Transcript Objectives - Metlink Teaching Weather and Climate

Lesson 3
Evaluate the reliability of
different methods of
measuring climate change.
How is it done?
One of the ways of finding out about
past climates is to look at the remains of
plants. Often these are preserved or
fossilised and they can be thousands or
even millions of years old.
The shape of the leaves, the nature of
the leaf margin (the outside of the leaf)
and the features of the leaf cuticle can all
be used to provide estimates of mean
annual temperature, temperature range
and water availability. This method has
been developed over a number of years
and there is a clear relationship between
the temperature when the leaves were
growing and the percentage of smooth
leaves found in a group of leaves. The
group of leaves is called an ‘assemblage.’
Problems
The main problem with this method is that the plants may
have evolved to be quite different from their ancestors.
The ecology may also be different to how it was in the
past. Plants from the southern hemisphere are not as well
documented as plants in the northern hemisphere. Finally,
it is important to collect a sample of leaves that represent
the plants in that area.
More on measuring the climate in the news:
http://www.newscientist.com/article/dn986-fossils-leaves-reveal-climate-model-errors.html
Tree rings
Graph shows temperature data based on tree rings from several studies
How is it done?
In areas of the world where there are large variations
between summer and winter climate, many trees form clear,
annual growth rings around their circumference. The
thickness of these rings depends on many things including
temperature, water availability, light levels, which insects
were around and how long the growing season was. The rings
can also be affected by variation in the concentration of
gases in the atmosphere. By studying these tree rings,
scientists have access to a year-by-year record of the climate
stretching back hundreds, and sometimes thousands, of
years. Tree rings are most useful in cold climates where
temperature most strongly limits growth. Fossilized trees can
also be used, providing data which goes back even further.
Tree ring data closely follows other methods of measuring
temperature data. However, since about 1980, trees appear
to have stopped responding to an increase in temperature.
Problems
Sometimes it is difficult to identify which climate factor is responsible
for differences in growth. Is it wind, temperature, rainfall or sunlight
which may vary due to changes in cloud cover.
There are also non climatic factors which effect tree growth including
disease outbreaks, soil quality, pests, competition, genetic differences
and human impact. Some of these problems can be overcome by
collecting sufficient samples, but large fossil samples are not always
available.
Finally, there is not always a “linear” increase in growth. You might
expect that the more water there is, the more a plant grows. This
would be a linear effect. However, too much water can also stop the
plant growing.
More on measuring the climate in the news:
http://www.ncdc.noaa.gov/paleo/pubs/briffa2001/briffa2001.html
http://en.wikipedia.org/wiki/Dendroclimatology
How is it done?
Most water molecules are made of 2 atoms of
hydrogen with an atomic mass of 1 and one
atom of oxygen with an atomic mass of 16.
About 1 molecule in 500 contains the heavier
isotope of oxygen, 18O. Occurring even less
often is water containing 1 atom of deuterium
(D) which is hydrogen with an atomic mass of
2. Elements with the same atomic number but
different mass numbers are called isotopes.
These isotopes are stable (not radioactive) and
do not decay.
Measuring the relative proportions of them
can given an indication of temperature.
Rain and snow are formed from water which
has evaporated from an ocean, condensed as a
cloud and then fallen to the Earth. Heavier
molecules have lower vapour pressures which
means that when water evaporates the vapour
is depleted in these molecules and when the
vapour condenses out the condensate is
enriched in them. As the air moves from the
warm oceans towards the poles, water
enriched in the heavier isotopes condenses
out. As they do so the water vapour becomes
more depleted in the heavier isotope. The
result of this is that when the air arrives in
Antarctica the amount of water remaining and
the proportion of the water which contains the
heavier isotopes is mainly temperature
dependent.
Graph shows temperature data based on ice core samples
Problems
Most data is collected from Antarctica. This may not be
representative of temperature across the globe.
The carbon dioxide concentrations can be measured from air bubbles
trapped in the ice. Air can naturally moves in and out of the top 60100 meters of snow and ice. However, air that circulates deeper than
this gets trapped in tiny bubbles. The air bubbles are NOT the same
ages as the surrounding ice and this can confuse the results when
scientists analyse samples because they have to match up the
temperature data with the atmospheric data.
