Carbon

Most are aware the role carbon, specifically in carbon dioxide, plays in global warming. What is important is not to designate carbon as something inherently harmful. In fact, without carbon, life would not be possible. So lets take a look at carbon and how it fits into the big picture on Earth.

Carbon is created in the nuclear fusion of stars.  When sun-like stars become red giants, their cores fuse helium and beryllium into carbon atoms.  When a massive star goes supernova, the explosion disperses the matter created by that star into the universe and is recycled into new stars and planets. Remember the old song lyric, “We are stardust?” That is literally the case. The matter that makes up most of our bodies was produced in the fusion reaction of an ancient generation star.

So what is carbon? Lets take a look at the image below:

Credit: Alejandro Portis/Wiki Commons.

First, note the number of protons in the nucleus equals the number of electrons orbiting the nucleus. Protons have a positive charge and electrons have a negative charge. The fact that there are equal numbers of both means the atom is electrically neutral. Also, note that there are four electrons in the outside orbital shell. This shell can fit a total of eight electrons. Thus, the carbon atom can form molecules with other elements by sharing four electrons in the outer shell with the other element. Atoms like to have their outside shells filled, or as many a high school chemistry teacher has said, are “happy” when those outer shells are filled.

Carbon Based Life

The study of organic chemistry is often treated as a course onto itself. What is important to understand is that life on Earth is carbon based. The bonds that a carbon atom can form with hydrogen, oxygen, and nitrogen atoms make it the backbone of organic molecules that life consists of.  Carbon atoms have the ability to form long complex chains of molecules to create carbohydrates, lipids, proteins, and nucleic acids (such as DNA).

Nature likes to recycle. As noted above, carbon was formed in stars and recycled in new stars. Carbon is recycled on Earth as well. Ever hear of the term fossil fuels? That is because the fuel we use is carbon based. And those carbon based fuels are extracted from the Earth. How did those carbon based fuels get there? From the dead remains of plant and animal life that existed on Earth millions of years ago.

Hydrocarbons

The fuel we use in our day-to-day lives are based on hydrocarbons. The term is derived from the molecular structure of these fuels based on molecules composed of carbon and hydrogen atoms. For example, natural gas is mostly methane which is a simple hydrocarbon based on one carbon atom sharing an electron with four hydrogen atoms. Hence, methane’s molecular formula is CH4. On the other hand, gasoline is formed by long chains of carbon-hydrogen bonds designated as C11H24 or C12H26. An example of some hydrocarbons is shown below:

Credit: United States Geological Survey.

Why do hydrocarbons make an excellent fuel source? There are a multitude of reasons. Hydrocarbons produce a lot of energy and can be controlled during combustion. Economically, fossil fuels are easy to store and transport. That also makes gasoline difficult to replace as not only do new automobile engines need to be designed, but a new infrastructure would need to be built to replace the current refinery-pipeline-gas station system. While great strides are being made in alternative fuel sources, fossil fuels will be a significant player in the economy for the foreseeable future.

To see why this is a concern, we’ll take a look at a simplified version of the carbon cycle below.

Credit: U.S. Department of Energy Genomic Science Program/http://genomicscience.energy.gov

Note how the use of fossil fuels results in a net intake of 6 billion (Gt=giga tons, giga = 1 billion) tons of carbon into the atmosphere. Carbon is recycled between the land, oceans, and atmosphere. Why do fossil fuels emit more carbon into the atmosphere than absorbed back into land? The reason is, it takes millions of years to form fossil fuels but only a few months to extract and burn it. It’s the same if you run more water into a bathtub than the drain can take away. So, what happens to that carbon when fossil fuels are burned and released into the atmosphere?

Carbon Dioxide

To understand how carbon dioxide is formed, lets take a look at an oxygen atom below:

Credit: Greg Robson/Wiki Commons.

Note that oxygen has 6 electrons in the outer shell that can hold eight electrons. Remember, the carbon atom has 4 empty spots in its outer shell to share. That being the case, two oxygen atoms will combine with a single carbon atom so that the outer shells of the oxygen atoms will be completely filled with eight electrons and are “happy”.

Methane is the simplest of the hydrocarbon fuels. What happens when methane is burned for energy?

Oxygen is used as a catalyst to burn methane as follows:

CH4 (methane) + 2O2 -> CO2 (carbon dioxide) + 2H2O + energy

Note that each side of the equation contains 1 carbon atom, 4 hydrogen atoms, and 4 oxygen atoms. When fossil fuels are burned for energy, carbon dioxide is released in the exhaust and into the atmosphere.

Greenhouse Gases

The composition of the Earth’s atmosphere is as follows:

Nitrogen:                    78%

Oxygen:                      21%

Argon:                         0.9%

Carbon Dioxide:        0.03%

Methane:                    0.00017%

How is it that trace gases such as carbon dioxide and methane play a dominant role in the greenhouse effect but nitrogen and oxygen do not? That is a matter of the molecular structure of each substance. Before we get into that, lets take a look at the role greenhouse gases have on Earth’s ability to support life.

