Evolution of the Atmosphere: Composition, Structure and Energy

Suggested Readings:

• Ahrens, C. Donald, "Meteorology Today", 6th ed., Brooks/Cole Publishing Co., 1999.
• Kump, L. R., and J. F. Kasting, and R. G. Crane, "The Earth System", Prentice-Hall, 1999.
• Ruddiman, W. F., "Earth's Climate", W.H. Freeman, 2000.

Driving Questions:

• How did the atmosphere evolve into what it is today?
• What gases in the atmosphere are important to life and how are they maintained?
• What natural variations occur in atmospheric constituents and what are the important time scales for change?
  
  

1. Evolution of the Present Atmosphere

Earlier in the course we learned that the evolution of the atmosphere could be divided into four separate stages:

1. Origin
2. Chemical/ pre-biological era
3. Microbial era, and
4. Biological era.

and the first three steps were discussed in detail. The composition of the present atmosphere however required the formation of oxygen to sufficient levels to sustain life, and required life to create the sufficient levels of oxygen. This era of evolution opf the atmosphere is called the "Biological Era."

The Biological Era - The Formation of Atmospheric Oxygen

The biological era was marked by the simultaneous decrease in atmospheric carbon dioxide and the increase in oxygen due to life processes. We need to understand how photosynthesis could have led to maintenance of the ~20% present-day level of O₂.

The build up of oxygen had three major consequences that we should note here.

Firstly, Eukaryotic metabolism could only have begun once the level of oxygen had built up to about 0.2%, or ~1% of its present abundance. This must have occurred by ~2 billion years ago, according to the fossil record. Thus, the eukaryotes came about as a consequence of the long, steady, but less efficient earlier photosynthesis carried out by Prokaryotes.

Figure 1. Photolysis of water vapor and carbon dioxide produce hydroxyl and atomic oxygen, respectivey, that, in turn, produce oxygen in small concentrations. This process produced oxygen for the eraly atmosphere before photosynthesis became dominant.

Oxygen increased in stages, first through photolysis (Figure 1) of water vapor and carbon dioxide by ultraviolet energy and, possibly, lightning:

H₂O -> H + OH

produces a hydroxyl radiacal (OH) and

CO₂ -> CO+ O

produces an atomic oxygen (O). The OH is very reactive and combines with the O

O + OH -> O₂ + H

The hydrogen atoms formed in these reactions are light and some small fraction excape to space allowing the O2 to build to a very low concentration, probably yielded only about 1% of the oxygen available today.

Secondly, once sufficient oxygen had accumulated in the stratosphere, it was acted on by sunlight to form ozone, which allowed colonization of the land. The first evidence for vascular plant colonization of the land dates back to ~400 million years ago.

Thirdly, the availability of oxygen enabled a diversification of metabolic pathways, leading to a great increase in efficiency. The bulk of the oxygen formed once life began on the planet, principally through the process of photosynthesis:

6CO₂ + 6H₂O <--> C₆H₁₂O₆ + 6O₂

where carbon dioxide and water vapor, in the presence of light, produce organics and oxygen. The reaction can go either way as in the case of respiration or decay the organic matter takes up oxygen to form carbon dioxide and water vapor.

Life started to have a major impact on the environment once photosynthetic organisms evolved. These organisms fed off atmospheric carbon dioxide and converted much of it into marine sediments consisting of the innumerable shells and decomposed remnants of sea creatures.

Cumulative history of O2 by photosynthesis through geologic time.

While photosynthetic life reduced the carbon dioxide content of the atmosphere, it also started to produce oxygen. The oxygen did not build up in the atmosphere for a long time, since it was absorbed by rocks that could be easily oxidized (rusted). To this day, most of the oxygen produced over time is locked up in the ancient "banded rock" and "red bed" rock formations found in ancient sedimentary rock. It was not until ~1 billion years ago that the reservoirs of oxidizable rock became saturated and the free oxygen stayed in the air.  The figure illustrates a possible scenario.

