1. How and why has climate changed in the geological past
Key idea : The earths climate is dynamic
• The earth's climate has undergone huge changes in the past 100 million years
• The changes have been particularly rapid in the last 2.5 million years, known as the quaternary period - with
numerous transitions from warm periods to cold periods
Methods used to reconstruct past climates
Methods used to reconstruct past climates :
Sea floor sediment fossils of tiny sea creatures called foraminifera, which accumulate in sea floor
sediments, can be used to reconstruct past climates. Their chemical composition
indicates the ocean temperatures when they formed.
Ice cores from polar regions contain tiny bubbles of air - records of the gaseous composition
of the atmosphere in the past. Scientists can measure the relative frequency of
hydrogen to oxygen atoms with stable isotopes. The colder the climate the lower
frequency of these isotopes.
Lake sediment pollen analysis identifies past vegetation types which infers the paleoclimatic
conditions.
Diatoms are single celled algae found in lakes with cell walls made of silica, they
record evidence of past climates in their shells (like foraminifera)
Varves are tiny layers of lake sediment comprising alternating light and dark bands.
The light bands are formed from coarser sediments, indicating high energy, melt
water run off in spring and summer. and the darker bands, made up of fine
sediment, show deposition occurring in the winter months
Tree rings Study of tree rings, dendrochronology. Annules vary in width each year depending
on temperature conditions and moisture availability
Fossils Plants and animals require specific environmental conditions to thrive. Some, such
as coral reefs, are highly sensitive to temperature, water depth and sunlight. Where
they exist in the fossil record
Fluctuations in the Earth's climate in the past
• Over hundreds of millions of years the earths climate has fluctuated between greenhouse and icehouse
conditions.
• During greenhouse conditions - co2 conc high - high global temps - high sea levels
• During icehouse conditions - co2 conc low - low global temps - large parts of the continental surface are
covered in ice
• Glacial and interglacial periods occur in icehouse phases but on much smaller timescales
• Most recent is the pleistocene, lasted for around 1 million years
• past climate reveal periods of greenhouse and icehouse earth including :
, Long term changes - transition to colder to colder global temps
• During mid cretaceous period
- 100 million years ago
- average global temperatures were around 6-8 degrees higher than today
- Sub tropical conditions were extended from antarctica to alaska
- And there were no polar ice caps
- Very warm phase
- Co2 levels were 5 times higher than today
- Lasted for 10 million years
- At this time continents had a very different configuration, which affected ocean circulation
and the earths energy budget
• paleocene - eonce
- 55million years ago
- Another spike in global temperatures
- Average global temperatures peaked around 23 degrees
- at a time when Pangaea was splitting into separate continents and lasted for 200,000 years.
- It is suspected that huge amounts of carbon were released into the atmosphere and oceans in
the form of carbon dioxide and methane.
- During this period of warming, The globe warmed 5 to 9°C (9 to 16°F). Most ecosystems were
able to adapt to this.
• Oligocene
- Colder conditions
- Which continued into present day
- Change was related to a major decreases in atmospheric greenhouse gases - co2
Glaciation of Antarctica
• Antarctica is the largest glacial system on the planet and the whole continent is covered by a huge ice sheet
– there is 25-30 x 106 km3 of glacial ice.
• The thickness is such that a few tops are exposed of the mountain ranges that are under the ice.
• Despite its current conditions, evidence from the fossil record shows that the continent experienced
subtropical conditions.
• It was around 35 million years ago that Antarctica rapidly descended into a permanent icehouse state.
• Changes in atmospheric CO2 and tectonic processes explain the transition of Antarctica from sub-tropical
conditions to an icehouse state:
1. 35 million years ago, the CO2 levels dropped rapidly from 1000-1200 ppm (parts per million) to
600-700ppm.
2. Continental drift can also explain the change in temperature – Antarctica moved towards the south
pole and away from South America and Australia which isolated it. An ocean current known as the
Antarctic Circumpolar Current then insulated the warmer water further north.
