Wed., Aug. 22 notes

Wednesday Aug. 22, 2007

Hurricane Dean moved onshore again this morning into mainland Mexico (we watched a short satellite loop from the University of Wisconsin - Madison Tropical Cyclone site) . Earlier in the week the storm hit the east coast of the Yucatan Peninsula as a category 5 hurricane. It then quickly weakened to category 1 strength as it crossed the peninsula. It moved back out over the Gulf of Mexico, but was over warm water again for only a short time and strengthened to category 2 before moving over mainland Mexico. A nice graphic from the Weather Underground site shows the path of Hurricane Dean.

A new reading assignment was also mentioned in class.

We listed the 5 most abundant gases in the atmosphere in class on Monday. Here is a list of several more important gases.

Water vapor, carbon dioxide, methane, nitrous oxide (N₂O = laughing gas), chlorofluorocarbons, and ozone are greenhouse gases. We'll discuss the greenhouse effect a little bit more later in class today and will learn more about it actually works when we get to Chapter 2.

Carbon monoxide, nitric oxide, nitrogen dioxide, ozone, and sulfur dioxide are some of the major air pollutants. We'll cover 2 or 3 of these in class on Friday and early next week. You may have heard or read about an incident earlier this week where carbon monoxide from a malfunctioning hot water heater sickened 23 Virginia Tech students in an apartment complex. Carbon monoxide concentrations indoors can easily and rapidly reach fatal levels. Carbon monoxide levels in the atmosphere are much lower but can still represent a health hazard.

Ozone in the stratosphere (a layer of the atmosphere between 10 and 50 km) is beneficial because it absorbs dangerous high energy ultraviolet (UV) light coming from the sun. Without the protection of the ozone layer life as we know it would not exist on the surface of the earth. Chlorofluorocarbons are of concern in the atmosphere because they destroy stratospheric ozone. In the troposphere (the bottom 10 kilometers of the atmosphere) ozone is a pollutant and is one of the main ingredients in photochemical smog.

Signup sheets for the experiments (or scientific paper or book reports) were passed around in class today. If you haven't yet made up your mind, you can sign up in class on Friday or in the next week or two. You aren't locked into a choice that you made today either. Distribution of the materials for the first experiment will begin on Friday (Aug. 24).

Today we will look at the concern over increasing concentration of carbon dioxide in the earth's atmosphere and the worry that this might lead to global warming and climate change. We consider CO₂ because it is probably the best known of the greenhouse gases; most of what we will say about CO₂ applies to the other greenhouse gases as well.

This is a complex and contentious subject and we will only scratch the surface. Much of what we covered today was found on pps. 1-4 in the photocopied Class Notes.

The natural greenhouse effect (i.e. the greenhouse effect that would be present on earth without the influence of humans) 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.

An increase in concentrations of greenhouse gases in the atmosphere, due to human activities, could enhance the greenhouse effect and cause additional warming. This then 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.

Some of the evidence for increasing CO₂ concentration is shown in the two graphs below.

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.

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 2007, 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 and the bubbles are small. 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.

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 addes 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 weather) is evidence of the production of water vapor during the combustion.

Photosynthesis removes CO₂ from the air (photosynthesis adds oxygen to the air). CO₂ also dissolves in ocean water.

We can use this information to better understand the yearly variation in atmospheric CO₂ concentration (the "wiggles" on the Keeling Curve). The figure below wasn't shown or discussed in 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 begin to remove carbon dioxide.

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.

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 somewhat confusing figure also not shown in class on Wednesday requires some careful examination.

1.    Underlined numbers show the amount of carbon stored in "reservoirs." For example 700 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 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). If these were the only processes present, the atmospheric concentration (700 units) wouldn't change.

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 rate 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 small imbalance 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?

*units: Gtons (reservoirs) or Gtons/year (fluxes)
Gtons = 10¹² metric tons. (1 metric ton is 1000 kilograms or about 2200 pounds)

The remainder of this CO₂ and global warming topic has been moved to the Fri., Aug. 24 notes.