Transcript L11 Natural Causes of Climate Change

To recap
• Give 2 examples of research
methods that show long term
historical climate change?
• How reliable are these?
• Give 2 ways of measuring medium
term climate change?
• How reliable are these?
• What about short term?
Recap
•On your whiteboards reproduce an
annotated drawing of the greenhouse
effect
Learning Objectives
• Know that there are both
human and physical causes for
climate change
• Understand the evidence for
and against the view that
climate change is unprecedented
• Be able to attempt an exam
style question using knowledge
from previous lessons
Is climate change natural or
human induced?
•Last lesson we focussed upon how we
have the data to justify claims of
climate change.
•We also discussed the human causes
of climate change
•Today we are looking at the views
that climate change is a natural event
The natural argument!
•There are 3 main ways that scientists
believe are affecting the worlds
climate naturally
•1 Variation in the earths orbit
•2 Variation in solar output
•3 Volcanic eruptions and cosmic
causes
1. Variations in the Earth’s Orbit
• Milankovitch Cycles
• A Serbian physicist working at the
beginning of the 20th century.
• He identified 3 variations in the Earth’s
orbit around the Sun
Milutin Milankovitch 1879 1958
Earths orbit!
• Every 100,000 yrs the Earths orbit
changes from spherical to elliptical,
changing solar input
Tilt of the Earth
• The Earth’s axis is tilted at 23.5o, this
changes over a 41,000 yr cycle between
22o & 24.5o, affecting solar input,
especially in higher latitudes.
Wobbly axis!
• The Earth’s axis wobbles, so which way
the hemispheres are facing to the sun
when closest to the sun varies over
21,0000 yrs. Affecting solar input.
• Until recently
Milankovitch’s
had no real
support from
past climate
history.
• However,
from
measuring ice
cores these
now support
this theory!
• Milankovitch Cycles
• Many scientists argue that the Milankovitch cycle may
have been just enough to trigger a major global climate
change, but that climate feedback mechanisms are needed
to sustain it.
• KEY WORD:
• Feedback effects are those that can amplify a change
and make it bigger (positive) or smaller (negative).
• An e.g. of positive feedback is snow and ice cover.
Small increase in snow and ice raises surface area
reflecting more solar energy back into space.
Resulting in further cooling
• An e.g. of negative feedback s cloud cover. As GW
occurs, more evaporation occurs increasing cloud
cover, which in turn may reflect more solar rays back
into space diminishing effects of the warming.
2. Variations in Solar Output
• The sun’s output is not constant is also varies.
A variety of cycles have been detected, most
are short term, the most obvious is due to
sun Spot activity – 11yrs
• The effect of sunspots
is to blast more solar radiation
towards the earth
• Some scientists have suggested
that around 20% of 20th Century
warming may be because of solar
output variation
3. Volcanoes
• Major eruptions eject
material into
stratosphere.
• The sulphur dioxide
forms a haze of
sulphate aerosols,
which reduces the
amount of sunlight
received at Earth’s
surface
• The eruption of
Tambora led to the
year without a summer
in 1816 as global
temperatures dipped
by 0.4-0.7 degrees C
Unprecedented Global warming?
•What does this phrase mean??
•Never been seen before!
•So is Global warming unprecedented?
•Not unprecedented as we have had changes
before where periods of time have been warmer.
•However, the quote from the IPCC in 2007 sums
up the current views on Global warming:
•“The observed widespread warming of the atmosphere and
ocean, together with ice mass loss, support the conclusion
that it is extremely unlikely that global climate change of
the past 50 yrs can be explained without external forcing,
and very likely that it is not due to known natural causes
alone”
So anthropogenic causes seem to be
confirmed! Note down some of the
recent facts to emphasize this!
•The level of CO2 in the
•11 of the 12 warmest years on
atmosphere
is
far above the
•Satellite
observations
since
1993
record occurred between 1995
‘natural’
level
and
continues
to level
suggest
an
annual
rise
in
sea
and 2006
rise.
of 3.1mm, andrises
a decline
Arctic
•Temperature
have in
been
sea
ice
of
2.7%
per
decade
recorded on all continents since
1970