1.A. - How and why has the climate changed in the geological past? Flashcards
1
Q
how do we normally expect the climate to change?
A
- normally there are seasonal/diurnal weather changes
- the climate doesn’t shift that much and changes are more long term
- climate change is imperceptable and changes happen slowly
2
Q
how and when has the climate changed?
A
Climate change can be assessed across a variety of timescales:
- long term (or geological) - over hundreds of thousands to millions of years
- medium term (or historical) - within the last few thousand years
- short term (or recent) - within the last few decades
3
Q
what is the difference between icehouse and greenhouse conditions?
A
- over geological periods, the earth’s cliamte has fluctuated between greenhouse and icehouse conditons.
- during greenhouse conditions, carbon dioxide, sea levels and global temps are all higher than avg
- while the opposite is true in ice house conditions
4
Q
what is the Palaeocene-Eocene maximum?
A
- at this time the continents had a very different configuration, which affected ocean circulation and the earth’s energy budget
- a further, but short lived spike in global temps occurred 55 mill years ago.
- this is known as the Palaeocene-Eocene thermal maximum
- where the avg. global temps peaked around 23C
5
Q
the glaciation of antartica
A
- antartica is covered by an ice cap that extends to sea level.
- it is the largest glacial system on the planet
- Yet 40 mill years ago, the fossil record shows that the continent experienced sub-tropical conditions
- the decent of antarctica into a permanent icehouse state occurred rapidly around 35 mill years ago because of changes in atmospheric CO2 and tectonic processes.
6
Q
what characterises quaternary glaciation?
A
- last 2.6 mill years
- cyclic changes of climate, with long glacial periods interrupted by shorter, warmer interglacials
- in the past 450,000 years there have been 4 major glacial episodes and 4 interglacials
- the most recent glacial, the Devensian, reached its maximum around 20,000 years ago when approx 1/3 of the continental surface was covered by snow and ice
7
Q
what characterises the holocene?
A
- the current geological period is known as the Holocene
- it began at the end of the last glacial 11,700 years ago
- the holocene is, therefore, an inter-glacial period, a brief interlude separating glacials which dominated 90% of the quaternary
- during the Holocene ice sheets and glaciers have shrunk and sea level has risen by over 100m+
- ice sheets have disappeared from the continental surface (excluding Antarctica and Greenland)
- warming has not been continuous. there have been several relatively short 6000yr episodes of warming and cooling
8
Q
what is the quaternary period?
A
- the quaternary period is a period that spans the last 2.6 million years (we are still in it)
- it is characterised by cyclical changes in climate
- the cold periods are known as glacials and the warmer periods are known as inter-glacials
- ice sheets have advanced from the poles to latitudes of 40 degrees
9
Q
what are glacials?
A
a prolonged cold climatic phase lasting for tens of thousands of years and causing continental glaciation in middle and high latitudes
10
Q
what is the length of glacial/interglacial cycle?
A
- typically, glacials have lasted for around 100,000 years
- interglacials for 10,000-15,000 years.
- cylce is every 100,000 years and the earth’s orbit around the sun drives this
11
Q
what is the holocene period?
A
- the holocene is a gradual warming period that occurred at the end of the Pleistone and is now the current epoch we are experiencing
- however, it too has been characterised by some fluctuations in temperature
- the medieval warm period and little ice age are examples of this.
12
Q
A
13
Q
how can we describe/characterise the medieval warming period and little ice age?
A
- there have been several relatively short lived episodes of warming and cooling. 6000 years ago temperatures were 1-2°C higher than todays and an early medieval warm period occurred between 1100 and 1300. linked to solar radiation.
- this was followed by the “little ice age” (1550 to 1850) when average global temperatures fell by 1°c and sent winters in europe into a deep freeze. Frost led to fairs held on the thames.
14
Q
what is short term?
A
- short term is over a period of hundreds of years to decades if we are investigating very recent time scales
- this period (Holocene) has been characterised by less extreme ranges in temperature
- but there has been a medieval warming period when greenland was settled by Norse people and a little ice age when frost fairs were held on the thames
15
Q
what do climate scientists believe is responsible for recent climate change?
A
- the vast majority of climate scientists believe that climate change in the past 200 years has been driven by HUMAN ACTIVITY
- indeed, the influence of people on the global climate has become dominant, so much so that many scientists believe we have entered a new geological period called the anthropocene
16
Q
what is the anthropocene?
A
the influence of people on the global climate has become dominant, so much so that many scientists believe we have entered a new geological period called the anthropocene
17
Q
what are the 4 main ways we can reconstruct past climates?
A
- tree rings
- fossils
- pollen and spores
- oxygen isotopes in sea floor deposits/ice cores
18
Q
how can tree rings help us reconstruct past climates?
A
- trees grow wider as well as taller as they grow
- each year adds on rings
- the rings are the cambium layer, a ring of cells that lies between the wood and bark and from which new bark and wood cells originate.
