Only a portion of these notes on Climate Change was covered in class.  This is a big, complex, and contentious subject and we will only scratch the surface. 

Quite a bit of information needs to be added to p. 3a in the photocopied ClassNotes; we will break it up into several smaller pieces for clarity.

Point 1.  Carbon dioxide is probably the best known of several greenhouse gases (water vapor is a more important greenhouse gas).  Much of what we say about CO2 applies to the other greenhouse gases as well.

Point 2.   Atmospheric CO2 concentrations are increasing.  This is generally accepted as fact.  We'll look at some of the evidence below.

Point 3.   The basic worry is that increasing greenhouse gas concentrations will cause global warming.  This is a hypothesis though many (perhaps the vast majority of) scientists regard this as fact and believe that enhanced greenhouse warming is already underway.

      Before we look at enhancement of the greenhouse effect, it is important to first understand that the greenhouse effect has a beneficial side.   You might refer to this as the natural greenhouse effect (i.e. one that has not been affected or influenced by human activities)

Point 3a.  If the earth's atmosphere didn't contain any greenhouse gases, the global annual average surface temperature would be about 0o F.  That's pretty cold
Point 3b.  The presence of greenhouse gases raises this average temperature to about 60o F.


Point 4.   The concern is that increasing atmospheric greenhouse gas concentrations might cause some additional warming.  This might not sound like a bad thing.  However a small change in average temperature might melt polar ice and cause a rise in sea level and flood coastal areas.  Warming might change weather patterns and bring more precipitation to some areas and prolonged drought to places like Arizona.  Nasty tropical diseases (such as malaria & dengue fever) might spread into more temperate areas.

Now some of the data that show atmospheric carbon dioxide concentrations are increasing.


The "Keeling" curve shows measurements of CO2 that were begun (by a graduate student named Charles Keeling) in 1958 on top of the Mauna Loa volcano in Hawaii (the air there is "clean" and not affected by nearby cities and other sources of pollutants).  Carbon dioxide concentrations have increased from 315 ppm to about 385 ppm between 1958 and the present day.  The small wiggles (one wiggle per year) show that CO2 concentration changes slightly during the course of a year (it also probably changes slightly during the course of a day). 

You'll find an up to date record of atmospheric CO2 concentration from the Mauna Loa observatory at the Scripps Institution of Oceanography site.

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 2010 say, go back and measure the amount of CO2 in the atmosphere in the past?  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 thin layer of solid ice.  The ice contains small bubbles of air trapped in the snow, samples of the atmosphere at the time the snow originally fell.  Scientists are able to date the ice layers and then take the air out of these bubbles and measure the carbon dioxide concentration.  This isn't easy, the layers are very thin, the bubbles are small and it is hard to avoid contamination.

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 significant amounts of CO2 to the atmosphere.  Concentrations of several of the other greenhouse gases have been increasing in much the same way CO2 has. 


In order to understand why atmospheric carbon dioxide concentration is increasing, and before we look at what the earth's temperature has been doing during this period, we will try to understand better how man has been able to change atmospheric CO2 concentrations.


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, a human activity also adds CO2 to the atmosphere.  Deforestation, 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.

CO2 is removed from the atmosphere by photosynthesis (the equation is shown above).  CO2 also dissolves in the oceans.

The ? means your instructor is not aware of an anthropogenic process that removes significant amounts of carbon dioxide from the air.  Though carbon sequestration (the capture/removal of CO2 from the air and storage) is something that is being considered to lessen or prevent global warming.

We are now able to better understand the yearly variation in atmospheric CO2 concentration (the "wiggles" on the Keeling Curve).

We assume that the release of CO2 to the air remains constant throughout the year (the straight brown line above).  Photosynthesis (the green curve) will change.  Photosynthesis is highest in the summer when plants are growing actively.  It is lowest in the winter when many plants are dead or dormant. 

Atmospheric CO2 concentration will decrease as long as the rate of removal (photosynthesis) is greater than the rate of release (blue shaded portion above).  Your bank account balance will drop as long as you spend more money than you deposit.  The minimum occurs at the right end of the blue shaded portion where removal once again equals release.

