Friday, Aug. 25

Distribution of the Experiment #1 materials began in class today.  This weekend would be a perfect time to start the experiment.      Once you have collected all of your data, return the materials and pick up the supplementary information sheet.

Some additional reading in Chaper 12 was assigned.  We'll begin covering air pollutants on Monday.
Here is a brief review of what we covered last Wednesday and where we will be headed today.


Atmospheric CO2 concentration is increasing (and has been increasing since the mid 1700s).  A look at the carbon cycle shows us why this is true.

This somewhat confusing figure requires some careful analysis.

1.    Underlined numbers show the amount of carbon stored in "reservoirs."  For example 700 units* of carbon are stored in the atmosphere (mostly in the form of CO2, but also CH4, CFCs and other gases; note that carbon is found in each of those molecules).  The other numbers show "fluxes," the amount of carbon moving into or out of a reservoir per year.  Respiration and decay add 113 units* of carbon to the atmosphere every year.  Photosynthesis (primarily) removes 113 units every year.
2.    Note the natural processes are in balance (over land: 113 units added and 113 units removed, over the oceans: 90 units added balanced by 90 units of carbon removed from the atmosphere every year). and won't change the atmospheric concentration.
3.    Anthropogenic (man caused) emissions of carbon into the air are small compared to natural processes.  About 5 units are added during combustion of fossil fuels and 1-2 units are added every year because of deforestation (when trees are cut down they decay and add CO2 to the air, also because they are dead they aren't able to remove CO2 from the air by photosynthesis)

The rates at which carbon is added to the atmosphere by man is not balanced by an equal rate of removal (2 or 3 units are removed every year, highlighted in yellow in the figure.  The ? refers to the fact that scientists still don't know precisely how or where this removal occurs). This will slowly cause the atmospheric CO2 concentration to increase.

4.    In the next 100 years or so, the 7500 units of carbon stored in the fossil fuels reservoir (lower left hand corner of the figure) will be added to the air.  The big question is how will the atmospheric concentration change and what effects will that have?

*units: Gtons (reservoirs) or Gtons/year (fluxes)
Gtons = 1012 metric tons. (1 metric ton is 1000 kilograms or about 2200 pounds)

So here's what we have learned so far: 
CO2 concentration was fairly constant between 1000 AD and the mid 1700s.  CO2 concentration has been increasing since the mid 1700s. 
The concern is that this might cause global warming.  So what has the temperature of the earth been doing during this period? 
The next two figures (found on p. 3 in the photocopied notes) address this question.




This first figure shows how the average global annual surface temperature has changed over the past 130 or 140 years.  This is based on actual measurements of temperature made at many locations on land and sea around the globe. 

Temperature appears to have increased 0.7o to 0.8o C during this period.  The increase hasn't been steady as you might expect given the steady rise in CO2 concentration; temperature remained constant or even decreased slightly between 1940 and 1975 or so.

It is very difficult to detect a temperature change this small over this period of time.  The instruments used to measure temperature have changed.  The locations at which temperature measurements have been made have also changed (imagine what Tucson was like 130 years ago).  Average surface temperatures naturally change a lot from year to year.  The year to year variation has been left out of the figure above so that the overall change could be seen more clearly (click here to see a different version of this figure that does show the year to year variation and the uncertainties in the yearly measurements).

Now it would be interesting to know how temperature was changing prior to the mid-1800s.  There aren't enough reliable measurements to be able to do that directly.  Scientists must use proxy data.

When you can measure something like temperature directly you might be able to look for something else or measure something else whose presence or concentration depended on the temperature at some time in the past.

Here's an example.

Example: Imagine trying to measure how many students live in a particular house

ILet's say you want to determine how many students are living in a house near the university.  You could walk by the house late in the afternoon and count the students if they were outside.  That would be a direct measurement. There could be some errors in your measurement (some students might be inside the house).

If you were to walk by early in the morning it is likely that the students would be inside sleeping.  In this case though you might look for other clues that might give you an idea of how many students lived in that house.  You would use these proxy data to come up with an estimate of the number of students inside the house.

In the case of temperature scientists look at tree rings.  The width of each yearly ring depends on the depends on the temperature and precipitation at that time that ring formed.  They analyze coral.  Coral is made up of calcium carbonate, a molecule that contains oxygen.  The relative amounts of the different oxygen isotopes depends on the temperature that existed at the time the coral grew.  Scientists can analyze lake bed and ocean sediments.  The types of  plant and animal fossils that they find depends on the water temperature at the time.  They can even use the ice cores.  The ice, H2O, contains oxygen and the relative amounts of various oxygen isotopes depends on the temperature at the time the ice fell from the sky as snow.

Using these proxy data scientists have been able to estimate average surface temperatures for 100,000s of years into the past.  The next figure shows what temperature has been doing since 1000 AD.


The blue portion of the figure shows the estimates of temperature derived from proxy data.  The orange portion are the instrumental measurements made between about 1860 and the present day.  There is also a lot of year to year variation and uncertainty that is not shown on the figure above (click here or see Figure 14.4 in the text for a more accurate representation of this curve).

It appears that there has been a significant amount of warming that has occurred in just the last 150 years or so.  Many scientists believe that this warming is a result of the increase in atmospheric greenhouse gas concentrations.  Others suggest that this change in temperature might be just a natural change in climate and is not due to anthropogenic release of greenhouse gases.  Mother Nature has produced much larger changes than we see here though usually on a much longer time scale.  We'll briefly look at natural changes in climate that have occured in the near and distant past in class next Monday.
As mentioned early distribution of the Expt. 1 materials  started in class today.

With this and the other experiments you will receive most or all of the materials you need to complete the experiment, a description of what should go into your report, instructions that tell you how to perform the experiment, and a data collection sheet.

The object of Experiment #1 is to measure the percentage concentration of the oxygen in air.  Basically you moisten a piece of steel wool, stick the steel wool into a graduated cylinder, and turn the cylinder upside down and immerse the open end in a cup of water.

As the next figure shows you need to use a small piece of flexible tubing so that water enters part way into the cylinder so that the water level can be read on the cylinder scale.

Be sure to remove the tubing once the water level can be read on the cylinder scale.  The air sample in the cylinder is now sealed off from the rest of the atmosphere.  The oxygen in the air sample will react with the steel wool to form rust.  As oxygen is removed from the air sample, the air sample volume changes.

The reaction between the oxygen and the steel wool sometimes happens in a day or two.  Other times it may take several days.  You will periodically need to record the time and the air sample volume ( you read the water level on the cylinder scale).  Be sure you do not lift the open end of the cylinder out of the water.  That would break the seal and you would need to restart the experiment.

Eventually the air sample volume will stop changing; all of the oxygen has been removed from the air sample and the experiment is over.
You will receive a supplementary information sheet when you have returned your materials.  You don't have to return the rusty piece of steel wool - throw it away.  Don't worry about trying to clean the rust stains off the inside of the cylinder.