Lecture i & ii: Evidence Of Climate Change & Natural Causes of CC

Question 1

Define climate

Answer 1

Mean atmospheric condit n of an area measured over substantial period time

Question 2

Define climate variability

Answer 2

Variat n/ change in mean state & other statistical property of atmosphere (precipitat n, insolat n, humidity, etc) eg standard deviat n & occurence of extremes of climate on all spatial & temporal scales beyond that of indiv weather events

Question 3

Describe Quaternary period

Answer 3

• most recent geological period in earth’s history
• started w Pleistocene Epoch abt 2.5 million years AGO
• Holocene Epoch (which we currently live in) followed abt 11 700 yrs AGO
• Pleistocene —> rapid growth & retreat of ice sheet (unstable). End of last glacial maximum marked start of Holocene
• Holocene —> period of relative warming (interglacial). Oni some events of cooling, drought but otherwise quite stable
• prior to year 1900, most climate changes r considered natural (not caused by anthropogenic factors)

Question 4

Name sources of evidence of climate change since the last ice age

Answer 4

Main
- Ice cores
- Ocean/Marine Sediment cores

Extra
- Paleolakes & River Terraces
- Sea-lvl change (fr coastlines & seas)
- Biological (Tree ring dating)
- Contemporary Recorded evidences

Question 5

Explain evidence from ice cores

Answer 5

• scientists drilled, extracted ice cores fr Greenland & Antartic ice sheets
• used isotope ratios determine climatic changes since last Ice Age (>10 000 yr ago)
  eg data show CO2 conc vv low during glacials (~180ppm) & higher during warmer interglacials (~280ppm)
  now, at 420ppm

Question 6

Explain evidence from ocean/maritime sediment cores

Answer 6

• radiometric dating of foraminifera skeletons
• nature of O isotope -> give us idea on temp, salinity, nutrient content in ocean

isotope ratio change w climatic variat n
- light 16O water molecules evaporate more readily but condense less readily vs 18O water molecules
=> lesser 16O in ice core mean higher rate evaporat n, indicate higher temp
- high temp, more 18O water molecules (high isotope ratio 18O to 16O)
- in cold climate, more 16O vs heavy 18O water molecules (low isotope ratio 18O to 16O)
=> in glacial period, 16O evaporate more readily, move polewards, b locked up in ice sheets, so more 18O in oceans (decrease 16O to 18O ratio)
=> in interglacial period, ice sheet melt, release 16O into ocean, increase ratio of 16O to 18O

• results partly match pattern provided in Milankovitch cycles
• cores reflect there were 8 glacial built up on abt 100 000 year cycle -> similar to Milankovitch’s eccentricity theory (cool phase)
• decrease in ice vol occur at 23000 and 41000 year intervals (same as precession & tilt frequencies (warm) )

Question 7

Explain biological evidence

Answer 7

• diff width of tree ring contain annual record of climate condit n of past, as tree growth greatly influenced by climatic condit n eg temp, precipita n
  eg warm, wet in tropical region favour growth -> larger tree ring
  BUT
  drought year -> narrow tree-ring

Question 8

Explain evidence from current day coastlines and sea levels

Answer 8

• sea lvl began rise ~19 000 year ago, reached present lvl abt 7000 year ago

Two processes account for global rise of sea lvl:
1. thermal expans n ocean
2. Melting land based ice sheets

• global sea lvl hv rise btw 10-20cm over past century, continue at accelerated rate in coming years
• rising sea lvl is real, current danger for 2 major types of area:
  world mega deltas & low lying countries
  eg Maldives in Indian Ocean, Kiribati in Central Pacific Ocean

Question 9

Define climate change

Answer 9

signi rise in global temperatures, particularly contemporary CC due to enhanced gh effect, causing damage in econ, social, environ, political components
OR
any significant change in statistical property of atmostphere eg temp, precipitat n, humidity, etc, lasting for extended period of time

Question 10

Name natural causes of climate change

Answer 10

1. Milankovitch cycles (orbital forcing theory)
2. Sunspot activity
3. Internal Feedback Mechanism

Question 11

Describe Milankovitch cycle

Answer 11

• affect amt solar radiat n & where it reach earth
  => determined by 3 parameters:
• orbit eccentricity
• obliquity
• precess n of equinoxes

Question 12

Explain orbit eccentricity (Milankovitch cycles)

