Teacher Copy, Level 3
Name_________________
What Do Trees Know About Rain?
Featured scientist: Alison O’Donnell from The University of Western Australia.
Co-produced with Science Journal for Kids.
Research Background:
Did you know that Australia is the driest inhabited continent in
the world? Because it is so dry, we need to be able to predict
how often and how much rain will fall. Predictions about future
droughts help farmers care for their crops, cities plan their water
use, and scientists better understand how ecosystems will
change. The typical climate of arid northwest Australia consists
of long drought periods with a few very wet years sprinkled in.
Scientists predict that climate change will cause these cycles to
become more extreme – droughts will become longer and
periods of rain will become wetter. When variability is the norm,
how can scientists tell if the climate is changing and droughts
and rain events today are more intense than what we've seen in
the past?
Bark
Most rec
ent year
Wet year
To make rainfall predictions for the future, scientists need data
on past rainfall. However, humans have only recorded rainfall in
Australia for the past 100 years. Because climate changes
slowly and goes through long-term cycles, scientists need very
long term datasets of rainfall.
The answer to this challenge comes from trees! Using
dendrochronology, the study of tree rings, scientists get a
window back in time. Many tree species add a ring to their trunks
every year. The width of this ring varies from year to year
depending on how much water is available. If it rains a lot in a
year, the tree grows relatively fast and ends up with a wide tree
ring. If there isn’t much rain in a year, the tree doesn't grow much
and the ring is narrow. We can compare the width of rings from
recent years to the known rain data humans have collected.
Then, assuming the same forces that determine tree ring width
are operating today as in the past, we can go back and interpret
how much rain fell in years where we have no recorded rainfall
data. This indirect information from tree rings is known as a
proxy, and helps us infer data about past climates.
Dry year
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Teacher Copy, Level 3
For this study, the scientists used cypress-pine, or Callitris columellaris. This species is
able to survive in Australia’s dry climates and is long lived enough to provide data far
back in time. Fortunately, scientists don't have to cut down the trees to see their rings.
Instead, they use a corer – a hollow metal drill with the diameter of a straw. They drill it
through the tree all the way to its core, and extract a sample of the tissue that shows all
the tree rings. The scientists took 40 cores from 27 different cypress-pine trees. The
oldest trees in the sample were more than 200 years old. Next, they developed a
chronology where they lined up ring widths from one tree with all the other trees, wide
with wide and narrow with narrow. This chronology gives them many replicate samples,
and data going back all the way to the 19th century! Next, they used a dataset of rainfall
from rain gauges that have been set out in Australia since 1910. They then take this
precipitation data and overlay it with the tree ring widths since 1910. For tree rings
before 1910, they then project back in time using this formula:
Rainfall = 125.850 + (104.973 x ring width)
Check for Understanding: After reading the introduction, students should be able to:
• Describe why it is important to be able to make predictions about rainfall in Australia.
• Describe why long-term datasets on weather are important tools for climate scientists.
Because weather is so variable, only by looking very far back can we determine if climate
is changing or if there is just a short-term fluctuation.
• Explain how dendrochronology can give us a window into the climate of the past.
Scientist Alison coring a cypress pine
A cypress pine tree growing in Australia
Data Nuggets developed by Michigan State University fellows in the NSF BEACON and GK-12 programs.
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Teacher Copy, Level 3
Scientific Questions: What do tree rings tell us about the past climates of Australia?
What do we learn about modern droughts and rain events by comparing them to what
we’ve seen in the past?
What is the hypothesis? Find the hypothesis in the Research Background and underline
it. A hypothesis is a proposed explanation for an observation, which can then be tested
with experimentation or other types of studies.
Scientific Data:
Teacher Note: Students will use the rainfall formula to complete the “Reconstructed Rainfall”
column in the data table. The table below only provides data for every 10 years. The full
dataset, with data from every year, is available online (link in Teacher Resources box at end
of activity) for students to practice graphing using Excel.
