Good work speaking up and spreading the word about global warming.
People who "reject" or "deny" the science of this are being absurd, and I believe it's therefore pointless to argue with them. You're fighting from a losing position.
It seems to me that scientists should present the case in terms of "here's how much real estate we're going to lose" and "here's which farming areas will cease to produce" and get down to brass tacks on that.
If you start saying that best case, or if things slow down a little bit, we're still going to lose the New York Financial District, most of Florida, part of LA, etc., and if things keep going as-is we lose everything up to Washington Square Park (or all of Manhattan), all of Florida, San Francisco becomes an archipelago, etc.
Artists' renderings would be good, too.
You could even send mailings and announcements to the people whose homes and businesses will be terminated by climate change.
If the politicians keep denying, at last now they're denying about details, and it starts to be more about arithmetic and less about predicting the result of processes. If politicians and pundits start having to say 2 + 2 = 5, some people might start questioning them.
Also, if politicians know that in the next CERTAIN number of decades their districts are going to be under water or unfarmable, their constituents are going to start asking some serious questions.
Further, scientists should make PLANS for how to deal with this, e.g., where should the financial center of the United States and the world be relocated to? It could be like choosing the next Olympics. Would it even be the United States? Where will we get our food? Which new roads will we have to build to get around the inundated areas? Where will all of the displaced populations have to be relocated to? On what schedule? Which of our military resources, bases, ports, etc., will have to be closed and/or moved?
Shift the debate. Shift it away from whether it's happening, and TO whether we're going to build dikes and levees or relocate people, and to where, which trade-offs, which sacrifices. Politicians can talk about that stuff.