Answer 12

• ~100 000yr cycle
• As eccentricity increase (more elliptical), diff in Earth’s dist fr Sun at orbit’s closest & furthest points oso increase, affecting solar rad n hit earth, & thus severity of seasons
  Eg. if winter in N. Hemis occur when the dist btw Earth & Sun is at max, winter will b more severe (the greater the eccentricity, the stronger the effects on seasons)

Question 13

Explain obliquity (tilt of the Earth, Milankovitch Cycle)

Answer 13

• now tilted 23.5 deg, varies fr approx 21.1 to 24.5 deg in 41 000yr cycle; affect seasons
• greater tilt angle, greater contrast btw summer & winter temp
• bcos greater tilt, more extreme season as each hemis receive more solar rad n during summer & less during winter
  => favour melt retreat ice sheets
• places at higher latitudes receive larger insolat n than areas close to equator
• as obliquity decrease, milder the seasons
• warmer winters and cooler summers
• over time, ice sheet build up (reflect more solar rad n)
• promote more cooling

Question 14

Explain precession of equinoxes (axial wobble, Milankovitch cycle)

Answer 14

• is change in orientat n of earth’s rotational axis
• due to grav force of sun, moon causing earth to bulge at equator, earth wobble like spin top, affecting its rotat n
• makes seasonal contrasts more extreme in one hemis, less in the other
• Vega/15014 (13 000yr ago) Earth reach furthest pt fr sun during S hemis winter, so slightly colder than N. hemis
• Polaris/2014 (present)
• Now (2020), perihelion (nearest to sun) occur at Winter Solstice (winter in N hemis)
• axial precess n make S hemis summer hotter, moderate N hemis winter
• BUT, in 13 000 yr, axial precess n cause reversal of these condit n (more extreme solar rad in N. Hemis, moderate S. hemis seasons)
  vs
  aphelion (furthest fr sun) -> opp effects

Question 15

Explain limitation of orbital forcing theory (Milankovitch cycle)

Answer 15

• other factors can oso influence CC
• predict n of CC based on orbital change oni no correspond to CC based on other expl n or evidence
  Eg
  1. Temp change needed for massive glacial expan n & retreat greater than changes caused by orbital forcing alone
  2. Orbital changes shd create smooth changes, but many short-term changes which r abrupt, show “saw-tooth” patterns

Question 16

Explain sunspot activity (solar variability)

Answer 16

• presence of dark regions forming on sun’s surface (peak every 11 years)
  => affects amt solar energy emitted
• sunspots connected to other events eg solar flares (sudden release of energy fr sun)
• bigger grp of sunspots, more intense solar weather tend to be
  => increase total amt insolat n earth receives, lead to higher global temp

Eg few sunspots seen from 1645 to 1715, causing normally ice free rivers to freeze and snow fields to remain year round at lower altitudes (little ice age)

Question 17

What is the difference between positive and negative feedback loops?

Answer 17

Positive: strengthen, enhances/accelerates to cause larger temp changes

Negative: reduces/supresses/dampens change

Question 18

Explain albedo and sea level change

Answer 18

1. (decrease temp) ice sheet cause increase in earth surface albedo
   => further drop in temp, create +ve feedback cycle (expans n/ growth ice sheets)
   - growth ice sheets cause sea lvl drop
   => easier for ice to flow fr land further onto continental shelves
   - sea lvl drops as ice covers ocean
   - perpetuates +ve feedback mechanism
   => cause further drop in temp & whole cycle repeats itself

2.( Increase in temp)
- increase amt cloud cover
- cause high albedo, prevent solar rad n fr reaching earth surface
- reduce incoming solar rad n & limit warming (stabilising climatic condit n)

Sea level changes: evidence from current-day coastlines

• Account for Holocene climate changes.
• Ice sheet melting and water returning to the oceans caused global average sea levels to rise around 19,000 years ago and reached their present level about 7,000 years ago.
• Much land was drowned during the first 5,000 years of the Holocene, resulting in most global coastlines being only a few thousand years old.
• Coastal geomorphological features like salt marshes, beaches, and spits are relatively young.

Eg southern North Sea between Britain and the Netherlands was once dry land. Today, it’s a shallow sea of 20 m deep.
Teeth of woolly mammoths and stone tools were found in the landscape, suggesting it was once occupied by non-marine inhabitants.