We have briefly mentioned the difference between reducing (electron-rich) and oxidizing (electron hungry) substances. Oxygen is the most important example of the latter type of substance that led to the term oxidation for the process of transferring electrons from reducing to oxidizing materials. This consideration is important for our discussion of atmospheric evolution, since the oxygen produced by early photosynthesis must have readily combined with any available reducing substance. It did not have far to look!

We have been able to outline the steps in the long drawn out process of producing present-day levels of oxygen in the atmosphere. We refer here to the geological evidence.

Banded Iron Formations

When the oceans first formed, the waters must have dissolved enormous quantities of reducing iron ions, such as Fe²⁺. These ferrous ions were the consequences of millions of years of rock weathering in an anaerobic (oxygen-free) environment. The first oxygen produced in the oceans by the early prokaryotic cells would have quickly been taken up in oxidizing reactions with dissolved iron. This oceanic oxidization reaction produces Ferric oxide Fe₂O₃ that would have deposited in ocean floor sediments. The earliest evidence of this process dates back to the Banded Iron Formations, which reach a peak occurrence in metamorphosed sedimentary rock at least 3.5 billion years old. Most of the major economic deposits of iron ore are from Banded Iron formations. These formations, were created as sediments in ancient oceans and are found in rocks in the range 2 - 3.5 billion years old. Very few banded iron formations have been found with more recent dates, suggesting that the continued production of oxygen had finally exhausted the capability of the dissolved iron ions reservoir. At this point another process started to take up the available oxygen.

Red Beds

Finally after another 1.5 billion years or so, the red bed reservoir became exhausted too (although it is continually being regenerated through weathering) and oxygen finally started to accumulate in the atmosphere itself. This signal event initiated eukaryotic cell development, land colonization, and species diversification. Perhaps this period rivals differentiation as the most important event in Earth history.

The oxygen built up to today's value only after the colonization of land by green plants, leading to efficient and ubiquitous photosynthesis. The current level of 20% seems stable.

The Oxygen Concentration Problem.

Why does present-day oxygen sit at 20%? This is not a trivial question since significantly lower or higher levels would be damaging to life. If we had < 15% oxygen, fires would not burn, yet at > 25% oxygen, even wet organic matter would burn freely.

The Early Ultraviolet Problem

The genetic materials of cells (DNA) is highly susceptible to damage by ultraviolet light at wavelengths near 0.25 µm. It is estimated that typical contemporary microorganisms would be killed in a matter of seconds if exposed to the full intensity of solar radiation at these wavelength. Today, of course, such organisms are protected by the atmospheric ozone layer that effectively absorbs light at these short wavelengths, but what happened in the early Earth prior to the significant production of atmospheric oxygen? There is no problem for the original non-photosynthetic microorganisms that could quite happily have lived in the deep ocean and in muds, well hidden from sunlight. But for the early photosynthetic prokaryotes, it must have been a matter of life and death.

It is a classical "chicken and egg" problem. In order to become photosynthetic, early microorganisms must have had access to sunlight, yet they must have also had protection against the UV radiation. The oceans only provide limited protection. Since water does not absorb very strongly in the ultraviolet a depth of several tens of meters is needed for full UV protection. Perhaps the organisms used a protective layer of the dead bodies of their brethren. Perhaps this is the origin of the stromatolites - algal mats that would have provided adequate protection for those organisms buried a few millimeters in. Perhaps the early organisms had a protective UV-absorbing case made up of disposable DNA - there is some intriguing evidence of unused modern elaborate repair mechanisms that allow certain cells to repair moderate UV damage to their DNA. However it was accomplished, we know that natural selection worked in favor of the photosynthetic microorganisms, leading to further diversification.

Fluctuations in Oxygen

The history of macroscopic life on Earth is divided into three great eras: the Paleozoic, Mesozoic and Cenozoic. Each era is then divided into periods. The latter half of the Paleozoic era, includes the Devonian period, which ended about 360 million years ago, the Carboniferous period, which ended about 280 million years ago, and the Permian period, which ended about 250 million years ago.