Finally, when water melts and runs into an area that is being sampled
it can effect the results. There are also differences in different areas.
More on measuring the climate in the news:
http://en.wikipedia.org/wiki/Ice_core#Paleoatmospheric_sampling
http://www.ipcc.ch/ipccreports/tar/wg1/068.htm
How is it done?
Coral Reefs, like the Great Barrier reef
found in Australia are areas of
enormous biodiversity. However, Coral
reefs are very sensitive to changes in
the sea water and the climate. Even
rivers that run into the sea can damage
corals because of the silt that they
carry can block the sunlight.
Coral is a living marine creature which
has a hard exo skeleton. A reef is
formed as the skeletons of millions of
organisms are joined together by
calcium carbonate.
Just like the way a tree produces
growth rings as it grows, so do the
skeletons of coral. Therefore, Scientists
can look at the bands formed in the
coral skeleton to work out what the
climate was like thousands of years
ago.
More on measuring the climate in the news:
http://www.ncdc.noaa.gov/paleo/outreach/coral/
http://www.ipcc.ch/ipccreports/tar/wg1/068.htm
Graph shows the warm events since
1600AD. Warm events are identified by
changes in coral growth rate.
Image shows a coral Reef.
Problems
Coral only grows in areas where there sea temperature is in the range of
16-25°C. It is also difficult to exactly match the growth rings to a
specific year.
Corals may also grow in areas isolated from the sea, such as in a lagoon
and these may not show true measurements. There can also often be
differences of 0.3% between different colonies of coral. Finally, the coral
can be eroded by the sea or by other creatures and this can effect the
thickness of their skeletons.
There is not as much research into using corals to measure the
temperature of the climate and very few studies that show temperature
data for more than 100 years. Therefore scientists tend to use other
measurements of their exoskeletons, such as isotope ratios.
How is it done?
Bore Holes are dug to allow people to extract minerals, gases
or oil. The walls of the hole are reinforced so it does not
collapse. A well is an example of a bore hole.
There are over 600 bore holes used to measure climate data
and some of these are as deep as 1000 meters.
Scientists can lower thermometers into the bore holes and
measure the temperature of the surrounding rock.
They know how much heat rises from the Earth’s core and
they take this away from their reading. This gives them an
indication of the surface temperature.
This data strongly corresponds to more recent readings taken
from the surface. Therefore scientists can extrapolate
backwards. A depth of 150 meters shows temperatures 100
years ago and a depth of 500 meters can show as far back as a
1000 years.
Graph shows temperature data based on bore holes. The jagged line
shows surface temperature data. The grey shaded area represents 2
standard deviations (i.e. the range of likely temperatures). The further
back scientists measure, the less certain they are of the accuracy of their
measurements.
Problems
Bore Hole data may be effected by the coverage on the land above with
snow, trees etc. In the UK snow coverage is less significant and
temperatures match other data well. In the US, there have been more
recent changes in land use (agriculture etc) and there are differences of 12°C compared to other measurements.
There have generally been less measurements made in the southern
hemisphere.
Comparison of bore hole data with other methods
More on measuring the climate in the news:
http://en.wikipedia.org/wiki/Proxy_(climate)#Boreholes
http://www.unisci.com/stories/20011/0227012.htm
http://climateaudit.org/2010/04/05/ipcc-and-the-law-dome-graphic/
• Summarize your method
• How does the data obtained from this method
compare with what you already know?
• Has the data been inferred or is it a direct
measurement of the temperature?
• What are the possible problems with this data
collection?
• Can you think of any other problems with this
method?
Method
Reliability
Not at all reliable
Ice Cores
Tree Rings
Leaf shape
Coral
Bore Holes
Very Reliable
Graphs shown - Clockwise
Leaf data
Bore Holes
Ice Cores
Tree Rings
Coral
Which method do you
think is most suitable
and why?
Give one advantage and
one disadvantage of
SOMEONE else’s method
List three methods to
measure climate change