I'm having trouble finding the primary source for the 39% figure. Medium doesn't directly link it. They link to this source when they quote the 39% figure but that source doesn't have the figure.
I skimmed over the sources linked in that source and still couldn't find the figure.
I'm just wanting the original study. Sharing your sources isn't this hard.
Edit: I found a study that has a similar figure, but it required independent searching. Medium needs to do better with their citations.
Climate anxiety in children and young people and their beliefs about government responses to climate change: a global survey
Hickman, Caroline et al.
The Lancet Planetary Health, Volume 5, Issue 12, e863 - e873
https://www.thelancet.com/journals/lanplh/article/PIIS2542-5196(21)00278-3/fulltext
Here is the table where subjects were asked: “Does climate change make you feel any of the following?”
"I am hesitant to have children" has a collective response of 39.1%, and it's not the most popular response compared to the other negative feelings listed.