The CO2 concentration will increase when release exceeds removal (red shaded section).  The highest CO2 concentration occurs at the right end of the red shaded portion.

To really understand why human activities are causing atmospheric CO2 concentration to increase we need to look at the relative amounts of CO2 being added to and being removed from the atmosphere (like amounts of money moving into and out of a bank account and their effect on the account balance).  A simplified version of the carbon cycle is shown below.  We didn't have time in class to go through all of this.

Here are the main points to take from this figure:

1.    The underlined numbers show the amount of carbon stored in "reservoirs."  For example 760 units* of carbon are stored in the atmosphere (predominantly in the form of CO2, but also in small amounts of CH4 (methane), CFCs and other gases; anything that contains carbon).  Notes that the atmosphere is a fairly small reservoir.

    The other numbers show "fluxes," the amount of carbon moving into or out of the various reservoirs ( actually just into and out of the atmosphere ).  Over land, respiration and decay add 120 units* of carbon to the atmosphere every year.  Photosynthesis (primarily) removes 120 units every year.
2.    Note the natural processes are in balance (over land: 120 units added and 120 units removed, over the oceans: 90 units added balanced by 90 units of carbon removed from the atmosphere every year). If these were the only processes present, the atmospheric concentration (760 units) wouldn't change.
3.    Anthropogenic (man caused) emissions of carbon into the air are small compared to natural processes.  About 6.4 units are added during combustion of fossil fuels and 1.6 units are added every year because of deforestation (when trees are cut down they decay or are burned and add CO2 to the air, also because they are dead they aren't able to remove CO2 from the air by photosynthesis)

The rate at which carbon is added to the atmosphere by man is not balanced by an equal rate of removal: 4.4 of the 8 units added every year are removed (highlighted in yellow in the figure).  This small imbalance (8 - 4.4 = 3.6 units of carbon are left in the atmosphere every year) explains why atmospheric carbon dioxide concentrations are increasing with time.

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 dug up or pumped out of the ground and burned.  That will add 7500 units of carbon to the air.  The big question is how will the atmospheric concentration change and what effects will that have on climate?

*don't worry about the units.  But here they are just in case you are interested:
 Reservoirs - Gtons
 Fluxes - Gtons/year
 A Gton = 1012 metric tons. (1 metric ton is 1000 kilograms or about 2200 pounds)

So here's where we're at.
Atmospheric CO2 concentration was fairly constant between 1000 AD and the mid 1700s.
CO2 concentration has been increasing since the mid 1700s.
The obvious question is what has the temperature of the earth been doing during this period?  In particular has there been any warming associated with the increases in greenhouse gases that have occurred since the mid 1700s?

We must address the temperature question in two parts.

First part:
Actual accurate measurements of temperature (on land and at sea)


This figure (top of p. 3c in the photocopied ClassNotes) is based on actual measurements of temperature made using reliable thermometers at many locations on land and sea around the globe.  The left side of the figure shows how average temperatures at various time compare with the 1961 to 1990 average.  The vertical axis on the right side of the plot shows actual global average surface tempeature values.

Temperature appears to have increased 0.7o to 0.8o C during this period.  The increase hasn't been steady as you might have expected given the steady rise in CO2 concentration (and assuming that increasing CO2 is what's causing the earth to warm); temperature even decreased slightly between about 1940 and 1970.

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).  About 2/3rds of the earth's surface is ocean and measurements were pretty sparce during much of this time period (sea surface temperatures can now be measured using satellites). 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 trend could be seen more clearly.  The figure below does show the year to year variation (dotted black line) and the uncertainties (green bars, note how the uncertainty is lower in recent years) in the yearly measurements.


These data are from the NASA Goddard Institute for Space Studies site.
These temperatures are compared to a different, 1951-1980, thirty year  mean. 
Temperatures prior to about 1930 were colder than the 1951-1980 mean and temperatures after 1980 were warmer.

Here's another plot of global temperature change over a slightly longer time period
from the University of East Anglia Climatic Research Unit



The overall tendency seems to be the same in both cases.  

Second Part
Now it would be interesting to know how temperature was changing prior to the mid-1800s.  This is similar to what happened when the scientists wanted to know what carbon dioxide concentrations looked like prior to 1958.  In that case they were able to go back and analyze air samples from the past (air trapped in bubbles in ice sheets). 

