The Rome Statute is the governing document which sets out the existing international Crimes Against Peace. Ecocide was to be included in earlier drafts, until it was removed in 1996. You can read about the history of the law of Ecocide, which dates back to 1972 in The Human Rights Consortium research paper, called Ecocide is the missing 5th Crime Against Peace.
The proposed amendment to the Rome Statute was submitted to the United Nations in March 2010. A full legal reasoning for a law of Ecocide and the definition was laid out – you can read what was submitted in Chapters 5 & 6 of Polly Higgins’ first book, Eradicating Ecocide. Polly Higgins proposed as an amendment to the Rome Statute that Ecocide be legally defined as
the extensive damage to, destruction of or loss of ecosystem(s) of a given territory, whether by human agency or by other causes, to such an extent that peaceful enjoyment by the inhabitants of that territory has been or will be severely diminished.
By amending the Rome Statute, many countries can become signatories. There are currently 122 countries that are signatories to the Rome Statute. Any amendment to the Rome Statute requires the support of a two-thirds majority of the states parties (which now counts as 82 state parties).