Wednesday Sep. 9, 2009
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A couple of songs, "Stairway to Heaven" and "Diablo Rojo" from Rodrigo y Gabriela were played before class today.

The first 1S1P Assignment of the semester is now online.  We'll discuss this a little more on Friday.

We'll see how much of the material on climate change we can finish before the Practice Quiz.

Here's about where we left off last Friday.
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 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 2109 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 2009 when the air was sealed in the bottle.  You couldn't, in 2109, use the air in the bottle to determine what the temperature of the air was when it was originally put into the bottle in 2009.

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. This is about as far as we got in class on Wednesday. 

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





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