- each year a new cambium is created leaving the previous one in place
- how large the cambium’s cells grow in each year, measured as the width of each ring, depends on temperature and moisture, how warm/cool/dry/wet each year’s seasons were.
- during dry years the cambium’s cells are smaller and thus the layer is thinner than during wet years.
- not all trees can be measured or used without additional analytical techniques: not all trees have cambiums that are created annually
- in tropical regions, for example, annual growth rings are not systematically formed, or growth rings are not tied to years, or there are no rings at all.
- evergreen cambiums are commonly irregular and not formed annually.
19
Q
what is the cambium layer in tree rings and what does it show?
A
- the rings are the cambium layer, a ring of cells that lies between the wood and bark and from which new bark and wood cells originate.
- each year a new cambium is created leaving the previous one in place.
- at its most basic, during dry years the cambium’s cells are smaller and thus the layer is thinner than during wet years.
20
Q
what are the cons of using tree rings as a method to reconstruct past climates?
A
- not all trees can be measured or used without additional analytical techniques: not all trees have cambiums that are created annually
- in tropical regions, for example, annual growth rings are not systematically formed, or growth rings are not tied to years, or there are no rings at all.
- evergreen cambiums are commonly irregular and not formed annually.
21
Q
how can fossils, particularly fossil corals, be used as a method to reconstruct past climates?
A
- some plants and animals can be very sensitive to climate and do not adapt easily to change
- for example, coral reefs live in tropical waters. they need a particular temperature, a specific depth of water and the right amount of light.
- if the depth of the water changes just a fraction, they cannot survive.
- therefore where fossil corals are found it is possible to estimate fairly precisely the environment they lived in by assuming that they needed the same conditions as those that thrive today.
22
Q
how can pollen and spores be used as a method to reconstruct past climates?
A
- plants produce pollen and spores that are particularly useful in helping to determine climate.
- they are tiny with a resistant outer case and are produced in millions
- this means they can be covered in mud quickly and are more easily preserved as fossils than large animals.
- each plant has different shaped pollen or spore so when the fossil is put under a microscope it is possible to identify the type of plant it came from.
- different plants are adapted to different climates therefore looking at all the types of pollen present in a layer of rock can be a good indication of the climate at the time they were living.
23
Q
what are pollen/spores?
A
- plants produce pollen and spores that are particularly useful in helping to determine climate.
- they are tiny with a resistant outer case and are produced in millions
- each plant has different shaped pollen or spores so when the fossil is put under a microscope it is possible to identify the type of plant it came from.
24
Q
how can ice cores be used as a method to reconstruct past climates?
A
- oxygen is most commonly found as Oxygen-16, but it has a heavier isotope, oxygen-18
- in water the relative amount of each type of oxygen varies with the temperature
- rain and snow contains water high in oxygen-16 (as it is lighter and more easily evaporated) so when it forms ice at the poles (and does not return to the sea) the ocean water is left with a high proportion of oxygen-18.
- shells and sediments carry the isotopic signature of the water in which they were formed.
- if those sediments are high in oxygen-18 it will indicate that the shells were formed at a time of colder temperatures as the sea water will have higher proportions of oxygen-18
- therefore, by analysing the ratio of oxygen-16 to oxygen-18 recorded in rocks and sediments on the sea floor we can find evidence of the climate at the time they were formed.
25
Q
what isotope is oxygen most commonly found as?
A
- oxygen is most commonly found as Oxygen-16, but it has a heavier isotope, oxygen-18
- in water the relative amount of each type of oxygen varies with the temperature
26
Q
how do antartic/greenland ice cores show our past climate?
A
- because the snow or rain that forms the ice near the poles contains higher proportions of the lighter oxygen-16 in colder periods.
- this is because the water vapour is increasingly depleted of oxygen-18 as it moves to higher latitudes and left in the oceans.
- the ice cores contain an abundance of information about the climate because as the glacial ice forms it traps wind-blown ash, bubbles of atmospheric gas and radioactive substances.
- the variety of climatic proxies is greater than in any other natural recorder of climate, such as tree rings or sediment layers.
27
Q
what was the younger dryas?
A
- COLD period
- reversed climate warming conditions and trends = brief cold spell
- concerning as temp dropped off suddenly
28
Q
what did the younger dryas show?
A
- that in the earth’s system there is rapid cooling and warming.
- within feedback systems something triggered that concerns climate scientists
- cold loving plant pollen was buried under warm loving pollen, showing how the climate switched from cold to warm VERY quickly.
- by analysing the chemical signature of ice its possible to determine the temperature at the time of the snowfall
- not much snowfall in the younger dryas. temperature jumped 5C in 1 year
29
Q
why was the younger dryas concerning?
A
- doesnt fit with cyclical slow climate change = short/sharp drop off where the earth cooled down rapidly = difficult to cope with.
- will this happen in the future?
