Carbon Dioxide in the Atmosphere

The CO₂ concentration in the atmosphere has been increasing since the mid 1800s. The increase correlates well with burning of fossil fuels. Thus, humans appear to have an effect.

1. In the atmosphere as CO₂ gas. From here it exchanges with seawater or water in the atmosphere to return to the oceans, or exchanges with the biosphere by photosynthesis, where it is extracted from the atmosphere by plants. CO₂ returns to the atmosphere by respiration from living organisms, from decay of dead organisms, from weathering of rocks, from leakage of petroleum reservoirs, and from burning of fossil fuels by humans.
   
   
2. In the hydrosphere (oceans and surface waters) as dissolved CO₂. From here it precipitates to form chemical sedimentary rocks, or is taken up by organisms to enter the biosphere. CO₂ returns to the hydrosphere by dissolution of carbonate minerals in rocks and shells, by respiration of living organisms, by reaction with the atmosphere, and by input from streams and groundwater.
   
   
3. In the biosphere where it occurs as organic compounds in organisms. CO₂ enters the biosphere mainly through photosynthesis. From organisms it can return to the atmosphere by respiration and by decay when organisms die, or it can become buried in the Earth.
   
   
4. In the Earth's lithosphere as carbonate minerals, graphite, coal, petroleum. From here it can return to the atmosphere by weathering, volcanic eruptions, hot springs, or by human extraction and burning to produce energy.

For example, carbon stored in the Earth in sedimentary rocks or as fossil fuels only re-enters the atmosphere naturally when weathering and erosion expose these materials to the Earth's surface. When humans extract and burn fossil fuels the process occurs much more rapidly than it would occur by natural processes. With an increased rate of cycling between the Earth and the atmosphere, extraction from the atmosphere by increased interaction with the oceans, or by increased extraction by organisms must occur to balance the input. If this does not occur, it may result in global warming.

Global Warming

Average global temperatures vary with time as a result of many processes interacting with each other. These interactions and the resulting variation in temperature can occur on a variety of time scales ranging from yearly cycles to those with times measured in millions of years. Such variation in global temperatures is difficult to understand because of the complexity of the interactions and because accurate records of global temperature do not go back more than 100 years. But, even if we look at the record for the past 100 years, we see that overall, there is an increase in average global temperatures, with minor setbacks that may have been controlled by random events such as volcanic eruptions or El Niño events (See figure 19.7 in your text). Records for the past 100 years indicate that average global temperatures have increased by about 0.7^oC. While this may not seem like much, the difference in global temperature between the coldest period of the last glaciation and the present was only about 5^oC.

In order to predict future temperature changes we first need to understand what has caused past temperature changes. Computer models, called Global Circulation Models have been constructed to attempt this. Although there is still much uncertainty, most of these models agree that if the greenhouse gases continue to accumulate in the atmosphere until they have doubled over their pre-1860 values, the average global temperature increase will be between 1 and 5^oC by the year 2050. This is not a uniform temperature increase. Most models show that the effect will be greatest at high latitudes (near the poles) where yearly temperatures could be as much as 16^oC warmer than present.

Again, because of the large number of uncertainties involved in the computer models scientists are reluctant to rely on the models. Still, there is a consensus that average temperatures have increased over the last 100 years, and that if these increases are due to the added input of greenhouse gases into the atmosphere, then temperatures will continue to increase at a rate of about 0.3^oC per decade. This will lead to average temperatures about 1 degree warmer by the year 2025 and about 3 degrees warmer by the year 2100. 

• Global Precipitation changes - A warmer atmosphere will lead to increased evaporation from surface waters and result in higher amounts of precipitation. The equatorial regions will be wetter than present, while the interior portions of continents will become warmer and drier than present.

• Changes in vegetation patterns - because rainfall will distributed differently, vegetation will have to adjust to the new conditions. Mid latitude regions are likely to be more drought prone, while higher latitude regions will be somewhat wetter and warmer than normal, resulting in a shift in agricultural patterns.
  
  
• Increased storminess - A warmer, wetter atmosphere will favor tropical storm development. Hurricanes will be stronger and more frequent.

• Changes in Ice patterns. - Due to higher temperatures, ice in mountain glaciers will melt. But, because more water will be evaporated from the oceans, more precipitation will reach the polar ice sheets causing them to grow.

• Reduction of sea ice - Sea ice will be greatly reduced to the increased temperatures at the high latitudes, particularly in the northern hemisphere where there is more abundant sea ice. Ice has a high albedo (reflectivity), and thus reduction of ice will reduce the albedo of the Earth and less solar radiation will be reflected back into space, thus enhancing the warming effect.

• Thawing of frozen ground - Currently much of the ground at high latitudes remains frozen all year. Increased temperatures will cause much of this ground to thaw. Organic compounds and gas hydrates in the frozen ground will be subject to decay, releasing more methane into the atmosphere and enhancing the greenhouse effect. Ecosystems and human structures currently built on frozen ground will have to adjust.
  
  
• Rise of sea level - Warming the oceans results in expansion of water and thus increases the volume of water in the oceans. Along with melting of mountain glaciers and reduction in sea ice, this will cause sea level to rise and flood coastal zones, where much of the world's population currently resides.

