Edexcel AS Geography

Edexcel AS GeographyUnit 1 –

World at RiskClimate Change & it’s causes – Part A

Aims • Is global warming a recent short term

phenomenon or should it be seen as part of long term climate change?

• What evidence is there of long-, medium-, and short- term climate change?

• What are the human and natural causes of climate change?

• What is the evidence for and against the view that current climate change is largely as a result of human activity?

Climate is constantly changing

• Climate - the average condition of precipitation, temperature, pressure and wind over a 30 year period.

• Climate change - is any significant long term trend (of at least several decades) or shift in the average value for any climatic element (eg rainfall, drought, storminess)

Climate time scales

• Climate change can be assessed across long, medium and short timescales.

• NB Not all scientists use exactly the same time frames for each!

Long term Climate Change (Geological timescale)

Long-term climate change• Taking temperature as an indicator of climate, there have

been huge changes over Earth’s history• Temperatures have been much higher at some points in the

past than they are today• Looking at more detailed data for the last 400,000 years (on

the previous slide) you can see significant fluctuations• Climate shifted between cold glacial periods that lasted

around 100,000 years and warmer interglacial periods that lasted around 10,000 years.

• We are in an interglacial period now.

Medium/Long-term Climate Change

• The last glacial period ended around 18,000 years ago.

• The warming of the climate after this was fast at times, but it wasn’t constant – about 13,000 years ago the climate switched back to cooler conditions for about 1000 years

• Around 5,000 years ago, temperatures were 1-2°C higher than today.

Medium-term Climate Change

Short-term Climate Change (recent)

• Global warming is the term used for the rapidly increasing global average temperature on earth over the last century – there has been a sharp rise in temperature when you look at the last 1,000 years (see graph on next slide)

Short-term Climate Change

Short-term Climate Change• The overall pattern over the last century shows a

general increase in temperature, but the pattern has not been constant.

• Global temperature rose steadily from the early 20th century until the 1940s (although they fluctuated annually), then dropped back down.

• Scientists thought there would be another glacial period, but temperatures have risen rapidly again since the 1970s (this is global warming)

Global warming

• You need to think about global warming changes in the context of all the other climatic changes that have occurred

• It is important to remember that climate change is not fully understood – but the changes in temperature (and the speed at which they are occurring) over the last century are worrying because they are thought to be unprecedented

Exam Hint

In an exam question you may be asked how we know there have been climatic changes – you would be expected to describe and explain a variety of techniques pointing out their strengths &/or weaknesses.

Evidence for Long-term Climate Change

• 1. Ice core analysis• 2. Ocean core analysis• 3. Pollen analysis

Read P 42-44 Philip Allan

Evidence for Long-term Climate Change 1. Ice Cores – for Carbon and Oxygen

• Scientists drill deep into ice sheets to extract cores of ice in Greenland and Antarctica.

• Ice sheets are made up of layers of ice – one layer is formed every snowfall and compressed by later falls.

• So the ice at the bottom of the core is really old• They can analyse the gases trapped when the ice formed to tell

what the temperature was each year.• Then they can figure out how temperature has changed over time.• A core has been extracted from the East Antarctic ice sheet to a

depth of 3200 m and has been used to show temperature for the last 800,000 years.

a) Carbon analysisTechnique• Air bubbles trapped in the ice contain atmospheric CO2.• Low concentration of carbon dioxide occur in cold glacial periods.• High concentrations are found in interglacial periods.• So this allows a time series of CO2 levels in the atmosphere to be

reconstructed. Findings• CO2 levels have varied between 180ppm and 280 ppm over the

last 800,000 years. • It is clear that atmospheric CO2 levels now are greater than at

any time in the last half million years.• In 2007 they were 383 ppm.

Carbon dioxide

• Reliability?• The number of CO2 sequences that correlate well

with each other suggests this is quite reliable

b) Oxygen analysisTechniques• The ice itself preserves a record of oxygen isotopes. • The 18O/16O ratio provides a record of ancient water temperature.• Water 10 to 15 °C cooler than present represents glaciation. As colder temperatures spread

toward the equator, water vapor rich in 18O preferentially rains out at lower latitudes. • The remaining water vapor that condenses over higher latitudes is subsequently rich in 16O.• Precipitation and therefore glacial ice contain water with a low 18O content. • Since large amounts of 16O water are being stored as glacial ice, the 18O content of oceanic

water is high. • Water up to 5 °C (9 °F) warmer than today represents an interglacial, when the 18O content

of oceanic water is lower. • A plot of ancient water temperature over time indicates that climate has varied cyclically,

with large cycles and harmonics, or smaller cycles, superimposed on the large ones. This technique has been especially valuable for identifying glacial maxima and minima in the Pleistocene.

Findings• The oxygen and carbon dioxide isotope records

correlate well. • When carbon dioxide levels were low so was the

sea level.• Around 20,000 years ago as the ice sheets

reached their maximum extent in the glacial climate, sea level was 130m lower than today.

Reliability?• As with carbon dioxide sequences the number of

oxygen isotope records that broadly agree on sea level changes is high and the correlation between with carbon dioxide levels is good.

• Pollen from plants is often preserved in sediment (taken as cores from peat bogs and lake beds).

• The preserved pollen can be identified and dated to show when it was released and the ecology of the past.

• Scientists know the conditions that plant lives in now. • When they find preserved pollen from similar plants, it

indicates that conditions were similar when that pollen was produced.

• Different plant species have characteristic pollen shapes that can be identified.

