Friday Jan. 12, 2007

Signup sheets for the experiments (or scientific paper or book reports) were passed around in class today.  If you haven't yet made up your mind, you can sign up in class next Wednesday.  You aren't locked into a choice that you made today either.  Distribution of the materials for the first experiment will begin next Wednesday (Jan. 17).


Today we will look at the concern over increasing concentration of carbon dioxide (and other greenhouse gases) in the earth's atmosphere and the worry this might lead to global warming and climate change.

Carbon dioxide is one of several greenhouse gases (H2O, CH4, N2O, CFCs are some of the others). 
The natural greenhouse effect (i.e. the greenhouse effect that would be present on earth without the influence of humans) is beneficial .  The average global annual surface temperature on earth without greenhouse gases  would be about 0o F.  The presence of greenhouse gases raises this average to about 60o F.

An increase in concentrations of greenhouse gases in the atmosphere due to human activities could enhance the greenhouse effect and cause global warming.  This then could have many detrimental effects such as melting polar ice and causing a rise in sea level and flooding of coastal areas, changes in weather patterns and changes in the frequency and severity of storms.

Some of the evidence for increasing CO2 concentration is shown in the two graphs below.

The "Keeling" curve shows measurements of CO2 that were begun in 1958 on top of the Mauna Loa volcano in Hawaii.  Carbon dioxide concentrations have increased from 315 ppm to about 375 ppm during this period.  The small wiggles show that CO2 concentration changes slightly during the year. 

Once scientists saw this data they began to wonder about how CO2 concentration might have been changing prior to 1958.  But how could you now,  in 2007, go back and measure the amount of CO2 in the atmosphere in 1900 say?  Scientists have found a very clever way of doing just that.  It involves coring down into ice sheets that have been building up in Antarctica and Greenland for hundreds of thousands of years.



As layers of snow are piled on top of each other year after year, the snow at the bottom is compressed and eventually turns into a layer of solid ice.  The ice contains small bubbles of air trapped in the snow at the time it originally fell.  Scientists are able to date and then take the air out of these bubbles and measure the carbon dioxide concentration.  This isn't easy, the layers are very thin and the bubbles are small.  It is hard to avoid contamination.  A book, The Two-Mile Time Machine, by Richard B. Alley discusses ice cores and climate change.  This is one of the books available for checkout should you decide to write a book report instead of an experiment report.



Using the ice core measurements scientists have determined that atmospheric CO2 concentration was fairly constant at about 280 ppm between 1000 AD and the mid-1700s when it started to increase.  The start of rising CO2 coincides with the beginning of the "Industrial Revolution."   Combustion of fossil fuels needed to power factories began to add CO2 to the atmosphere.




Carbon dioxide is added to the atmosphere naturally by respiration (people breathe in oxygen and exhale carbon dioxide), decay, and volcanoes.  Combustion of fossil fuels, an human activity also addes CO2 to the atmosphere.  Deforestation should have been mentioned also.  Cutting down and killing a tree will keep it from removing CO2 from the air by photosynthesis.  The dead tree will also decay and release CO2 to the air.

The chemical equation illustrates the combustion of propane (the gas ordinarily found in an outdoor grill).  Propane reacts with oxygen to produce carbon dioxide and water vapor.  The steam cloud that you sometimes see come from a rooftop vent or the tailpipe of an automobile is evidence of the production of water vapor during the combustion.  Burning natural gas (CH4 = methane) is similar:

CH4 + 2O2 ---> CO2 + 2H2O + energy

Photosynthesis removes CO2 from the air.  CO2 also dissolves in ocean water.

Someone brought up the point that the top of a volcano (a natural source of carbon dioxide)  would seem an odd place for Charles Keeling to have made his measurements.  The Mauna Loa volcano is dormant.

We can use this information to better understand the yearly variation in atmospheric CO2 concentration.

Atmospheric CO2 peaks in the late winter to early spring.  Many plants die or become dormant in the winter.  With less photosynthesis, more CO2 is added to the atmosphere than can be removed.  The concentration builds throughout the winter until the rate of photosynthesis increases and brings things back into balance in the spring.

Similarly in the summer the removal of CO2 by photosynthesis exceeds release.  CO2 concentration decreases throughout the summer and reaches a minimum in late summer to early fall.



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? 

Accurate measurements of temperature are available only from the past 150 years or so.  The figure below shows how global average surface temperature has changed during that time period.


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't 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.

Let'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 still 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 that case you might look for other clues (such as the number of empty bottles in the yard) 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 (
Mother Nature has produced much larger changes than we see here though usually on a much longer time scale) and is not due to anthropogenic release of greenhouse gases.

There is general agreement that
    Atmospheric CO2 concentration is increasing and that
    The earth is warming

Not everyone agrees
    on the Causes of the warming or
    on the Effects that warming will have on weather and climate