Use the rainfall formula to complete the partial data table below:
Year
Tree Ring
Width (mm)
1800
1810
1820
1830
1840
1850
1860
1870
1880
1890
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
2010
0.539
2.054
1.723
1.814
2.188
0.532
0.942
0.782
1.164
0.651
1.451
1.672
0.378
0.432
0.600
0.433
1.288
0.726
2.147
0.197
3.902
0.122
Instrumental Reconstructed
Rainfall (mm)
Rainfall
x
x
x
x
x
x
x
x
x
x
x
115.20
193.80
163.80
121.80
148.40
320.80
160.30
420.40
199.20
627.60
102.30
182.43
341.46
306.72
316.27
355.53
181.70
224.73
207.94
248.04
194.19
278.17
301.36
165.53
171.20
188.83
171.30
261.06
202.06
351.23
146.53
535.45
138.66
Data Nuggets developed by Michigan State University fellows in the NSF BEACON and GK-12 programs.
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Teacher Copy, Level 3
What data will you use to answer the question?
Independent variable: Year
Dependent variables: Mean tree ring width (mm) & Amount of
rainfall (instrumental & reconstructed)
Teacher Note: After completing the data table, students may wonder, why do the numbers
differ for the Instrumental and Reconstructed Rainfall columns? The instrumental rainfall
represents how much rain actually fell and was recorded by instruments, like rain gauges,
while the reconstructed rainfall represents our best estimate of how much it rained based on
tree ring width. These two numbers are not the same because tree rings are not a perfect
way to estimate rainfall – other things affect tree ring widths besides the amount of water
available to a tree, for example if the tree was growing in sun or shade, if the tree got a
disease in a particular year, or if there was a forest fire. To help account for this, scientists
use tree cores from many trees (in this study they cored 27) and complex mathematical
equations to account for all of the variability between trees. But, the width of tree rings will
never be a perfect approximation for rainfall. In a perfect world we’d have instrumental
rainfall measurements for every year because these are going to be the most accurate, but
when we don’t we can use tree ring width as a proxy.
Ask students to look at the data in the table or graph, paying specific attention to years when
we have data from both rain gauges and tree rings. Does it appear that the Reconstructed
Rainfall estimates are a good approximation for the Instrumental Rainfall?
Below are graphs of the full dataset:
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1800
1805
1810
1815
1820
1825
1830
1835
1840
1845
1850
1855
1860
1865
1870
1875
1880
1885
1890
1895
1900
1905
1910
1915
1920
1925
1930
1935
1940
1945
1950
1955
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
Mean Tree Ring Width (mm)
4.5
Year
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Teacher Copy, Level 3
700
Reconstructed Rainfall
500
400
300
200
100
0
1800
1805
1810
1815
1820
1825
1830
1835
1840
1845
1850
1855
1860
1865
1870
1875
1880
1885
1890
1895
1900
1905
1910
1915
1920
1925
1930
1935
1940
1945
1950
1955
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
Rainfall (mm)
600
Instrumental Rainfall (mm)
Year
* Horizontal dark lines represent the average tree ring width and rainfall across all
recorded years (1800-2012). Use these lines to determine which years have above
average rainfall or above average tree ring widths.
* Vertical light gray bars represent decades, or intervals of 10 years.
Interpret the data:
What trends, changes, or differences do you see in the table or on the graph?
In the past 20 years the rainfall has been mostly above the
average level of rainfall. In the past, it looks like these
periods only lasted about 10 years. There is a lot of
variability around the mean in both graphs. There does not
appear to be an overall trend of increasing rainfall over time.
Make a claim that answers the scientific questions.
Tree rings give us the ability to look back in time and have an
estimate for the amount of rain that fell before humans were
collecting data on rainfall in Australia. Compared to what we’ve
seen in the past, today rainfall events are lasting longer and
are wetter than any time in the past.
Data Nuggets developed by Michigan State University fellows in the NSF BEACON and GK-12 programs.
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Support your claim using data as evidence. Reference specific parts of the table or
graph.
Using this long-term dataset we can see that the last two
decades (1995-2012) have been unusually wet. The average
precipitation was 310 mm in these years, compared to an average
of 229 mm for the previous two centuries. In fact, five of the
ten wettest years in the past century occurred in the last two
decades (1995, 1997, 1999, 2000, and 2006).
The long-term data also tells us that not only is it wetter, but
rainfall cycles are changing too. The tree ring data
demonstrated a historical long-term rainfall pattern of droughts
lasting 10-30 years, followed by wet periods lasting 5-10 years.
