Scientific consensus occurs when the vast majority of the available evidence independently converges towards the same general conclusion within the margin of error and this typically convinces the vast majority of active research experts within the area. Contrary to popular belief, scientific consensus is not a popularity contest, but an assessment of published scientific evidence.
This does not mean that all scientists regardless of expertise agree on all possible details and aspects of an area, but that a particular model is so well-supported by the evidence that it would be very unreasonable to reject it. Scientific consensus forms over time as evidence is uncovered, analyzed and published in the scientific literature and general agreement about the facts emerge, often in association with high-quality systematic reviews, meta-analyses and reports by large scientific organizations.
No one can be an expert on all issues, so one has to start somewhere. Scientific consensus positions are not always right, but it is the most well-supported position right now and backed up by considerable amount of evidence. It is probably vastly more likely to be true that some ranting YouTube video or blog post filled with conspiracy theories. Sometimes, the scientific community is forced to change their consensus position based on new and emerging evidence. However, the former consensus position is disproved by smarter scientists who work with better and more accurate methods, not by ideological bloggers or shrieking radio hosts with an axe to grind.
When the scientific consensus changes, it does not change arbitrarily, but typically incorporate new discoveries into already existing models, slightly modifying them. This is because of the correspondence principle, which says that a new model has to agree with an older models in areas where they are both applicable. That is why Newtonian mechanics is still around even though models such as general relativity or quantum mechanics are more accurate outside everyday experience than Newtonian mechanics. These two more advanced models still make the same predictions as Newtonian mechanics in the areas where they both can be applied. New discoveries have been made in evolutionary biology in the late 1900s, but this did not completely replace basic evolutionary biology concepts such as common descent or natural selection, only providing a larger framework and more detailed explanations.
Scientists almost always disagree with each other on some issue or another. This is because there is always a debate about the fine details of a model, but this does not mean that the validity of the model itself is in doubt. The question should not be about whether or not there is disagreement, but whether or not the accumulated knowledge is enough to overturn the residual uncertainties and disagreement about details that always exists.
Proponents of pseudoscience typically appeal to certain “gotcha” claims where they trot out some supposedly unexplained phenomena and ask proponents of science to explain it and if they cannot, the pseudoscience proponents declare victory. Examples of this includes the evolution of the human eye, how jet fuel melts metal or abusing short-term weather trends against climate science. They fail to realize that scientists have already thought about these issues. In the end, we have to acknowledge that scientists can be mistaken, but what is more likely, that the community of scientists are wrong or that uninformed critics that lack even a basic understanding of the issues are mistaken?
Another common trick by proponents of pseudoscience is that they cite various protest lists signed by scientists that reject the consensus position in e. g. evolution, climate, HIV as the cause of AIDS or the collapse of the twin towers. However, these researchers are typically either not scientists or not scientists who work in a relevant field. The number of scientists they have gathered also has to be weighed against the number of scientists who support the consensus position, which is many orders of magnitude larger. This is, however, conveniently ignored by anti-science activists. In fact, this technique is so common that it has been given the name of “magnified minority”.
When proponents of pseudoscience cannot lift their own ideology to the position of science, they attempt to denigrate science to the level of irrational ideology. This is done by falsely calling science a religion or falsely claiming that scientific consensus is just an appeal to authority or popularity. This is wrong, since the consensus is just a proxy for the position that is most well-supported by the evidence. It is this evidence that is being appealed to when science defenders appeal to consensus, not authority or popularity. Another tactic is to accuse all scientists in a particular field of being paid or bought by some corporation. This line of argument does not and cannot work, since such vast conspiracies would never be able to sustain itself without leaks.
So how do you learn what the consensus position is or find out more information about it? As stated previously, no one can be an expert on all subjects, but we can identify reliable experts that provide summaries of the current state of some scientific field or issue. This can be done by reading summaries provided by e. g. the Centers for Disease Control and Prevention (CDC), National Institute for Standards and Technology (NIST), National Aeronautics and Space Administration (NASA) and similar sources. These are not always right, but they are decidedly more often right than random bloggers or social media posts. It might be tempting to just do a few searches in article databases such as PubMed, but it is way too easy to search millions and millions of papers and find something that superficially appears to support one’s position. Instead, one has to look deeper and more broader to get reliable knowledge, and scientific and medical organizations can provide that level of knowledge.
More seasoned defenders of science might read several systematic reviews in high-impact journals and then write down their own summaries of scientific questions, experimental and statistical methods, basic results as well as strengths and limitations of specific research designs. This is a wonderful way to gain reliable knowledge from the scientific literature on issues from vaccine safety to climate change, but it require a deeper understanding of the scientific area as well as stronger ability for critical thinking, since nonsense occasionally gets published in the scientific literature.
The general idea is that when something gets published in the scientific literature, it is the time that the discussion begins, not ends. Once a scientific consensus develops, it is time to accept the general picture that science provides and continue researching the details to expand our realm of knowledge.
Scientific consensus positions are often seen as looming threats to different pseudoscientific movements. That is why e. g. climate deniers and anti-GMO activists spend so much time and money attacking mainstream climate science and vaccine research. If they can spread fear, uncertainty and doubt, they think they will be able to undermine the science. They could not be more wrong. By pointing out the difference between debates about some particular detail of a scientific explanation and debates about the entire model, one can more easily counter deceptive strategies by the merchants of doubt.