Everything lives or dies by this one sentence
A thesis statement is not a topic announcement. 'This essay is about climate change' is not a thesis — it's a subject. A thesis makes a claim: a specific, arguable position that reasonable people could disagree with. 'Climate change requires immediate policy response because economic inaction costs more than mitigation' — that's a thesis. It has a position, a reason, and it's arguable. Every paragraph serves this claim.