Mankind Is the Main Cause of Global Warming

Correlation and CausationConsider the Cenozoic era (the last 65 million years). Overall, solar activity increased 0.4% over this period. "Because Earth absorbs about 240 W/m^2 of solar energy, that brightness increase is a forcing of about 1 W/m^2. This small linear increase of forcing, by itself, would have caused a modest global warming through the Cenozoic Era. " The CO2 levels caused a much higher forcing. In contrast, atmospheric CO2 during the Cenozoic changed from at least 1000 ppm in the early Cenozoic to as small as 170 ppm during recent ice ages. The resulting climate forcing, as can be computed accurately for this CO2 range using formulae in Table 1 of Hansen et al. (2000), exceeds 10 W/m^2. It is clear that CO2 was the dominant climate forcing in the Cenozoic. "[1]In fact, the temperature changes correspond to the CO2 changes. ". .. there is a close correlation between Antarctic temperature and atmospheric concentrations of CO2 (Barnola et al. 1987). The extension of the Vostok CO2 record shows that the main trends of CO2 are similar for each glacial cycle. Major transitions from the lowest to the highest values are associated with glacial-interglacial transitions. During these transitions, the atmospheric concentrations of CO2 rises from 180 to 280-300 ppmv (Petit et al. 1999). The extension of the Vostok CO2 record shows the present-day levels of CO2 are unprecedented during the past 420 kyr. "[2]This graph shows the CO2-temperature correlation over the last 700,000 years: Excess CO2 traps heat. Considering the recent global warming, satellites measure less heat escaping out to space, at the particular wavelengths that CO2 absorbs heat, thus finding ". .. direct experimental evidence for a significant increase in the Earth's greenhouse effect. " In other words, the Earth is retaining more of the heat that it receives from the sun that it received from before. This excess heat manifests itself through global temperature increases. "If less heat is escaping to space, where is it going? Back to the Earth's surface. Surface measurements confirm this, observing more downward infrared radiation. A closer look at the downward radiation finds moreheat returning at CO2 wavelengths,leadin to the conclusion 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. '"[3][4][5]Evidence of Mankind's Influence on the Recent Global Warming TrendIt would be rather coincidental if the recent rise in global warming happened to start just around the time that humans started to emit large quantities of greenhouse gases. However, there is direct evidence as well, in addition to the already established correlation between temperature and CO2. Now, it is known that CO2 levels are increasing. "In pre-industrial times over the last 10,000 years, CO2 was relatively stable at around 275 to 285 parts per million. Over the last 250 years, atmospheric CO2 levels have increased by about 100 parts per million. " CO2 levels are increasing at a level not seen in at least 500,000 years, if not longer. [6]Here is a graph showing CO2 concentrations over the last 10000 years: [7][8]Now, carbon has several isotopes, three of which are C12, C13, and C14. C14 is radioactive, and has a half life of around 5,730 years. The CO2 emissions that humans make come from fossil fuels. Thus, if humans were the cause of the excess CO2, we'd except the C14/C12 ratio to decline, because, while there is a relatively constant amount of C14 in the atmosphere due to continuous replenishment (when a neutron collides with a regular nitrogen atom, a C14 atom and a proton result), the amount of C14 in the Earth has steadily decreased because it is not replenished, and since fossil fuels come from the ground, almost pure C12 is being ejected into the atmosphere. Indeed, it is observed that the C14/C12 ratio has declined. And thus, if humans were the cause of the excess CO2, we'd except the C13/C12 ratio to decline, because plants find it easier to take in carbon dioxide with a C12 atom than one with a C13 atom, implying that plants have a lower C13/C12 ratio than the atmopshere and, since animals eat plants (either by directly eating then or eating something that eats plants), also implying that fossils also have a lower C13/C12 ratio than the atmopshere, implying that we'd see a decline in the C13/C12 ratio as well. Indeed it is also observed that the C13/C12 ratio has declined. [9][10][11]This can be observed in the graph below (this is the C13/C12 ratio over the last 650 years; the C14/C12 ratio graph was too big to put in the debate): The evidence that this excess CO2 is the cause of the recent global warming is voluminous. One piece of evidence is a comparison of