External causes of climate change Flashcards
How does Interstellar dust affects radiative transmissivity of interplanetary space?
Influences cloud formation on entry to atmosphere which leads to greater reflection.
What has the Solar System having crossed Galaxy arms four times in last 520 Million years caused?
Alternating warm and cold periods.
What do molecular dust clouds called?
Decreased solar radiation which leads to a decreased temperature of 5-7 degrees.
What do stellar models predict?
a star’s luminosity is proportional to the nth power of its mean molecular weight, where n = 6 or 7.
How do stars produce energy?
Nuclear fusion - conversion of hydrogen to helium which leads to an increase in mean molecular weight.
Faint young sun paradox?
-Ice age conditions today.
-Oldest known rocks >3.7 Gy old, in Greenland show sedimentary features, demonstrating oceans not frozen.
-Evolution of life during first billion years suggests Earth surface was quite warm.
- Could have been warm due to intense greenhouse effect.
What are the three variations in earths orbital geometry?
-Obliquity
-Precession of the equinoxes
-Eccentricity
What caused Pleistocene ice age?
-caused mainly by periodic changes in distribution of solar radiation at Earth’s surface from variations in Earth’s orbital geometry.
-Milutin Milankovitch (1941)
What’s obliquity and what does it determine?
-Angle of tilt of earth from plane perpendicular to orbital plane.
-Varies from 22.1 degrees to 24.5 degrees - currently 23.5 degrees.
-Determines latitudes of polar circles and tropics
What happens when angle of obliquity increases? And what does it cause?
-Summer radiation receipts at poles increase
-Winter radiation receipts at poles decrease
-Little effect at low latitudes
-Affects equator to pole temperature gradient as it controls strength of atmospheric circulation which has seasonal contrast.
What are the two components of the precession of the equinoxes?
-‘Wobble’ in axis of rotation
-Orbit ‘swings around’ in space
Features of the precession of the equinoxes?
-Affects timing of the extreme earth sun distances.
-Today - Perihelion (Earth closest to the sun) during Northern Hemisphere winter.
-11,000 years ago - perihelion (Earth closest to the sun) during Northern Hemisphere hotter summers colder winters
-Greatest effect in low latitudes
What is eccentricity? Features?
-Degree of egg shapedness of earth’s orbit.
-Varies from near circular to markedly elliptical.
-Influences the effect of the precession of the equinoxes.
-Maximum difference in receipt of solar radiation (perihelion and aphelion (earth furthest away from sun)) is 30 %.
What are the conditions of orbital configurations (position of earth to sun) that favours ice sheet growth?
-Minimum obliquity
-High eccentricity
-Aphelion during NH summer
(These conditions induce northern hemisphere glaciation)
Consequences of orbital configurations (position of earth to sun) that favours ice sheet growth?
-low levels of summer radiation especially at poles.
Low ice melt
increased equator to pole temperature gradient
Which leads to increased poleward transport of moisture which feeds snowfall
-Warm NH winters leads to evaporation from oceans.
Causes expansion of NH ice cap
What is the evidence of an Orbital Signature in the Palaeoclimatic Record?
-O18O of calcareous marine fauna incorporated in sediments show dominant 100,000 y cycle.
What supports the evidence of an Orbital Signature in the Palaeoclimatic Record?
-Other proxy climate records that show spectral peaks consistent with effects of orbital forcing:
-Land-based sediments
-Pollen records
-Ice cores
-Coral reef sea level records
Problems with Orbital Signature being in plaeoclimatic record?
-Dominance of 100,000 y cycle
-Eccentricity important only in emphasising precession effects
-Simultaneous termination of last glaciation in NH and SH
Why is internal amplification of orbital forcing important in terms of CO2?
-CO2 trends closely follow deuterium isotope temperature record in Antarctic ice core.
-This leads to positive feedback as CO2 changes reinforce and amplify temperature change.
Why is internal amplification of orbital forcing important in terms of sea level changes?
-Sea level changes explain synchronicity of NH and SH deglaciation.
-Increased summer insolation and temperature at N Pole leads to substantial ice melt.
-Increased atmospheric CO2.
-Sea-level rise and greenhouse warming lead to melting and break-up of Antarctic marine ice shelf.
Co2 changes involve changes in marine primary production?
Increased production leads to utilisation of dissolved CO2 leads to atmospheric CO2 (‘draw-down’).
How could changes in marine primary production be driven by nutrient availability?
-Stronger atmospheric circulation (enhanced equator to pole temperature gradient).
-Enhanced upwelling.
-Enhanced atmospheric transport of minor nutrient elements (e.g., Fe).
