Thu., Aug. 24 notes

Thursday, Aug. 24, 2006

I was just joking when I said that we should start class at 7:45 next Tuesday, at 7:30 next Thursday, and at 7:15 am the Tuesday after that. Class will start at 8 am next week just like it did this week.

No, not very much. Not much physics, math, biology, or geology either. But, stay open minded, you might find that these subjects are more interesting than you might have thought and not necessarily that difficult either.

Here's another student question that was asked last Tuesday.

Does the dew point temperature have anything to do with relative humidity? They are related in the sense that they both tell you something about moisture in the air.

In the figure above the air temperature changes from 75 F in the morning to 95 F in the afternoon. The air's temperature (as we will see when we get to Chapter 4 later in the semester) determines how much water vapor the air can potentially contain.

The dew point temperature remains constant in the figure above. The dew point is a measure of how much water vapor is actually in the air, so in this example the actual amount of water vapor in the air doesn't change during the course of the day.

The relative humidity tells you how close the air is to being "filled to capacity" with water vapor.

If the early morning temperature had been 65 F, the same as the dew point, the relative humidity would have been 100%. It would have been foggy.

The relative humidity really tells you whether a cloud or fog or dew is about to form. The RH also gives you an idea of how well your evaporative cooler will work (it cools more effectively when the RH is low). It is also hard for your body to cool by perspiring when the RH is high (see heat index on p. 86 in the textbook).

Still another question from a student that came to my office

Many people think that the term monsoon just means thunderstorm. We will learn a fair amount about thunderstorms in this class.

The term monsoon really means a seasonal change in the direction of the prevailing winds (we'll learn a little bit about what causes that too). For most of the year winds in the Arizona come from the west and are dry. For two or three months in the summer the winds pick up an easterly component and are moister. When there is sufficient moisture thunderstorms can form. In an average year Tucson gets about half of its yearly precipitation during the summer monsoon season. The website maintained by the Tucson office of the National Weather Service has a lot of additional information about the summer monsoon.

There is a tropical storm (Debby) off the east coast of the US and a strong hurricane (Ileana) off the west coast. You can learn more about these tropical systems and see their predicted paths at the National Hurricane Center webpage.
This was a good time to introduce the Saffir-Simpson scale used to rate hurricane strength or severity.

With sustained winds of 100 MPH, hurricane Ileana is currently a category 2 hurricane (winds were up to 120 MPH yesterday). The hurricane center expects continued weakening. Moisture from tropical storms and hurricanes is sometimes pulled into southern Arizona. This can lead to an increase in thunderstorm activity and heavy rainfall.

At this point, 20 or 25 minutes into the class period, we covered some new material found on p. 1 in the photocopied Class Notes.

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

Increasing the concentrations of greenhouse gases in the atmosphere could enhance the greenhouse effect and cause global warming. This 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.

The evidence for increasing CO₂ concentration is shown in the two graphs below

The top "Keeling" curve shows measurements of CO₂ that were begun in 1958 on top of the Mauna Loa volcano in Hawaii. Carbon dioxide concentrations 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 2006, go back and measure the amount of CO₂ in the atmosphere in 1906? 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. 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 280 ppm between 1000 AD and the mid-1700s when it started to increase. The start of rising CO₂ coincides with the "Industrial Revolution." Combustion of fossil fuels needed to power factories began to add CO₂ to the atmosphere.

The figure above lists processes that add CO₂ to and remove CO₂ from the atmosphere.
We can use this information to better understand the yearly variation in atmospheric CO₂ concentration seen on the Keeling curve.

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.

Some of the release and removal processes listed above are more important than the others. We can get an idea of what the dominant processes are by looking at the next figure which shows the "Carbon Cycle."

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.

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.

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 (the word instruments was added after class). 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. We'll briefly look at changes in climate that have occured in the near and distant past in class next Tuesday.