Fri., Jan. 25 notes

Friday Jan. 25, 2008

Today we will finish the section on air pollutants and will start a short section on carbon dioxide and global warming. Next week we will start to cover pressure and the vertical structure of the atmosphere found in the middle of Chapter 1. The reading assignments page has been updated.

Class began with the acid rain demonstration that we didn't have time for last Wednesday.

Today and next Monday we will take a brief look at the current concern over increasing concentrations of carbon dioxide in the earth's atmosphere and the worry that this might lead to global warming and climate change. This is a big, complex, and contentious subject and we will only scratch the surface.

You'll find this at the top of p. 1 in the photocopied Classnotes. The highlighted numbered points were added after class.

1. Carbon dioxide is one of several greenhouse gases. Much of what we say about CO₂ applies to the other greenhouse gases as well.

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

3. Before we look at enhancement of the greenhouse effect, it is important to understand first that the greenhouse effect is beneficial.

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
3b. The presence of greenhouse gases raises this average temperature to about 60^o F.

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 less to places like Arizona.

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

The "Keeling" curve shows measurements of CO₂ that were begun in 1958 on top of the Mauna Loa volcano in Hawaii. Carbon dioxide concentrations have increased from 315 ppm to about 380 ppm between 1958 and the present day. The small wiggles (one wiggle per year) show that CO₂ concentration changes slightly during the year.

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 2008, 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.

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 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.

Shown below are some more carefully drawn graphs of changing carbon dioxide, methane, and nitrous oxide concentrations during the past 1000 years 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)

Now 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 (or burning the tree) will keep it from removing CO₂ from the air by photosynthesis. The dead tree will also decay and release CO₂ to the air.

The chemical equation illustrates the combustion of a fossil fuel. The by products are carbon dioxide and water vapor. The steam cloud that you sometimes see come from a rooftop vent or the tailpipe of an automobile (especially during cold wet weather) is evidence of the production of water vapor during the combustion.

Photosynthesis removes CO₂ from the air (in some respects, photosynthesis is the opposite of combustion, photosynthesis manufactures fuel and adds oxygen to the air). CO₂ also dissolves in ocean water.

Note: your instructor is not aware of an anthropogenic process that removes large 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). The figure below was redrawn after class.

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 and reaches a peak value in late winter - early spring. Plants come back to life at that time and start to remove the "excess" CO₂.

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.

With careful measurements you could probably also observe a daily variation in atmospheric CO₂ concentrations.

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. This figure (not shown or discussed in class on Friday) differs somewhat from the one on p. 2 of the photocopied Classnotes. Copies of this updated figure were distributed in class.

This figure requires some careful examination.

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; carbon is found in each of those molecules). The other numbers show "fluxes," the amount of carbon moving into or out of the atmosphere every year. 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 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 added 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: Gtons (reservoirs) or Gtons/year (fluxes)
Gtons = 10¹² metric tons. (1 metric ton is 1000 kilograms or about 2200 pounds)