Essays / Thesis statements on global warming? [26]

How to write a thesis statement about global warming

I have to write a research paper about global warming. The topic is "United states posture on global warming: past, present, future"
However, I'm having trouble setting up a thesis statement on that topic. Any suggesstions or ideas?

Essays / Thesis statements on global warming? [26]

Do you know how to put all 3 time periods in one sentence? That seems a bit complicated to write a thesis statement for all 3 of them.