We do not need a global state because it would inevitably lead to centralised authoritarian control over the mass population which would become a collective instead of individuals. This would lead to a system similar to communism because the masses would be equal as slaves while the elite do what they want. Who knows what kind of strict laws could be implemented under the guise of 'protecting the planet'. Climate change is a big problem but the best way to tackle it is to create awareness amongst everyone so we all individually do our part instead of being forced and controlled.
We do not need global government, Because capitalists are incompetent at leading us and slowing climate change. What we really need is socialist world of cooperative economies with economic planning and anarchist idea of mutual aid. Capitalism, By its very nature, Cannot tackle or stop climate change, No matter how green their intentions are. Under socialism products would be produced for direct use rather than a commodity.