week 8

Question 1

the limits of the (direct) observational record

Answer 1

• direct observations of the climate are best
• but the records only go back to mid 18th century
• coverage is poor, especially outside europe/usa

Question 2

what is a paleoclimate proxy

Answer 2

• physical, chemical and biological materials preserved within the geologic record that can be correlated with observations of modern climate or environmental conditions
• paleoclimate proxies span a wide range of timescales and record different conditions

Question 3

what are paleo-climate reconstructions

Answer 3

• paleoclimate proxies can be used to develop paleoclimate reconstructions using an understanding (model) of their correlations with climate conditions
• paleoclimate reconstructions - model / theory-informed estimates of past climate conditions

Question 4

what can we learn from tree rings

Answer 4

• provides a record of how well the tree grew each year
• bad weather (too dry, too cold) limits growth and is recorded as a narrower ring and vice versa
• by combining tree ring records from many trees, a continuous record of conditions can be built

Question 5

what can we learn from ice-cores

Answer 5

• snowfall accumulates each year on the ice-sheets and glaciers
• the longest continuous records are 130 Kyr in Greenland and 800Kyr in Greenland and 800Kyr in Antarctica
• the ice cores record snowfall but also:
• trapped atmospheric bubbles
• isotopic ratios in the water

Question 6

what can we learn from sediment cores

Answer 6

• within the cores, scientists study:
• pollen
• plant remains
• microfossils
• algae
• charcoal
• elemental composition and stable isotopes

Question 7

describe paleoclimate modeling

Answer 7

• climate models can be used to simulate the climate of the past
• appropriate boundary conditions must be specified:
• continental arrangement, orography, etc.
• land surface conditions, e.g. location of ice sheets, etc.
• greenhouse gas concentrations
• then temperature, rainfall, etc. can be simulated and compared to paleoproxies

Question 8

how are detailed reconstructions of the climate produced

Answer 8

combining paleoclimate proxy records with sophisticated paleoclimate model simulations allows detailed reconstructions of the climate of the past to be produced

Question 9

what is the faint young sun paradox

Answer 9

• stellar observations reveal that stars like out sun become brighter with time
• the early earth therefore received less sunlight
• however, there is evidence that for much of earth’s history it was warmer than today

Question 10

what is the weathering thermostat

Answer 10

• volcanism releases CO2 into the atmosphere and on geological timescales weathering of rocks captures CO2 in ocean sediments
• if there is an increase in CO2, the planet warms and the hydrological cycle intensifies. this speeds up the rate of weathering driving CO2 and temperatures back down over 100s KYrs, regulating the Earth’s temperature

Question 11

describe plate tectonics and the climate

Answer 11

• the merging and separating of the plates shapes volcanism, the key source of CO2 on geological timescales
• and where land is shapes weathering:
• at low-latitudes: temperature and rainfall are higher, speeding up weathering which captures CO2
• at high-latitudes: temperature is lower and there is land available for ice-sheets to grow
• bunched into large continents: the interiors are dry, reducing weathering
• the continental distribution also shapes ocean circulation, another key regulator of climate

Question 12

what explains the ice ages

Answer 12

• the sun is strengthening only very slowly and continents are fixed - so they can’t drive this
• CO2 is partly responsible for the swings in temperature but it is not large enough and temperature change leads CO2 at times
• another thing to explain is the quasi-regular cycling in and out of ice ages

Question 13

“weather” on the sun

Answer 13

• the sun appears constant but it isn’t
• the sun’s atmosphere varies in time, experiencing weather and seasons in a way analogous to Earth

Question 14

describe minima in sunspot activity

Answer 14

• there have been sporadic observations of sunspots for centuries, which show various minima in sunspot activity
• giving the correlation between sunspots and the sun’s luminosity during the solar cycle this suggests the sun’s output was consistently lower
• the Maunder Minimum was a prolonged period of very low sunspot activity in the 17th century that is correlated with the latter part of the ‘little ice age’

Question 15

describe the solar cycle

Answer 15

• the sun goes through an ~11-year cycle of activity
• the sun’s luminosity varies quasi-sinusoidally with time
• this luminosity variation is very closely correlated with:
• the number of sunspots
• the solar flare activity