Key idea : The earths climate is dynamic
• The earth's climate has undergone huge changes in the past 100 million years
• The changes have been particularly rapid in the last 2.5 million years, known as the quaternary period - with
numerous transitions from warm periods to cold periods
Methods used to reconstruct past climates
Methods used to reconstruct past climates :
Sea floor sediment fossils of tiny sea creatures called foraminifera, which accumulate in sea floor
sediments, can be used to reconstruct past climates. Their chemical composition
indicates the ocean temperatures when they formed.
Ice cores from polar regions contain tiny bubbles of air - records of the gaseous composition
of the atmosphere in the past. Scientists can measure the relative frequency of
hydrogen to oxygen atoms with stable isotopes. The colder the climate the lower
frequency of these isotopes.
Lake sediment pollen analysis identifies past vegetation types which infers the paleoclimatic
conditions.
Diatoms are single celled algae found in lakes with cell walls made of silica, they
record evidence of past climates in their shells (like foraminifera)
Varves are tiny layers of lake sediment comprising alternating light and dark bands.
The light bands are formed from coarser sediments, indicating high energy, melt
water run off in spring and summer. and the darker bands, made up of fine
sediment, show deposition occurring in the winter months
Tree rings Study of tree rings, dendrochronology. Annules vary in width each year depending
on temperature conditions and moisture availability
Fossils Plants and animals require specific environmental conditions to thrive. Some, such
as coral reefs, are highly sensitive to temperature, water depth and sunlight. Where
they exist in the fossil record
Fluctuations in the Earth's climate in the past
• Over hundreds of millions of years the earths climate has fluctuated between greenhouse and icehouse
conditions.
• During greenhouse conditions - co2 conc high - high global temps - high sea levels
• During icehouse conditions - co2 conc low - low global temps - large parts of the continental surface are
covered in ice
• Glacial and interglacial periods occur in icehouse phases but on much smaller timescales
• Most recent is the pleistocene, lasted for around 1 million years
• past climate reveal periods of greenhouse and icehouse earth including :
, Long term changes - transition to colder to colder global temps
• During mid cretaceous period
- 100 million years ago
- average global temperatures were around 6-8 degrees higher than today
- Sub tropical conditions were extended from antarctica to alaska
- And there were no polar ice caps
- Very warm phase
- Co2 levels were 5 times higher than today
- Lasted for 10 million years
- At this time continents had a very different configuration, which affected ocean circulation
and the earths energy budget
• paleocene - eonce
- 55million years ago
- Another spike in global temperatures
- Average global temperatures peaked around 23 degrees
- at a time when Pangaea was splitting into separate continents and lasted for 200,000 years.
- It is suspected that huge amounts of carbon were released into the atmosphere and oceans in
the form of carbon dioxide and methane.
- During this period of warming, The globe warmed 5 to 9°C (9 to 16°F). Most ecosystems were
able to adapt to this.
• Oligocene
- Colder conditions
- Which continued into present day
- Change was related to a major decreases in atmospheric greenhouse gases - co2
Glaciation of Antarctica
• Antarctica is the largest glacial system on the planet and the whole continent is covered by a huge ice sheet
– there is 25-30 x 106 km3 of glacial ice.
• The thickness is such that a few tops are exposed of the mountain ranges that are under the ice.
• Despite its current conditions, evidence from the fossil record shows that the continent experienced
subtropical conditions.
• It was around 35 million years ago that Antarctica rapidly descended into a permanent icehouse state.
• Changes in atmospheric CO2 and tectonic processes explain the transition of Antarctica from sub-tropical
conditions to an icehouse state:
1. 35 million years ago, the CO2 levels dropped rapidly from 1000-1200 ppm (parts per million) to
600-700ppm.
2. Continental drift can also explain the change in temperature – Antarctica moved towards the south
pole and away from South America and Australia which isolated it. An ocean current known as the
Antarctic Circumpolar Current then insulated the warmer water further north.
