Wed., Jan. 18, 2006

The Experiment #1 materials were distributed in class.

Handouts concerning the Donora Pennsylvania air pollution disaster and SO₂ pollution in Russia were distributed in class.

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The carbon cycle shows the rates at which carbon (mostly in the form of CO₂) is added to and removed from the atmosphere.  It also shows various places where carbon is stored (underlined numbers).

The atmosphere contains only about 700 units of carbon (mostly CO₂ but also some CH₄, methane).  The deep oceans contain 38,000 units of carbon, 50 million units are stored in sedimentary rock.  There are 7500 units of carbon in the form of fossil fuels waiting to be dug up. 

Natural processes such as respiration and decay add 113 units of carbon to the atmosphere every year.  113 units are removed by photosynthesis.  The oceans add and remove 90 units of carbon per year.  Notice the natural processes are in balance - they would not change the atmospheric CO₂ concentration.

Activities of man such as burning fossil fuels and deforestation add a total of 6 to 7 units (5 + 1 or 2 units) of CO₂ to the atmosphere every year.  This is fairly small compared to the CO₂ added by natural processes, however the manmade contributions are not balanced by equal rates of removal.  About one-half of what man adds every year is removed.  Exactly how this is done is not known.  It is this imbalance that is causing atmospheric CO₂ to increase.

There are 7500 units of carbon in the form of fossil fuels that will probably be burned in the next 100 years or so.  This is 7500 units of carbon that will be added to the atmosphere.  You can see that this could have a big effect on atmospheric CO₂ concentration.  There is a lot of research being done at present to try to figure out how the atmospheric concentration will change over time, and also how changing atmospheric concentration of CO₂ (and other greenhouse gases) will change climate.

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I tried to improve the presentation of some of the following material in the T Th section of the class.   Some of the following figures have been borrowed from the T Th class and were not shown in class on Wednesday.

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The Keeling curve shows us that the atmospheric CO₂ concentration has been increasing since 1958.  What was happening before that?  How can we determine atmospheric concentrations in the past anyways?



Ice has been accumulating in Antarctica and Greenland for hundreds of thousands perhaps millions of years.  The ice sheets are two miles thick in some locations.  As this ice was deposited (in the form of snow) small volumes of the ancient atmosphere were also trapped in bubbles in the ice.  Scientists have drilled into the ice sheets and removing cores of ice.  They have then been able to extract and analyze the gases in the bubbles.  They are actually able to measure CO₂ concentrations in past atmospheres.

A small portion of what they have found is shown in the following figure (the top of p. 3 in the photocopied notes).


The red portion of the graph above shows the Keeling curve measurements that were made starting in 1958.  The blue portion shows CO₂ concentration measurements based on ice cores.  You can see that CO₂ concentration was fairly constant up until 1750 or so.  That is about the time of the start of the "Industrial Revolution."  Burning fossil fuels to power machines and factories began adding larger amounts of CO₂ to the atmosphere.

The figure below shows that other greenhouse gases (methane [CH₄] and nitrous oxide [N₂O]) have also been increasing since the start of the industrial revolution.


This figure is from "Climate Change 2001: The Scientific Basis," published by the Intergovernmental Panel on Climate Change (www.ipcc.ch).



OK if greenhouse gas concentrations have been increasing since about 1750, What has the global average surface temperature been doing during this same time period? The overall change in shown on the bottom part of p. 3 in the photocopied notes.



This figure doesn't show the average temperature rather how the average temperature has changed over the past 130 or 140 years.  The change is relative to the 1950 to 1980 30-year average (the dotted line).  Between 1860 and about 1920 the global average temperature was about 0.3^o C cooler than the 1950 to 1980 average.  The temperature rose between 1920 and 1940.  The 1940 to 1970 section is somewhat puzzling.  The exact cause of this slight cooling is unknown.  Temperatures have increased from 1970 to the present day.  Many scientists think that this second period of warming can be attributed to increasing greenhouse gas concentrations.

Detecting such a small overall change in temperature for the earth is difficult.  Instruments and locations at which measurements were made have changed (imagine how Tucson has changed in the last 130 years or so).  There is also a considerable amount of year to year variation.  About 70% of the earth is covered by oceans and measurements over oceans are more difficult to make.

The graph above was smoothed to show the overall change.  The top figure below shows the actual year to year variation (red bars) and gives you an idea of the uncertainly in the yearly average temperature measurements (black vertical lines).


The lower curve shows estimates of global average surface temperature for times before 1860.  This is more difficult.  This is not based on actual measurements of temperature.  We can't directly measure the temperature of the air trapped in the bubbles of polar ice (like can be done with CO₂ concentration), the temperature has changed.  Scientists have come up with other techniques to try to indirectly determine temperatures in the past.  We will probably discuss some of this work later in the semester.  [These two figures are also from the Intergovernmental Panel on Climate Change report].