Edexcel AS Geography
Unit 1 –
World at Risk
Climate 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 12°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 analysis
Technique
• 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 analysis
Techniques
• 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.
Evidence for Long-term Climate Change
Pollen Analysis
• 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.
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.
Evidence for Long-term Climate Change
Sea Level 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 Medium-term Climate Change
Historical Records
• 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).
Findings
They 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 nonclimatic factors like disease and conflict
Evidence for Medium-term Climate Change
Tree Rings
• 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.
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.
Evidence for Medium-term Climate Change
Retreating Glaciers
• 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.
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.
Evidence for Short-term Climate Change
Weather 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.
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.
Evidence for Short-term Climate Change
Ice Response
• 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.
Recent changes in global ice cover
Evidence for Short-term Climate Change
Ecosystem Changes
• 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
Exam Question
Outline the sources of evidence for long-term climate change (10 marks)
• 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 analyse
these gases to tell what the temperature was when the
ice was formed.