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.’

ProblemsThe 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

Leaf Shape

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.

ProblemsSometimes 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

Tree Rings

Tree ringsGraph shows temperature data based on tree rings from several studies

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 whichmeans 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.

ProblemsMost 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 60-100 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

Ice Core Data

Graph shows temperature data based on ice core samples

Coral Reefs

Graph shows the warm events since 1600AD. Warm events are identified by changes in coral growth rate.Image shows a coral Reef.

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.

ProblemsCoral 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.

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

Bore Holes

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.

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.

ProblemsBore 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 1-2°C compared to other measurements. There have generally been less measurements made in the southern hemisphere.

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/ Comparison of bore hole data with other methods

• 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?

Part 1 – Discussion points

PlenaryMethod Reliability

Not at all reliable Very Reliable

Ice Cores

Tree Rings

Leaf shape

Coral

Bore Holes

Graphs shown - Clockwise Leaf dataBore HolesIce CoresTree RingsCoral

Graph Summary

How do the methods compare?

Usi

ng th

e Da

ta

Give one advantage and one disadvantage of

SOMEONE else’s method

List three methods to measure climate change

Which method do you think is most suitable

and why?

Assessment Triangle