The time of the year when I note the Geneva Conventions already does address the environment:
Methods or means of warfare that are intended or may be expected to case widespread, long-term and severe damage to the environment are prohibited.
- (Protocol I, Art. 35, Sec. 3)
Attacks against crops, livestock and agricultural areas needed for food production for the civilian population are prohibited. [...] Attacks against civilian drinking water installation and irrigation works are prohibited.
- ( Protocol I, Art. 54 , Sec. 2)
(Quotes from genevaconventions.org)
But should the convention be stricter? Should it be more often enforced? Yes.