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Global Warming vs Greenhouse Effect
Global warming has become a highly politicized topic not just in the United States but around the world. Debates about global warming from the Internet to the floor of Congress rarely have much to do with actual science, but instead hinge on wild conspiracy theories involving leaked emails, UN sustainability proposals, and Al Gore's power bills.
However, at the heart of all this debate and denialism are some very basic principles of physics and chemistry commonly known as the greenhouse effect. While climate is extremely complicated and is controlled by a great many factors, the greenhouse effect plays a critical role in regulating the climate - as well as making our planet habitable.
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The Greenhouse Effect Explained
We often think of the Earth as being in the habitable zone of the Sun - a comfortable distance at which water can remain in a liquid state. However, liquid water is only possible on Earth due to the greenhouse effects of the atmosphere.
If the Earth had no atmosphere, its temperature would be a chilly -0.4° Fahrenheit (-18° Celsius). The molecules in the blanket of air surrounding our planet absorb radiation of certain incoming wavelengths, reflect others, and allow still other wavelengths to pass through. Most of the radiation from the Sun comes in the form of ultraviolet and visible light, with lesser amounts in the infrared and other portions of the spectrum. Ozone and oxygen high in the atmosphere block much of the ultraviolet radiation from reaching the ground. Visible light mostly passes through, with some scattering in the lower wavelengths, toward the blue end of the spectrum (making the sky blue). Infrared light mostly passes through the atmosphere, warming the surface.
The Earth then emits thermal radiation in the near-infrared part of the spectrum. Greenhouse gases such as H2O (water vapor), CO2 (carbon dioxide), CH4 (methane), and O3 (ozone) absorb this thermal radiation and then re-radiate it in different directions, sometimes out into space and sometimes back down to the ground. It is the portion that is re-radiated to the ground that creates the greenhouse effect, and keeps our planet at a temperature suitable for liquid water and for life.
When atmospheric levels of greenhouse gases remain steady, the planet stays in a state of equilibrium between incoming and outgoing energy. When the concentration of greenhouse gases in the atmosphere changes, however, it can lead to a runaway greenhouse effect, in which positive feedback mechanisms throw this equilibrium off-balance.
Runaway Greenhouse Effect: The Paleocene–Eocene Thermal Maximum
Geologic history provides us with a good example of a runaway greenhouse effect in the Paleocene-Eocene Thermal Maximum (PETM), a period of rapid warming (on a geologic time scale) that occurred around 55 million years ago. Average global temperatures rose by 6° Celsius (10.8° F) over a 20,000 year period, eventually cooling to normal temperatures after 150,000-200,000 years.
Paleobiologists have been able to reconstruct the climate of this period by studying sediment deposits and the fossilized shells of marine organisms. These retain a chemical signature from the seawater around them, which in turn tells researchers about ocean temperatures at the time. During the PETM, ocean temperatures rose drastically, from a 4 degree sea surface temperature rise near the equator to an 8 degree rise in the deep ocean at higher latitudes.
The consequences of this sudden temperature increase were a change in ocean currents, rise in sea levels, a shift in precipitation patterns, and acidification of the oceans. The effects on life at the time were mixed. There was a mass extinction of 35-50% of deep-sea plankton during this period, and extinction of a number of small mammal and coastal plant species. On the other hand, there was a great diversification of other mammals at the time, and the orders leading to modern-day cattle, horses, deer, and primates appear in the fossil record at this time. This was also a period of migration, as temperate-zone plant species are found at much higher latitudes than before, and fossils of tropical fish are found in the Arctic.
The exact cause of the PETM is still a mystery to paleogeologists, but the evidence points to a massive release of carbon as a triggering event. This then led to a positive feedback loop as warming temperatures caused more carbon dioxide and other greenhouse gases to be released into the atmosphere, intensifying the warming even further. When the planet finally cooled to a stable equilibrium temperature 200,000 years later, it was a very different place.
Books on Global Warming
Global Warming: More Than Just Temperature Change
The Paleocene-Eocene Thermal Maximum saw a raise in temperatures of 6 degrees Celsius over about 20,000 years. Current worst-case climate models show our planet undergoing a similar average temperature rise by the end of this century due to the increased concentration of carbon dioxide and other greenhouse gases in the atmosphere since the Industrial Revolution.
While some species did show remarkable adaptability during the geologically rapid thaw of the PETM, we cannot expect to see this during the geologically-instantaneous warming of the current era. The consequences for the biosphere will be drastic, and we should expect to see far more extinctions and disruptions to the precipitation patterns we rely on for growing crops and raising livestock.
Though climatologists may disagree over some of the details of how the climate will change due to the introduction of an extra 117 ppm of CO2 into the atmosphere, basic physics and chemistry demonstrate that this will intensify the Earth's greenhouse effect and warm the planet. This basic fact is beyond dispute. How we as a species will work to solve this climate crisis and mitigate its consequences, on the other hand, is still an open question.
Sources and Further Information
350.org is building a global grassroots movement to solve the climate crisis. Our online campaigns, grassroots organizing, and mass public actions are led from the bottom up by thousands of volunteer organizers in over 188 countries.
RealClimate: Climate Science from climate scientists
- Why Carbon Dioxide Is a Greenhouse Gas: Scientific American
In making a case against CO 2 as a greenhouse gas, the Galileo Movement relies on irrelevant facts while omitting pertinent ones
- IPCC Working Group 1: What is the Greenhouse Effect?
The Sun powers Earth’s climate, radiating energy at very short wavelengths, predominantly in the visible or near-visible (e.g., ultraviolet) part of the spectrum.
- CHAPTER 7. THE GREENHOUSE EFFECT
Introduction to Atmospheric Chemistry, by Daniel J. Jacob, Princeton University Press, 1999
- www.palaeontologyonline.com | The Paleocene–Eocene Thermal Maximum
Article" The Paleocene–Eocene Thermal Maximum" by Phil Jardine published on www.palaeontologyonline.com with in the Patterns in Palaeontology category.... Download PDF by Phil Jardine*1 Introduction: The Paleocene–Eocene Thermal Maximum (PETM) is one