To appreciate greenhouse gases on Earth, we’ll take a look at a place without greenhouse gases, the Moon. The Moon is the same distance from the Sun as the Earth and provides a baseline to examine. Below is a comparison of average temperature on the Moon and on Earth:

Moon: 00 F

Earth: 600 F

In other words, without greenhouse gases, the average temperature of the Earth would be the same as the Moon at 00 F.   At that temperature, water on Earth would be frozen and human life would not exist. The point here is that greenhouse gases are not “bad”. In fact, we need those gases to survive. However, too much of a good thing can be a bad thing and that includes greenhouse gases.

What makes a gas a greenhouse gas?

That question can be answered by looking at the molecular structure of the gases that exist in the Earth’s atmosphere. Some molecules, such as carbon dioxide, have molecular bonds that can stretch and vibrate, while others, such as nitrogen and oxygen, have molecular bonds that are rigid. In addition, the molecules whose bonds can vibrate are choosy at which frequencies they vibrate. To understand this better, take a look at the electromagnetic (EM) spectrum below:

Credit: NASA

Note that radio, microwaves, infrared, light, ultraviolet, x-rays, and gamma rays are all forms of EM radiation. What differentiates the various types of EM radiation are the wavelengths. The shorter the wavelength, the more energy the EM radiation has. That is why gamma rays are very damaging to life and we must be careful not to overexpose ourselves to x-rays and ultraviolet rays. Greenhouse gases only absorb radiation in the infrared range. What exactly is infrared radiation?

As you can tell from the image above, our eyes can only detect a small part of the EM spectrum. Infrared radiation is one form that we cannot see but can feel as heat. The vibrational motions of atoms and molecules produce infrared radiation and all objects radiate in the infrared. In fact, humans radiate most strongly in the infrared as does the planets, including Earth. Night vision goggles are basically infrared sensors. Detecting heat from objects at night allow us to see those objects in the dark.  Below is an image of a cat in infrared:

Credit: NASA/IPAC

Note the yellow areas on the infrared image. These are the warmest areas of the cat. The nose, which is dark, is the coolest area of the cat.

As sunlight strikes the Earth’s surface, the ground warms and radiates the energy back into the atmosphere as heat or infrared radiation.

What happens when infrared radiation encounters a greenhouse gas? The gas molecule absorbs the infrared energy and converts it to kinetic energy via vibration of molecular bonds. The molecule then stops vibrating and reconverts the kinetic energy back into the atmosphere as infrared energy where surrounding carbon dioxide molecules repeat the process. This prevents the infrared radiation from entering the upper atmosphere and escaping into space.  In essence, increasing greenhouse gases is like throwing an extra blanket on the Earth.*

The impact of the greenhouse effect is twofold. One, it traps heat in the lower atmosphere. This increases global temperature near the surface. Second, by preventing heat from escaping into the upper atmosphere, it cools the stratosphere.  This provides us with a key diagnostic tool to test if greenhouse gases are causing increasing surface temperature. If the increase in surface temperature originates from another forcing such as solar irradiance, then both the lower and upper atmosphere would become warmer. So how does the evidence look? The answer is below:

Credit: NASA Earth Observatory.

As the lower atmosphere has warmed the upper atmosphere has cooled. A good portion of the upper atmospheric cooling is due to ozone loss. The less ozone there is, the less ultraviolet radiation is absorbed in the stratosphere. However, the loss of ozone has not been enough to explain all the stratospheric cooling. The rest is caused by the greenhouse effect. You’ll note the two short-term spikes in stratospheric temperatures around 1983 and 1992. These were generated by volcanic ash ejected into the upper atmosphere from two separate explosions. The aerosols reflect sunlight and heat the stratosphere. However, the effect lasts on the order of 2-3 years and should not be confused with long-term trends.

Carbon Isotopes

All carbon atoms come with six electrons and six protons.  Where they differ is in the amount of neutrons in the nucleus.  Most carbon atoms have six neutrons, about 1% have seven neutrons and one out of a trillion will have eight neutrons.  Plant life produces carbon dioxide that favor the common six neutron configuration.  As fossil fuels consist of the remnants of past life on Earth, burning it produces less of the heavier seven and eight neutron carbon atoms than natural processes.  If the increase in atmospheric carbon dioxide is a result of the burning of fossil fuels, we would expect it to have a higher ratio of lighter six neutron carbon atoms.  Indeed, the amount of six neutron carbon to seven neutron atoms has increased since 1850, and are at their highest levels in at least 10,000 years.

Thus, the theoretical model meets data, meaning the best explanation is climate change is caused by human made greenhouse gases, especially carbon dioxide.

*Gavin Schmidt uses this analogy in his book Climate Change.  As Schmidt notes, like most analogies this is not perfect.  Under a blanket, heat is generated by the person using it.  In the atmosphere, the energy is received above from the Sun in the form of light and transformed and radiated by the ground in the form of infrared radiation.

**Image atop post is NASA computer model on the global distribution of carbon dioxide.  Credit:  NASA’s Goddard Space Flight Center/B. Putman

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