I agree with the gist of your argument, but I wouldn't phrase it as "totally solvable". Limiting it to 2 degrees is possible, limiting it to 3 is likely. Both are bad, both are locally catastrophic but neither are globally catastrophic.

I like my phrasing "millions will die". That works on numerate people, but most people can't really conceptualize the difference between a million and a billion. Any other suggestions?