L26

Question 1

Why does geological history matter? (6 reasons)

Answer 1

1. It establishes the link between CO2 and temperature
2. It provides context for the current climate crisis
3. It allows the sensitivity of climate models to CO2 to be tested
4. It demonstrates the central role of life in the Earth system
5. It shows how Earths environment was radically different in the past
6. It suggests how life adapts to major environmental challenges

Question 2

Biological proxies of past temperature?

Answer 2

• Tree rings
• Macrofossils

Question 3

Geochemical proxies of past temperature

Answer 3

• Benthic foraminifera that carry a sea temperature record in their shell isotopic record
  • Gives rise to zacho oxygen isotope curve

Question 4

Palaeo CO2 proxies

Answer 4

• Instrumental record for CO2 only goes back 70 years. Before that it needs to be inferred from proxies

Biological and geochemical
* Properties of plants / fossils that respond to CO2
* Using either density of stomata or isotopic property
* Assumptions have to be made about how co2 relates to property

Question 5

CO2 and temperature are coupled over geological timescales

Answer 5

• Periods of uncoupling imply other mechanisms involved but generally they are coupled
  • Myocene uncoupling is an area of research figuring how the earth system is behaving

Question 6

It provides context for the current climate crisis

Answer 6

• Current temperatures are higher than they have been for at least one thousand years
  • Often used in media to portray the current state of climate
  • Geological record estimates have high uncertainty
  • General pattern of cooling over time and CO2 reduction

Question 7

Comparison with past climatic events

Answer 7

• paleocene - eocene thermal maximum (PETM) - 56 million years ago
  
  • Massive injection of greenhouses gases into the atmosphere, doubled to tripled atmospheric CO2
  • Rapid release but much slower than current emission release

Source uncertain:
* CO2 volcanism through coals?
* CH4 from oceans or permafrost?

Question 8

Impacts of past climate change

Answer 8

• Origin of carbon may have been biological eg volcanism
  • Led to a 5c excursion in temperature
  • Short lived but on a geological timescale
  • Crash in abundance and diversity of dinoflagellates (marine phytoplankton)

Question 9

It allows the sensitivity of climate models to CO2 to be tested

Answer 9

The climate system is not in equilibrium, how much warming is in the pipeline?

Equilibrium climate sensitivity (ECS) may be inferred using past climates

• Compare temperatures of the last glacial maximum with the holocene using oceanic isotopic proxies
  • Calculate an ECS value of 3.4c for a doubling of CO2

Question 10

Past climates are increasingly used as analogs for future windows

Answer 10

• As climate change progresses we go further back into the past climate
  
  • Under various future climate emissions this essential pushes back into past climates
  • Often discussion of what type of climate we want to avoid that has happened previously on Earth

Question 11

Equilibrium sea level rise during the Pliocene

Answer 11

-Rock record shows ~17 m sea level rise accompanying 2-3 oC temperature rise (compared with pre-industrial period)

-Implies almost complete loss of Greenland and West Antarctic ice sheets

- Most sea level rise due to warming ocean expanding

- Tipping cascade feedbacks by glaciers melting on land will further cause sea level rise as they are on land not in the sea

- 2-3 temperature rise over rock record

Question 12

It demonstrates the central role of life in the Earth system

Answer 12

• Life is a major player, not a passenger in the Earth system
  • Life plays fundamental roles in the geological oxygen and carbon cycles, influencing atmospheric o2 and co2 on multi- million year timescales

Question 13

Evolutionary innovations

Answer 13

• Major innovations in plant life have caused big changes in atmospheric oxygen and carbon dioxide
  • These include photosynthesis, invasion of land by plants, lignin- degrading fungi
  • Oxygen is somewhat an inverse pattern of CO2 rise

-Photosynthetic life is responsible for oxygenating the atmosphere

Question 14

It shows how Earth’s environment was radically different in the past

Answer 14

• Past worlds were a very different environment
  • Varying o2, co2, greenhouse effects

Question 15

Palaeo- environmental data important for interpreting the fossil record

Answer 15

Green Sahara events during the Holocene and Pleistocene

• Humid climate across the Sahara region 11,500 to 5,000 years ago. Lakes, grasslands and forests.

Ancient human paintings show animals no longer present in the Sahara desert region.

Question 16

Hothouse Earth

Answer 16

Ice-free Antartica during the Mesozoic

Question 17

Snowball Earth

Answer 17

Intervals when Earths surface was frozen

Question 18

Climate system has been dynamic throughout Earth history

Answer 18

• Extreme changes
• Climate change is not as bad for the planet as it is for humans

Question 19

It suggests how life adapts to major environmental challenges

Answer 19

• Plant adaptations to low atmospheric CO2
• Animal adaptations to rising atmospheric CO2

Question 20

How the evolution of oxygenic photosynthesis in cyanobacteria caused the Earth to freeze

Answer 20

No oxygen (O2)
CO2 was less than 10x present value
High methane (CH4) concentrations

Sun was 30% less bright than today (=faint young sun)
Paradox is that Earth should’ve frozen but it didn’t
Enhanced greenhouse effect prevented this

Question 21

Oxygen producing photosynthesis

Answer 21

Evolved once in the history of life (2.45 billion years ago)

First photosynthetic organisms were prokaryotic cyanobacteria

Endosymbiosis gave rise to all other eukaryotic algae and plants

Question 22

Release of oxygen is documented in the geological record

Answer 22

• Sulphur isotopes in ancient rocks show when this oxygenated the atmosphere
  • Sulphur isotopes from 2.5ma to present are stable

Question 23

Mass- independent fractionation

Answer 23

• Happens when there is no ozone layer
  • Ultra violet light causes MIF of Sulphur
  • When oxygen accumulated in the atmosphere forms an ozone layer and protects form UV, MIF stops

All of life was suddenly in a crisis as O2 is damaging to anaerobic organisms

Question 24

Oxygen destroyed…

Answer 24

Earth’s methane greenhouse

• This was good as methane has a high climate warming potential

Question 25

Weakened greenhouse effect led to an ice-albedo temperature feedback

Answer 25

• Opposite to current sea ice albedo effect today
  • Tipping point where Earth gets covered in ice
  • Runaway destabilising positive feedback

Question 26

Plate reconstruction

Answer 26

• It is clear there are glaciers at the equator
• • At least three snowball earth events caused by weakened greenhouse effect
  • Current hypothesis is linked to carbon burial associated with marine phytoplankton
  • On each occasion, volcanic CO2 production strengthened the atmospheric greenhouse effect to thaw the snowball
  • This carries on despite earth freezing
  • There is eventually enough co2 to reverse cooling effect and sea ice melts

Question 27

Photosynthetic organisms have entrained feedbacks in the Earth system for billions of years

Answer 27

• Evolution of photosynthesis in cyanobacteria oxygenated the atmosphere 2.45 bn years ago
  • Feedbacks between marine photosynthetic microbes and the greenhouse effect radically altered Earth’s climate
  • Ancient example of how life is a major and intrinsic part of the Earth system