A letter of opinion is not a study

As the Rumour Detector explains, an opinion, even when published by a scientist, does not have the same value as a study conducted by a scientist.

This is because in science, data, unlike opinions, can be verified by other scientists, thereby permitting the conclusions of a study to be confirmed or invalidated.

A two-step verification

Before agreeing to an opinion expressed in a letter of this kind, the Rumour Detector proposes two quick, simple steps.

The first step is to analyze the content of the letter. You don't need to be a science expert to do this: you only have to ask yourself whether the statements made in the letter are based not on opinions, but on verifiable data published in studies that, ideally, are referenced in the letter itself.

The second step is to check whether the scientists who signed the letter are experts in the field. This step is particularly easy in the case of academics who have a web page on their institution's website. Another advantage of studies is that, in addition to the data they provide, they are usually signed by experts in the field. In the September opinion letter, however, less than 3%  of the signatories had  expertise in the fields of climatology or meteorology.

If, after this preliminary examination, you are still not convinced of the validity of the statements presented in the letter, you may need to take a closer look at its content. This is what the Climate Feedback website did: it pointed out, for example, that the letter claims that climate models ignore the benefits that an increased amount of CO2 would have on plant growth, which is not true. "Climate models do include the carbon cycle (...), and these include the effects of increased atmospheric CO2 on vegetation," Climate Feedback says. The letter also suggests that "very few studies" claim that the current warming is caused by humans, which is also false: "Most papers on climate change do not state that recent warming is chiefly anthropogenic because the anthropogenic driver is noncontroversial."

500 compared to how many scientists??

Incidentally, what do 500 scientists represent out of all the people who can claim the title of "scientist"? It should be remembered that this term is not a title reserved for members of a professional order, such as doctors, lawyers or notaries. Anyone working in research, health or engineering can call themselves a scientist.

According to the U.S. Bureau of Labor, there were about 750,000 practicing “physicians and surgeons, 21,600 “physicists and astronomers,” 40,000 biologists and microbiologists, 31,000 geologists, 84,000 chemists and 1.6 million engineers in the United States in 2018.

And these totals only represent those who were working in 2018: to arrive at a more precise order of magnitude for the number of "scientists," we would have to add to these figures millions of retired scientists, since a large number of the signatories to the September letter are retired. Note that these figures apply only to the United States, whereas the September list is international.

Finally, it is relatively easy to find people with a scientific background who defend some surprising ideas. For example, in 2001, the Discovery Institute, an American creationist group that rejects the theory of evolution, published a letter signed by "100 scientists who contest Darwinism" (since then, a few hundred other names have been added to the list). In response, the National Center for Science Education launched a petition, signed by "1400 scientists named Steve who support the theory of evolution.”