-Fall in sea level during glacial periods leads to exposure of nutrient-rich continental shelf sediments leads to erosion.
What do sunspot numbers directly correlate with?
Solar magnetic activity.
How have we recorded solar activity?
-Observing sunspots
-Measuring variable amplitude
-Cosmogenic nucleotide data
How do cosmogenetic nuclides show solar variability?
-Cosmic radiation leads to neutron bombardment of atmospheric N atoms in upper atmosphere which creates 14C.
-14C is oxidised rapidly to 14CO2 which diffuses and mixes in atmosphere.
-Cosmic radiation penetration modulated by solar magnetic activity.
- Overall, low sunspot number leads to low solar magnetic activity leads to increased cosmic radiation penetration leads tp increased 14C.
True or false? There is correlation between cold and warm periods and low and high solar activity?
True.
Indirect effects of cosmic radiation intensity?
-Ionisation.
-Formation of radiacals.
-More intense=more low level cloud coverage.
-Less intense=less low level cloud coverage.
How does increased cosmic radiation lead to a lower surface temperature?
Increased cosmic radiation means low solar activity which leads to increased low level cloud coverage which leads to decreased surface temperature.
How could cosmic radiation intensity potentially explain the increase in solar activity over the last 100 years?
in last 100 years there has been a 8% decrease in cosmic radiation intensity which has caused a 2% decrease in cloud cover which has caused a 0.5% increase in solar radiation receipts?
What does a long residence time of materiel in stratosphere mean?
-Eruptions must be strong enough to get material into stratosphere
-A long residence of material in stratosphere means a great hemispheric or global influence.
How does materiel in stratosphere affect radiation balance? And what does this lead to?
-Sulfur dioxide absorbs UV radiation which produced heat.
-Sulfur dioxide also quickly gets converted to the aerosol sulfate which reflects and scatters solar short wave radiation.
-Dust also absorbs terrestrial long wave radiation.
-Leads to warming of stratosphere and cooling of troposphere
What are the emperical studies?
-Hubert Lamb’s ‘Dust Veil Index’ of Volcanic Activity
Based on eye-witness accounts (volcanic blasts, colourful sunsets, hazy skies)
-Tree Growth-Ring Chronologies
Narrow growth rings associated with colder conditions following volcanic eruptions
Discrepancies, so not particularly reliable
-Ice Core Data
High acidity levels shows well known and unidentified eruptions
Compared with ice core temperature record
Dating imprecision limits observance of clear cause and effect
What is the superposed epoch analysis? And what is it restricted to?
-Surface temperature trends around time of volcanic events composited to maximise signal to noise ratio.
-Period for which surface temperature trends are available
How are record of eruption scales and dates compiled for superposed epoch analysis?
-Observational chronologies (historical records)
-Geological evidence (volcanic deposits)
-Ice core dust/acidity levels
-Pollen records (dust layers)
What’s the eruption selection criteria for superposed epoch analysis?
-Evidence consistent
-Well-dated events
-VEI greater or equal to 5 and/or DVI greater or equal to 300
-Small number of events selected (NH = 5; SH = 4)
Features of Standardisation of Temperature Data to Remove Seasonal Bias for superposed epoch analysis?
-Data for each eruption selected for months -60 to +48 around eruption dates.
-Each monthly temperature subtracted from long-term mean for month.
-Each monthly value divided by long-term standard deviation for month.
Process of compositing data for superposed epoch analysis?
-Standardised temperature data expressed for each month as a departure from the monthly mean for the 5 years preceding eruption date.
-Data for each eruption composited for months -24 to +48 around eruption dates.
Results of superposed epoch analysis?
-Rapid response in NH with little lag (0.3 degrees in summer, 0.6 degrees in winter).
-Differences in hemispheric response reflect differences in land/sea distribution.
-Individual eruptions can cause significant cooling up to 2-3 years after event.
Details of study of Mt Pinatubo?
-Globally averaged radiation balance at top of atmosphere was –3 W per square metre.
-Average reduction in surface temperature of 0.5 degrees.
-Reduced global precipitation (reduced radiation receipts lowered SST and affected evaporation).
-Long term effects
Temperature and precipitation anomalies vanish in 7 y.
Sea ice extent and volume anomalies vanish in 10 y.
Changes in interior ocean temperature, sea level and strength of overturning circulation persist for decades to a century.
The Impact of Volcanic Activity on Expected Anthropogenic Warming?
(Satellite and other ground-based observations)
-Aerosol distributions and optical properties
-Estimates of radiative forcing and temperature changes
-Cumulative volcanic forcing effect currently offsets one third of anthropogenic warming