• Changes in the hydrologic cycle - With new patterns of precipitation changes in stream flow and groundwater level will be expected.

• Decomposition of organic matter in soil - With increasing temperatures of the atmosphere the rate of decay of organic material in soils will be greatly accelerated. This will result in release of CO₂ and methane into the atmosphere and enhance the greenhouse effect.
   
• Breakdown of gas hydrates - This is basically solid water with gas molecules like methane locked into the crystal structure.  They occur in oceanic sediments and beneath frozen ground at the high latitudes.  Warming of the oceans or warming of the soil at high lattitudes could cause melting of the gas hydrates which would release methane into the atmosphere.  Since methane is a greenhouse gas, this would cause further global warming.
   

Global Warming in the Past.

From our study of glaciations in the past we know that climate can change as result of natural processes, both becoming warmer and colder than present. Although these climatic fluctuations appear to be caused by eccentricities in the Earth's orbit, it is interesting to note that during glaciations in the past the concentrations of greenhouse gas concentrations in the atmosphere were lower, atmospheric dust was higher, and the Earth's albedo was higher, all of these factors could have contributed to cooler climates. Similarly, during past interglacial episodes, the atmosphere contained less dust, higher concentrations of greenhouse gases, and the Earth had a lower albedo, all of which contribute to warmer climates. The questions that remain to be answered are:

• Are there higher concentrations of greenhouse gases and lower dust concentrations in the atmosphere due to the warmer temperatures or did they cause the warmer temperatures?  

• Are these differences simply due to orbital variations, or is there some other natural self regulating process that allows for cycles?  

• How do human affect these cycles?

Over the past 100 million years, geologists have been able to reconstruct CO₂ concentrations in the atmosphere and average atmospheric temperature based on a wide variety of geologic and geochemical evidence. From this reconstruction, it appears that temperature was much higher than present during the Mid-Cretaceous, during the Eocene, and during the Pliocene. We will next look at what might have caused these periods of global warming.

During this period we note the following observations: 

• The rate of production of new oceanic crust between 120 and 90 million years ago (mid Cretaceous) was nearly twice the rate prior to and after that time.

• Large volcanic plateaus were emplaced in the ocean basins. The total volume of these eruptions of basalt are unknown, as some may have been subducted, but many are greater than 10 million km³. (The Ontong Java plateau of the southwestern Pacific alone has a volume of ~ 55 million km³.

• The time interval during which these volcanic plateaus were emplaced correlate with:

• A long interval of normal magnetic polarity.
  

• A peak in oceanic paleotemperatures. 
  

• A peak in world-wide formation of petroleum. 

• Deposition of oxygen depleted sediments like black shales. 

• A peak in sea level stands, which became 100 to 200 m higher than present. 

This information can be interpreted in the following manner:

• Magnetic polarity remained constant because a superplume originated at outer core/mantle boundary taking with it a large amount of heat. This resulted in increasing the Temperature gradient in the core and thus resulted in vigorous convection in the core, which then became resistant to magnetic polarity changes. (Convection currents in the core are what are thought to cause the Earth's magnetic field. If the rate of convection is high, then it is more difficult to change the polarity of the magnetic field). 
  
  
• CO₂ released from the magmas erupted on the ocean floor by these plumes resulted in a super green house effect, causing mid Cretaceous climates to increase to 10 to 12^o C above current average global temperatures.
  
  
• Increased ocean temperatures resulted in an increase in productivity of marine life which resulted in the formation of increased formation of petroleum.
  
  
• Increased global temperatures resulted in sluggish circulation of ocean water which resulted in oxygen depleted waters and the deposition of Carbon-rich black shales. These shales were preserved because shallow seas flooded the continents.
  
  
• The large volume of basalts erupted on the ocean floor displaced sea water resulting higher stands of the sea.

During Eocene we note the following: 

• Fossils of alligators are found on Elsmere Island at 78^o North Latitude

• Tropical vegetation and tropical marine organism fossils occur up to 45 to 55^o North and South Latitude, about 15^o higher than today.

• Estimates of atmospheric CO₂ concentrations show values between 2 and 6 times current values.

The increased CO₂ concentrations have been attributed to a large scale metamorphic event that occurred as a result of the continent-continent collision that began to uplift the Himalayas, and other metamorphic events that occurred in the Mediterranean region and the circum-Pacific region during the Eocene. Such metamorphic events, particularly in the upper parts of the metamorphic areas where greenschist metamorphism would occur, would release large amounts of CO₂ into the atmosphere.

This example shows how the rock cycle itself, aided by tectonic processes could affect atmospheric conditions.

Unfortunately, the complexity of the processes are not completely understood. This has major political implications. For example, scientists are uncertain about the reliability of models that attempt to predict future conditions. This uncertainty is taken by some political factions as a denial that an event like global warming will take place. Most scientists, however, agree that global warming is highly possible, but they are unwilling to say that it will definitely occur. Politicians want, or expect you to want, exact answers. The real question, however, is whether or not we should be preparing for such events to avoid disaster if it does occur, or, since we can't be certain, just wait until the disaster has occurred and we can do nothing about it.