The phrase "serious threat" means that something or someone poses a significant danger or risk that should be taken seriously. It implies that there is a potential harm or negative consequence that needs to be addressed or prevented. Full definition
Climate change increasingly poses serious threats to the stability of our economic system. (wemeanbusinesscoalition.org)
Climate change is one of the most serious threats facing global society. (climatecare.org)
Worse, bugs and unnoticed security flaws, even ones that may have once seemed trivial, can be serious threats in the Net - connected era. (entrepreneur.com)