Global Warming is Man-Made

He is one of the best debaters on the site and is very knowledgeable on this subject. It is a pleasure to debate him. ==There exists a scientific consensus== Before I continue, I would like to note this argument in and of itself does not prove that global warming is caused by man. Actual evidence should be the driving force behind this debate. The reason I chose to put this in my argument is simple: the science is not clear because of consensus, but a consensus exists because the evidence is clear. This evidence will be discussed later. A study published in Science reviewed the ISI web of science in order to take a survey of relevant climate literature as to what the causes of climate change are. The study failed to find a single paper which was in opposition to the consensus position, that the main driver of climate change is anthropogenic. 75% of the papers supported the consensus position, whereas 25% had no position (they were focused on things other than forgings, like impacts or paleoclimate) [1]. The study also noted how many organizations have come out supporting the idea of anthropogenic climate change. The IPCC, the American Meteorological Society, American Geophysical Union, and the American Association for the Advancement of Science have all concluded that the evidence for AGW is overwhelming [2]. NASA furthers this argument, noting “most of the leading scientific organizations worldwide have issued public statements endorsing this position”. They also argue “[n]inety-seven percent of climate scientists agree that climate-warming trends over the past century are very likely due to human activities” [3]. A 196 page report representing 13 governmental agencies, and written by 28 authors from scientific institutions, has stated “[t]he global warming of the past 50 years is due primarily to human-induced increases in heat-trapping gases” [4]. ==Climate sensitivity== As Roy is well aware, this is the key point in all of climate science (we have discussed this before in a PM). For those who are unaware, climate sensitivity is defined as how much warming is produced by a doubling of CO2. Both skeptics and AGW supporters agree that the direct effect of doubling CO2 is one degree C, the debate is over feedbacks [5]. First, what is a feedback? Climate skeptic Roy Spencer explains it fairly simply. He describes a forcing as how much temperature change occurs. A feedback determines how large the change in temperatures will be. Feedbacks refer to how clouds and other warming/cooling mechanisms change in response to a forcing (i. e. CO2, TSI, CRs). He also explains positive and negative feedbacks. Take a car, for example. The temperature forcing would be the sun beating down on the car. Opening the window slowly would cool the car, over time, and be a negative feedback. On the other hand, if you were to close the window, the car would warm (due to an enhanced greenhouse effect), which would be a positive feedback [6]. So the question is whether or not the climate is dominated by negative or positive feedbacks. There is even the possibility of there being no feedbacks, as the positive and negatives could cancel each other out. Research by climate skeptics published in Climate Research has demonstrated that climate sensitivity, based on model means, is 1.9 degrees C. Using differing methods of empirical approaches, they conclude that climate sensitivity is 1 – 1.6 degrees C [7]. Although these estimates shy towards having no net-amplifier, the no-amplification scenario still supports a strong influence of man on climate. In fact, to put an answer numerically, about 50% of the warming would be caused by man if there was no net-feedback [8]. Assuming no amplifier, research has found that “the largest contribution to the 20th century warming comes from anthropogenic sources” [9]. They find that of the ~0.7 degree C temperature rise in the 20th century (some estimates slightly higher), about 0.4 degrees of that are due to anthropogenic forgings, or about 57% of the warming (personal calculation from these skeptical studies and personal correspondence with their authors). A significant body of research notes that climate sensitivity is likely much higher. A review of the evidence in 2008 finds that mean climate sensitivity is 3 degrees C, with a range of 2 – 4.5 degrees C. Based on multiple lines of evidence, we see *almost* every which way to measure climate sensitivity leads to approximately the same result: a doubling of CO2 will lead to 3 degrees C of temperature rise [10]. The following graphic from the study corroborates this: The gray is the range (2 – 4.5 degrees C), the line in the center is on 3 degrees C, and the circles in the lines are the ‘very likely’ ranges. Most of them rest on the 3 degree mark, with some under and some above. Therefore, we can conclude that from these lines of evidence, mean sensitivity is about 3 degrees C. Another study replicates the above conclusions. The upper limit for climate sensitivity is around 4 degrees C, and certainly below 6 degrees C. 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 [11]. The following graph [12] again details the results of many different studies. The best estimates generally center around 2.5 – 3 degrees C. Some evidence published in 2012 claims sensitivity is on the low end of the spectrum, however, still note that “[h]umanity is . . . responsible for the most recent period of warming from 1976 to 2010” [13]. The fact is, climate sensitivity is high and the climate system is predominated by positive feedback mechanisms. Whether it be due to changes in clouds, increased water vapor, or long-term albedo changes. With a climate sensitivity of 1 (no positive or negative forcings), anthropogenic CO2 still causes up to 50% of the warming. With a climate sensitivity of 3 degrees C, humanity is the main driver of climate from about 1976 through present (see, e. g. , [13]). ==Paleoclimate== There is significant evidence from paleoclimate records that CO2 can have an effect on changes in the climate. Climate skeptic and paleoclimatologist Bob Carter emphasizes the importance of paleoclimate data. He compares climate to a piece of string. The current instrumental data (1850 – present) is a very short period of time, according to Carter. And when the ‘string’ is lengthened, we see a lot of climactic variability. Instead of looking at the recent past, we should look at the entire temperature record in order to get an accurate picture of the climate [14]. I will provide evidenc, contrary to what Carter believes, lengthening the string supports AGW theory. Over the Cenozoic Era, which began 66 million years ago, we see clear warming and cooling cycles caused by changes in CO2 concentrations. The sun increased slightly over that time period, whereas temperatures cooled. CO2, however, fell steadily through that time period. Plate tectonics was also accounted for. With natural forcings an unlikely cause, “CO2 was the dominant climate forcing in the Cenozoic” [15]. When you lengthen the ‘string’ over the course of the entire Phanerozoic (500 million years; begins at the Cambrian) there is still evidence of CO2 driving multiple climate changes. The GEOCARB study, published in 2001, is one of the definitive proxy records for CO2 concentrations within the last 500 million years. The study notes that “over the long term there is indeed a correlation between CO2 and paleotemperature” [16]. Evidence from the Vostok ice cores (specifically 240,000 before present), proves that CO2 has had an effect on temperatures. Although the initial forcing was orbital changes in the sun, CO2 “plays . . . a key role in amplifying the initial orbital forcing” and “the CO2 increase clearly precedes the Northern Hemi sphere deglaciation”. The study also notes that these forcings “are also at work for the present-day and future climate” [17]. There is also proof that it is warmer now than it has been since the MWP. In fact, it is warmer now than any year since 1400 AD [18]. Newer reconstructions continue to support this finding, and even extends the dataset. It finds that the Roman warm period (RWP) and the MWP as much cooler than today, and claims to have replicated previous ‘hockey stick’ studies [19]. The results are depicted below. Looking into the geologic past, we see strong evidence in favor of the idea that carbon dioxide can increase temperature. It logically follows that if humans continue to emit CO2, and other greenhouse gasses, that human emissions will increase temperatures. Conclusion: It is clear as to *why* a scientific consensus exists. There is overwhelming evidence that 1) climate sensitivity is likely high, 2) even if it wasn’t, anthropogenic forces still account for 50% or more of the warming (from CO2 alone, not counting methane and other greenhouse gases), and 3) that when you ‘lengthen’ the piece of string, there is still ample evidence ample evidence supporting the fact that CO2 can drive temperatures. The consensus itself does not prove AGW—but the evidence proves the consensus. CO2 is the main driver of recent temperature changes. Sources: . http://www.debate.org...