An extensive and documented reply is given at Quora from John Walker, former Laboratory Medical Director/Pathologist (1984-2011). Excerpts in italics with my bolds.(red text is link)

Perhaps you really mean “Why are there so many catastrophic anthropogenic CO2 global warming (CAGW) skeptics?”

There are very few individuals who are skeptical that the climate changes. But there are millions and millions of individuals (and growing), who are quite skeptical that human emissions of CO2 are causing apocalyptic global warming, including many scientists, climate scientists, Nobel Laureates, and other highly educated individuals.

The reason for this is multi-factorial and very voluminous. The following presents condensed summaries of 12 of the reasons that so many individuals have become highly skeptical of the theory of CAGW. Even though it is rather long, it represents only a small portion of the information, studies and references engendering skepticism of this unproven assemblage of hypotheses. Most of it is taken from my 250+ page treatise on the fallacies of the theory of CAGW.

1 . First and foremost is the fact that there is currently NO experimental evidence validating the theory of CAGW. Rather CAGW is a collection of unproven/unvalidated hypotheses, which can only be accepted by faith. However, most of these hypotheses have been shown to contain fallacies and/or misinformation.

2 . The “science” behind the theory of CAGW has not been sufficiently rigorous, non-biased, or open, and, crucially, does not comply with the tenets of the scientific method since it is not subject to potential falsification by testing/experiment.

3 . The theory of CAGW is based entirely upon:

a . Atmospheric CO2 versus temperature correlation studies (which are not proof of cause and effect, are partially based upon fictitious/manipulated/estimated temperature data [as in the “hockey stick” graph and altered NASA/NOAA/CRU data], and actually do not correlate all that well):

b . Partially altered, manipulated, selective, imprecise, incomplete, extrapolated and unverified/fictitious temperature data (as revealed by Climategate, the “Hockey Stick” confutation, and other sources), with frequent measurements selected from urban concrete and asphalt hot spots, naturally producing higher temps, which increase in number over time due to continued urban growth).

“Government reports, writers of opinion pieces, and bloggers posting graphs purporting to show rising or record air temperatures or ocean heat, are misleading you. This is not actual raw data. It is plots of data that have been ‘adjusted’ or ‘homogenized’ (i.e., manipulated) by scientists – or it is output from models that are based on assumptions, many of them incorrect. UK Meteorological Office researcher Chris Folland makes no apologies for this. ‘The data don’t matter. We’re not basing our recommendations [for reductions in carbon dioxide emissions] upon the data. We’re basing them upon the climate models’.” Climate: The Real ‘Worrisome Trend’ (Part I: Faulty Science) – Master Resource

c . Unreliable computer models (based upon partially altered, manipulated, selective, imprecise, incomplete, extrapolated and unverified/fictitious temperature data, woefully inadequate/incomplete input data regarding thousands of climate parameters, and “educated guesses” about the climate sensitivity to atmospheric CO2), which can be programmed to reveal whatever result the programmer desires, and many of which have already been proven incorrect or exaggerated.

d . Insufficient understanding of the role and relative magnitude/sensitivity of CO2 as a “greenhouse” gas, and the unproven (and many would say ludicrous) hypothesis that the earth’s atmosphere (with all its enormity, complexity, multiple layers, convection, layers of exceedingly cold air [as low as -60F] and even colder adjacent outer space [-455F] as well as extremely hot (high kinetic energy) upper layers, huge underlying oceans with complex currents and temperature fluctuations, varying molecular compositions, stratospheric ozone [which absorbs both UV and IR radiation], variable humidity, massive heat-absorbing evaporative processes, extensive cloud formations, variably intense winds, the jet stream, varying barometric pressures, cosmic ray effects, and NO glass ceiling or walls) functions identically to a glass-enclosed greenhouse. (Yes, that does seem rather ludicrous!)

5 . Promoters of the theory of CAGW falsely claim there is a 97% “consensus” among climate scientists that the theory is true. Indeed the 97% figure is false and based upon poorly contrived surveys/studies by CAGW promoters and “peer-reviewed” (i.e., “pal-reviewed”) by other CAGW promoters. If one reads the original papers where the 97% figure was contrived, it is quite easy to see how poorly designed and biased these surveys were. All of these surveys/studies have been debunked by multiple statistical analyses and better defined and controlled surveys and studies, revealing that less than half of climate scientists believe in the theory of CAGW.

