Describe Earth’s energy balance
- Incoming shortwave radiation from the Sun
- Outgoing long wave radiation
- Energy balance equation:
S_0/4 * (1-alpha)=epsilonsigmaT^4
S_0= solar constant (avg intensity in upper atmosphere)
Alpha = albedo (0.3)
Epsilon = emissivity (0.62)
Sigma = Stefan’s constant
- Lower latitudes receive greater insolation, driving atmospheric circulation
What is radiative forcing and what affects it?
delta F = S_0(1-alpha)-4epsilonsigma*T^4
* Difference between incoming energy from the Sun and emitted by Earth
Solar constant
* Sunspots - greater solar activity
* Faint young sun paradox - Sun was 30% fainter when Earth formed but liquid water present
Albedo
* Snow/ice/clouds have high albedo, water/vegetation/soil has low albedo
* Sulfate aerosols (volcanic and anthropogenic) increase albedo
Emissivity
* Decreased by absorption of IR by greenhouse gases (water vapour, CO2, CH4, CFCs)
List six climate feedback loops and explain them.
Water vapour
* Positive feedback: water vapour is a GHG, absorbs IR causing Earth to warm, increased evaporation (fast - years)
*Negative feedback: water vapour condenses to form high-albedo clouds, causing cooling (fast - years)
CO2
* Positive feedback: CO2 increases causing Earth to warm, sea temperatures rise, solubility decreases so CO2 released into the atmosphere (fast - surface ocean (decades), slow - deep ocean (centuries))
Silicate weathering
* Negative feedback: high temp increases rate of weathering which consumes CO2, leading to cooling (slow - millennia)
Ice
* Positive feedback: high temp causes ice to melt, decreasing avg albedo of Earth, leading to more warming and melting (slow - centuries)
Vegetation
* Increased CO2 means increased rate of photosynthesis in vegetation, hence decreased CO2 and cooling (slow - centuries)
What is climate sensitivity?
- Temperature change for given forcing (lambda)
*delta T = lambda * delta F
lambda = dT/dE = (d/dT(epsilonsigmaT^4))^-1
* Ignores feedbacks
What is the carbon cycle?
- Carbon stored in reservoirs and can move between them (box model)
- Residence time: average lifetime of C in reservoir (total mass/flux out). Slow feedbacks = longer residence time
- Reservoirs by size: rocks and sediments, deep ocean, land biosphere, surface ocean, atmosphere
- Biosphere-atmosphere exchange - photosynthesis
*Atmosphere-ocean exchange - CO2 dissolves to form DICs - Atmosphere-ocean-lithosphere exchange: Volcano emits CO2, acid rain dissolves rocks, DICs run into sea, marine organisms make shells, seafloor subducted
What are anthropogenic carbon sinks?
- Increase in CO2 less than anthropogenic emissions
- Uptake by oceans (ocean acidification) and biosphere
How can palaeoclimate be reconstructed?
- Use of proxies
Ice cores
* Depth proportional to age (oldest core 800ka - recent climate only)
* Trapped air bubbles used to analyse atmosphere (CO2 conc)
* Gas younger than ice around it (air in snow and firn exchanges with atmosphere)
* Deuterium ice isotopic ratio is a temperature proxy (lighter=warmer)
*d18O also used
Marine sediment cores
* Foraminifera (CaCO3 shells) - d18O ratio
* Benthic - deep ocean temp proxy. Planktic - surface ocean temp and glaciation proxy
* Increased temp decreases d18O
* Glaciation increases d18O as 16O more likely to evaporate, then precipitated and locked away as ice
Describe the trends in climate through the Cenozoic
- Early Cenozoic ice free, about 10C hotter than today
- First glaciation 34Ma
- Trend of cooling and increased glaciation
- Evidence: decrease in CO2 (proxy data), albedo feedback, opening of the Drake passage (cold currents around Antarctica)
- CO2 concentrations varied with glacial/interglacial cycles (800ka) however conc never as high as today
Describe the recent trend in Northern Hemisphere glaciation and its causes
- Intensification 2.6Ma
Milankovitch theory
* Summer insolation controls whether glaciers advance/retreat, affected by orbital parameters
Obliquity
* Tilt of Earth’s axis varies over time (22-24.5deg, period 40ka)
* Increase in obliquity increases summer insolation at poles (poles closer to Sun)
Precession
* Axial precession - rotation of Earth’s rotational axis (period 25ka)
* Elliptical precession - rotation of Earth’s orbit around the sun
* Changes time of year of aphelion/perihelion
* If perihelion coincides with N Hemisphere summer, increased insolation
Eccentricity
* Increased eccentricity means smaller distance to Sun at perihelion, so increased summer insolation
* Only effect which changes solar constant
Evidence
Deep sea d18O data