Evidence for Long-term Climate Change Pollen Analysis

Findings• Pollen sequences show that the ecosystems have

changed in the past in response to climate change.

• In the UK tundra was present in the glacial periods and forest gradually colonised areas in the interglacial conditions.

Reliability?• Accurate pollen reconstructions rely on good

preservation of the pollen.• Long pollen sequences are rare.• Vegetation change may lag behind the climate

change.

• Sea level is affected by things like the volume of water stored as ice

• Past sea levels are shown by raised beaches (formed when sea levels were higher).

• Raised beaches can be dated.• They can indicate that less water was stored as

ice (i.e. It was warmer)

Evidence for Long-term Climate Change Sea Level Change

• Historical records can indirectly indicate different conditions in the past

• They are called Proxy records.• They include artistic and photographic

sources, time series data (such as grape harvest dates), and written descriptions, diaries and records (such as the Greenland sagas).

Evidence for Medium-term Climate Change Historical Records

FindingsThey seem to clearly indicate two different climatic

periods in the past:

• The Medieval Warm Period (sometimes called the Medieval Climate Optimum) – dating from around 800-1300AD. The end of this period coincides with the Black Death. There was an unusually warm North Atlantic climate.

• The Little Ice Age – from about 1400-1850, but with colder spells around 1770 and 1850.

The Thames Frost fair

Victorian ramblers on a swiss glacier

Burgundy grape harvest dates

Reliability?• These sources did not set out to record the climate• They must be used with care• They are usually local• Long time series data are rare• It is difficult to use them to generalise• Often extremes are recorded rather than the norm• Much of the data has been lost• Grape harvest could be influenced by non-climatic

factors like disease and conflict

• A new tree ring is formed each year as a tree grows. • Trees are sensitive to changes in temperature, rainfall and

sunlight.• If conditions that year were good, the tree ring produced

will be thick.• Narrow rings are produced in years of climatic stress.• Scientists can take cores and count the rings to find the age

of a tree. They then look at the thickness of each ring to see what the climate was like each year.

• Tree rings can reliably show and date climate change up to 10,000 years ago.

• Living trees, such as the Bristlecone pines in W USA are as old as 4500 years.

Evidence for Medium-term Climate Change Tree Rings

Reliability?• The accuracy of the tree ring records is good but

they are very localised.• It is difficult to determine the relative importance

of temperature, precipitation, sunlight and wind.

• Valley glaciers in places like the Alps grow and shrink in response to the climate.

• Changes in a glacier can be tracked by examining secondary records such as old paintings, photographs and maps.

• Scientists can also tell how big a glacier was and how far it extended by looking at the position of rocks (snout and morraine) deposited by it. These rocks can be dated to show when they were deposited.

• The distance of the rocks from the current glacier indicates climate change– E.g. If the front of the glacier is now miles away from the rocks it indicates that temperatures have increased over that period of time.

Evidence for Medium-term Climate Change Retreating Glaciers

Findings• Evidence suggests that the majority of glaciers

reached their recent maximum extent in 1850.• This correlates well with the Little Ice Age and

colder temperatures in the 17th and 18th centuries.

• Most glaciers have retreated since 1850.

Reliability?• Reliable results date back to about 1880.• Before this time the record is patchy and relies on

more proxy historical records.

• Details of weather conditions have been constantly collected since 1861.

• These can be used to show detailed climate changes over the short time period they have been collected.

Evidence for Short-term Climate Change Weather Records

Findings• They show that the near surface air temperature rose

by 0.74 degrees C between 1900 and 2000• The warming trend has been almost constant since

1960.• 11 of the world’s hottest 12 years happened between

1995-2006• Oceans have warmed to depth of 300om• Warmer oceans cause problems for temperature

sensitive species like corals• There are fears about ocean acidificiation – pH of the

ocean has decreased from 8.25-8.14 since 1750 (likely due to increased CO2 dissolved)

• Instrumental records show that global sea level has increased.

• Between 1961 and 2003 it rose by 1.8mm per year.• By 1993-2003 it was rising 3.1mm per year. • Most is thought to be associated with thermal

expansion.• Thermal expansion is the increased volume of the

oceans due to their higher temperatures. • It accounted for about 60% of sea level rise in the

late 20th century.• The ice melt from glaciers and ice caps so far is

thought to have a lesser effect on sea levels.

• Ice is found in glaciers, as ice caps on mountain ranges, as ice sheets in Greenland and Antarctica and as sea ice in the Winter in high Northern latitudes.

• In a warming world, ice might be expected to melt.

Evidence for Short-term Climate Change Ice Response

Recent changes in global ice cover

• Changes in temperature affect the availability of food and shelter

• This affects what species live in an area• So scientists can use changes in how species

are distributed to indicate changes in the climate

Evidence for Short-term Climate Change Ecosystem Changes

Exam Question

• To get full marks you need to outline (describe and explain) a number of sources of evidence for long-term climate change, e.g. Ice cores, pollen analysis and indicators of sea-level change

• Structure the answer to have an introduction, then separate sections outlining each of the sources of evidence and finish with a conclusion

• In your introduction, define the key terms of the question such as ‘climate change’ and ‘long-term’, e.g. ‘Climate change is any significant change in the weather of a region over a period of at least several decades.’

• For each source of evidence describe it and then explain how it’s used to determine past climate change, e.g. Scientists can drill deep into ice sheets (huge masses of ice) to extract cores of ice. When the ice was formed, gases were trapped inside it. Scientists can analysethese gases to tell what the temperature was when theice was formed.

Outline the sources of evidence for long-term climate change (10 marks)