Question 16

describe the orbit of the earth

Answer 16

• the earth has a slightly elliptical orbit, with an eccentricity (e) of 0.0167 (1.7%)
• at perihelion, the earth is closest to the Sun (rp) and at Aphelion, it is farthest (ra)
  rp = r0 x (1-e)
  ra = r0 x (1+e)
  where r0 is the average Earth-Sun distance
• the intensity of incoming sunlight is inversely proportional to the square of the Earth-Sun distance, so it isn’t constant
• in NH summer it is ~3.5% weaker and in NH winter it is ~3.5% stronger

Question 17

how to calculate sunlight incident on a surface

Answer 17

the solar energy (l) incident on a plane is given by:
I = I0 x cos (thetaz)
I0 = solar constant
thetaz = zenith angle

Question 18

describe the earth’s axis

Answer 18

• the earth spins around an axis that runs through its poles
• the earth has an axial tilt or obliquity of 23.4 degrees relative to its orbital plane
• this xis maintains a constant orientation as the earth rotates around the sun
• at the summer solstice, the NH is maximally tilted towards the sun and at the winter solstice it is maximally tilted away

Question 19

describe the tropics of cancer and capricorn

Answer 19

• mark the limit of where the sun can appear directly over head (thetaz = 0)
• the tropic of cancer is at 23.4 degrees north, tropic of capricorn is at 23.4 degrees south

Question 20

describe the arctic and antarctic circle

Answer 20

• they mark the limit of where the sun can continuously appear above (summer) or below (winter) the horizon. appear directly over head (thetaz=0)
• the arctic circle is a 66.6 degrees North (90-23.4) and Antarctic is at 66.6 degrees South

Question 21

describe solstices, equinoxes and the declination angle

Answer 21

• the declination angle is the angle between the sun’s rays and the plane of the Earth’s equator. it is equal to 0 degrees at the equinoxes and +- 23.4 degrees and the solstices
• during the equinoxes, the night and day have equal length and the sun passes directly overhead at the equator

Question 22

describe insolation over the year as a function of latitude

Answer 22

• at the equinoxes, insolation at each latitude is simply determined by the cosine of latitude
• at the June solstice, the NH is maximally tilted towards the Sun
• at the December Solstice it is maximally tilted away from the sun

Question 23

describe annual-average incoming solar radiation

Answer 23

• the annual-average incoming solar radiation reaching the top of the atmosphere varies as a function of latitude, being greatest in the tropics and less than half as large at the poles
• absorption in the atmosphere and reflection by clouds reduces the amount of solar radiation reaching the surface

Question 24

why is the earth’s orbit not constant

Answer 24

• the eccentricity, axial tilt and other aspects of the Earth’s orbit vary over time, driven by the mechanics of spinning objects and gravitational interactions with other planets

Question 25

describe eccentricity variations

Answer 25

- the current eccentricity (e) of earth is 0.0167 but it varies from a min of 0.005 to a max of 0.057 - the variations have different periods. the 413Kyr cycle is the greatest but there are also 95Kyr and 125Kyr cycles - these variations are driven by gravitational interactions with Jupiter and Saturn

Question 26

describe axial tilt/obliquity variations

Answer 26

- the earth's current obliquity is 23.4 degrees but it varies from 22.1 degrees to 24.5 degrees - the primary mode of obliquity variability is 41 Kyr - it was at it's maximum obliquity about 10,700 years ago and will reach its minimum in about 9800 years

Question 27

describe axial precession

Answer 27

- the earth's axis does not have a constant alignment, it precesses over time - this cycle has a more-or-less constant period of 26Kyr - this changes the relationship between the solstices and the earth's perihelion and aphelion

Question 28

describe the impact of axial precession

Answer 28

- currently the perihelion falls in mid-winter and near to the NH Winter Solstice. this means that the NH summer recieves less sunlight than the SH summer - as the axis of the Earth precesses around the date of Perihelion changes - around 10Kyrs ago, the situation was reversed and Perihilion occured in NH summer which received more sunlight

Question 29

why is the northern hemisphere more prone to snow and ice

Answer 29

- the NH has more land than the SH - land has a lower heat-capacity than the ocean so warms and cools more over the seasons - the intense sunlight and warmth of the summer melts the snow and ice away each year - which regrows in the winter

Question 30

describe ice-albedo feedback

Answer 30

- ice and snow have a higher albedo than land and ocean, so with more around, it is colder which reduces melt, promoting growth

Question 31

describe ice-elevation feedback

Answer 31

temperatures are lower at higher altitudes so as an ice sheet grows, its surface temperature drops, reducing melt