The real question is whether there will be more biomass and more photosynthesis in response to more CO2, not whether individual trees grow more. The climate skeptics don't say tree trunks will get wider, they say there will be more plant biomass.
And also this is adding in confounding variables and saying more CO2 could change precipitation in certain places, and thus cause lower growth. Other studies that show more growth with more CO2 are using greenhouses to control for confounding variables.
And even if you are trying to answer the question in terms of a holistic analysis, you can't just study one forest in Canada and then make any kind of conclusion.
You'd need to study trees from most of the major forests, and look at the density of the foliage in each.
Speculating that plant biomass might help mitigate global warming doesn't really deserve the label "climate skeptic." It's already confirmed that rising CO2 affects crop growth rates.
For instance, the Saharan desert seems to be greening.
I hadn't heard about that, and my first few Googles came back without any good, definitive articles; do you have more information?
The study is about sub saharan Africa, mostly the African savanna, and the identified drivers are human causes (decline in burned area, changes in herbivores density). They explicitly exclude CO2 levels increase as a driver.
> CO2 fertilization effects explain most of the greening trends in the tropics
It's unrelated to the specific positives and negatives.
Not so much in reasonably well-run parks or woodlands.
The problem is exacerbated by the fact that special interest groups fund these misleading studies and are widely propagated by media with a particular interest.
And it's been proven to be true many times.
I guess what the study is trying to address is the positive aspect of CO2 rising enough to counter the negative aspect of CO2 rising to the plant biomass.
( disclaimer I believe in global warming with all my heart)
On top of that, plant growth can be reduced by heat stress. They also have to contend with insects and disease, and may not have good defenses when climate change has brought invasive organisms to the area.
"climate change is generating warmer, drier conditions that could make them grow less in many places"
Overall, conditions are definitely getting warmer on average, and many places are also getting drier, sometimes with occasional very heavy rainfalls that wash away topsoil.
Another factor I didn't mention is forest fires, brought about by drier conditions, as we've been seeing in California.
It's important, because the fertilizer for anti-science attitudes exploited by groups like climate denialists is a false impression of radical flip-flops in science conveyed by popular media which uses exaggerated contradiction to overplay the impact of stories and draw eyeballs.
This is a subject of interest to scientists, but not to the general public, because nothing being studied here changes the central message in the slightest, which is that we are undoubtedly headed for climate apocalypse and we are accelerating into it rather than decelerating.
Even the one neat CO2 trick I remember was really a hack to preserve water: some desert plants have a mechanism to store a day’s worth of CO2, allowing them to close pores and prevent evaporation.
Anyway, if you’re looking for a counterexample, I’d switch environments altogether and try again in bacteria, algae, and maybe grasses. While I doubt there would be much of a difference to trees, the first two are at least a few degrees further away from them in the deity’s org chart. All three categories also strike me as possibly being more relevant than trees in terms of total biomass, although that’s just a hunch that may be completely wrong.
If it's "as close to the nature right before apes became humans", it's easy to answer (though hard to justify on any grounds other than quasi-religous belief in Gaia or something). If it's "planet with more plants and animals" it could be quite different, and any honest scientist would answer it "we don't really know, but here are some considerations..."
I once ran the numbers for O2, which is proportional, and it came out to roughly 10m of high in the atthmosphere. I. e. climb a tree in the countryside and you’re at O2 pressure similar to a city.
This is not surprising, as we already know trees and plants consume CO2 to produce cellulose and other plant materials.
This is heavily exploited in greenhouses which are CO2 enrichened to produce better yields - albeit used mostly for growing vegetables, not trees.