Question 19

Explain volcanic eruptions

Answer 19

1. hv short term cooling effect on earth climate (2-3yr)
   - release huge qty of volcanic ash, gas & sulfate aerosols (fine suspended particle) into atmos
   => increase earth albedo (reflected rad n) & fall in insolat n reaching earth surface
   - causes decline in avg temp at earth surface
2. Emiss n of ghg eg CO2, CH4, CO into atmos
   => act as thermal blanket in atmos, re-emit long-wave rad n back into earth surface
   -> increase global temp
   -> enhance global warming (+ve feedback mechanism, amplifies temp increase)

Eg
• Temperature Impact of Eruptions: According to USGS, Several eruptions in the past century have caused a decline in the average temperature at the earth’s surface for periods of one to three years.
• Mount Pinatubo Eruption: The climactic eruption of Mount Pinatubo on June 15, 1991, injected a 20-million ton sulfur dioxide cloud into the stratosphere, causing a significant cooling effect on the earth’s surface (1 deg C max, 0.5 deg C avg across years)
• Stratospheric Aerosol Disturbance: The Pinatubo cloud was the largest sulfur dioxide cloud ever observed in the stratosphere since satellite observations began in 1978, causing a significant aerosol disturbance.

Question 20

Explain Thermohaline circulation (THC)

Answer 20

• ~ 500yr cycle
• start in cold polar region

under normal conditions,
- cold, saltier water fr north oceans sinks (gulf stream) like giant conveyer belt
- warm water fills up space left by cold water
- warm surface water fr S. Hemis move toward N. Pole
- N. America, Europe experience warmth

in contemporary CC
- North ice sheet melt, dilute water
- harder for water (less cold, less dense) to sink
- THC stops, so no gulf stream to warm N. America, Europe
- cause cooling in North Atlantic region

Question 21

Describe evidence of Thermohaline circulation in the past causing temp variability

Answer 21

• Less warm water moved fr equator Atlantic Ocean to higher latitudes
• cause cooling of regional climate in Artic at N. Polar region => Younger Dryas event (sudden cooling occur ard 12900 yr ago)
• sudden cooling produced drop in temp (4-10 deg C) in Greenland & glaciers advanced over much of temperate N.Hemis (even tho interglacial expected)
• Global climate condit n became unstable w change to rainfall, including monsoon rainfall

Question 22

Explain implication of thermohaline circulation causing temperature variability in the future

Answer 22

• Increased warming, esp in Artic, may cause more melt of polar ice caps
• influx of large amt fresh water cools, reduces salinity of sea water => sea water bcome less dense
• cld slow or shut down THC
  eg THC supposed keep Europe warm, BUT, if there is global warm, regional cooling wld occur (-ve feedpack loop)
• oso reduced capacity for ocean to store carbon, heat (cold water btr at dissolve gases eg CO2)
• W weak THC, ocean absorb less CO2
  => reduce downward flux of carbon & more heat loss to atmos
  => drive even more rapid CC + damaging effects to aquatic (lack of nutrients) & terrestrial ecosystems

Question 23

Define temperature, solar radiation, insolation

Answer 23

temp - degree of hot or coldness of item

solar rad n - e-magnetic pwr/energy radiating/emitted fr sun

insolat n - incoming solar rad n fr sun reaching earth, felt as heat energy

Question 24

Explain evidence from Paleolakes and river terraces

Answer 24

Evidence from Paleolakes and River Terraces

Paleolakes, ancient lakes, and lacustrine deposits (lake sediments that form terraces along former shorelines) in river valleys and lakes worldwide provide insights into past climates. River terraces, often found on valley sides or floodplains, can be vertical steps (e.g., River Meuse, Europe).

The highest terrace is typically the oldest, while lower terraces are younger. This sequence can be complex, and the study of their age and composition suggests possible sea-level changes and precipitation regime shifts. River terraces are rich in plant and animal remains.

Examples of Pleistocene climate change include the two largest paleolakes, Lake Bonneville and Lake Lahontan, in the Basin and Range Province of the western USA. Lake Bonneville was eight times larger than its present-day remnant, the Great Salt Lake, and covered over 50,000 km² at its greatest extent, reaching depths of 300 m. Today, the Great Salt Lake is the fourth largest saline lake in the world, with an area of approximately 4,400 km².

Lake-level records from Lake Bosumtwi in Ghana indicate fluctuations over millennia, suggesting prolonged droughts.