How to lower pandoc verbosity using env. var (when it is invoked via nbconvert)?

Question

The --atx-headers command is deprecated (in favor of --markdown-headings=atx), as pandoc noisily reminds us, displaying this warning almost 40 times in a row:

[WARNING] Deprecated: --atx-headers. Use --markdown-headings=atx instead.
[WARNING] Deprecated: --atx-headers. Use --markdown-headings=atx instead.
[..]
[WARNING] Deprecated: --atx-headers. Use --markdown-headings=atx instead.
[WARNING] Deprecated: --atx-headers. Use --markdown-headings=atx instead.

Unfortunately the old option is still used by jupyter nbconvert --to asciidoc for invoking pandoc.

So how to change pandoc output verbosity to "ERROR" (that will suppress this and other warnings) while invoking nbconvert, e.g. by using env. variables (the only method that comes to mind in this situation)? The docs indicate that pandoc seems to lack such an option.