Global warming is mostly anthropogenic

Transient Climate Sensitivity The Earth has warmed approximately .8ºC since 1870 (1). I allege that at least 75%, or .6º, of this warming was caused by CO2. We can evaluate the truth of this statement by determining the transient climate sensitivity value which would be required to produce .6ºC of warming. In essence, transient climate sensitivity is a constant which determines how much warming we can expect to see immediately following a change in atmospheric CO2. Transient sensitivity is lower than equilibrium sensitivity, due to the thermal inertia of ocean heat uptake. Fully equilibrating ocean temperatures would require thousands of years. Therefore, transient climate sensitivity is more relevant to determining near-term climate change than equilibrium climate sensitivity. Transient climate sensitivity is equal to the observed temperature change divided by the radiative forcing which produced the change (2). So, to calculate transient climate sensitivity we first need to estimate CO2’s radiative forcing. We can use the following formula to do so (3): ∆F = 5.35 ln(C/Co) C is the current concentration of atmospheric CO2, which is approximately equal to 390 ppm. Co is the pre-industrial concentration of 290 ppm. Therefore, CO2’s radiative forcing equals about 1.6 W/m2. As I mentioned above, transient climate sensitivity can be calculated by dividing the magnitude of the observed temperature change (.6) by CO2’s radiative forcing (1.6). Doing so gives us a value of .375 °C/[W/m2] for transient climate sensitivity when three fourths of global warming is anthropogenic. If this were the correct value, we would expect to see a 1.39ºC rise in temperatures immediately following a doubling of CO2 concentrations (2). However, climate models which are constrained by observational evidence indicate that we would actually see between 1.5º and 2.8ºC of warming (4). There is a 5% chance that this range is not correct (4), but this small of a percentage is hardly notable. Thus, we can say with confidence that CO2 can account for more than 75% of the observed warming trend. This is because a temperature change is the product of CO2’s radiative forcing and transient climate sensitivity. For only .6º of global warming to be anthropogenic, climate sensitivity would have to be unrealistically low. Equilibrium Climate Sensitivity Equilibrium climate sensitivity is the global mean temperature change that eventually results from a doubling of atmospheric CO2. This temperature change can only be achieved over thousands of years. Thus, equilibrium sensitivity is quite larger than transient climate sensitivity, which I defined above. My opponent believes that equilibrium climate sensitivity is 2ºF (1.1ºC) for doubled CO2. However, this value is inconsistent with the study I presented above. This research article determined that transient climate sensitivity is unlikely to be lower than 1.5ºC. Therefore, it’s virtually impossible that equilibrium climate sensitivity is as low as my opponent alleges. My opponent claims that 1.1ºC of warming for doubled CO2 is consistent with palaeoclimate evidence. To back up this suggestion, he cites an opinion article which concluded as follows: “The notion that the earth's climate is dominated by positive feedbacks is intuitively implausible, and the history of the earth's climate offers some guidance on this matter. About 2.5 billion years ago, the sun was 20%-30% less bright than now, and yet the evidence is that the oceans were unfrozen at the time, and that temperatures might not have been very different from today's. Carl Sagan in the 1970s referred to this as the Early Faint Sun Paradox. It turns out that increased thin cirrus cloud coverage in the tropics readily resolves the paradox. In present terms this means that they would diminish rather than enhance the impact of CO2.”For this claim to be accurate, an increase in thin cirrus cloud cover would have to provide a warming effect, and a decrease would have to provide a cooling effect (5). However, satellites indicate that the opposite of this statement is true (6-7). As Su et al. concluded, “The observations show that the clouds have much higher albedos and moderately larger longwave fluxes than those assumed by Lindzen et al. As a result, decreases in these clouds would cause a significant but weak positive feedback to the climate system, instead of providing a strong negative feedback.” Therefore, an increase in cloud coverage cannot explain the “Early Faint Sun Paradox.” Rosing et al. found an alternate solution to this alleged paradox. As they noted, “Our model calculations suggest that the lower albedo of the early Earth provided environmental conditions above the freezing point of water, thus alleviating the need for extreme greenhouse-gas concentrations to satisfy the faint early Sun paradox” (8). If these conclusions are correct, the “Early Faint Sun Paradox” cannot be used to determine climate sensitivity. Nevertheless, estimates of the Earth’s climate sensitivity can be deduced from other palaeoclimate data. For example, Hergerl et al. used temperature reconstructions from the past millennium to quantify the Earth’s climate sensitivity. They determined that the Earth would warm between 1.5ºC and 6.2ºC (2.7 and 5.6ºF) if CO2 concentrations were doubled (9). Climate ModelsMy opponent claims that climate models overestimate the Earth's climate sensitivity. However, this is not true. As it turns out, estimates of climate sensitivity from models are consistent with estimates from paleoclimate data (2). Atmospheric Temperature Profiles My opponent claims that, “CO2 global warming advocates do not claim that CO2 is causing the increased radiative reflectivity and the cooling troposphere.They claim that CO2 is increasing water vapor in the atmosphere which in turn produces the temperature profile.” My opponent appears to have confused the troposphere with the stratosphere. As I demonstrated in round one, the troposphere is expected to warm as carbon dioxide increases. Nevertheless, tropospheric warming cannot prove that climate change is anthropogenic.Meanwhile, as the greenhouse effect intensifies, the stratosphere is expected to cool. When CO2 traps more heat in the troposphere, less infrared radiation can reach the upper atmosphere. Therefore, if the increase in CO2 is responsible for the current warming, we would expect to see cooling above the tropopause. This has been observed by several scientific researchers. Cooling in the stratosphere actually suggests that global warming is caused by an intensifying greenhouse effect. For example, if global warming were caused by the sun, we would expect to see warming in all layers of the atmosphere, including the stratosphere. The Scientific Consensus Here is the full paper of the study I presented in round one.http://littleurl.net... The authors “compiled a database of 1,372 climate researchers and classified each researcher as either convinced by the evidence for anthropogenic climate change or unconvinced by the evidence for ACC.” In total, 903 of these scientists were convinced and 472 were unconvinced. However, this study did not attempt to determine how many researchers supported the IPCC’s conclusions. Instead, the authors of this study analyzed “the distribution of credibility of dissenting researchers relative to agreeing researchers and the level of agreement among top climate experts.” They concluded as follows: “The [unconvinced] group comprises only 2% of the top 50 climate researchers as ranked by expertise, 3% of researchers of the top 100, and 2.5% of the top 200. This result closely agrees with expert surveys, indicating that 97% of actively publishing climate scientists agree with the tenets of ACC.” References http://tinyurl.com... http://tinyurl.com... http://tinyurl.com... http://tinyurl.com... http://tinyurl.com... http://tinyurl.com... http://tinyurl.com... http://tinyurl.com... http://tinyurl.com...