Samenvatting van de hoorcolleges van het vak 'Global Change' gegeven door professor Huybrechts als titularis en andere gastdocenten aan de Vrije Universiteit Brussel.
Difference between weather and climate
- Weather: atmospheric condition at a given time and a given place
Can be measured: air temperature, precipitation, clouds, air pressure, wind speed,
atmospheric humidity
Has a chaotic component: ‘Lorenz butterfly’: if a butterfly flaps its wings somewhere in
the Amazon, this can be the cause of a thunderstorm in Australia 14 days later
o Cannot be predicted more than 10 to 14 days ahead
- Climate: mean and extreme conditions of the atmosphere, ocean, sea, ice, etc, over a longer
period of time (e.g. 30 years)
Easier to model and project than the weather
- Climate is the statistic of the weather
Interactions are crucial
- Lithosphere = crust
- Cryosphere = frozen water in all its different forms
- Hydrosphere = water
- Atmosphere
- Biosphere = living organisms
The Earth’s climate system: is becoming easier to model
Climate system has to abide to the rules of physics
- Radiation balance of the earth: incoming radiation = outgoing radiation
Q ( 1 – α) = εσT4
The only true variable is the temperature T (earth’s mean global temperature)
o For current radiative fluxes: T = 14.8 °C
o Without greenhouse effect (epsilon = 1): T = -18.6 °C
▪ Most powerful way to effect climate change
▪ Greenhouse effect is important: life on earth is not possible without it
o Weaker sun (Q -1%): T = 14.1 °C
▪ Not a big effect: sun is not the cause of climate change
o Change albedo (alpha) effect (reflectivity of the earth): T = 6.3°C
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,Exam: 1 question per topic
Greenhouse effect
- Smooth lines: Planck curves of radiation (theoretical)
- Jagged line: Actual radiation emitted by the Earth
- Contribution to natural greenhouse effect
H2O vapor: 60%
o Is a feedback, not a forcing
CO2: 26%
o Amount of CO2 is added by anthropogenic actions: is a forcing
o Effect of doubling the CO2 concentration
▪ Outgoing low wave radiation (L) lowers: 4 Wm² extra is trapped
▪ Temperature needs to rise: 1.2 °C to counter it
▪ Feedbacks kick in (water vapor): temperature rises around 2.5 °C
o
O3: 8%
CH4, NO2: 6%
b. Changes in human and natural drivers of climate
Changes of atmospheric concentrations of carbon dioxide, methane and nitrogen oxide
- First measurements of greenhouse gasses started in 1958 on Hawaii
Carbon dioxide concentration increase = 50% since pre-industrial times (from 315 ppm in
1958 to 415 ppm in 2021): same effect as ppm between last glacial time (LGM) and the
1800s (180 ppm to 280 ppm): rise is exponential
Increasing concentrations (CO2) is due to the combustion of fossil fuels: when CO2
concentration is rising, the O2 concentration is decreasing: burning consumes O2 and
releases CO2
o 2 lines: dark line is southern hemisphere: mean is lower: anthropogenic activity
that produces CO2 is mainly in the northern hemisphere
o Rigid line: due to summer/winter
2
,Exam: 1 question per topic
Fate of anthropogenic CO2 emissions:
- Sources: 86% fossil fuels, 14% deforestation
- Sinks: 46% in atmosphere, 31% in terrestrial biosphere, 23% in oceans
- Without the land and ocean sink: ppm would be around 600 ppm instead of 410 ppm
Role of aerosols: suspended particles in the air
- Direct effect by scattering and absorbing shortwave and longwave radiation
- Indirect effect by modifying the radiative properties, amount and lifetime of clouds
- Net cooling effect on climate change
- Lower life span than CO2: few years instead of centuries
Radiative forcing of the climate: positive radiating forcing due to human activities
Share of human activities to the enhanced greenhouse effect: fossil fuel combustion is the biggest
component (71%)
c. Observations of changes in climate: temperature
Rise of global mean surface temperature: has been measured (thermometers)
- Warmest years so far: 2016 and 2020
- 20 of the 21 warmest years since 1880 occurred in the 21st century
- Temperature has risen 1,1°C
- In the last 50 years observed temperature has increased unprecedented in at least 2000 years!
‘Hockeystick curve’
3
, Exam: 1 question per topic
- Belgium: Ukkel: total warming of 2.5°C
Land is warming more than the ocean: larger temperature rise on the land than the global
mean average
d. Attribution of climate change
Human influence has warmed the climate: unequivocal
- Black line is observed: can only be reached when human drivers are involved
Observed warming to date has been driven by greenhouse gas emissions, roughly a third of which had
been masked by cooling of aerosol emissions
e. Impacts of climate change on the cryosphere and sea level
Components of the cryosphere
- Cryosphere = ‘early warning system’
- Located in remote places but its changes have global consequences (ice caps at the poles,
permafrost, glaciers, seasonal snow cover …)
Changes in snow and ice affect the global radiation balance (decreases albedo: warms up)
and affect weather and climate elsewhere, including in Europe
Melting of land ice raises global sea level
Decrease of seasonal snow cover
- Northern hemisphere snow cover extent decreased 1.1% per decade for April over the 1922 –
2018 period
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