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The world needs to come together as one and tackle the climate change issue.
Literal
World [subject-が] one-body-becoming climate-change [genitive-の] issue [to-に] tackle [genitive-の] necessity [subject-が] exists.
An editorial/advocacy sentence. 一丸となる ('to become as one,' 'to come together as a unified whole') is a set idiom: 一丸 literally 'one lump/mass,' + となる ('become'). The nuance is of collective effort joining forces against a common challenge — common in political appeals and group mobilization rhetoric. ~に取り組む ('to tackle X,' 'to engage with X,' 'to address X') takes the issue being addressed with に, not を — the target is framed as the destination of the effort. 必要がある ('there is a need to') is the standard polite expression of necessity.