looking at the evidence

what is evidence showing

• levels of carbon dioxide and other greenhouse gases in the atmosphere are increasing

• mean global temperatures are increasing (global warming)

• the global climate seems to be changing

• extreme weather events such as flooding, droughts and hurricanes are increasing

what types of evidence can be used to support global warming is taking place and human activities responsible for climate change? 

• Records of atmospheric carbon dioxide levels

• Records of average global temperatures

• Records of changing plant communities gained from sampling of pollen grains preserved in peat over time

• Records of tree growth gained by analysing the rings in the trunks of trees; known as dendrochronology

why co2 levels have fluctuated

• Atmospheric carbon dioxide levels have fluctuated throughout Earth's history due to events such as volcanic eruptions and the weathering of limestone rocks

  • Scientists know this from having analysed the gas composition of bubbles formed in ancient ice cores

    • Ice is deposited as water freezes over time, so the deeper into the ice you go, the older it is

• Since the industrial revolution, however, atmospheric carbon dioxide levels have risen to their highest in Earth's history

how temperature records show evidence for climate change

• Thermometers can be used to measure air temperature, and thermometer records from different places around the world over extended periods of time can be put together to show average global temperature change over time

• 1998 Intergovernmental Panel on climate change collected lot of data to produce a graph of temperature of Northern Hemisphere and update this regularly.

• Data of measured temperatures only since mid 1800s.

• Temperature indications can be taken from other data called temperature proxies.

• These include – tree rings, corals, ice cores and peat bog

data.

how frozen isotopes and temperature records can show evidence fro climate change

• Antarctic and Greenland ice cores used as temperature proxies. Scientists drill deep down into the ice and analyse air trapped at different layers.

• Allows estimation going back thousands of years. Oxygen isotopes in melted ice (proportion of O18 and O16) reflect air temperature at the time the ice layer was formed.

• Cores can also estimate atmospheric carbon dioxide level. Analysis of air in the ice cores for over 300,000 years. Shows the ice ages and the interglacial warm periods.

what are interglacials

the relatively warm period between ice ages

what is dendrochronology?

a type of temperature proxy. The dating of past events using tree ring growth. 

what happens to width of trees

• Trees increase in width as they get older by cell division of one particular layer in their trunks

• If there’s a lot of moisture they grow quickly in early summer in temperate regions these cells are large. If conditions were difficult then the new cells produced are

smaller.

• Cells stop dividing between one spring and the next so easy to see small rings then larger rings in more favourable conditions.

• Light coloured rings are produced by fast growth during warmer spring and summer months and dark coloured rings form as a result of slow autumn growth, meaning that one light ring and one dark ring together represent a full year's growth in a tree

How dendrochronology works + limitations?

• By counting the number of rings an approximate age of the tree or piece of wood.

• Analysis of the width of tree rings can provide a measure of climate during each year of growth

  • Taking cores from the trunks of older trees can provide samples that go back over hundreds of years

what is dendroclimatology?

• Dendroclimatology is the study of what dendrochronology can tell us about the past climate.

• This is due to the fact that growth of trees is dependent on many factors including

sunlight availability, temperature, carbon dioxide levels and amount of rainfall.

• New science, may be useful in future. If rings are similar then climate was similar in those years but if rings are wider was it due to warmer weather, more rain, more light or more carbon dioxide. It is likely due to a combination of factors eg weather warmer, wetter, sunnier, more CO2

Limitations of using dendroclimatology?

• If conditions of surroundings vary a lot during a year then the tree will produce more than one ring. Growth in trees also dependent on many factors including amount of sunshine, temperature, carbon dioxide levels and amount of rainfall

• So values are approximate.

• many people have questioned the validity of using tree-ring data to determine past temperature conditions and this has weakened some of the findings on climate change

coral reefs as temperature proxies

• data from coral reefs can be used to confirm the evidence from trees, as the proportions of different isotopes taken up by the coral vary as the sea temperatures change and this gives another valuable proxy record of climate change

what are peat bog or peatland

• Under waterlogged and acidic conditions partly decomposed dead plant matter accumulates and becomes compacted under its own weight over time; this compacted, partially decomposed plant matter forms peat

  • The place where peat accumulates is known as a peat bog, or peatland

  • the peat is very acidic, cool and anaerobic, which prevents bacteria from decomposing organic material

why/how peat can be used as temperature proxies

• Peat builds up in layers, meaning that layers of peat at the top of a bog are recently formed and the peat become older as you dig down into a bog

• Peat cores can be taken from a bog and the layers can be analysed to assess the pollen grains ,moss spores and platn tissues that have become trapped in the peat and preserved

• Pollen grains from peat samples can be observed under a microscope, and because the pollen grains of each plant species are unique to that plant, the plant species that were growing around the bog at different points in time can be identified

• Different plant species grow under different climatic conditions, so the plants present at different times can be used a measure of the climate at that time

  • E.g. an increase in the number of plant species that grow better in warmer climates combined with a decrease in the number of plant species that grow better in cooler climates indicates a gradual warming of the climate

Explain how studying pollen grains in peat bogs can provide evidence of climate change

• pollen preserved in peat bogs

• a plant species can be identified from its pollen

• climate affects the type of plants growing

• depth of peat correlates with period of time since pollen was produced

• changes in pollen over time indicate changes in climate

how far back can peat bogs give us info about climate change

• Peat growth rate depends on the prevailing conditions.

• Undisturbed peat bogs and lake sediments can give us clear and unbroken record of the climate. This can give us a continuous record from about 10,000 years ago which gives clear evidence of periods of warming and cooling.

how to increase data reliability from temperature proxies?

• we can use both dendrochronology and peat bogs dating to confirm radiocarbon dating

• We can date the wood or peat bogs samples of known age from radiocarbon measurements using remains of plant and pollen grains.

• Indication of the climatic conditions at the time those plants were alive which link to recorded events such as flood or extreme cold weather.

• We can compare results to give a form of calibration.

• this gives scientists clear reference points which they can use to determine the accuracy of their estimations of age, making the data considerably more reliable