warming in the troposphere and stratosphere. Because the CO2 is in the upper troposphere, the troposphere temperature would increase, while the stratospheric temperature would decrease, because there would be less heat reaching the stratosphere. "Computer model estimates of the ‘human influence’ fingerprint are broadly similar to the observed pattern. In sharp contrast, model simulations of internal and total natural variability cannot produce the same sustained, large-scale warming of the troposphere and cooling of the stratosphere. "[4][12][13]This graph shows this: [4]Another piece of evidence is the frequency of cold days and nights. Because the sun only shines in the day time, is the sun was causing global warming, the days would warm faster than the nights, while if greenhouse gases were causing global warming, this wouldn't be observed. It is the latter's prediction that is observed. "What we observe is a decrease in cold nights greater than the decrease in cold days, and an increase in warm nights greater than the increase in warm days. "[4][14]This can be shown in the below graph: [4]Overall, the evidence shows that human-emitted greenhouse gases are the cause of global warming. CO2, in addition to other gases like methane and nitrous oxide are being emitted by humans in very large amounts, and this is manifesting itself in an increase in the average global temperature. Climate Sensitivity and FeedbacksClimate sensitivity is the amount the temperature would rise if the CO2 concentration were doubled. Obviously, if there's a large climate sensitivity, then increases in CO2 have large effect. It is known that the climate sensitivity is around 1 degree C. However, this can be amplified through feedbacks. Positive ones amplify the sensitivity, while negative ones diminish the sensitivity. The evidence overwhelmingly comes down on the former, that positive feedbacks are happening. Increases in CO2 cause temperature increases, which are amplified by water vapor and the effect on clouds. "Since the radiative effects associated with the buildup of water vapor to near-saturation levels and the subsequent condensation into clouds are far stronger than the equilibrium level of radiative forcing by the non-condensing GHGs, this results in large local fluctuations in temperature about the global equilibrium value. "[15]This can be shown in the below graph: Further, increases in CO2 affect the carbon cycle in this way: Global warming can result in the death of vegetation (due to droughts) and the warming of the ocean. Both of these further reduce the maximum absorption of the Earths carbon cycle, thus resulting in even more CO2 being released into the atmosphere. And with this, CO2 increases even more. In other words, CO2-caused temperature increases are amplified by positive feedbacks and the mechanics of the carbon cycle. So, the positive feedback amplifies the climate sensitivity. How much it is amplified can be determined through study. Using a Bayesian statistical approach, which is “the dominant [method] in the literature”, these findings support the notion of climate sensitivity as maximum 4 degrees C, a mean of 3 degrees C, and likely not lower than 3 degrees. [16]The graph below gives a statistical analysis: [17]The mean is around 3 degrees C. The CO2 that humans emit thus has an effect of 3 degrees C per doubling of CO2. This can be shown by the fact that CO2 concentrations have increased from around 275 ppm to around 400 ppm. This is an increase of around 40%. This should manifest itself with a temperature increase of a little less than 1.5 degrees C. Indeed, temperatures have increased around this amount over the last 150 years. The anthropogenic-forcing climate models thus match observations. [18]In other words, in addition to the direct evidence of how the Earth is warming, the climate models based on a greenhouse gas cause to global warming explain almost perfectly the recent global warming. This is a lot of evidence for a human case to the recent global warming. ConclusionGreenhouse gases cause global warming because of their heat trapping abilities. Humans have been emitting vast amounts of greenhouse gases over the last 150 years, and this shows itself on the CO2 measurements. The atmospheric warming pattern and greater heating at night are evidence that the recent global warming is caused by those human emitted greenhouse gases. These increases are amplified through the water vapor and cloud positive feedbacks and the positive feedback that arises through the climate cycle. The climate sensitivity ends up being around 3 degrees C. Finally, the CO2-temperature record shows that the two correlate with remarkable correlation. SourcesSources in comments.