“Claims that a ‘consensus’ exists among climate experts regarding the causes of the modest warming of the past century are contradicted by thousands of independent scientists.” – International Climate Science Coalition Core Principles

The fact is that tens of thousands of scientists, including climate scientists and many Nobel Laureates, do NOT accept the theory of CAGW:[Numerous examples are provided in linked article]

6 . Thus, in essence, CAGW promoters are demanding we accept their conclusions based upon consensus and faith (normally antithetical to most modern liberals’ thinking), just as theocrats and other religious fundamentalists argue. But the inability to follow the rigorous scientific method by the use of repeatable double blind, controlled experiments for validation does not justify acceptance of a theory without such experiments simply because they cannot be performed. It may be fine to accept beliefs by consensus or even by faith on personal or other matters which do not materially affect other people. But those pushing the theory of CAGW are demanding draconian changes affecting everyone on the planet, such as diverting tens of trillions of dollars from solving known existing existential problems (poverty, hunger, violence, war, infectious disease, cancer research, pollution and over-fishing of our oceans, lack of adequate sanitation, education and clean water, etc.) in order to “fight” an unproven future potentially existential problem with costly methods which have not been proven effective, replacing capitalism and democracy with global socialism and authoritarian one world government, and redistributing global wealth. Such actions would be premature, irresponsible, illogical, socialistic, cruel and lead to massive morbidity and mortality!

7 . In addition to the “97% consensus” falsehood, CAGW promoters and alarmists have promulgated many other lies, failed predictions (for both catastrophic global cooling and global warming) based upon their flawed computer models, abundant misinformation and disinformation. If the theory of CAGW is true, why the need to prevaricate? Anyone who is aware of this widespread sophistry must become skeptical of the theory. [Many examples are given in the linked article]

8 . Another clue that the theory of CAGW is fallacious is the fact that many promoters and alarmists so frequently resort to ad hominen attacks or demand that skeptics be banned from discussions. The former is another logical fallacy, which is used when the promoter has no real evidence to back up his/her claim and is unable to respond in a logical and respectful manner. They feel cornered because of their lack of intelligent retort. They hope that such attacks will make the skeptic afraid to make further comments.

Banning and refusing to hear/discuss information contrary to one’s dogmatic belief is characteristic of a fundamentalist who has been indoctrinated, often with propaganda. It is characteristic of religious fanaticism, not science. While it is prohibited under Quora policy, you will discover that some CAGW alarmist authors just can’t stop themselves from indulging in this fallacious and destructive tactic.

Again, this engenders more skepticism in their beliefs.

9 . Climategate. Climategate was a notorious event initiated by leaked emails in 2009 (with a second batch released in 2011) allegedly revealing the deceit and deception practiced by a prominent group of British (Climatic Research Unit or CRU) and American climate researchers (including Michael Mann of Penn State) who promote the theory of CAGW and supply much of the climate and temperature data and reports to the IPCC. The latter gives this group tremendous influence regarding the UN’s climate change agenda.

“There are three threads in particular in the leaked documents which have sent a shock wave through informed observers across the world. Perhaps the most obvious, as lucidly put together by Willis Eschenbach (see McIntyre’s blog Climate Audit and Anthony Watt’s blog Watts Up With That ), is the highly disturbing series of emails which show how Dr Jones and his colleagues have for years been discussing the devious tactics whereby they could avoid releasing their data to outsiders under freedom of information laws.

“But the question which inevitably arises from this systematic refusal to release their data is – what is it that these scientists seem so anxious to hide? The second and most shocking revelation of the leaked documents is how they show the scientists trying to manipulate data through their tortuous computer programmes, always to point in only the one desired direction – to lower past temperatures and to ‘adjust’ recent temperatures upwards, in order to convey the impression of an accelerated warming. This is what Mr McIntyre caught Dr Hansen doing with his GISS temperature record last year (after which Hansen was forced to revise his record), and two further shocking examples have now come to light from Australia and New Zealand.

