[0]:https://www.scientificamerican.com/article/does-global-warmi... [1]:https://journals.plos.org/plosone/article?id=10.1371/journal...
There is roughly a 1% decrease in nutrient density for every 6.25ppm increase of CO₂ in the atmosphere. From 1945 to 1985 atmospheric CO₂ increased by about 35ppm, so there should be about a 5% decrease in nutrient density. According to the meta-study, rats increased their calorie intake by roughly 6%.
¹ Caveat: Allison is a notorious "PR scientist" whose career has been made out of helping the fast- and processed- food industries deflect any blame or accountability for the health and safety of their "products." The hypothesis of the paper is that the obesity epidemic is being caused by viruses and pollutants (without offering any evidence), and has nothing to do with McDonald's. This "viruses and pollutants" fairy tale can then be used as a deflection in fast-food PR and expert testimony in class-action lawsuits and government investigations.
Is there any evidence of wild animals living far from humans getting fat as well?
If the latter are not affected, it would be reasonable to think that the former are fat because they are exposed to human junk food.
Junk food itself is a natural consequence of a food sector that has to grow in order to please its investors. Sell as much food as possible to whoever will buy it.
It can't shove food down people's mouth, but it can produce food that's deceptively tasty and manipulate our instincts.
There is a paper from 2010 about obesity in marmots: https://www.nature.com/news/2010/100721/full/news.2010.366.h...
"Feral rats. Our sample consisted of 6115 (2886 males, 3229 females) wild Norway rats (Rattus norvegicus) that were captured in the central alleys of high-density residential neighbourhoods using single-capture live traps, while rural rat populations were sampled from parklands and agricultural areas in areas surrounding the city [12,13], between the years 1948 and 2006."
I can't cross-check the methodological validity since neither articles are available online (the second one appears to be, but if you follow the links you end up with a n unrelated 1975 article on beach voles :-). No info is given on the distance between the traps and human habitations. Also, no idea on how comparable the sample from 1949 is with the one from 1989. They were not written by the same authors, and the 1989 authors don't cite the 1949 paper (https://scholar.google.com/scholar?cites=1484756086713821658...).
Had they compared rodent populations from places that the obesity epidemics has yet to reach (read: poor countries), it would be more convincing.
For marmots, the proposed hypothesis was that the longer summers let them eat more food and shortens the hibernation period during which they usually lose weight.
If you want to know whether atmospheric CO2 has an impact on rodent weight, you can devise a controlled experiment where they live in, and are fed with plants grown in controlled atmospheres. No idea if such a study has been performed.