Earth's Climate System (Lectures 42-47) Flashcards
Earth’s energy balance equation
S₀(1-α) * πr^2 = σT^4 * 4πr^2 S₀(1-α)/4 = σT^4 S₀ is solar constant α is albedo σ is Stefan Boltzmann's constant
In the energy balance equation for Earth, what is assumed about Earth?
It is a blackbody radiator
How does the energy balance equation change when considering the Earth as a grey body radiator?
RHS * ε
Where ε is emissivity
What is the average albedo for the Earth?
What is the emissivity for the Earth?
- 3
0. 62
Define a greenhouse gas
One that absorbs infrared radiation
Name the key greenhouse gases
Water vapour Ozone CO2 Methane Nitrous oxide CFCs
How does incoming solar radiation vary with latitude?
Lower latitudes receive more radiation
Why do lower latitudes receive more energy from the Sun?
Rays are more perpendicular to the surface at the equator
Rays must travel through a greater thickness of atmosphere in polar regions
What are the two mediums by which heat is transported from the equator to higher latitudes?
Where are they most important?
Ocean - carrying heat out of the tropics
Atmosphere - latitudes greater than 20
What are the three atmospheric circulation cells?
Hadley - 0 to 30 latitude
Ferrel - 30 to 60 latitude
Polar - 60 to 90 latitude
Persistent wind belts are caused by which two factors?
How are these wind belts affected by rotation?
Atmospheric pressure gradients - air flows from high to low pressure
Acted on by the Coriolus force (from Earth’s rotation)
Deflected to the right of motion in the northern hemisphere, to the left in the southern hemisphere
What is the effect of persistent winds applying a torque to the surface oceans?
Clockwise gyres in the northern hemisphere
Counterclockwise gyres in the southern hemisphere
What are greenhouse gases good at absorbing, what are they poor at absorbing?
Long-wavelength IR emitted by Earth
Short wavelength radiation emitted by the Sun
What are the three potential ways to change the average surface temperature of Earth?
Change the intensity of solar radiation
Change the albedo
Change the emissivity
What is the solar constant?
The power per unit area at the top of the Earth’s atmosphere
What are sunspots caused by?
When do they increase?
Intense magnetic fields that inhibit convection and form areas of lower surface T
During active periods when output is greater
What is the faint young sun paradox?
How is this countered?
The sun was 30% dimmer at the Earth’s formation 4.5 B years ago so Earth’s T should have been below freezing but water existed
Greenhouse gas levels were much higher
What is the rough albedo of water, sea ice and fresh snow?
Water: 0.05
Sea ice: 0.4
Fresh snow: 0.85
What is the current trend of September sea ice in the Arctic ocean?
It’s a consequence of what?
Steady decline
Global warming
Why does replacing sea ice with open water generate a strong positive feedback loop?
Decreases albedo
Increases absorption of heat by seawater
Temperature rise so more sea ice melts
What is a snowball Earth?
What is the tipping point?
Entire earth covered in ice/snow
Ice reaches 30 degrees latitude
What is the general albedo of clouds, what do they reflect?
High albedo
Reflect incoming short-wave radiation
How do clouds act as both a negative and positive feedback for global warming?
Which is the stronger of the two?
Increased cloudiness leads to cooling by increasing albedo
Clouds are made of water vapour, a strong greenhouse gas, increased water vapour absorbs IR
Negative feedback is slightly greater
How do volcanic eruptions affect albedo and global temperature?
SO2 injected high into the atmosphere
Sulfate aerosols reflect incoming radiation and increase albedo
Temperature decreases
What is emissivity?
What does it relate to?
The proportion of outgoing energy lost to space relative to that retained by the atmosphere
Greenhouse gas levels
What is the net radiative forcing from anthropogenic greenhouse gas emissions since the Industrial Revolution?
+1.6 W/m^2
The equation for the steady-state temperature change
ΔT(s) = λΔF
λ is the climate sensitivity
ΔF is the radiative forcing
How is λ, the climate sensitivity, worked out?
Differentiate grey body energy with respect to T, then divide 1 by it
How is the total temperature change worked out?
ΔT(total) = ΔT(s) + ΔT(fast feedbacks) + ΔT(slow feedbacks)
What are considered as fast feedbacks?
What are considered as slow feedbacks
Water vapour, clouds, sea-ice (<100 years)
Ice sheets, vegetation (>100 years)
What does the climate sensitivity value mean?
A temperature change of x degrees for every 1 W/m^2 of radiative energy gained/lost by the Earth
What is Charney’s estimate for doubling CO2 concentration?
Total sensitivity including fast feedbacks to be 3 degrees C
What is a reservoir, what are they measured in?
What is flux, what is it measured in?
Storage of a mass of carbon, Gt of C
Movement of masses of carbon, Gt of C per year
Define residence time
Define steady state
How is residence time calculated?
The average amount of time that a mass resides in a reservoir
Input flux equals the output flux
Mass of reservoir / input or output flux
What are the 5 main carbon reservoirs in order of size?
