Fri., Sep. 4 notes

Friday Sep. 4, 2009
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Three songs from the Doors today before class: "Love Me Two Times", "Soul Kitchen", and "Love Her Madly."

At this point the Tucson Weather Service webpage is showing a 60% chance of heavy rain for Saturday. It looks like the UA football home opener might be wet.

I added a slightly different explanation of how particles in the air can affect visibility to the end of the Wed., Sep. 2 online notes. We reviewed this quickly at the start of class.

We spent the period today and will spend part of class next Wednesday (before the Practice Quiz) looking at the current concern over increasing greenhouse concentrations in the earth's atmosphere, global warming, and climate change. 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 0^o F. That's pretty cold
Point 3b. The presence of greenhouse gases raises this average temperature to about 60^o 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) might spread into more temperature 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. 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 2009 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 (volcanoes was added after class).

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. 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.

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

In the bottom curve we assume that the release of CO2 to the air remains constant throughout the year. Photosynthesis 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, I added most of the information after class.

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)