According to recently developed geochemical models, oxygen levels are believed to have climbed to a maximum of 35 percent and then dropped to a low of 15 percent during a 120-million-year period that ended in a mass extinction at the end of the Permian. Such a jump in oxygen would have had dramatic biological consequences by enhancing diffusion-dependent processes such as respiration, allowing insects such as dragonflies, centipedes, scorpions and spiders to grow to very large sizes. Fossil records indicate, for example, that one species of dragonfly had a wing span of 2 1/2 feet.

Geochemical models indicate that near the close of the Paleozoic era, during the Permian period, global atmospheric oxygen levels dropped to about 15 percent, lower that the current atmospheric level of 21 percent. The Permian period is marked by one of the greatest extinctions of both land and aquatic animals, including the giant dragonflies. But it is not believed that the drop in oxygen played a significant role in causing the extinction. Some creatures that became specially adapted to living in an oxygen-rich environment, such as the large flying insects and other giant arthropods, however, may have been unable to survive when the oxygen atmosphere underwent dramatic change.

2. Composition of the Present Atmosphere

Comparison to Other Planets

The overall composition of the earth's atmosphere is summarized below along with a comparison to the atmospheres on Venus and Mars - our closest neighbors.

	VENUS	EARTH	MARS
SURFACE PRESSURE	100,000 mb	1,000 mb	6 mb
	COMPOSITION
CO2	>98%	0.03%	96%
N2	1%	78%	2.5%
Ar	1%	1%	1.5%
O2	0.0%	21%	2.5%
H2O	0.0%	0.1%	0-0.1%
	(more on Mars)	(more on Earth)	(more on Mars)

The variations in concentration from the Earth to Mars and Venus result from the different processes that influenced the development of each atmosphere. While Venus is too warm and Mars is too cold for liquid water the Earth is at just such a distance from the Sun that water was able to form in all three phases, gaseous, liquid and solid. Through condensation the water vapor in our atmosphere was removed over time to form the oceans. Additionally, because carbon dioxide is slightly soluble in water it too was removed slowly from the atmosphere leaving the relatively scarce but unreactive nitrogen to build up to the 78% is holds today.

Current Composition

The concentrations of gases in the earth atmosphere is now known to be (ignoring water vapor, which varies between near zero to a few percent):

CONSTITUENT	CHEMICAL SYMBOL	MOLE PERCENT
Nitrogen	N2		78.084
Oxygen	O2		20.947
Argon	Ar		0.934
Carbon Dioxide	CO2		0.035
Neon	Ne		0.00182
Helium	He		0.00052
Methane	CH4		0.00017
Krypton	Kr		0.00011
Hydrogen	H2		0.00005
Nitrous Oxide	N2O		0.00003
Xenon	Xe		0.00001
Ozone	O3		trace to 0.00080

The unit of percentage listed here are for comparison sake. For most atmospheric studies the concentration is expressed as parts per million (by volume). That is, in a million units of air how may units would be that species. Carbon dioxide has a concentration of about 350 ppm in the atmosphere (i.e. 0.000350 of the atmosphere or 0.0350 percent).

Greenhouse Gases

Click to interactively explore Selective Absorbers (install "Shockwave")

Radiative Properties

Objects that absorb all radiation incident upon them are called "blackbody" absorbers. The earth is close to being a black body absorber. Gases, on the other hand, are selective in their absorption characteristics. While many gases do not absorb radiation at all some selectively absorb only at certain wavelengths. Those gases that are "selective absorbers" of solar energy are the gases we know as "Greenhouse Gases."

The interactive activity to the right allows you to visualize how each greenhouse gas selectively absorbs radiation. Wien's Law states that the wavelength of maximum emission of radiation is inversely proportional to the object's temperature. Using that law we know that the wavelength of maximum emission for the Sun is about 0.5 µm (1 µm = 10^-6 m) and the wavelength for maximum emission by the Earth is about 10 µm. In the activity to the right see where the greenhouse gases absorb relative to those two important wavelengths.

Sources and Sinks

Greenhouse Gases (apart from water vapor) include:

• Carbon Dioxide
• Chlorofluorocarbons (CFCs)
• Methane
• Nitrous Oxide
• Ozone

and each have different sources (emission mechanisms) and sinks (removal mechanisms) as outlined below.