“The third shocking revelation of these documents is the ruthless way in which these academics have been determined to silence any expert questioning of the findings they have arrived at by such dubious methods – not just by refusing to disclose their basic data but by discrediting and freezing out any scientific journal which dares to publish their critics’ work. It seems they are prepared to stop at nothing to stifle scientific debate in this way, not least by ensuring that no dissenting research should find its way into the pages of IPCC reports.”

10 . The IPCC, which is the primary authority driving the CAGW agenda is a political body, not a scientific body. It’s originating mission was to find human causes of climate change.

“It is to specifically find and report a human impact on climate, and thereby make a scientific case for the adoption of national and international policies that would supposedly reduce that impact.

[Thus, the IPCC has been directed to attribute the cause, or at least significant portions of the cause, of climate change to human influences. If it does not make claims of significant human influence, it’s function would be obviated and its members likely out of their UN jobs!]

“The IPCC is also designed to put political leaders and bureaucrats rather than scientists in control of the research project. It is a membership organization composed of governments, not scientists. The governments that created the IPCC fund it, staff it, select the scientists who get to participate, and revise and rewrite the reports after the scientists have concluded their work. Obviously, this is not how a real scientific organization operates.

11 . Much of the motive behind the promotion of the theory of CAGW is driven by money, power, and politics. Socialists, globalists and radical environmentalists are using the fear of CAGW to convince the world to replace capitalism with authoritative global socialism. The climate change industry now exceeds $1.5 trillion. If “cap-and-trade” legislation is ever passed in the US, as Al Gore, Goldman-Sachs, and other wealthy investors hope, they could potentially make $trillions via the buying and selling of carbon credits on a commodities exchange. Gore and Goldman tried desperately to get such legislation passed during the Obama administration. They were major investors in the Chicago Climate Exchange, which would have been the commodities exchange for carbon credits.

12 . There are better alternative theories regarding the mechanisms that drive the earth’s climate. Most of the theories derive from the observation that the earth’s climate goes through multiple, well-defined cycles (and cycles within cycles) of warming and cooling, and have done so for millennia. They generally involve various changes in total solar radiation reaching the earth and the adiabatic heating of the earth’s atmosphere due to atmospheric pressure. The theory of Cosmoclimatology is gaining credence among many climate scientists and astrophysicists.

Many other theories about the cause of climate change also involve solar influences. Think of the extreme temperature changes that are caused by changes in the amount of solar radiation the earth receives. Just the variation in the tilting of the earth leads to 4 seasons with temperatures varying from over 100 F to minus 20 F (or even colder) between summer and winter months in many locations. Temperatures increase dramatically just by moving towards the equator from higher or lower latitudes, due to differences in solar radiation. Day and night temperatures can easily differ by as much as 30 F or more, all in a 12 hour span. Temperatures on a sunny summer day can drop by 5 F in a matter of seconds if a cloud passes overhead. Compare that to the claimed increase in global temp of 1.4 F over 150 years supposedly caused by anthropogenic CO2.

In addition to the multiple periodic clusterings creating grand solar minima and maxima, there are multiple additional cyclic changes of solar activity, which are being elucidated with continuing climate research (another reason to stop making the absurd, counter-productive and pseudoscientific claim that the “science is settled”). There are centennial and bicentennial cycles of grand solar minima and maxima, along with many other cyclic processes of longer time intervals related to celestial changes:

There have been numerous glacial cycles, each lasting an average of 100,000 years. They coincide with the Milankovitch Eccentricity cycle of the earth’s orbit around the sun. Within each cycle is a period of marked global cooling (lasting from 70k to 90k years in which immense glaciers cover much of the land surface, and much of the ocean surface freezes. The cold periods are followed by interglacial (warm) periods lasting from 10k to 30k years. Some climatologists believe that the Earth is on the downward slope of the current interglacial period and headed towards the next ice age, which could arrive in the next several thousand years. During this downward slope, global temperatures are expected to slowly decrease with intermittent warmer and cooler trends. Milankovitch cycles – Wikipedia

And there is so much more but not enough time and space to present it all.