Dec. 29, 2017

Earth’s atmosphere is about 300 miles thick, but 80% of the atmosphere is within 10 miles of the earth’s surface. Above 10 miles the atmosphere thins out into space. But all the water vapor and weather occurs near the earth’s surface. The thin blue line above the earth's surface shows just how thin the atmosphere really is.

The earth's atmosphere contains trace gases, some of which absorb heat. These gases (water vapor, carbon dioxide, methane, ozone, and nitrous oxide) are referred to as "greenhouse gases." They all are molecules composed of more than two component atoms, bound loosely enough together to be able to vibrate with the absorption of heat. The major components of the atmosphere (N2 and O2) are two-atom molecules too tightly bound together to vibrate and thus they do not absorb heat and contribute to the greenhouse effect. The earth's greenhouse effect is what makes this planet suitable for life as we know it.

Our moderate temperatures are the result of having just the right kind of atmosphere. The Goldilocks Principle can be summed up neatly as "Venus is too hot, Mars is too cold, and Earth is just right." The fact that Earth has an average surface temperature comfortably between the boiling point and freezing point of water, and thus is suitable for our sort of life, cannot be explained by simply suggesting that our planet orbits at just the right distance from the sun to absorb just the right amount of solar radiation. A Venus-type thick atmosphere would produce hellish, Venus-like conditions on our planet (870 degrees Fahrenheit); a thin Mars atmosphere would leave us shivering in a Martian-type deep freeze (-135 degrees Fahrenheit).

Although our atmosphere filters out high frequency energy that would damage tissues of living things, most of the energy from the sun that reaches the surface is visible light frequencies. Absorbed by land, oceans, and vegetation at the surface, the visible light is transformed into heat and re-radiates in the form of invisible infrared radiation. If that was all there was to the story, then during the day earth would heat up, but at night, all the accumulated energy would radiate back into space and the planet's surface temperature would fall far below zero very rapidly. The reason this doesn't happen is that earth's atmosphere contains molecules that absorb the heat and re-radiate the heat in all directions. This reduces the heat radiated out to space. Called greenhouse gases because they serve to hold heat in like the glass walls of a greenhouse, these molecules are responsible for the fact that the earth enjoys temperatures suitable for our active and complex biosphere.

So, in this very thin atmosphere humans have been releasing carbon mostly due to energy production and agri-business practices. CO2 emissions from humans have increased from about 5 billion tons per year during the 1950’s to over 35 billion tons per year today. To think that will not have an effect on global systems is hubris.

Greenhouse gas levels have varied over geologic time, along with temperature and climate changes often related to mass extinction events. But during most of the time that humans have been on the planet, greenhouse gases and temperatures have remained in stable ranges for human habitation.

For more details of the history of carbon and temperatures there are several web sites for further study: