CO2_global_warming

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; much 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.

The natural greenhouse effect (i.e. the greenhouse effect that would be present on earth without the influence of humans) is beneficial . The average annual global 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 (causing a rise in sea level and flooding of coastal areas), changing weather patterns and 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.

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.

The figure below (from the IPCC 3rd Assessment Report: Climate Change 2001. The Technical Summary report can be found online) shows the increase of CO2 and also methane and nitrous oxide.

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

We are now able 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.

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 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 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?

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

Here's a review of what we learned last Wednesday
Atmospheric CO₂ concentration was fairly constant between 1000 AD and the mid 1700s. CO₂ concentration has been increasing since the mid 1700s (other greenhouse gas concentrations have also been increasing). The concern is that this might enhance or strengthen the greenhouse effect and cause global warming.

What has the temperature of the earth been doing during this period? There is a two part answer to that question.

First part:
Accurate direct measurements of temperature are available only from the past 150 years or so. The figure below (top of p. 3 in the photocopied Class Notes and also Fig. 14.7 in the text) shows how global average surface temperature has changed during that time period.

This is based on actual measurements of temperature made (using thermometers) at many locations on land and sea around the globe.

The graph doesn't actually show temperature. It shows how much different temperatures at various times beween 1860 and 2000 were compared to the 1961-1990 average. Temperature appears to have increased 0.7^o to 0.8^o C during this period. The increase hasn't been steady as you might have expected given the steady rise in CO₂ concentration; 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. 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 and the uncertainties in the yearly measurements.

These data are from the NASA Goddard Institute for Space Studies site. Temperatures here are compared to the 1951-1980 mean. The green bars are estimates of uncertainty.

The following figure compares temperature changes in the northern and southern hemispheres

The period of cooling between about 1940 and 1970 is present only in the northern hemisphere data.

2nd 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 (trapped in bubbles in ice sheets).

That doesn't work with temperature.

Imagine putting some air in a bottle, sealing the bottle, putting the bottle on a shelf, and letting it sit for 100 years. In 2108 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 2008 when the air was sealed in the bottle. You couldn't, in 2108, use the air in the bottle to determine what the temperature of the air was when it was originally put into the bottle in 2008.

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 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 when the students might 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 one of the 8 am NATS 101 classes). 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 (atoms of oxygen with different numbers of neutrons in the nucleus) 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.

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. This is for the northern hemisphere only, not the globe.

The blue portion of the figure shows the estimates of temperature derived from proxy data. The red portion are the instrumental measurements made between about 1860 and the present day. There is also a lot of year to year variation and uncertainty that is not shown on the figure above.

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 (Mother Nature has produced much larger changes than we see here though usually on a much longer time scale), or might be do to other human activities that affect climate (changing land use).

We've only considered a small part of a large debate that involves science, economics, and politics.

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 or
    on the Effects that warming will have on weather and climate in the years to come