth.
Difference is heat trapped in atmosphere
- Valleys = areas in spectrum where specific gases in
atmosphere absorb the heat radiation
- If we add more gases ® valleys deeper
1
,Contribution to natural greenhouse effect: 60% H2O vapour, 26% CO2, 8% O2, 6% rest (CH4 & NO2)
® H2O = largest contributor → BUT is not cause of human-driven climate change because:
- Short lifespan: condenses and rains out after few days (CO2 stays much longer in atmosphere)
- Feedback effect, not forcing: T↑ ® water vapour↑ ® T↑↑ ® …
Effect of doubling CO2-concentration:
a Initial state
b CO2 x2 ® greenhouse effect traps more heat ® L↓
c TS↑ + restoring of radiative balance
d Climate feedback (e.g. H2O↑) ® TS↑
Changes in human and natural drivers of climate
Anthropogenic Sources of Greenhouse Gas Emissions
CO2 Fossil fuels, cement, deforestation
CH4 Agriculture waste
NO2 Fertilizers, livestock
- Result detailed measurements in Hawaii: CO2 concentrations have
increased by more than 50% since pre-industrial times
Result atmospheric CO2 during last 3,6 million years: current cCO2 = first
time since 2 million years ago
- Humans have tipped natural carbon cycle out of balance:
• blue arrows = natural annual CO2-transfers (photosynthesis…)
• brown arrows = anthropogenic annual (largest = fossil fuel
combustion)
Main human activity contributing to greenhouse effect = fossil fuel
combustion (71%)
- Increase in CO2 = parallel with decrease in O2 → because CO2 = C + O2
Fate of Anthropogenic CO2-emissions (2014-2023 The cumulative contributions to the global carbon
average) budget from 1850 to 2023
2
,Aerosols
- Direct effect: scatter and absorb radiation
- Indirect effect: modify radiative properties, amount and lifetime of clouds
® They have a net cooling effect ® but a short lifetime (few years)
Radiative forcing
What? A measure of the net change in energy balance of Earth system (in W/m2)
1750-2019: anthropogenic radiative forcing = +2,72 W/m2 (mainly increased from 1970)
Observations of changes in climate: temperature
- Warming Earth’s surface 1880-2025:
• + 1,3° C
• 1st warmest year = 2024; 2nd warmest year = 2025
• 25 of the 26 warmest years in 21st century
- Climate spiral = animation with monthly temperature anomalies
- Warming in Belgium since mid-19th century: +3° C
- Warming in last 1000 years = “hockeystick”
- Comparison on high scale: global mean surface T last decade =
• Highest T of Holocene
• Comparable to T of last interglacial period
Attribution of climate change
- Observed warming only reproduced in simulations including human influence ® human in cause
- Observed warming is caused by greenhouse gas emissions ® 1/3 = masked by aerosols
Impacts of climate change: the cryosphere and sea level
- Cryosphere mostly located in remote places but has global consequences:
• Changes in snow & ice ® affect global radiation balance + weather & climate everywhere
• Melting of land ice ® sea level↑
- Snow cover extent (SCE) = 2D surface area of Earth covered by snow at a specific time
→ SCE in Northern Hemisphere (1922-2018): decreased 1.1% per decade
- Current Arctic sea ice coverage levels annual mean and late-summer values since at least 1850:
are the lowest
3
, - Importance glaciers:
• Freshwater storage & supply (especially in summer in arid regions)
• Reflect solar energy back to space (albedo)
Polar ice sheets
Greenland ice sheet Antarctic ice sheet
- Volume: 7,4 m sea level equivalent - Volume: 58,3 m sea level equivalent
- Maximum thickness: 3300 m - Maximum ice thickness: 4776 m
- First ice: ͂7 m/y ago - First ice: ͂35 m/y ago
- Full ice sheet: 3 m/y ago - Permanent ice sheet since: 14 m/y ago
1. Evolution of Greenland ice sheet:
Small central thickening & much larger marginal thinning (mainly along major glaciers in SE and
W)
2. Evolution of Antarctic ice sheet:
- Small central thickening & peripheral thinning (mainly in Amundsen
Sea sector of West Antarctic Ice Sheet) => overall mass lost
- Causes of mass losses of West Antarctic outlet glaciers:
• Mainly: ice-shelf basal melt
• Locally: strong surface melt followed by ice-shelf disintegration
(Ice shelf = a floating sheet of ice permanently attached to a land mass)
3. Statistics for 2 ice sheets (up to 2021)
- Since 1990s: mass lost at increasing rate. Why?
• Increased surface melting
• Increased marine ice discharge in response to a warming ocean
- Mass-loss rate 1992-1999 compared to 2010-2019
Ocean warming (thermal expansion) 39%
• Antarctic ice sheet: x3 Land water storage 17%
• Greenland ice sheet: x6 Changes in glaciers 17%
Greenland ice sheet 17%
Current rate of sea level rise: 3,7 mm/y ® contributions: Antarctic ice sheet 10%
4
