Global warming is mostly anthropogenic

I will begin round two by defining a few additional terms: Isotope: One of two or more atoms with the same atomic number that contain different numbers of neutronsInfrared radiation: Electromagnetic radiation which we humans perceive as heatTroposphere: The lowest portion of Earth’s atmosphereClimate forcing: An externally imposed perturbation in the radiative energy budget of Earth’s climate system Now, on with the debate! Carbon dioxide emissions are increasing rapidly. In 1870, the amount of carbon dioxide in the atmosphere was about 290 parts per million (ppm). As of April 2011, the amount of carbon dioxide in the atmosphere had risen to approximately 390 ppm (1). Very carefully calibrated measurements have confirmed that human activities are the primary cause of this increase, as I will demonstrate below. There are two primary carbon isotopes: carbon-12 and carbon-13. Carbon dioxide produced from burring fossil fuels and clearing forests has a unique isotopic composition. This is because plants prefer lighter isotopes, that is they prefer carbon-12 over carbon-13. Fossil fuels like coal or oil are derived from ancient plants. Thus, when we burn fossil fuels, we’re releasing more of the lighter Carbon-12 into the atmosphere. If human activities are the primary cause of the observed increase in carbon dioxide concentrations, we would expect to see the ratio of Carbon-13 to carbon-12 fall. Indeed, this is just what we observe. Measurements from the atmosphere (2), corals (3), and sea sponges (4) indicate that this ratio has declined greatly over the past 100 years. Without a doubt, the rise in atmospheric carbon dioxide is almost entirely anthropogenic. As I will demonstrate, several lines of evidence indicate that the current warming trend is caused by this increase in carbon dioxide. 1) More Infrared radiation is returning to the Earth: If carbon dioxide is responsible for global warming, more infrared radiation should be returning to the surface. Before presenting further evidence which affirms the resolution, I will provide a simple explanation of why this is so. The Earth emits a large amount of infrared radiation into the atmosphere. Some of this heat is absorbed by carbon dioxide molecules, which then reemit infrared radiation in all directions. Some radiation spreads downward and ultimately comes back into contact with the Earth’s surface, where it’s absorbed. However, some of the radiation spreads upward. In the upper layers of the troposphere, carbon dioxide molecules are sparse, and this heat energy can eventually escape to space. When additional carbon dioxide molecules are added to these layers, significantly more thermal infrared radiation from lower levels will be absorbed. Therefore, the place from which heat energy finally leaves the Earth will shift to higher layers. These layers are very cold, since temperatures decline greatly with altitude in the troposphere. Notably, the amount of energy emitted by an object is proportional to its temperature. Thus, these high altitude layers will emit significantly less radiation than the layers below, thereby retaining a large amount of heat energy. As these layers accumulate heat, they will warm and emit an increasing amount of infrared radiation. Thus, more radiation will spread downward and reach the surface, causing the surface to warm as well. Measurements from ground stations indicate that more infrared radiation is indeed returning to the Earth (5-6). When we take a close look at the spectrum of this radiation, we can determine how much each greenhouse gas is contributing to the warming effect. From these results, one scientist concluded that, “This experimental data should effectively end the argument by skeptics that no experimental evidence exists for the connection between greenhouse gas increases in the atmosphere and global warming” (6). 2) The upper atmosphere is cooling: As greenhouse gases trap more heat in the troposphere, less infrared radiation reaches the upper atmosphere, beginning with the stratosphere. Therefore, if the increase in carbon dioxide is responsible for the current warming, we would expect to see cooling in the upper atmosphere. This has been observed by several scientific researchers (7-9). 3) The tropopause has risen: Theoretically, global warming would also lead to an increase in the tropopause height. The tropopause is the atmospheric boundary between the troposphere and the stratosphere. Because the troposphere has warmed and the stratosphere has cooled, the tropopause has risen several hundred meters over the past three decades (10). 4) Oceanic heat content is increasing: Ocean heat content is increasing, especially in the upper layers. This specific pattern of ocean warming, with heat penetrating from the surface, can only be explained by greenhouse gas warming. As Barnett et al. concluded, “[The increase in ocean heat content] cannot be explained by natural internal climate variability or solar and volcanic forcing, but is well simulated by two anthropogenically forced climate models. We conclude that it is of human origin, a conclusion robust to observational sampling and model differences. Changes in advection combine with surface forcing to give the overall warming pattern” (11). 5) Observed precipitation trends can best be explained by increasing carbon dioxide: Zhang et al. showed that models do a much better job of matching the observed precipitation trends when anthropogenic climate forcings were included. As they stated, “Anthropogenic forcing has had a detectable influence on observed changes in average precipitation within latitudinal bands, and that these changes cannot be explained by internal climate variability or natural forcing. We estimate that anthropogenic forcing contributed significantly to observed increases in precipitation in the Northern Hemisphere mid-latitudes, drying in the Northern Hemisphere subtropics and tropics, and moistening in the Southern Hemisphere subtropics and deep tropics. The observed changes, which are larger than estimated from model simulations, may have already had significant effects on ecosystems, agriculture and human health in regions that are sensitive to changes in precipitation” (12). 6) Most scientists believe that global warming is anthropogenic: Several studies have surveyed climate scientists who are actively publishing climate research. All of these studies found that over 97% of climate experts are convinced that humans are the primary factor changing global temperature (13-14). I look forward to my opponent's response! References: http://www.esrl.noaa.gov... http://bluemoon.ucsd.edu... http://www.webpages.uidaho.edu... http://mgg.rsmas.miami.edu... http://www.agu.org... http://ams.confex.com... http://journals.ametsoc.org... http://www.ufa.cas.cz... http://www.agu.org... http://www.math.nyu.edu... http://www.sciencemag.org... http://www.cccma.ec.gc.ca... http://www.pnas.org... http://tigger.uic.edu...