Had the global community taken more decisive action against climate change decades ago, we might have avoided the current environmental crisis, which【, as most scientists now point out,】 is posing a significant threat to our biodiversity.

【　】の部分は挿入されているだけなので削除してみます
Had the global community taken more decisive action against climate change decades ago, we might have avoided the current environmental crisis, which is posing a significant threat to our biodiversity.

これで少しは英文の骨格が少しははっきりしませんか?