However, recently these wet cycles are longer. Instead of wet
periods lasting 5-10 years, there has been a wet period now in
arid northwest Australia for 20 years.
Explain your reasoning and how the data supports your claim. Connect the data back to
the climate of Australia and scientists’ predictions that climate change will cause
droughts to last longer and periods of rain to be wetter.
Based on the data, the climate in arid northwest Australia is
wetter today than it was in the past. The tree ring data tells
us that not only is it wetter on average, but yearly cycles are
changing as well. Though in the past 20 years there are still
some dry years scattered in, there is a much higher frequency of
wet years in this period that makes the average rainfall higher
than in the past.
What do the data from this study tell us about the scientist’s hypothesis?
The tree ring data supports the hypothesis that arid northwest
Australia’s climate is changing. Rain events today are more
intense than what we've seen in the past, and wet periods last
longer than they did historically.
Your next steps as a scientist: Science is an ongoing process. What new question do
you think should be investigated? What future data should be collected to answer your
question?
Scientists are still trying to figure out why it has been
raining so much lately. One way to do this is to look for other
climatic patterns that are varying along the same timescales as
the rain in Australia and see if they are correlated. The
scientists in this study looked for causes such as air pollution
coming from Asia (pollution particles can seed rain clouds) and
Data Nuggets developed by Michigan State University fellows in the NSF BEACON and GK-12 programs.
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Name_________________
the El Niño Southern Oscillation. Neither of these could fully
explain the rainfall pattern.
Scientists now believe that the abnormally high rains seen in
Australia are a result of a recent anomaly in sea level air
pressure and wind patterns in the Southern Ocean, which has been
related to the Antarctic Ozone Hole – one example of climate
change. As we continue to burn fossil fuels and increase the
concentration of greenhouse gasses in the atmosphere, this
anomaly will likely continue. As a result, the scientists expect
that the rainfall in arid northwest Australia will also continue
to increase.
Meta Moment: As a way to conclude this activity, discuss with students the value of
long-term data. What is the value of long-term data collection for climate scientists? Is it
really necessary to collect data for so long? Could we learn this same information by
collecting data for a shorter time span? As an exercise, have students look at the data
only 100 years at a time by covering up first the left side of the graph, and then the right
side. Would students make the same claim with only 100 years of instrumental data,
without the dendrochronology data? Is there anything we learn from the
dendrochronology data that we cannot determine from the instrumental data alone?
Data Nuggets developed by Michigan State University fellows in the NSF BEACON and GK-12 programs.
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Additional teacher resources related to this Data Nugget:
There is one scientific paper associated with the data in this Data Nugget. The
citation and PDF are below:
• O'Donnell AJ, Cook ER, Palmer JG, Turney CSM, Page GFM, Grierson PF
(2015) Tree Rings Show Recent High Summer-Autumn Precipitation in Northwest
Australia Is Unprecedented within the Last Two Centuries. PLoS ONE 10(6):
e0128533. doi:10.1371/journal.pone.0128533
o http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0128533
• The same paper, written at a middle/high school reading level by Science Journal
for Kids, can be found here:
o http://www.sciencejournalforkids.org/what-do-trees-know-about-rain.html
The original dataset is in Excel and can be used for a digital graphing exercise with
your students! The Excel sheet with the full, original dataset can be found here:
o http://datanuggets.org/wpcontent/uploads/2016/01/AustraliaTreeRingRainfallData.xlsx
This Data Nugget was created in collaboration with Science Journal for Kids. They
adapt cutting edge peer-reviewed environmental science research for students (and
their teachers). Their original activities can be found on their website:
• http://ScienceJournalForKids.org/
• Videos demonstrating dendrochronology:
o How to take a core: https://youtu.be/G71ISbIRCAc
o How tree rings can reveal weather patterns:
https://youtu.be/xmZO7aRgcW4
o Dendrochronology: https://youtu.be/AlMfqzihNTE
• Modern rainfall data can be found at: Climate Explorer http://climexp.knmi.nl/
Data Nuggets developed by Michigan State University fellows in the NSF BEACON and GK-12 programs.
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