That doesn't work with temperature.

To understand why, imagine putting some air in a bottle, sealing the bottle, putting the bottle on a shelf, and letting it sit for 100 years.  In 2110 you could take the bottle down from the shelf, carefully remove the air, and measure what the CO2 concentration in the air had been in 2010 when the air was sealed in the bottle.  You couldn't, in 2110, use the air in the bottle to determine what the temperature of the air was when it was originally put into the bottle in 2010.

With temperature you need to use proxy data.  You need to look for something else whose presence, concentration, or composition depended on the temperature at some time in the past.

Here's a proxy data 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 when the students would likely be outside and count them.  That would be a direct measurement (this would be like measuring temperature with a thermometer). There could still be some errors in your measurement (some students might be inside the house and might not be counted, some of the people outside might not live at the house).

If you were to walk by early in the morning it is likely that the students would be inside sleeping (or in 8 am section of NATS 101).  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 a variety of things.'
They could look at tree rings.
The width of each yearly ring depends on the depends on the temperature and precipitation at the time the ring formed. 
They analyze coral. 
Coral is made up of calcium carbonate, a molecule that contains oxygen.  The relative amounts of the oxygen-16 and oxygen-18 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 depend 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.

Here's an idea of how oxygen isotope data can be used to determine past temperature.



The two isotopes of oxygen contain different numbers of neutrons in their nuclei.   Both atoms have the same number of protons.



During a cold period, the H2O16 form of water evaporates more rapidly than the H2O18 form.  You would find  relatively large amounts of O16 in glacial ice.  Since most of the H2O18 remains in the ocean, it is found in relatively high amounts in calcium carbonate in ocean sediments.  Note also the drop in ocean levels during colder periods when much of the ocean water is found in ice sheets on land.



The reverse is true during warmer periods.

Using proxy data scientists have been able to estimate average surface temperatures for 100,000s of years into the past.  The next figure (bottom of p. 3c in the Classnotes) shows what temperature has been doing since 1000 AD.  This is for the northern hemisphere only, not the globe.



The major portion of the figure shows the estimates of temperature (again relative to the 1961-1990 mean) derived from proxy data.  The instrumental measurements were made between about 1850 and the present day.  There is also a lot of year to year variation and uncertainty that is not shown on the figure above.

Many scientists would argue that this graph is strong support of a connection between rising atmospheric greenhouse gas concentrations and global warming.  Early in this time interval when CO2 concentration was constant, there is little temperature change.  Temperature only begins to rise in about 1900 when we know an increase in atmospheric carbon dioxide concentrations was underway.

There is historical evidence in Europe of a medieval warm period lasting from 800 AD to - 1300 AD or so and a cold period, the "Little Ice Age, " which lasted from about 1400 AD to the mid 1800s.  These are not clearly apparent in the temperature plot above.  This leads some scientists to question the validity of this temperature reconstruction.  Scientists also suggest that if large changes in climate such as the Medieval warm period and the Little Ice Age can occur naturally, then maybe the warming that is occurring at the present time also has a natural cause.

The so-called Year Without a Summer occurred in 1816, toward the end of the Little Ice Age.  The unusally cold summer temperatures were apparently caused by a very large volcanic eruption the year before.  Here's a short explanation of how volcanoes can cause short term climate changes.



Here's the figure that the sketch above was based on





Note the considerable year to year variation.

from
Climate Change 2001 - The Scientific Basis
Contribution of Working Group I to the 3rd Assessment Report of the
Intergovernmental Panel on Climate Change (IPCC) 

Here's a comparison of several additional estimates of temperature changes over the past 1000 years or so




 This is from the University of East Anglia Climatic Research Unit again. 

Here's a brief summary that tries to separate fact from hypothesis in the debate over climate change:

SUMMARY
There is general agreement that
    Atmospheric CO2 and other greenhouse gas concentrations are increasing and that
    The earth is warming

Not everyone agrees on
    the Causes (natural or manmade) of the warming,
    how much Additional Warming there will be or how quickly it will occur, or
    the Effects that warming will have on weather and climate in the years to come