Layers of the atmosphere (click to enlarge)

3. Structure of the Atmosphere

In large measure, the atmosphere has evolved in response to and controlled by life processes. It continues to change as a consequence of human activities, but at a rate that is far in excess of the rate of previous evolutionary change. The atmosphere controls the climate and ultimately determines the quality of life on Earth. We will begin our discussion with a brief review of the composition and structure of the present-day atmosphere. Then we will discuss the major events in the evolution of the atmosphere that led to its current state. We will discuss some important tools along the way that will prove useful in many settings.

• The word "structure" is used in atmospheric physics to mean the vertical profile of particular variables of interest (such as temperature, density, pressure, etc.)
  
  
• Atmospheric structure is subdivided into four thermal layers or "-spheres" that are divided by transition regions or "-pauses". The nomenclature used dates back to the 1950's and is based on the measured temperature profile of the atmosphere. Figure 1 illustrates the temperature profile and the names used for the different regions.

As discussed earlier, the ground heats up due to the absorption of visible light from the Sun. The warm ground, in turn, heats the atmosphere via the processes of conduction, convection (turbulence) and infrared radiation. As we move upwards from the ground, we might expect temperature to drop off according to the R-squared law. This happens (more or less) for a while, but the declining thermal structure reverses at the tropopause and increases to a new maximum at the stratopause. In the mesosphere, the temperature drops to the lowest values seen anywhere in the atmosphere. Above the mesosphere, the temperature rises again in the thermosphere. Eventually, the temperature reaches a maximum value at very high altitudes (see Figure above).

The reason for the strange-looking temperature profile is quite simple. Regions of high temperature are heated by different portions of the solar radiative output.

The stratosphere is heated by the absorption of ultraviolet (UV) light by ozone.

The thermosphere is heated by the absorption of extreme ultraviolet (EUV) light by other atmospheric constituents (primarily molecular and atomic oxygen and molecular nitrogen).

In this course we will be mostly concerned with the troposphere--the region where we live--and its variations. The stratosphere, however, also plays a major role in global change and evolution, as we will soon see. Although some scientists think the mesosphere and thermosphere might also play key roles in the story of global change, the question is still not resolved.

We have so far only considered the vertical variation of temperature. Other atmospheric variables also vary with altitude. Since the atmosphere is a gaseous envelope, it is compressible. This means that density and pressure both decrease exponentially with altitude. The lowest regions are weighed down by the mass of the overlying atmosphere, becoming compressed and therefore more dense. Jet aircraft flying in the low-density stratosphere have to pressurize their hulls due to the compressibility of the atmosphere. Figure 3 shows the variation of density and pressure with altitude.

The temperature profile shown in Figures 1 and 2 plays a significant role in controlling atmospheric turbulence. We all know that the troposphere is a turbulent place to live: we experience wind gusts, cloud formation and severe weather. The fact that temperature drops with altitude in the troposphere leads to atmospheric instability. In the stratosphere, on the other hand, the temperatures rise with altitude, leading to a very stable region. It is for this very reason that we are able to drink cups of coffee in jet aircraft. One of the many gifts showered on us by ozone is the ability to fly commercial aircraft in relative comfort!

4. Summary

We developed a few useful tools for the study of biogeochemical cycles. These include the concepts of the reservoir, fluxes, and equilibria.

• Atmospheric evolution progressed in four stages, leading to the current situation. The atmosphere has not always been as it is today - and it will change again in the future. It is closely controlled by life and, in turn, controls life processes. Complex feedback mechanisms are at play that we do not yet understand.
  
  
• Oxygen became a key atmospheric constituent due entirely to life processes. It built up slowly over time, first oxidizing materials in the oceans and then on land. The current level (20%) is maintained by processes not yet understood.
  
  
• Sometime just before the Cambrian, atmospheric oxygen reached levels close enough to today's to allow for the rapid evolution of the higher life forms. For the rest of geoogic time, the oxygen in the atmosphere has been maintained by the photosynthesis of the green plants of the world, much of it by green algae in the surface waters of the ocean.
  
  
• Selective absorbers in our atmosphere keep the surface of the earth warmer than they would be without an atmosphere.

Copyright © Regents of the University of Michigan