What are their rough sizes?
Atmosphere - 600 Gt Surface ocean - 900 Gt Biosphere - 2100 Gt Deep ocean - 37100 Gt Sediments and rocks - 10,000,000 Gt
What is the main process of carbon exchange between the atmosphere and biosphere?
The two processes almost balance, where does some carbon get transported to?
Photosynthesis - atmos to bio
Respiration - bio to atmos
By water into rivers and then oceans
In the northern hemisphere, why does CO2 concentration rise in the winter and fall in the summer?
Lower photosynthetic rates in winter
Respiration > photosynthesis
CO2 rises
What is the law and the equation for it by which CO2 dissolves in seawater?
Henry’s law
p = k(H)C
p is partial pressure, C is concentration, k(H) is a T varying constant
Outline the process by which CO2 dissolves in water to form carbonate ions
What are the favoured pHs of these species?
CO2 + H2O H2CO3 HCO3(-) + H(+) CO3(2-) + 2H(+)
CO2 below 6, HCO3(-) at 8.2, CO3(2-) above 10
What is chemical weathering?
Which is the most common weak acid?
The chemical breakdown of rock by a weak acid
Carbonic acid - H2CO3
Outline the atmosphere-ocean-lithosphere chemical weathering cycle
Weathering
Carbonate deposition - shells
Metamorphism - subducted rock
By how much has the atmospheric reservoir increased due to fossil fuels and land use?
How much carbon does deforestation add to the atmosphere reservoir every year?
165 Gt of C
1.6 Gt
What was the pre-industrial CO2 concentration?
What is it now?
280 ppm
400 ppm
How has atmospheric CO2 concentration been measured since 1958?
Why is it used?
Mauna Loa observatory
A 4km high volcano that is far from any point sources of CO2
Before 1958, what is used to obtain measurements of CO2 concentration?
Air trapped in polar ice cores
How is air trapped in polar ice cores?
What is the gas age-ice age difference?
As snow is compressed to ice, air bubbles can be sealed off from the atmosphere
The air in snow and firn can exchange with the atmosphere, so the ice is slightly older than the air
What is the equation for atmosphere increase of CO2?
Atm. increase = emissions - ocean uptake - land uptake
How much C does the ocean uptake every year?
2.2 Gt of C
What is the process of ocean acidification?
CO2 dissolving seawater forms carbonic acid and decreases ocean pH
What is the temperature-CO2 solubility feedback?
Increased CO2 -> ocean warms -> CO2 solubility decreases ->
Of the CO2 added since the industrial revolution, how much remains in the atmosphere, what has removed it and by how much?
40% remains
30% removed by oceans
30% removed by the biosphere
What is the problem with increasing CO2 concentration and biosphere uptake rates?
CO2 removed by photosynthesis
Photosynthesis rates increase slower as CO2 increases
What is a proxy?
Why are they used?
A substitute for an environmental variable
Can’t measure atmospheric and oceanographic variables directly in the past
Why are forams useful proxies?
What can they measure from them?
They precipitate CaCO3 from the seawater they live in
Oxygen isotopes can be derived and their ratio
What does the oxygen isotope ratio in forams reflect?
What will be the difference between benthic and planktic forams?
The temperature and the oxygen isotope ratio of the water at the time
Planktic show surface, benthic show seafloor
A 1‰ change in δO-18 corresponds to what T change?
4 degrees change
What is the paleotemperature equation with two unknown values in?
T = 16.9 - 4.0(δ(c) - δ(w))
T is an unknown paleotemperature
δ(c) is measured in forams
δ(w) is an unknown constant
Why do polar regions have a very low oxygen isotope ratio?
Water with O-16 evaporates faster than water with O-18
Water vapour is enriched in lighter isotopes
As it is transported to higher latitudes heavier isotopes are lost
So oxygen isotope value of ice is very low
What happens to the oxygen isotope value during glacial periods, why is this seen in forams?
As continental ice volume increases, δO-18 increases
Due to lower T and increased O-18/O-16 in seawater
What has been the general trend in climate throughout the Cenozoic?
Increasing δO-18 values
Deepwater cooling and increased continental ice volume
If Antarctica were to melt, what would be the sea level rise? For Greenland?
60m rise
6m rise
If all ice melted, what would be the δO-18 value change?
-1.2‰
Using the DeConto model, what is the threshold for glaciation wrt. CO2 concentration?
Major Antarctic glaciation didn’t occur until below 700 ppm of CO2
Major Northern Hemisphere glaciation below 300 ppm
What allowed Antarctic glaciation to occur?
Why?
When did this occur?
Opening of the Tasman Seaway and Drake passage
The Antarctic circumpolar current developed, isolating Antarctica in a ring of cold water
31 or 32 Ma
When did the intensification of Northern Hemisphere Glaciation occur?
Initiation at 2.8 Ma
What has the Quaternary been marked by a lot of?
