Transcript File - AHA Geography Resources

Why has the temperature
changed throughout history?
How do we
know?
Why has the temperature changed
throughout history?
It can be changed through feedback effects
Feedback effects are those that can amplify a change and
make it bigger (positive) or smaller (negative).
positive feedback is
snow and ice cover.
Small increase in snow
and ice raises surface
Albedo reflecting
more solar energy back
into space. Resulting in
further cooling
negative feedback
s cloud cover. As
global warming
occurs, more
evaporation occurs
increasing cloud
cover, which in turn
may reflect more
solar rays back into
space diminishing
effects of the
warming
Know –
there are
both human and
physical causes for
climate change
Understand –
why some people
view current
climate change
as
unprecedented
Skills – explanation,
comprehension,
note taking
A*-A
Justify
B-C
which
cause,
Explain
human or
D
how natural
natural,
they think variations
Describe
affect the the natural
is the
cause of climate and reasons of
climate
assess
our
change
which
changing
hazards
climate.
1. Variations in the Earth’s Orbit
Milankovitch Cycles
• A Serbian physicist working at the beginning
of the 20th century developed a theory to
explain periods of glaciation in recent history.
• He identified 3 variations in the Earth’s orbit
around the Sun
Milutin Milankovitch
This might explain the small variations in the
earth’s temperature such as in the Middle ages.
Until recently studying previous
temperatures was not really possible, but
with Ice core analysis it is now possible
and there is support for his theory.
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
21,000 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.
• 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
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 0.40.7°C
• MT St Helens in 1980 and Mt
Pinatubo in 1991 released
huge quantities of volcanic
dust which blocked out
sunlight, and also caused
increases in cloud cover and
rainfall for 1-2years.
US geological survey says
by contrast humans
produce 150x the amount
of CO2 emitted by
volcanoes.
Why has the temperature changed
throughout history?
Is the global warming we see now unprecedented –
“never seen before”?
 Sir David Attenborough
 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 years 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!
•11 of the 12 warmest years on
record occurred between 1995
•The
level
of
CO2
in
the
and 2006
atmosphere is far above the
•Temperature
rises
have
been
‘natural’ level and continues to
rise. recorded on all continents since
1970
•Satellite observations since 1993
suggest an annual rise in sea level
of 3.1mm, and a decline in Arctic
sea ice of 2.7% per decade
Plenary
Hint: use years ad
average number of
sunspots per year to
quote in your
answer.
Hint: look for tends
in fluctuation. Does
it go up, down or
vary?