How many specifically?
Glacial-Interglacial cycles
52 glacial and 52 interglacials
Define insolation
The incident solar radiation on the Earth
Changes in Earth’s orbital geometry leads to changes in what? This leads to what positive feedback loop?
Changes in the seasonal distribution of insolation as a function of latitude
Glacial-interglacial climate change -> CO2, albedo feedback -> amplified by other processes
James Croll developed a theory of the effects of orbits on climate cycles, what did he emphasise?
Decreases in winter insolation favoured snow accumulation
Positive ice-albedo feedback amplified the insolation decrease
What did Milankovitch hypothesise?
Why did he say this?
Summer radiation is the determinant factor whether ice sheets grow or melt
Always cold enough to preserve ice in winter, so summer controls if all that ice melts
What is the equilibrium line of a glacier?
When would a glacier grow?
When would a glacier melt back?
Accumulation equals ablation
Winter accumulation > summer ablation
Summer ablation > winter accumulation
What are Earth’s orbital parameters, and what is the distribution of their effect?
Obliquity - seasonal
Eccentricity - effects total radiation but small
Precession - seasonal
What are the cardinal points during Earth’s rotation around the Sun?
Winter solstice Perihelion (closest to Sun) Spring equinox Summer solstice Aphelion (furthest from Sun) Autumn equinox
What is obliquity? What is the current value? What does it range between? What is it's period? What is the effect on insolation?
The tilt of Earth's rotational axis 23.5 degrees 22.1 degrees to 24.5 degrees 41000 years Lower obliquity, lower insolation at poles, higher at the equator (stronger effect at the poles)
What is eccentricity? Current value? What has it ranged between? Periods? Effect on insolation? Why is it important?
Variation in the ellipticity of the orbit
0.017
0 (circular) to 0.06
100 and 400 kyrs
0.5 W/m^2 change from min to max value
Modulates the amplitude of the precession cycle
What is precession?
What is the period?
What is the combined effect of axial and elliptical precession?
The wobble of the Earth’s rotational axis
25,700 years
Change the time of year of the aphelion and perihelion, called the precession of the equinoxes
What are the extreme positions in the precession of the equinoxes?
Perihelion coincides with the summer solstice
What is the effect of precession on insolation?
What are the dominant periods?
Affects insolation at all latitudes
19, 22 and 24 kyrs
How does eccentricity modulate precession?
Greater eccentricity, the larger an effect of precession on seasonal insolation
Eccentricity = 0 means no difference between perihelion and aphelion so minimised effect of precession
What are the ideal orbital configurations for ice growth and decay? What would be the net effect?
Growth: high eccentricity, low tilt, high Earth-Sun distance -> less seasonal contrast
Decay: high eccentricity, high tilt, low Earth-Sun distance -> warmer summers + colder winters
What does the Milankovitch theory predict about insolation?
Insolation at 65 degrees N in summer is the primary forcing of glacial to interglacial cycles
What are the unresolved problems for the Milankovitch theory?
Cyclicity is close to 100 kyrs but insignificant changes in radiative forcing at this period
Middle Pleistocene transition change from 41 to 100 kyr periodicity occurred without major changes in orbital forcing
Stage 11 problem, the most prominent transition occurs at a time of minimal forcing
What are the important feedbacks that can amplify the orbital forcing into a strong climate response?
Albedo
Greenhouse gases
What evidence gave the same periodicities as those predicted by the Milankovitch hypothesis?
Oxygen isotope data from deep-sea sediment cores
When did the Holocene begin?
What marked it?
11,500 years ago
Current interglacial period
When was the last glacial maximum?
20,000 years ago
What was the last interglacial called?
Why was it warmer than the Holocene?
Eemian
Not greenhouse gases but insolation in Northern Hemisphere summer was stronger
How was sea level different during the Eemian compared to the Holocene?
Where is this data from?
What is this sea-level difference attributed to?
6-9m higher than today at its peak
Fossil corals found above sea level
3m from Greenland as it didn’t all melt, the rest from West Antarctica
What are Dansgaard-Oeschger (DO) events?
What is the explanation for them?
Swings between warmer (interstadial) and colder (stadial) states
Changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC)
How does the Gulf Stream work?
Transports warm, salty water to subpolar North Atlantic
Water cools and heats the atmosphere
Water is denser and sinks to form North Atlantic Deep Water
Circulation increases northward heat transport and keeps EUW warmer than same latitude US
What disrupts the Gulf Stream?
Increased freshwater input from melting ice sheets
What was the drop in radiative forcing during the last ice age?
What temperature drop does the correspond to?
-5.7 W/m^2
2 degrees cooler
What caused the last deglaciation?
A rise in boreal summer insolation
During the last glacial how was the sea level different to now?
130m
How is sea level currently changing?
Rising at an accelerating rate, currently 3.3mm/yr
What are the Holocene greenhouse gas trends?
Methane steady rise for the past 7000 years
CO2 increased over the past 3000 years
